HUNTRIESER ET AL.: Airborne EULINOX measurements
Contribution of lightning-produced NOX to the European
and global NOX budget:
Results and estimates from airborne EULINOX
measurements
H. Huntrieser1, Ch. Feigl1, H. Schlager1, F. Schröder1, Ch. Gerbig2, P. van
Velthoven3, F. Flatøy4, C. Théry5, H. Höller1, and U. Schumann1
1
Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt,
Oberpfaffenhofen, Germany
2
Harvard University, Department of Earth and Planetary Science, Cambridge, MA,
USA
3
KNMI, Section of Atmospheric Composition, De Bilt, Netherlands
4
NILU, Kjeller, Norway
5
DMPH/EAG, ONERA, Atmospheric Environment Research Sec., Chatillon Cedex,
France
Abstract. Airborne chemical in situ measurements were performed in thunderstorms during
the field experiment EULINOX in July 1998. NOX (=NO+NO2) enhancements observed in the
upper troposphere, which were caused by lightning or/and convective transport of polluted
boundary layer air, were investigated in detail. Tracers for boundary layer air (CO2, CO, and
O3) were used to determine the fraction of lightning-produced NOX and convective
transported NOX in thunderstorms. The amount of lightning-produced NOX in comparison to
aircraft emissions in the upper troposphere was estimated on the European and global scale.
1. Introduction and Objectives
Nitrogen oxides released into the upper troposphere have a strong impact on the
chemistry in the atmosphere. Primarily this is due to the lower temperatures and the longer
lifetime of NOX at these altitudes in comparison to the conditions in the boundary layer.
Lifetimes of about 5 days can be expected in summer at midlatitudes in the upper troposphere
[Jaeglé et al., 1998]. During this time NOX acts as an important catalyst for the production of
ozone. In addition the abundance of the hydroxyl radical (OH) in the upper troposphere is
controlled by NOX [Liu, 1977] and thus the oxidation capacity of the atmosphere. Since all
this affects the abundance of important green house gases, the NOX concentration in the upper
troposphere is one of the factors controlling our climate.
The production of NOX by lightning is considered to be the most important NOX source
in the upper troposphere (see Table 1). However, the uncertainties are still large (2-20 Tg(N)
yr-1) since validations by in situ measurements are difficult to perform. Even less knowledge
exists about the (probably) second most important NOX source in the upper troposphere, the
upward transport of polluted planetary boundary layer (PBL) air. This transport is controlled
by the strong convection in the thunderstorm inflow area and by frontal lifting. In less than
one hour a thunderstorm can transport PBL air into the upper troposphere [Skamarock et al.,
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EULINOX, 1998-1999 - FINAL REPORT
1999]. This source is most important when thunderstorms move over urban areas. Over the
oceans this contribution can be neglected [Corbett et al., 1999]. To separate the mixture of
lightning-produced NOX and PBL-transported NOX in thunderstorm anvils, tracers for
boundary layer air like O3, CO and CO2 have been used in earlier studies [Dickerson et al.,
1987; Huntrieser et al., 1998; Dye et al., 1999].
Table 1.
Summary of Estimates for NOX Sources, Uncertainties, and Emission
Locations (after Lee et al., 1997)
Source
Emission
Uncertainty range
Principal location of emissions
-1
-1
Tg(N) yr
Tg(N) yr
Fossil fuel
22
13-31
Northern hemisphere midcombustion
Latitude continental surface
Biomass burning
7.9
3-15
Tropical continental surface
Soil microbial
7.0
4-12
Non-polar continental surface
production
Lightning
5.0
2-20
Tropical continent, Troposphere
Stratospheric
0.64
0.4-1
Stratosphere
decomposition of
N2O
Ammonia oxidation
0.9
0.6
Tropical continental surface
Aircraft
0.56
Northern hemisphere, Latitudes
30-60°N
Total
44
23-81
From the turbulent mixing in thunderstorms it follows that a mixture of lightningproduced NOX and NOX transported from the PBL can be observed as a NOX enhancement in
the thunderstorm anvil. During most anvil penetrations (reported up to now in the literature) a
rather smooth NOX enhancement (< 2 ppbv) has been observed. A survey of estimates of
lightning-produced NOX from field measurements carried out in the last 20 years has been
published recently by Huntrieser et al. [1998] (see Table 2 in this reference). Sometimes
narrow NO spikes (~4 ppbv) have been observed in the anvil region [Dickerson et al., 1987;
Luke et al., 1992; Stenchikov et al., 1996; and Huntrieser et al., 1998]. The spiky NO structure
has been attributed to the production by recent flashes [for example by Höller et al., 1999].
Recently, a large spike of 19 ppbv NO was measured in a thundercloud anvil during the field
experiment STERAO in Colorado [Dye et al., 1999; Stith et al., 1999].
To reduce of some of the uncertainties associated with lightning-produced NOX the
field campaign EULINOX („European lightning nitrogen oxides project“) was conducted in
July 1998. The experiment was performed at and coordinated by DLR (Deutsches Zentrum für
Luft- und Raumfahrt) Oberpfaffenhofen located near Munich in southern Germany. One of
the main objectives of EULINOX was to estimate the importance of lightning-produced NOX
in comparison to other NOX sources in the atmosphere. In this paper the lightning-NOX source
is quantified on the European and global scale based on thundercloud observations.
During the whole experiment (8 flights) about 10 different thunderstorms were
penetrated by the DLR research aircraft Falcon. In Section 2 the complex instrumentation,
which was used to study the chemistry and electrification in the thunderstorms, is described
briefly. The results of the in situ trace gas measurements are described in Section 3. An
estimate of the importance of lightning-produced NOX for the European and global nitrogen
budget using different analysis of the observed NOX enhancement in thunderclouds is given in
Section 4. Important findings of EULINOX are summarized in Section 5.
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HUNTRIESER ET AL.: Airborne EULINOX measurements
2. Work Performed
2.1. Instrumentation
The airborne measurements during EULINOX were performed with the DLR Falcon,
operating mainly in the upper troposphere, and a DO 228 for boundary layer observations.
This paper focus on the measurements from the Falcon since they cover all altitudes. Some
measurements from the ozone and NO instrument on the DO 228 are included in the vertical
profiles presented in this paper. Since the O3 and NO instruments on the Falcon mainly
measured above the boundary layer the DO 228 measurements have been added to give a
complete picture of the vertical distribution for these species.
The chemical and particle instrumentation is presented in Table 2. For a more precise
description of the instrumentation, which has been used in several previous field experiments,
see publications by Gerbig et al. [1996], Volz-Thomas et al. [1996], Schlager et al. [1997],
Schulte et al. [1997], Feigl [1998], Schröder [1998], and Ziereis et al. [1999]. Position,
altitude, temperature, humidity, pressure, and wind (u,v,w) were measured with the standard
Falcon Meteorological Measurement System.
Table 2.
Falcon Instrumentation Provided by DLR and FZJ
Species Technique
Detection Sampling
Limit
Rate
NO
Chemiluminescence (CL) 3 ppt
1s
NO2
photolytic converter + CL 15 ppt
5s
NOy
Au-Converter + CL
50 ppt
1s
O3
UV-Absorption
1 ppb
5s
CO
VUV-Fluorescence
2 ppb
3s
CO2
NDIR
1 ppm
1s
J(NO2) Filterradiometry
1E-4 s-1
1s
CN
1s
CNC (size 10 nm - 1 m) 10 nm
Accuracy
Group
5% (for 0.2 ppb)
10% (for 0.2 ppb)
15% (for 0.2 ppb)
5%
10%
5%
6%
-
DLR
DLR
DLR
DLR
FZJ
DLR
DLR
DLR
The total lightning activity (100 km around the DLR operational center), in terms of
intra-cloud (IC) and cloud-to-ground (CG) flashes, was registered by a measuring system
provided by ONERA. Two remote VHF interferometric stations measured the direction of
incoming VHF signal from lightning flashes at 114 MHz, with a 1 MHz bandwidth. The
antenna were located 50 km apart on a roughly northeast-southwest baseline, and they were
nearly equidistant from the DLR operational center at Oberpfaffenhoffen (OP), about 30 km
west of Munich (48°N, 11°E). A more detailed description of the interferometer can be found
in the ONERA contribution of this issue [Théry et al.]. In this study we also use observations
from the German lightning network LPATS (for description see Finke et al. this issue) which
registers mainly CG lightning.
