GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 18, GB4027, doi:10.1029/2004GB002294, 2004
Uptake of methanol to the North Atlantic Ocean surface
Lucy J. Carpenter, Alastair C. Lewis, and James R. Hopkins
Department of Chemistry, University of York, York, UK
Katie A. Read
Department of Chemistry, University of Leeds, Leeds, UK
Ian D. Longley and Martin W. Gallagher
Department of Physics, University of Manchester Institute of Science and Technology, Manchester, UK
Received 5 May 2004; revised 8 September 2004; accepted 23 September 2004; published 14 December 2004.
[1] An anticorrelation between atmospheric methanol (CH3OH) concentrations and wind
speed and a positive correlation between dimethylsulphide (DMS) concentrations and
wind speed have been observed at the coastal air monitoring site of Mace Head in Ireland,
during a period of cyclonic activity in which the averaged surface wind speed
changed substantially as a low-pressure system evolved over the northeast Atlantic. These
observations suggest a net air-to-sea flux of CH3OH. This conclusion is supported by the
good agreement between the wind speed dependencies of the measured gas
concentrations and theoretical predictions using wind-induced turbulent gas transfer
velocities of DMS and CH3OH calculated from a resistance model, embedded in a
photochemical box model. For a wind speed of 8 m s1, an ocean deposition rate of
methanol of between 0.02 and 0.33 cm s1 is calculated, with a best estimate of
0.09 cm s1, in good agreement with deposition rates used in global models and derived
from atmospheric budgets. The large uncertainty in the calculated deposition rates is
due almost entirely to the uncertainty in the degree of saturation of methanol in the
surface ocean, highlighting the critical requirement for measurements of methanol in
seawater. Owing to the dependence on wind speed, the deposition rates calculated showed
substantial range and the calculated contribution of ocean deposition to total loss of
CH3OH (ocean uptake and gas phase OH oxidation) varied from approximately 20% to
INDEX TERMS: 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339,
60%.
4504); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry;
KEYWORDS: air-sea exchange, atmospheric chemistry, methanol
Citation: Carpenter, L. J., A. C. Lewis, J. R. Hopkins, K. A. Read, I. D. Longley, and M. W. Gallagher (2004), Uptake of methanol to
the North Atlantic Ocean surface, Global Biogeochem. Cycles, 18, GB4027, doi:10.1029/2004GB002294.
1. Introduction
[2] Methanol has an almost ubiquitous presence throughout the depth of the troposphere [Singh et al., 2000] and is a
highly significant sink for hydroxyl radicals, exceeded only
by methane, carbon monoxide, and isoprene. Despite its
important effect on the atmospheric oxidative capacity [e.g.,
Singh et al., 1995; Monod et al., 2000], the global sources
and sinks of methanol are not well known. Its major
chemical source is methane oxidation, but it appears that
there are additional sizable terrestrial biogenic sources [e.g.,
Macdonald and Fall, 1993; Fall, 1999; Warneke et al.,
1999]. Methanol reacts with the hydroxyl radical both in the
gas phase and in cloud, giving a combined photochemical
lifetime in the surface boundary layer of about a week.
Recent evaluations of the methanol budget conclude that a
Copyright 2004 by the American Geophysical Union.
0886-6236/04/2004GB002294$12.00
major uncertainty is whether the ocean is a net source or
sink [Singh et al., 2000; Heikes et al., 2002]. Although the
consensus is that the net flux of methanol is into the ocean,
to date this phenomenon has not been observed directly.