2.2. Flight Experiment
During the EULINOX period from June, 29, to July, 26, measuring flights were
performed on 8 days. The single aircraft missions are briefly described in the Introduction
Section of this issue [Höller et al.]. Overall about 10 different thunderstorms were penetrated,
some of them several times. The thunderstorm cells had different characteristics. Some of
them were isolated, some of them imbedded in larger convective clusters or cold fronts. The
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EULINOX, 1998-1999 - FINAL REPORT
convective clouds were penetrated in different stages of development including the growing
stage characterized by a cumulus congestus cloud. In this stage almost no or very little
lightning is occurring (if lightning then only IC) and no anvil is visible. In the mature stage,
the so called cumulonimbus stage, an anvil develop and the lightning frequency usually
increases. However, depending on cloud top height almost no or a lot of lightning can be
observed. In the dissipating stage the distinct structure of the cloud edges disappears and the
cloud spreads out in the upper troposphere in a shape of a wide, thin cirrus shield and the
lower part of the cloud evaporates more and more after the rainout process. During the
missions all kind of convective clouds described above were penetrated. Observations for a
large variety of this type of clouds help to distinguish between NOX produced by lightning in
the cloud and NOX transported upward from the PBL by the convection. Furthermore, the age
of produced and/or transported NOX in thunderclouds can be roughly estimated. Therefore, we
analyzed in this study all cloud penetrations during EULINOX to get a broad spectrum of
conditions in convective clouds and we focused on the change in trace gas and particle
concentrations.
Especially, we were interested in the upper tropospheric NOX distribution in different
kind of clouds as described above. The measurements were separated according to events
observed on different atmospheric scales and divided into synoptic-scale signatures (> 100
km, convective cold fronts and thundercloud remnants in the shape of cirrus clouds), mesoscale signatures (1-100 km, convective clouds with and without lightning) and micro-scale
signatures (< 1 km, near lightning channels). Depending on the atmospheric scale different
NOX concentrations were measured with the highest concentrations on the micro-scale (fresh
NOX) and the lowest on the synoptic-scale as expected from the age of the NOX in the sample.
3. Observed NOX Signatures on Different Atmospheric Scales
3.1. Synoptic-scale NOX signatures
During the first EULINOX mission on July 1st, 1998, we performed a flight to the west
to probe an area close to Basel (Switzerland). The forward trajectories from KMNI predicted
that in this area we would find advected cloud remnants of a large thunderstorm cluster that
was active the day before over Spain and France. The main part of the flight was performed at
the tropopause and in the lower most stratosphere since it actually was a flight to test the
instrumentation for different ambient conditions. However, during the ascent into the
stratosphere we passed through an area (between 9 and 10 km) with enhanced NO and NOX
concentrations. In Figure 1 the vertical NOX distribution measured during 7 EULINOX flights
is shown in different colors. Here only „background“ concentrations are shown since all single
cloud penetrations have been excluded (this was done for all vertical profiles presented in this
paper). The thick black line indicate the mean vertical NOX distribution for the whole period
of EULINOX. The typical C-shape of the vertical NOX distribution can be noticed (high
concentrations in the boundary layer and increasing concentrations above the tropopause).
However, on some of the EULINOX days (green and blue lines) the concentration in the
upper troposphere is larger than expected from the typical C-shape profile. The question is
where does this additional NOX come from? Let us try to explain what caused the NOX
increase on July, 1st (blue line) since we started to discuss this day already.
4
HUNTRIESER ET AL.: Airborne EULINOX measurements
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Figure 1. Observed vertical NOX distribution (in ppbv) on single EULINOX mission days in
different colors (single cloud penetrations excluded). The black thick line is the mean vertical
NOX profile determined by averaging the NOX data from all EULINOX mission days.
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b)
Figure 2a-b. Vertical CO distribution (in ppbv) on July, 1 (a). The black line in (b) is the
mean vertical CO profile determined by averaging the CO data from a part of the EULINOX
mission days (July, 1, 3, 7, and 20). The horizontal bars indicate the standard deviation. The
red line shows the exceptional distribution on July, 21 (extremely polluted boundary layer).
The other trace gases like CO, CO2, and O3 can be used to find out more about the
unknown NOX source. Figure 2a shows the vertical CO distribution on July, 1st. The profile is
not complete since data are missing in the boundary layer. However, a CO increase is visible
between 9 and 10 km. The normal CO distribution decreases with altitude (like in the left
profile in Figure 2b). This indicates that the CO increase observed between 9 and 10 km must
be due to air transported from lower altitudes. The vertical profile in Figure 2a indicate that
the air must have its origin below 4 km. The most probably explanation for the upper
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EULINOX, 1998-1999 - FINAL REPORT
tropospheric CO increase is convective transport of lower tropospheric air. Similar conditions
were observed on July, 21st, (the red profile in Figure 2b). However, on this day very high CO
concentrations were measured in the boundary layer indicating strong pollution. We will
discuss the special conditions on this day later in the paper.
In Figure 3a-b vertical CO2 distributions are shown. On July, 1st, a small CO2 decrease
is visible between 9 and 10 km (Figure 3a), which again can be explained by convective
transport. The typical vertical profile of CO2 in the troposphere in summer (Figure 3b, left and
middle profile) shows an opposite distribution (increasing concentration with increasing
altitude) in comparison to CO shown before. However, above the tropopause the CO2
concentration also decreases with altitude. The left profile in Figure 3b is the average CO2
distribution with standard deviation during the field experiment LINOX which was conducted
in 1996 [Huntrieser et al., 1996]. In comparison the mean EULINOX profile shows a very
similar shape except that the CO2 level has increased by 5-10 ppmv due to the permanent
increase of anthropogenic CO2 emissions. The overall CO2 distribution is very pronounced
with distinct differences in concentration between the boundary layer and the free troposphere
(even if the standard deviation is included) which can be used to identify convective
transported air masses from the boundary layer into the upper troposphere. The explanation
for the low concentrations in the boundary layer is the CO2 uptake by vegetation
(photosynthese) which is active especially in the summer season, and at noon and in the
afternoon when the flights were performed [Figure 7 in Denning et al., 1996]. As a
comparison the vertical profile from July, 21st, is shown (Figure 3b, right profile). This day
was very unusually with no CO2 minimum in the boundary layer. Instead a huge maximum is
visible which indicate that the air was strongly polluted (as discussed above for CO).
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355
360
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380 345
350
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370
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CO2 (ppmv)
a)
b)
Figure 3a-b. Vertical CO2 distribution on July, 1 (a). The black line with black dots in (b) is
the mean vertical CO2 profile estimated by averaging the CO2 data from all EULINOX
mission days (except July, 21). The black line with white dots in (b) is the mean vertical CO2
profile determined by averaging the CO2 data from all LINOX mission days (field experiment
1996). The horizontal bars indicate the standard deviation. The red line shows the exceptional
distribution on July, 21 (extremely polluted boundary layer).
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HUNTRIESER ET AL.: Airborne EULINOX measurements
Finally, in Figure 4a and b the vertical O3 distribution is shown. On July, 1st, a small
O3 decrease is visible between 9 and 10 km (Figure 4a), which again can be explained by
convective transport. The mean EULINOX vertical O3 profile in the troposphere (Figure 4b)
shows a similar increasing tendency with altitude as in the case for CO2. In Figure 4b ozone
measurements from the second aircraft (DO 228) have been added to give a more complete
picture of the O3 distribution.
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O3 (ppbv)
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50
75
100
125
150
O3 (ppbv)
a)
b)
Figure 4a-b. Vertical O3 distribution (in ppbv) on July, 1 (a). The black line with white dots
in (b) is the mean vertical O3 profile estimated by averaging the O3 data from all EULINOX
mission days (Falcon measurements). The black line with black dots in (b) is the mean vertical
O3 profile determined by averaging the O3 data from all EULINOX mission days (DO 228
measurements). The horizontal bars indicate the standard deviation.
In order to explain where and how the expected convective transport took place and to
explain the trace gas signatures in 9-10 km we used 5-days backward trajectories calculated by
KNMI (Figure 5). The trajectories are tightly bounded when we follow them one day back
which indicates that the origin area of the air mass can be identified easily. 24 h before the air
sampled on the July, 1, flight originated from eastern Spain. The vertical motion of the
trajectories, not shown here, indicate a distinct lifting of the trajectories from the mid to the
upper troposphere 24-12 h before the flight.
7
EULINOX, 1998-1999 - FINAL REPORT
Figure 5. Backward trajectories calculated by KNMI for July, 1, 1998 (one day travel time is
marked with a cross).
The METEOSAT-IR image 24 h before the flight shows in the northern part of Spain
the development of several large thunderstorms (Figure 6a). The next hours until midnight the
convection grows and a huge convective complex covers most of northern Spain (Figure 6b).
High lightning frequency was registered.
a)
b)
Figure 6a-b. METEOSAT-IR images from June, 30, at 16 UTC (a) and July, 1, at 00 UTC
(b).
Chemical transport model (CTM) calculations by NILU support the assumption that
convective transport and NOX production by lightning over Spain perturbed the trace gas
composition in 9-10 km over mid-Europe on July, 1st (Figure 7a-b). In the upper panel of
Figure 7a the horizontal NOX distribution over western Europe in 250 hPa is shown two hours
later (18 UTC) than the flight mission (for more information about the CTM see Flatøy et al.
in this issue). An elongated area with enhanced NOX is visible west of Basel. The black line
indicate the position of the cross section that is presented in the lower panel in Figure 7a. The
vertical NOX distribution shows clearly the effect of the thunderstorm complex visible in the
satellite images. There is a strong upward pumping of polluted boundary layer air and a
8
HUNTRIESER ET AL.: Airborne EULINOX measurements
distinct outflow of this air in the upper troposphere. However, in the CTM also the NOX
enhancement due to production by lightning is included. Therefore, in both panels in Figure
7a we see the effect of anthropogenic NOX sources and the NOX production by lightning. The
Falcon flight track is added in the lower panel to the right as thin black line. The Falcon
penetrated the convective outflow area which explain the measured enhanced NOX
concentration of 0.2-0.5 ppbv. The CTM indicate concentrations in the same range, about 0.3
ppbv NOX. It is interesting to mention that the area with enhanced NOX penetrated by the
Falcon was cloud free and could not have been identified without the use of forward
trajectories. It appears that convectively lifted air by thunderstorm systems can be observed
many hours after the thunderstorms had their main activity and enhanced NOX can be
measured over an extended area (several 100 km). The PBL tracer signatures (CO2, CO, and
O3) seem to disappear faster since their relative concentration changes are lower.
In Figure 7b the horizontal NOX distribution in 600 hPa is shown in the upper panel.
The pattern is similar as discussed before for the 250 hPa level. The lower panel shows the
vertical cross section of the OH distribution. The convective pumping of polluted boundary
layer air and water vapour causes an OH increase in the outflow region in the upper
troposphere. Peroxides and acetone have been suggested to be major OH sources in the upper
troposphere [Jaeglé et al., 1997]. The OH concentration is about 10 times higher than normal
which probably has a large effect on the ozone concentration the next hours in that area.
The case study described above is import to discuss since most other EULINOX
thunderstorms have a smaller horizontal extent and therefore are not resolved by the CTM that
have a larger horizontal grid than the average thunderstorm size. For the same reasons the
trajectory calculations by KNMI are not able to treat the meso-scale air mass transport in
single thunderstorms. However, the dynamic of the thunderstorm cluster on July, 1st was
extended enough (horizontally) to show up in the ECMWF analysis data used for the CTM
and the trajectory calculations. Only the reproduction of the vertical motion of the trajectories
in the thunderstorm complex is probably to weak in comparison to reality. The strong updrafts
in thunderstorms can not be resolved by the ECMWF analysis. Usually a rapid vertical
transport from the boundary layer to the upper tropopause is expected. However the
trajectories show a vertical motion only from the mid troposphere to the upper troposphere in
this and most other thunderstorm cases known to us. The only case where we until now found
a very fast transport from the boundary layer up to the tropopause visible in the trajectories
was in the case of an hurricane (1999) which however is an order of magnitude larger than
thunderstorms.
In order to generalize the findings for the July 1st case we present the mean NOX
vertical profile obtained during EULINOX together with standard deviations (Figure 8a). The
C-shape mentioned before is visible. In addition large standard deviations are found in the
boundary layer, which can be explained by the varying anthropogenic surface sources, and in
the upper troposphere (8-10 km just below the tropopause), which probably is the result of
many convective events with lightning that dispersed in July 1998 and aircraft emissions.
Similar vertical profiles from the winter season (not shown here) does not show this high
variability in the upper troposphere. In Figure 8a the NOX variability observed in the upper
troposphere is not caused by the change in tropopause height since this is rather constant at
10-11 km during the whole month. Since this NOX enhancement in the upper troposphere is
clearly visible in the mean profile we can assume that convective transported and lightningproduced NOX has an important influence on the ozone seasonal distribution in this altitude
and latitude. This affect should be included in general circulation models (GCM) to give a
proper result regarding the effect of NOX on O3.
9
EULINOX, 1998-1999 - FINAL REPORT
a)
b)
Figure 7a-b. CTM calculations from NILU for July, 1, at 18 UTC. The upper panel in (a)
shows the horizontal NOX distribution in 250 hPa over western Europe and the lower panel in
(a) shows the vertical NOX distribution along the cross section indicated in the upper panel.
The upper panel in (b) shows the horizontal NOX distribution in 600 hPa over western Europe
and the lower panel in (b) shows the vertical OH distribution along the cross section indicated
in the upper panel.
The high NO/NOX ratio in 8-10 km during EULINOX (Figure 8b) also indicates that
NO has been produced recently by lightning (probably IC-lightning). Besides, the NO/NOX
ratio shows a second maximum in the mid troposphere between 4-5 km, which is close to the
main negative charge region of thunderstorms (see also Dotzek et al. in this issue). In addition,
the vertical distribution of lightning signals from the interferometer system (ONERA) shows a
maximum at this altitude.
10
HUNTRIESER ET AL.: Airborne EULINOX measurements
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Figure 8a-b. Average vertical NOX (a) and NO/NOX (b) distribution from all EULINOX
mission days. The horizontal bars indicate the standard deviation.
0
2000
4000
6000
8000
10000
CN (# stdcm-3)
Figure 9. Average vertical CN distribution from all EULINOX mission days. The horizontal
bars indicate the standard deviation.
The remaining EULINOX trace gases (CO, CO2, and O3) did not show similar large
deviations in the upper troposphere which also explain that the NOX variability is not due to
changes in tropopause height. Only the mean EULINOX condensation nuclei (CN)
distribution shows similar variability in the boundary layer and the upper troposphere (Figure
9). The variability in the upper troposphere is large and even seems to extend the
concentrations observed in the boundary layer which indicates that particles can be produced
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EULINOX, 1998-1999 - FINAL REPORT
in thunderclouds (probably due to lightning). This finding is also true for the vertical CN
profile from July, 1st (not shown here). Similar as for NOX (Figure 1) a tongue with enhanced
number of particles (almost 10000 particles per stdcm-3) was found between 9-10 km which
exceeded the upper boundary layer concentrations on July, 1st.