[3] In the ‘‘two-layer’’ approach of air-sea exchange
[Liss and Merlivat, 1986], the oceanic flux of a gas is
the product of the total gas transport velocity KT and the
partial pressure difference across the air-sea interface,
1
F ¼ KT Cw Cg =H ¼ 1=akw þ 1=Hkg
Cw Cg =H ; ð1Þ
where Cg is the well-mixed atmospheric boundary layer
concentration above the gas-phase film, Cw is the water
concentration below the aqueous-phase film, H is the
dimensionless Henry’s Law constant (H (Cg/Cw)eq), kw
and kg are exchange constants for liquid and gas phases, and
a is a chemical enhancement factor due to reactions in the
sea surface microlayer (assumed to be 1 for DMS and
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Table 1. Calculated Gas Transport and Deposition Velocities for
DMS and CH3OH
Wind
DMS
Speed, kw(= KT),
cm h1
m s1
4
6
8
10
12
14
2.91
6.34
11.08
17.13
24.50
33.18
CH3OH
kg,
KT,
vD,a
Range in vD
kw,
cm h1 cm h1 cm h1 cm s1 values,b cm s1
3.44
7.48
13.07
20.21
28.91
39.15
1800
2700
3600
4500
5400
6300
0.36
0.55
0.75
0.94
1.14
1.34
0.045
0.069
0.094
0.119
0.144
0.169
0.01 – 0.16
0.02 – 0.24
0.02 – 0.33
0.03 – 0.42
0.04 – 0.51
0.04 – 0.59
resistance. Assuming a neutrally stable atmospheric
surface layer, the total aerodynamic resistance to gas
transfer from a height z to the atmospheric surface layer,
governed by the turbulence intensity, can be estimated by
[Garland, 1977]
R1 ¼ U ð zÞ=u2* ;
a
Calculated for (1 S) = 0.1 and using a value of H of 2.2 104
[Singh et al., 2003].
b
See section 2.1 for a description of the calculation of the vD ranges.
CH3OH [Liss and Slater, 1974]). The liquid phase gas
exchange constant kw is calculated using semi-empirical
formulations involving wind speed and the temperaturedependent Schmidt number of the gas [Liss and Merlivat,
1986; Wanninkhof, 1992; Nightingale et al., 2000]. For
sparingly soluble gases the total gas exchange coefficient
can be approximated to kw, and such formulations have
been used to estimate sea-to-air fluxes of DMS [Ayers et
al., 1995; Turner et al., 1996] and ocean deposition of
halocarbons and other gases [Yvon-Lewis and Butler,
2002]. However, for very soluble gases such as methanol,
the gas phase resistance to transport dominates over the
liquid phase. The total gas exchange constant for the gas
phase is given by kg = 1/(R1 + R2), where R1 is the
aerodynamic resistance and R2 is the gas-phase film
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ð2Þ
where U(z) is the mean wind speed at the height z and u*
is the friction velocity. The gas-phase film resistance R2
depends upon the molecular diffusivity of the species and
can be parameterized as [Wesely, 1989]
R2 ¼ 5=u* Sc2=3 :
ð3Þ
Sc is the Schmidt number in air, equal to v/D where v is
the kinematic viscosity, and D is the molecular diffusivity
of the species. The total deposition velocity of a gas, vD,
is given by
vD ¼ ð1 S ÞKT =H;
ð4Þ
where S is the saturation of the gas in seawater, equal to
HCw/Cg.
[4] In this study, we investigate and assess the flux of
methanol across the air-sea interface by comparing measured concentrations of both methanol and the oceanic
tracer DMS, made during the North Atlantic Marine Boundary Layer Experiment (NAMBLEX) held at Mace Head
during 24 July to 3 September 2002, with calculations using
Figure 1. Time series of methanol and DMS concentrations observed during NAMBLEX. Also shown
on the top x axis are labels denoting the origin of air masses arriving at Mace Head during the campaign:
A, South Atlantic; B, Mid-Atlantic; C, Scandinavia/Greenland; D, Europe; E, Atlantic with coastal/local
influence; F, surface Atlantic cyclonic conditions.
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Figure 2. (a) [DMS], [CH3OH], and mean wind speed, and (b) [ethane] and [acetylene] measured
during 17– 20 August at Mace Head, Ireland.
a zero-dimensional box model and wind-speed-dependent
gas transport velocities.
2. Methods
2.1. Calculated Gas Transport and Deposition
Velocities
[5] The liquid phase gas exchange constant kw was calculated according to the Nightingale et al. [2000] parameterization appropriate for wind speeds over 3.6 m s1,
kw ¼ 0:23ðU Þ2 þ 0:1ðU Þ ðSc=600Þ1=2 :
ð5Þ
The Schmidt number of DMS was calculated from
Saltzman et al. [1993], assuming a sea surface temperature of 15C as deduced from SeaWiFS data. Sc for
CH 3OH was calculated using a molecular weight
correction of the DMS value. A constant and supersaturated DMS sea surface concentration of 4 nM was
assumed, as appropriate for northeast Atlantic summer
conditions [e.g., Kettle and Andreae, 2000].