One of the secondary objectives of EULINOX was to find the optimal tracer for
boundary layer air to be able to distinguish the amount of convectively transported NOX and
lightning-produced NOX in thunderclouds. Before EULINOX our group found that CO2 is
more suitable as boundary layer tracer than O3 in our region [Huntrieser et al., 1998]. During
EULINOX one goal was to investigate if CO can be used as an additional tracer for boundary
layer air observed in thunderstorm anvils in southern Germany. In Figure 10 single (a) and the
mean EULINOX (b) vertical CO profiles are presented. In comparison to the mean CO2
profile presented before in Figure 3b (middle profile) it is clear that CO is not useful as tracer
for southern Germany since the variability in time and space is very large which causes huge
standard deviations. Further no distinct difference between boundary layer air and free
tropospheric air is visible as for CO2. Thus, it was decided to use only CO2 as tracer for
boundary layer air in this study.
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CO (ppbv)
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CO (ppbv)
a)
b)
Figure 10a-b. Vertical CO distribution (in ppbv) on single EULINOX mission days in
different colors (a). The black thick line (b) is the mean vertical NOX profile estimated by
averaging the NOX data from all EULINOX mission days. The horizontal bars indicate the
standard deviation.
3.2. Meso-scale NOX Signatures
NOX signatures on the synoptic-scale give an indication of the extensive spatial
dispersion and age of the convective outflow containing enhanced NOX due to lightning
production and transport from the boundary layer. However, in order to quantify the amount
of NOX produced by lightning we have to analyse single convective cloud events on the mesoscale. All cloud penetrations available from EULINOX missions were analyzed to get a large
variety of convective clouds as described in Section 2. All together 29 penetrations of
convective clouds with and without lightning were investigated. The clouds developed on the
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HUNTRIESER ET AL.: Airborne EULINOX measurements
EULINOX mission days July 3, 7, 14, 17, and 20. Although several convective clouds were
penetrated on July, 21, this day was excluded from this study since the vertical trace gas
profiles were strongly influenced by very polluted conditions in the boundary layer as
described in Section 3.1. for CO and CO2.
The times for cloud penetrations were noted by the mission scientist on board the
Falcon. Furthermore, the registrations of horizontal wind speed (which decreases distinctly in
the cloud), CN concentration (which increases in the cloud), vertical wind speed (which
indicates well developed up- and downdrafts in the cloud), and J(NO2) photolysis frequencies
(which increases at cloud edges and decreases in the cloud) were used to identify the cloud
penetrations. To our knowledge for the first time J(NO2) measurements have been performed
extensively in thunderstorms. At the edges of convective clouds an increase in J(NO2) of up to
50% was registered due to light scattering (Figure 11). In the centre of the cloud the intensity
of sun light was reduced and J(NO2) decreased down to 50% of the values out of the cloud,
except in one case (not shown here) where J(NO2) increased near cloud center. This increase
was measured in an area where graupel and many short and high NO signals („NO spikes“)
were observed indicating close lightning. The additional light from flashes may explain the
J(NO2) increase in the cloud centre.
980720
-1
J(NO2) (s )
0.015
upward
downward
0.010
0.005
0.000
56000
57000
58000
59000
60000
UTC (s)
Figure 11. Time series of J(NO2) photolysis frequencies from the flight of July, 20, 1998
(upward looking sensor upper line and downward looking sensor lower line).
Typical trace gas signatures (NO, CO2, and CO) during penetrations of different
convective clouds in the upper troposphere on July, 3, are visible in Figure 12a. At least five
distinct minima can be observed in CO2 between 58000 and 62000 s (UTC) during the cloud
penetrations. Four of the minima indicate concentrations down to 363 ppmv. One minimum
reached even 358 ppmv. The vertical CO2 and CO profiles from July, 3, show very distinct
differences between the boundary layer and the upper troposphere which can be used to
identify boundary layer air convectively transported to the upper troposphere (Figure 12b-c).
The CO2 minima measured in the clouds indicate clearly that the air has been transported from
an altitude below 2.5 km or in one case even below 1.5 km. The air masses found in these
convective clouds have probably originated close to the border between the boundary layer
13
EULINOX, 1998-1999 - FINAL REPORT
and free troposphere (around 2 km) and reached almost undiluted, very rapidly the upper
troposphere.
F980703
12000
Altitude (m)
10000
980703
8000
6000
7
4000
2000
6
NO (ppbv)
CO2 (ppmv)
5
CO (ppbv)
350
355
360
365
370
375
380
CO2 (ppmv)
250
375
200
370
4
0
380
100
365
3
50
2
12000
360
1
10000
0
355
-50
350
-100
0
Altitude (m)
150
8000
58000
6000
60000
62000
UTC (s)
4000
2000
0
50
75
100
125
150
175
200
CO (ppbv)
b) top, c) bottom
a)
Figure 12a-c. Time series (a) of NO (black), CO2 (red) and CO (blue) from the flight of July,
3, 1998. Vertical CO2 (b) and CO (c) distribution on July, 3, 1998.
It is interesting to point out that the lightning activity was quite different in the clouds
shown in Figure 12a which can explain why the clouds with about the same CO2 minimum
concentration show very different NO concentrations. The first cloud penetration around
58000 s (UTC) was an intensive thunderstorm with high lightning frequency which caused the
high NO level (mean value 2 ppbv). The spiky NO structure (up to 6 ppbv) indicate that the
NO increase was produced by very fresh lightning. The second CO2 minimum close to 59300
s (UTC) shows a small NO enhancement and one single superimposed NO spike. This cloud
just started to be electrically active. The main part of NO observed in this cloud resulted from
transport from the boundary layer. The third CO2 minimum close to 59800 s (UTC) shows a
small NO enhancement and one single small superimposed NO spike. It is the same cloud that
was penetrated just before (and probably also the same NO spike). The mean NO
concentration in this cloud (dominated by boundary layer air) was 0.3-0.4 ppbv and much
lower than during the first cloud penetration. The fourth CO2 minimum close to 60200 s
(UTC) shows a rather distinct NO enhancement (mean value 0.6 ppbv) and at least one
superimposed NO spike. Lightning was observed in this cloud several times during the last
hour before the measurement and caused the enhanced NO concentration together with
transported NO from the boundary layer. The last CO2 minimum close to 61400 s (UTC) was
14
HUNTRIESER ET AL.: Airborne EULINOX measurements
not observed in connection with a cloud penetration. The former mentioned clouds were all
organized along a line and the last (fifth) observed CO2 minimum was observed downwind of
this line (close to the second and third cloud penetration) in a cloud free area during the ascent
from 7.5 to 9.7 km (just below the tropopause). Again we find unexpected high NO signatures
in a cloud free area as described in the synoptic-scale case. However, this time the cloud
outflow is very fresh since the nearby thunderstorms are active. The cloud outflow has a
vertical extension of 2 km and the highest NO concentrations were observed (0.4 ppbv) in the
middle of the outflow area (8.5 km). Not only the NO concentration but also the CO2
concentration is very similar in the unvisible cloud outflow as in the clouds (second and third
penetration). That indicates that the mixing of the cloud air mass with ambient air is very
slow. During the second and the third cloud penetration almost no lightning was active and we
can attribute the average NOX enhancement (0.5-0.6 ppbv) to transport of polluted air from the
boundary layer. We can use these values to estimate the amount of lightning-produced NOX in
the first penetrated thunderstorm since we observed similar CO2 concentrations in this storm
and the second (and third) which indicates that the air was originated from about the same
source region with similar NOX concentration in all cases. We can subtract the CO2/NOX
correlation in the clouds without lightning from the relationship in clouds with lightning to
estimate the amount of lightning-produced NOX. In the first thunderstorm an average NOX
concentration of 3.0 ppbv (lightning+transport) was observed. We then estimate that 80% of
the NOX observed in this thunderstorm resulted from production by lightning and only 20%
from the transport of boundary layer air. The same estimate we can make for the fourth
thunderstorm penetration. Here we observed an average NOX concentration of 0.9 ppbv from
which about 60% resulted from transport from the boundary layer and 40% from production
by lightning. This example shows how the trace gas concentrations observed in clouds without
lightning (CO2/NOX correlation) can be used to estimate the amount of lightning-produced
NOX in clouds with lightning.
During most EULINOX flights it was not possible to penetrate a convective cloud
more than 2-3 times because of air traffic control restrictions. However, during two flights
(July 17 and 21) we managed to follow a storm development over a longer time and to
penetrate each of these cloud systems 7 times. On July 17, we investigated an isolated squall
line near Bayrischer Wald (dimension of the system 30x60 km). The squall line was in a
dissipating stage and not very electrically active anymore. We penetrated the squall line in
three different altitudes. Repeted penetrations at the same altitude always showed similar NOX
concentrations. Therefore, the calculated NOX standard deviation is low for the penetrations at
the same altitude. At 7.6 km (top of the cloud) we measured 0.9 0.1 ppbv, at 7.3 km 1.4 0.1
ppbv, and at 6.7 km 0.8 0.1 ppbv NOX. The highest NOX concentration was observed at 7.3
km and in this altitude also the largest number of NO spikes were observed which indicate
that this region was close to a region where lightning was generated (probably IC lightning).
The 21 July case will be described in detail in the next section.
3.2.1. NO spikes and vertical velocity. Figure 13a shows the correlation between the
maximum anvil NO concentration and maximum vertical velocity observed in the 29
convective clouds under investigation. The 29 dots in Figure 13a are marked with different
colors (black or red) depending on if NO spikes were observed during the cloud penetrations
or not. The overall correlation is good with r=0.74, however, the data tends to spread out for
higher values. The average value of the maximum anvil NO is 3.8 ppbv and the average width
of the NO spikes is 1-1.5 km. It appears that NO spikes were only observed above a certain
vertical velocity. Lightning develops in regions with strong up- and downdrafts since the
friction between different cloud particles (water, ice, graupel particles) causes the charge
separation in the cloud. Since NO spikes are observed only in regions with maximum vertical
15
EULINOX, 1998-1999 - FINAL REPORT
velocities exceeding 3 m s-1, probably these are also the regions where lightning developed.
The number of NO spikes found in a convective cloud was also positively correlated with the
vertical velocity (not shown here) indicating that lightning caused the NO spikes.
Figure 13b shows the correlation between mean anvil NOX and the maximum vertical
velocity. Again the correlation is good and the separation between black and red dots is
apparent. In clouds without any NO spikes (black dots) no lightning occurred and the
observed NOX enhancement can be attributed to the transport of polluted boundary layer air
only. In comparison clouds with NO spikes (red dots) contain NOX from both transport and
production by lightning. By using the correlation in Figure 13b we find a transition zone
between clouds with and without lightning. Above vertical velocities larger than 1-3 m s-1 and
mean anvil NOX concentrations 0.5-0.9 ppbv clouds with lightning can be expected.
20
20
without NO spikes
correlation r = 0.74
with NO spikes
18
16
14
max w (m s-1)
max w (m s-1)
16
12
10
8
14
12
10
8
6
6
4
4
2
2
0
0
0
2
4
without NO spikes
correlation r = 0.70
with NO spikes
18
6
8
10
0
12
max anvil-NO (ppbv)
1
2
3
4
mean anvil-NOX (ppbv)
a)
b)
Figure 13a-b. Correlation between maximum anvil-NO (a), mean anvil-NOX (b) and
maximum vertical velocity.
3.2.2. Amount of lightning-produced and transported NOX in convective clouds.
In about 50% of the 29 investigated clouds lightning was observed. The clouds were divided
into two groups with and without lightning and the average NOX concentrations were
calculated (Table 3). The average NOX concentration in clouds without lightning was used to
estimate the ratio of transported and lightning-produced NOX in clouds with lightning. For an
average EULINOX thunderstorm about 60-70% of the measured anvil-NOX was produced by
lightning and about 30-40% was transported boundary layer NOX. In large EULINOX
thunderstorms the amount of lightning-produced NOX was found to reach up to 90%. These
values are close to the values inferred from the LINOX field experiment [Huntrieser et al.,
1998].
16
HUNTRIESER ET AL.: Airborne EULINOX measurements
Table 3.
NOX Concentration in 29 Convective Clouds Observed During EULINOX
mean standard deviation
minimum
maximum
Convective clouds
without lightning
NOX (ppbv)
Convective clouds
with lightning
NOX (ppbv)
0.4
0.2
0.9
1.3
0.5
3.0
0.2
0.7
3.2.3. Origin height of anvil air.
Mean trace gas concentrations in convective
clouds without lightning were analyzed to find out more about the origin height of anvil air
and about mixing. In addition to the mean NOX concentration in the upper part of these clouds
(0.4 0.2 ppbv ) we calculated the mean CO2 and O3 concentration (362 1 pp mv, 65 10
ppbv, respectively). In Figure 3b, 4b, and 8a the mean EULINOX vertical profiles for CO2,
O3, and NOX have been presented. By using these profiles we can make a rough estimate from
where the air samples observed in the cloud top regions originated. The most probable origin
height for the sampled air parcels (near cloud top) is 2 km (range 1.5-2.5 km) which is the top
of the boundary layer. These upward transported air parcels seem to be like protective cores
with almost no mixing with the ambient air except in a thin region near the cloud edges. The
low O3 concentration observed in the cloud top region indicate that almost no mixing with the
ambient air in the upper troposphere took place, otherwise the O3 concentration would be
higher (at least 80 ppbv) and not as low as in 2 km.
3.2.4. Global lightning NOX production rate. We can use the result presented in
Table 3 to estimate the global lightning NOX production rate as suggested by Chameides et al.
[1987]. The equations have also been used during the LINOX field experiment and discussed
in detail in Huntrieser et al. [1998]. The same parameter were estimated for EULINOX and
the following mean values were achieved: va-vs=7 m s-1, a=0.5 kg m-3, x=30 km, z=1 km,
FC=1.1x108 kg s-1. The average rate at which air is advected out of the anvil (FC) was then
multiplied with the average NOX concentration in the anvil due to production by lightning
(here 0.9 ppbv, the average EULINOX value) and the average number of thunderstorms
occurring at any instant globally (2000). Following the equations (1-3) presented in
Huntrieser et al. [1998] we estimate that the global NOX production rate due to lightning is
about 3 Tg (N) yr-1 (range 1-20 Tg (N) yr-1). Again a similar mean value 4 Tg (N) yr-1 was
obtained for the previous field experiment LINOX [Huntrieser et al., 1998]. The mean values
estimated from the LINOX and EULINOX field experiments are slightly lower than the mean
value 5 Tg (N) yr-1 presented in Table 1. We are aware of the fact that our measurements have
only been performed in the midlatitudes over Europe and that these thunderstorms are perhaps
not typical global thunderstorms. However, the flash frequency in the European thunderstorms
seems to be similar to the ones observed in thunderstorms midlatitudes over the United States.
An open question is, if this is also true for tropical storms? Further it is to assume that most of
the NOX that we measure in the upper troposphere when we penetrate thunderstorms result
from IC lightning (NO spikes). The amount of NOX produced by CG lightning is probably not
detected completely during the flights in the upper troposphere. Our conclusion from the
EULINOX data is that 3 Tg (N) yr-1 is probably the lower limit for the global NOX production
rate by lightning.