[6] The aerodynamic resistance R1 was derived by assuming a constant u*/U ratio of 0.358, as observed at Mace
Head at 20 m in ocean sector winds, and a modified version
of equation (2),
2
R1 ¼ U =u* :ð1=U Þ:
ð6Þ
Thus, small variations in u*, which would otherwise have a
large sporadic effect on R1, were smoothed out. This was
appropriate for our analysis because we used mean wind
speeds and gas concentrations (see sections 2.2 and 3). The
gas-phase film resistance R2 (equation (3)) was calculated
by assuming the ‘‘climatological’’ u*/U(20 m) ratio of
0.358 and multiplying by the relevant wind speed value to
obtain u*.
[7] The saturation S (equation (4)) is dependent upon the
assumed seawater concentration of methanol. Singh et al.
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Figure 3. Examples of 5-day back-trajectories and surface plots arriving at Mace Head during 17–
20 August. The crosses on the trajectories denote 6-hour time periods.
[2003] calculated 100 nM for the Pacific, and Galbally and
Kirstine [2002] calculated a mean value of 300 nM for the
Northern Hemisphere. Using these values and an average
Cg of 1 ppbv, methanol is between 48% undersaturated and
56% oversaturated. Clearly this represents a large uncertainty. Another estimate can be made using a steady state
approach, assuming that oceanic methanol production is
negligible compared to atmospheric input,
H Cw =Cg ¼ S ¼ k1 =ðk0 þ k1 Þ;
ð7Þ
where k1 is the rate constant for oceanic evasion = KT/zw
where zw is the ocean mixed layer depth (typically 75 m),
and k0 is the total average rate constant for destruction
within the oceanic mixed layer including chemical and
biological consumption (kd) and eddy diffusion from the
surface ocean layer to deeper layers, Ked, equal to (Dzkd)0.5
[Butler, 1994], where Dz is 5.4 103 m2 yr1 [Li et al.,
1984]. Galbally and Kirstine [2002] estimate an aquatic
lifetime (1/kd) of CH3OH due to OH attack only of
1000 years. The biological destruction rate of methanol in
seawater is unknown, but a 3-day lifetime for bacterial
methanol uptake has been estimated based on biological
turnover times for other marine metabolites [Heikes et al.,
2002]. The average lifetime over the mixed layer depends
on the extent and nature of any methanol-degrading
bacteria, a large uncertainty in budget considerations. A
lifetime of 1000 years gives an undersaturation value,
(1 S), of 0.03, and a lifetime of 10 years gives a value of
0.3. For the purposes of this study we used a value of 10%
undersaturation, similar to Singh et al. [2003]. Values of KT
and vD for different wind speeds are shown in Table 1. The
range in calculated values of vD is also shown; values were
estimated assuming a ±17% error in KT (arising from a ±5%
error in R2 and a ±25% error in R1, calculated from the
variability in the measured U/u* ratio) and a range of (1 S) values between 0.03 and 0.3; the overall error in the
estimated value of vD is dominated by the uncertainty in the
saturation of methanol in seawater.
2.2. Wind Speed
[8] Rather than use local wind speed, we used wind
speeds calculated from the ECMWF back trajectories (the
average over 5 days). The rationale for this was that the
westerly winds impacting Mace Head are influenced by
local coastal effects, for example, sea breezes and the effects
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Figure 4. Wind speed dependence of the averaged DMS (shaded squares) and CH3OH (black circles)
concentrations during 17– 20 August. The error bars indicate the 1s variation of the gas concentrations
over the averaging period; where these are not apparent, the error is less than the size of the symbol.
of an island, whereas the longer-term wind speeds reflect
the history of relatively long-lived trace gases in an air mass
more accurately. The back-trajectories revealed that during
the cyclonic period, the air-mass trajectories had been
largely at or near the surface (pressures between 1000 and
950 mbar) during the 5 days previous to arriving at the site.