17
EULINOX, 1998-1999 - FINAL REPORT
3.3. Micro-scale NOX Signatures
Micro-scale NOX signatures (sharp NO-spikes due to production by lightning) were
observed frequently on the last EULINOX mission day July, 21. A very intensive, isolated
thunderstorm developed in the evening west of Munich and the upper part of this storm was
penetrated 7 times at different altitudes (7-10 km) by the aircraft. In Figure 14a the IRMeteosat image from 18:00 UTC is shown. The investigated thunderstorm is marked by a
black box. It developed ahead of a larger cold front system extending from Scandinavia to
Spain. The July 21 thunderstorm was surveyed by the ONERA VHF mapper, from 14:00 UTC
until 22:10 UTC. More than 12 000 flashes were reconstructed for this period of time. Figure
14b shows the most interesting part of the Falcon flight track and the registered 2D density of
the VHF signals (between 17:40 and 18:10 UTC). It is obvious that the research aircraft
penetrated cloud areas with high lightning frequency. For further description of the electrical
and microphysical development of the storm see contributions by Théry et al. and Höller et al.
(this issue).
a)
b)
Figure 14a-b. METEOSAT-IR image of central Europe from July 21, 1998, at 18:00 UTC
(a). The thunderstorm investigated by the aircraft is marked by the black box. Ground
projection (90x90 km) of VHF sources detected by the ONERA system between 17:40 and
18:10 UTC (b). Each color corresponds to a 5 min period. Red letters: labels of electrical
cells. Black line : Falcon trajectory during this time, with an arrow every 5 min. Red squares:
interferometric stations. Blue square: operational center, at Oberpfaffenhofen (48°N, 11°E).
The time series of chemical and particle measurements for the entire flight are
presented in Figure 15. Between 17:30 and 18:45 UTC the isolated thunderstorm (diameter
40-50 km) described before was investigated by the aircraft (starting at 63500 s). The storm
penetrations were exceptional because they were performed more in the vicinity of the main
cell than in the main anvil region. Lightning was visible from the aircraft several times in the
thunderstorm. The highest NO mixing ratios (up to 25 ppb) were encountered during the pass
3 of the storm (see Figure 16). At the same time the aircraft nose was hit by a small lightning
stroke shown in Figure 17. A few minutes later the highest CN concentration of 35000
particles per standard cm3 was measured during pass 5 (see Figure 15).
18
HUNTRIESER ET AL.: Airborne EULINOX measurements
F980721
35
1e+14
370
30
360
15
350
10
340
1e+6
5
0
1e+8
CN (# scm-3)
1e+10
20
CO2 (ppmv)
NO (ppbv) & CO (0.1*ppbv)
1e+12
25
330
1e+4
320
68000
1e+2
-5
60000
62000
64000
66000
UTC (s)
Figure 15. Time series of Falcon chemical (NO, CO2, and CO) and particle (CN) data from
the flight of July 21, 1998. More than 100 NO spikes are visible. The Falcon penetrated an
isolated thunderstorm 7 times after 63500 UTC (s) which is clearly visible in NO.
F980721
35
1e+14
370
30
360
15
350
10
340
5
0
1e+8
CN (# scm-3)
1e+10
20
CO2 (ppmv)
NO (ppbv) & CO (0.1*ppbv)
1e+12
25
1e+6
330
1e+4
320
64800
1e+2
-5
64200
64400
64600
UTC (s)
Figure 16. Time series of Falcon chemical (NO, CO2, and CO) and particle (CN) data from
the flight of July 21, 1998. In comparison to Figure 15, which shows the whole flight, here
only the third penetration of the isolated thunderstorm is shown. Many sharp NO spikes are
clearly visible reaching a maximum concentration of 25 ppbv.
19
EULINOX, 1998-1999 - FINAL REPORT
Figure 17. A photography of the lightning stroke which hit the Falcon meteorological boom
at 17:55:48 UTC.
Figure 18 shows the vertical profiles of CN, CO2 and CO, which were used as tracers
for boundary layer air and anthropogenic sources, respectively. In comparison to the vertical
profiles presented in the previous sections here the single convective cloud penetrations have
not been excluded. Thus it is easier to compare trace gas concentrations observed during the
penetrations of the anvil region with boundary layer concentrations. The vertical profiles of
CO2 and CO were exceptional on July 21 with very high concentrations for this region in the
PBL (372 ppmv and 220 ppbv, respectively). Ahead of the approaching cold front (which
triggered the thunderstorms in the evening) the wind was turning to the east already in the
morning. Polluted air from Munich spread out over the area where the thunderstorm
developed in the evening. In fact similar high concentrations were observed in the upper part
(8-10 km) of the investigated thunderstorm (Figure 18) which indicate that the dilution rate
was low [also found by Poulida et al., 1996]. The chemical signatures found in the anvil
region seem to be strongly dependent on the conditions at the top of the boundary layer, a kind
of mirror effect between PBL and upper troposphere. The NOX enhancement during the
thunderstorm passes was coincident with an enhancement in CO and CO2 (Figure 15).
According to the tracer analysis the probed air masses experienced a fast uplifting.
The boundary layer O3 concentration shows relatively low variations in the
measurement region with 90 +/- 20 ppbv on July 21 (not shown). The observed values agree
quite well with ground based measurements of the Rothenfels (Reinluftstation of LFU). The
high O3 mixing ratios in the boundary layer (about 90 ppbv) indicate that ozone smog
conditions were predominant on July 21.
20
HUNTRIESER ET AL.: Airborne EULINOX measurements
-3
CN (# stdcm )
0.0e+0
2.0e+4
0
4.0e+4
6.0e+4
8.0e+4
CO (ppbv)
100
50
1.0e+5
150
1.2e+5
200
10000
Altitude (m)
8000
6000
4000
2000
0
355
360
365
370
375
380
385
390
CO2 (ppmv)
Figure 18. Vertical distribution of CN (left), CO2 (center) and CO (right) as observed on July
21, 1998. The planetary boundary layer (PBL) is discernible by high mixing ratios of CO, CO2
and a high CN-concentration (below 2.6 km).
3.3.1. Estimate of lightning-produced NO. The NO mixing ratio shows a very
spiky structure (Figure 16) which can be attributed to NO production by lightning. The
penetrations were performed in the upper part of the storm (7-10 km) where mainly fresh
intracloud (IC) lightning can be expected. In Figure 14b it was shown that the Falcon flight
was performed in a thunderstorm with high lightning frequency. During the flight more than
150 NO spikes were registered with diameters between ~170 and 2000 m (mean ~600 m,
median ~200-400 m). The diameter of the spikes could also be less, however, ~170 m is the
lower limit that can be detected by the used time resolution of the NO instrument (1 s).
The NO enhancement observed in the spikes (mean value 4.8 ppbv) can not only be
attributed to the production by lightning. The convective transport of polluted air from the
planetary boundary layer (PBL) also contributes to the NO enhancement. To distinguish
between these two NO sources two different tracers were used, CO2 and CO. The vertical
profiles of these trace gases were exceptional on July 21 (Figure 18) with extremely high
concentrations for this region in the PBL (372 ppmv and 220 ppbv, respectively).
To be able to subtract the transported part of NO in the spikes it was searched for a
correlation between NO and the two tracers CO2 and CO. Because of the strong convective
weather situation on this day (high dewpoint in lower altitudes) the NO instrument was just
operated above 5 km and no information is available below this altitude. However, an
indication of the NOX concentration in the PBL is available from other EULINOX mission
days (mainly <1.2 ppbv) and ground based measurements (Figure 8a). For this reason we tried
to identify the transported part of NO in Figure 15 during the anvil penetrations. It can be
21
EULINOX, 1998-1999 - FINAL REPORT
370
363
369
364
368
365
CO2 (ppmv)
CO2 (ppmv)
recognized that in most cases a smooth NO enhancement (this smooth enhancement is partly
also visible in the CO2 and CO signal) is superimposed by the spiky structure (Figure 16).
Since this smooth NO enhancement roughly follows the CO2 and CO signal it can mainly be
attributed to the part of NO transported from the PBL. Explicit all NO spikes were cut (NO
values above 1.2 ppbv) and the correlations between NO-CO, NO-CO2, and CO-CO2 were
studied (Figure 19a). A positive correlation was found between all pairs. CO2 turned out to be
the more useful tracer, since small structures in the NO signal could also be found in CO2. In
comparison the CO signal was extremely smooth during most anvil penetrations (caused by
the longer response time). It is also visible in the NO-CO2 correlation plot (not shown here)
that 1.2 ppbv NO seems to be a reasonable upper limit for PBL air (accumulation of values
left of this limit).
367
366
365
366
367
368
369
364
370
363
120
371
130
140
150
160
0.0
170
CO (ppbv)
a)
0.2
0.4
0.6
0.8
NO (ppbv)
1.0
1.2
1.4
b)
Figure 19a-b. Correlation plot between CO and CO2 (a) during the first penetration of the
thunderstorm on July, 21. A high positive correlation was observed (r=0.75). Mean correlation
between NO and CO2 (b) during the penetrations of the thunderstorm on July, 21. The
horizontal bars indicate the standard deviation.
Reliable CO2 and CO data were not available for all of the observed NO spikes. The
CO2 measurements were interrupted during the last part of the flight (Figure 15). The decrease
in the CO signal was often delayed (in comparison to the other trace gas signals) at the
moment when the research aircraft left the thunderstorm. Therefore, for every of the 7 anvil
penetrations this part of the CO signal was neglected for further analysis. Finally, because of
the several disadvantages mentioned above for CO, it was decided to use only CO2 as tracer
for PBL air. Reliable CO2 data were available for 92 of the observed NO spikes.
For CO2 intervals of 1 ppmv (between 364 and 370 ppmv) the average NO
concentration (with standard deviation) was calculated (Figure 19b). (The scaling of the CO2axis has been mirrored to be comparable with a vertical profile.) The strong increase in NO
between 366 and 367 ppmv gives a hint that this is the transition zone between the conditions
in the PBL and the free troposphere (compare with Figure 18). First the correlation between
CO2 and the mean NO value was used to estimate the transported part of NO in the spikes.
The calculated NO and the observed NO concentration (without spikes) was superimposed. It
turned out that the calculated NO concentration was too high in comparison to the observed
smooth NO signal (without spikes). The reason is probably that the average NO values in
Figure 19b contain a mixture of transported PBL-NO and aged lightning-NO. Therefore it was
decided to use the lower limit of the NO values (mean value minus standard deviation) to get
more reliable values for the transported NO part.
22
HUNTRIESER ET AL.: Airborne EULINOX measurements
A CO2 limit for PBL air was defined (>366.0-366.4 ppmv). (A more distinct limit is
difficult to define from the uncertainty in the measurements.) It was estimated that between 65
and 80% of the air in the upper part of the thunderstorm originated from the PBL. In the
spikes the ratio was even higher with values between 70 and 90%. The correlation between
NO and the tracer CO2 was used to subtract the part of PBL-NO in the spikes ( 0.6 ppbv).
The residue of 4.3 ppbv (mean value) was attributed to lightning (median 2 ppbv, range
between 1 and 24 ppbv). It is obvious that the NO contribution from the PBL ( 0.6 ppbv) is
minor (10-20%) in comparison to the lightning-produced part in these spikes. This statement
is only valid for the NO spikes and not if we consider the complete anvil region.
These estimates of the lightning-produced part of NO in thunderstorms can be used to
calculate the production rate of NO molecules per m flash [as proposed by Stith et al., 1999;
and others]:
P C [ NO]
D2
(1)
PNO
4 R T
where P is the pressure (Pa), C is the Avogadro constant (6.023x1023 molecules mol-1), [NO]
is the NO volume mixing ratio, D is the diameter of the spike (m), R is the universal gas
constant (8.314 J mol-1 K-1), and T is the temperature (K). The residue of 4.3 ppbv (mean
value) which was attributed to lightning (median 2 ppbv, range between 1 and 24 ppbv)
corresponds to an average production rate of 1x1022 molecules NO per m flash (median
0.1x1022, range 0.02-10x1022). In comparison to these results Stith et al. [1999] reported lower
values (only 14 NO spikes were investigated: mean value 0.25x1022 and range 0.02-0.7x1022
molecules NO per m flash).
By using an average flash length of 7 km [Liaw et al., 1990], the global flash rate of 65
-1
s [Mackerras et al., 1998] and the average NO production rate found in this study (1x1022
molecules NO per m flash) it was estimated that on an average 3 Tg(N) yr-1 (global value)
result from the lightning production (assuming that the investigated thunderstorm is a
typically global thunderstorm). If we use the median value for the NO production (0.1x1022
molecules NO per m flash) we estimate 0.3 Tg(N) yr-1. As a result of this study a value
between 0.3 and 3 Tg(N) yr-1 is most probable for the global NO production by lightning
[similar values as found in a model study by Levy et al., 1996; and a recent laboratory study
by Wang et al., 1998].
However, the combination of different flash lengths [5-50 km; see Coppens et al.,
1998; Stith et al., 1999], flash rates [40-120 s-1; see Turman and Edgar, 1982; Liaw et al.,
1990] and NO production rates (in this study 0.02-10x1022 molecules NO per m flash) shows
that the actually range can be very large (0.03-396 Tg(N) yr-1) if we make estimations based
on the NO production per m flash. However, these values are the most extreme values
possible and not realistic. In addition we have to consider that the investigated NO spikes can
contain NO from more than 1 flash. (In this study we assumed only 1 flash.) Then the NO
production rate is reduced. At the moment it is not possible to estimate how many flashes
produced the NO observed in one spike. For such an investigation the time resolution of the
NO instrument must be changed (from 1 s to at least 0.1 s).
Another problem is the rather low spatial resolution of the VHF interferometer signals.
For the 21 July thunderstorm these signals can only be estimated with an accuracy of about 1
km. Therefore it was not possible in this study to assign single NO spikes to single VHF
signals.
In Figure 20a-c all CN data referring to regions
3.3.2. Particle production.
characterized by lightning activity (measured for NO>0.5 ppbv) are shown for the July, 21,
23
EULINOX, 1998-1999 - FINAL REPORT
flight as scatter plots versus each tracer (CO and CO2) separately. The solid lines represent the
expected maximum CN-concentrations and the resulting interpolation that can be expected
when continental boundary layer (CBL) air masses are mixed with air from the free
troposphere (FT) to various amounts. Consequently, all data points below the line are in
agreement with mixing/dilution of uplifted CBL-air into the FT, while those CNconcentrations aloft cannot be explained by upward transport of CBL air alone. Further, the
maximum observed CN-concentrations aloft the line appear anti-correlated to both tracer
substances as indicated by the dashed lines. These observations show that substantial amounts
of new "excess" aerosol particles that evidently do not origin from upward transport appear in
the diameter range above 10nm within the investigated air masses.
However, the limited information on aerosol microphysics does not allow to identify
or distinguish certain processes that could have led to the rapid increase of detectable
particles. One possibility could be nucleation of new cluster-sized aerosols and/or enhanced
condensation growth of initially undetectable small particles caused by rapid uplift and
subsequently high supersaturations of condensable precursor species like H2SO4, H2O and
certain NMHCs inside the cumulonimbus clouds. An additional effect could be the presence
of high ion concentrations induced by the lightning which would generally enhance the
growth ability of molecular clusters as e.g. proposed by Yu and Turco [1999].
Lightning activity - as indicated by the NO signature - indeed seems to imply the
occurrence of new aerosol particles. Although the underlying physical processes must remain
unidentified in this context, the minimum relative amount of excess aerosols appearing in the
investigated atmospheric region can be roughly quantified. Simple calculations relating the
excess aerosol concentrations to the proposed underlying background yields a relative number
fraction of about 50% made up by the fresh aerosol (independently determined for both
tracers) as integral value for the whole atmospheric volume that has been sampled.