A correction to the back-trajectory wind speeds was made
by forcing agreement between the highest wind speeds both
calculated and measured (at 20 m) at the start of the cyclonic
period, and applying the reduction of 19% to the whole backtrajectory wind data set. Theoretically, the variation of mean
wind speed with height can be described using a power law
function [Seinfeld and Pandis, 1998],
U ð zÞ=U ðz1 Þ ¼ ð z=z1 Þp :
ð8Þ
The power p to which (z/z1) is raised depends upon the
surface roughness and Monin-Obukhov lengths, and for
neutrally stable atmospheres with a surface roughness of
103 m, appropriate for a smooth sea, p is about 0.05– 0.15
[Seinfeld and Pandis, 1998]. Thus a height of about 100 m
is calculated for a wind speed 19% higher than that at 20 m,
which is consistent with the trajectories of between 1000
and 950 mbar calculated by ECMWF. We note that
formulation (5) applies to wind speeds measured at 10 m
height (rather than 20 m), but assume any errors here are
negligible.
2.3. Model
[9] A box model was constructed from a subset of the
Master Chemical Mechanism (MCMv3.0) [Saunders et al.,
2003]. The model contained 83 reactions of DMS and
methanol oxidation chemistry. Gas-phase production of
methanol was mainly from the permutation reactions of
CH3O2 with other organic peroxy radicals, with a small
contribution from the photolysis of glycoaldehyde [Bacher
et al., 2001]. The concentrations of CH3O2 and of total
organic peroxy radicals were set to those predicted by the
full MCM after 10 days of transport from a continental
landmass. Photochemical destruction of methanol was
assumed to be solely from reactions with OH; cloud
reactions were not included in the model for the sake of
simplicity. NO2 levels were kept constant in the model at
200 pptv, giving rise to peak midday NO levels of 50 pptv,
consistent with previously reported clean air data at Mace
Head [Carpenter et al., 1997]. The model-predicted OH
concentrations peaked at noon at 3.5 106 molecule
cm3, in line with concurrent OH measurements in clean air
made using the FAGE technique (J. D. Lee et al., manuscript in preparation, 2004). The model was run for 20 days
to achieve steady state. Model runs were repeated for
different DMS evasion rates and CH3OH deposition velocities according to the wind speed.
2.4. Measurements
[10] The NAMBLEX field campaign was held at the
remote observatory Mace Head (53.3N, 9.9W), Ireland,
between 24 July and 3 September 2002. Mixing ratios of
C2– C7 hydrocarbons, DMS, and small oxygenated molecules including methanol were determined using a dual
channel Perkin Elmer GC-FID instrument coupled to a
thermal desorption sampling system. Sample volumes of
500 mL were preconcentrated on a two-stage carbon-based
adsorbent trap held at – 20oC before being transferred to the
GC by rapid heating to 400oC. Oxygenated volatile organic
species including methanol were separated on a 10-m
LOWOX column (Varian Inc.) and hydrocarbons and
DMS were analyzed using a 50-m Al2O3 PLOT column
(Varian Inc.) [see Hopkins et al., 2003]. The accuracy of
both the DMS and methanol measurements is determined
primarily by the uncertainties in the permeation tube
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Figure 5. Wind speed dependences of the calculated DMS (shaded line) and CH3OH (black line) gas
transport velocities.
method used for calibration, comprising weight measurement uncertainties and calibration gas dilution uncertainties.
The resulting accuracy is calculated to be ±10 ppt for DMS
and ±150 ppt for methanol. Precision, assessed from run-torun reproducibility of standards, is better than 5 ppt for
DMS and 20 ppt for CH3OH.
[11] All samples were collected at shoreline from a height
of 20 m through a stainless steel 3/400 manifold, pumped at
30 L min1. Other members of the NAMBLEX group,
although not reported explicitly in this paper, made an
extensive suite of measurements on other inorganic gases
and aerosols some of which have been used to provide
alternative confirmations of airmass origin and characteristics. Back-trajectory calculations were made by the British
Atmospheric Data Centre based on ECMWF reanalysis of
satellite wind field data.
3. Results
[12] Figure 1 shows the time series of methanol and DMS
mixing ratios during the NAMBLEX campaign and the
different air mass types arriving at the site. Three periods of
surface Atlantic cyclonic activity occurred during which the
mixing ratios of DMS and methanol exhibited an anticorrelation. Figure 2a shows measured CH3OH and DMS concentrations along with the mean wind speed during the
longest cyclonic event of 17 –20 August, a period during
which the averaged surface wind speed changed substan-
Figure 6. Comparison of modeled (lines) and measured (symbols) wind speed dependences of DMS
(shaded) and CH3OH (black) concentrations.