24
HUNTRIESER ET AL.: Airborne EULINOX measurements
a) top, b) middle, c) bottom
Figure 20a-c. Correlative scatter plots of CN (> 10nm) versus CO (a), CO2 (b) and NO (c)
observed inside and in the vicinity of the thundercloud investigated on July, 21. Considered
are all CN measurements above 8 km altitude at ambient temperatures between -32 and -42° C
where the NO concentrations exceeded 0.5 ppbv. Solid lines envelope the range of CN
concentrations that can be explained by transport and mixing of boundary layer air into the
free troposphere.
4. European and Global Lightning-NOX Emission Rates
The final step in the present study is to estimate the importance of European and global
lightning-NOX emissions in comparison to the other NOX emissions. The average European
flash density (2 flashes km-2 yr-1) published recently by Simpson et al. [1999] was used to
estimate the annual flash rate over Europe (0.7 flashes s-1) and to compare it to the global
flash rate (65 flashes s-1). We use the European flash rate (about 1/100 of the global flash rate)
to estimate the European NOX production rate by lightning and end up with 0.03 Tg(N) yr-1
25
EULINOX, 1998-1999 - FINAL REPORT
which is 1/100 of the global value estimated from EULINOX. In Table 4 the most important
NOX emissions on the European and global scale are listed. On both scales the anthropogenic
surface sources dominate clearly. However, in the upper troposphere different sources
dominate depending upon the investigated area. On the European scale aircraft emissions
dominate with 0.1 Tg(N) yr-1 in comparison to the production by lightning with 0.03 Tg(N) yr1
. The opposite relationship can be found on the global scale: lightning emissions dominate
with 3 Tg(N) yr-1 in comparison to aircraft emissions with 0.6 Tg(N) yr-1.
Table 4.
Summary of Estimates for NOX Sources on the European and Global Scale
Fossil fuel combustion
Soils
Biomass burning
Lightning
Aircraft
Europe1
[Tg(N) yr-1]
Global2
[Tg(N) yr-1]
6
1
0.01
0.03*
0.1**
22
7
8
3*
0.6
*
Estimates from EULINOX
Lee [1999] personal communication
1
Simpson et al. [1999]
2
Lee et al. [1997]
**
5. Summary and Conclusions
The EULINOX field experiment was performed in summer 1998 over Europe. In this
study the in situ observations in thunderstorms with the DLR research aircraft Falcon
(equipped with chemical, particle, and meteorological instrumentation) were analyzed in
detail. In addition lightning data from the German LPATS system and the French VHF
interferometer system (ONERA) were used for interpretation of the airborne data.
NOX signatures observed on different atmospheric scales were analyzed to estimate the
contribution of lightning-produced NOX to the European and global nitrogen budget. NOX
enhancements found on the synoptic-scale in cold fronts or remaining cirrus sheet after
convection, on the meso-scale in convective clouds with and without lightning, and on the
micro-scale in aged lightning channels were investigated. The highest NOX concentrations
were found on the smallest scale. On July, 21, several NO spikes with 25 ppbv were observed
(the highest value ever reported for thundercloud anvils in the literature). The spiky NO
signature in the thundercloud (on an average 20 spikes) was probably the result of high
lightning activity as observed with the VHF interferometer. However, not only NO was
produced in the thunderstorm, we also found indications of condensation nuclei production
(CN >10nm). More complete CN measurements (particles <10nm) would be useful to find out
more about the particle production in thunderstorms.
Almost 30 convective cloud penetrations were analyzed in detail to estimate the origin
of the NOX enhancement found in all of the clouds. The strength of the NOX increase in the
clouds was dependent on the frequency of lightning. In convective clouds without lightning
about 0.4 ppbv NOX was observed (only due to upward transport of polluted boundary layer
air). In comparison in clouds with lightning the mean NOX concentration reached 1.3 ppbv
26
HUNTRIESER ET AL.: Airborne EULINOX measurements
(due to transport and production by lightning). Tracers for boundary layer air (CO2, CO, and
O3) were used to estimate the origin height of air masses found in the upper troposphere in
these clouds. From the investigated tracers CO2 turned out to be the most useful tracer and the
CO2/NOX correlation in clouds without lightning was used to subtract the amount of
transported NOX from the boundary layer in clouds with lightning. The origin height of
convective air masses found in the upper troposphere was estimated to be close to the top of
the boundary layer (around 2 km). On July, 21, the boundary layer was extremely polluted and
this polluted air was transported upward very quickly by a thunderstorm that developed in the
evening close to Munich. Similar trace gas concentrations as in the polluted boundary layer
were found in the upper troposphere in the cloud outflow. This example shows how effective
and fast thunderstorms are in redistributing trace gases in the troposphere. The NOX
enhancement found in convective clouds with lightning was estimated to result mainly from
lightning (60-70%). About 30-40% was estimated to result from boundary layer NOX.
However, in clouds with high lightning frequency lightning-produced NOX dominated clearly
(90%). In these clouds also the NOX/NOY was very high exceeding 0.7 which indicate that NO
was produced recently by lightning.
The global lightning-NOX production rate was estimated by using two different
methods based on NOX produced per thunderstorm [Chameides et al., 1987] and NO produced
per lightning flash [Stith et al., 1999]. For the method introduced by Stith et al. the mean value
for the production rate was estimated to 1x1022 molecules NO per m flash for EULINOX.
Both methods end up with a mean value of 3 Tg(N) yr-1 for the global lightning-NOX
production rate. However, still many uncertainties are included and a range of 1-20 Tg(N) yr-1
is possible. For the second method even a larger range was obtained because of large
uncertainties in global flash rate and flash length. Therefore, the method based on NOX
produced per thunderstorm was preferred used in this study since the uncertainties are smaller.
The objective of this study was to estimate the importance of NOX produced by
lightning in comparison to NOX emitted by aircraft in the upper troposphere. On the global
scale the effect of lightning production with 3 Tg(N) yr-1 is more important than aircraft
emissions with 0.6 Tg(N) yr-1. On the European scale aircraft emissions dominate with 0.1
Tg(N) yr-1 and the production by lightning is minor with 0.03 Tg (N) yr-1. The different
importance of lightning-produced NOX depending on scale should be considered in future
climate models.
During EULINOX for the first time J(NO2) photolysis frequency measurements were
performed in thunderstorms. Until now these measurements have only been performed in
stratus and altocumulus clouds [Kelley et al., 1995]. In thunderclouds large J(NO2) changes
were observed. At the edges of the cloud J(NO2) increased by about 50% and in the center of
the cloud J(NO2) decreased by 50%.
After performing this field experiment it is recommended to conduct future
measurements in the tropics since most lightning is observed in the tropical region and on the
other hand pollution in the tropical boundary layer can in most cases be neglected [Benkovitz
et al., 1996] which simplifies the estimates of global lightning-produced NOX considerable.
Acknowledgments. Excellent support by the Falcon pilots (M. Scherdel and R. Welser) and
the ground crew is greatly acknowledged. We thank R. Marquardt, M. Fiebig, P. Stock, G.
Uhlemann, J. Baehr, A. Petzold (DLR) for the support during the campaign. We are also
grateful for the meteorological forecasts (from Deutscher Wetterdienst) and satellite images
(from METEOSAT) provided by A. Tafferner and H. Mannstein (DLR). Thanks to LfU
(Munich) for providing the chemical ground measurement data, and to P. Blanchet, G. Blanc
27
EULINOX, 1998-1999 - FINAL REPORT
(ONERA) for the preparation of the VHF interferometer data. This work was funded by the
European Community through the Environment and Climate programme (Contract No.
ENV4-CT97-0409).
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