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tially as a low-pressure system evolved over the northeast
Atlantic (typical air mass trajectories are shown in
Figure 3). The decrease in DMS and increase in CH3OH
concentration as the wind speed gradually subsides is
clearly apparent. Figure 2b shows the concentrations of
CH3OH along with the tracers ethane and CHCl3 during this
cyclonic period. Although the ethane variability and concentrations are consistent with background oceanic air, the
chloroform trace shows evidence that the trajectories had a
coastal influence during the morning of 19 and 20 August.
The data collected during these periods were removed for
subsequent analysis.
[13] Figure 4 shows the wind speed dependence of the
averaged DMS and CH3OH concentrations during 17 –
20 August (with coastally influenced data removed). The
positive correlation between [DMS] and wind speed and the
anticorrelation between [CH3OH] and wind speed suggested by Figure 2a is clearly confirmed. It is also apparent
that CH3OH shows a more linear dependence on wind
speed than DMS, whose concentration increases markedly
after a threshold wind speed of about 8 m s1. On the basis
of the premise that the source and sink terms of DMS and
CH3OH, respectively, are responsible for the wind speeddependence of their concentrations, a simple explanation for
the trends apparent in Figure 4 is that the DMS flux is
dependent almost entirely upon kW which increases nonlinearly with wind speed whereas the transport velocity of
methanol is largely controlled by the aerodynamic resistance (equation (6)); that is, KT is proportional to U, if U(z)/
u* is constant. Figure 5 illustrates the wind speed dependence of the derived transport velocities (as shown in
Table 1) for DMS and CH3OH.
[14] In order to form a complete picture of the interdependencies between gas concentration and wind speeds,
the atmospheric chemistry has to be accounted for. Figure 6
shows the wind speed dependence of the measured CH3OH
and DMS concentrations compared with the values predicted by the box model. The agreement is excellent for
DMS and good for CH3OH, although the model shows a
weaker dependence of [CH3OH] on wind speed than the
measurements. A repeat simulation with a higher value of S
(and hence higher vD) and, to compensate, a higher production rate of methanol, resulted in a steeper slope. As our
estimate of S is subject to a high degree of uncertainty, it
would certainly be possible to increase the deposition
velocities to obtain better agreement with the measurements; however, this would require higher atmospheric
methanol production rates than those calculated.
4. Conclusions
[15] We have shown evidence that the North Atlantic
Ocean is a sink of methanol. Although ocean deposition
of CH3OH has also been inferred by other recent modeling
studies [Galbally and Kirstine, 2002; Heikes et al., 2002;
Singh et al., 2003], this study is the first to observe a wind
speed dependence in observed concentrations consistent
with a net flux to the surface. The concurrent observed
increase in DMS concentrations with wind speed, and the
good agreement between model and measurements, adds
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weight to these conclusions. At a wind speed of 8 m s1, the
calculated deposition velocity of CH3OH was 0.09 cm s1,
in excellent agreement with other studies [Singh et al.,
2000; Heikes et al., 2002]. However, the absolute value is
crucially dependent upon the concentration of methanol in
the ocean and the rate of oceanic methanol destruction,
neither of which to our knowledge has been reported. The
rate of oceanic destruction is a particularly important uncertainty. Nevertheless, deposition to the ocean surface looks to
be an important sink for methanol, with an estimated average
marine boundary layer lifetime of 13 days against deposition, similar to the lifetime with respect to OH oxidation. The
calculated vD for methanol varied over a large range from
0.05 to 0.17 cm s1, depending on the wind speed, highlighting the requirement for an intrinsic calculation of windinduced turbulent gas transfer in atmospheric chemistry
models rather than use of constant values.
[16] Acknowledgments. We are grateful to Dwayne Heard (University of Leeds) for organization of the NAMBLEX field project and
John Methven (University of Reading) for calculating the back-trajectories. NAMBLEX was funded by the Natural Environment Research
Council.
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L. J. Carpenter, J. R. Hopkins, and A. C. Lewis, Department of
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