Atmospheric Research 71 (2004) 215 – 232
www.elsevier.com/locate/atmos
Field intercomparison of filter pack and impactor
sampling for aerosol nitrate, ammonium, and
sulphate at coastal and inland sites
Zhuoer Huang 1, Roy M. Harrison *, Andrew G. Allen,
Jonathan D. James, Rob M. Tilling, Jianxin Yin
Division of Environmental Health and Risk Management, School of Geography, Earth and Environmental
Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Abstract
An intercomparison has been performed on the coarse (>2.5 Am) and fine fraction ( < 2.5 Am)
mass concentrations of particulate nitrate, ammonium, and sulphate determined simultaneously by
filter pack and MOUDI samplers. Five intensive field campaigns have been carried out in summer
and winter seasons, both at coastal sites (Mace Head, Ireland, and Tenerife, Spain) and at an inland
site (University of Birmingham, West Midlands, UK). Comparison between particle sulphate
measurements shows that sulphate measurements are the same with both filter pack and MOUDI,
independent of sampling site or season. For both nitrate and ammonium, the MOUDI results are
observed to be usually less than those from the filter pack, especially in the case of polluted air
masses. During periods when the measured concentration products [NH3][HNO3] are low ( < 0.1
ppbv2), the ammonium concentrations obtained with the two samplers are matched very well with
each other, but for nitrate, the filter pack system provides 15% higher mass concentrations than the
MOUDI which are attributable to different inlet efficiencies. During more polluted periods, however,
when there are high levels of gaseous ammonia and nitric acid in the atmosphere (with the measured
concentration products [NH3][HNO3]>0.1 ppbv2), significantly negative sampling artefacts are
observed for both nitrate and ammonium concentrations obtained with the impactor relative to the
filter pack. Nevertheless, it is shown that both filter pack and MOUDI are capable of collecting
ammonium nitrate from polluted air masses although the absolute efficiency is unknown. From the
measurements obtained with the MOUDI in summertime at both coastal and inland sites, it is
observed that about 64% of collected particle nitrate is present in the coarse (>2.5 Am) mode; but in
wintertime, only about 29% of particle nitrate is found to reside in the coarse particles collected at the
* Corresponding author. Tel.: +44-121-414-3494; fax: +44-121-414-3709.
E-mail address: [email protected] (R.M. Harrison).
1
Present address: Guangzhou Environmental Monitoring Centre, 95 Jixiang Road, Guangzhou 510030, PR
China.
0169-8095/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.atmosres.2004.05.002
216
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
inland site. The lower efficiency of the MOUDI for ammonium nitrate relative to the Teflon filter is
in marked contrast to the reported higher efficiency of the Berner impactor than the Teflon filter for
sampling nitrate from polluted air.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Nitrate; Ammonium; Sulphate; MOUDI
1. Introduction
Particulate nitrate, ammonium, and sulphate are a subject of interest in the investigation
of atmospheric aerosols since these inorganic species are important anthropogenic
components of aerosols and precipitation (Harrison and Allen, 1990; Schaap et al.,
2002). Field measurements of nitrate, ammonium, and sulphate in atmospheric particles
are frequently reported in the literature from different regions of the world (e.g., Robarge
et al., 2002; Moya et al., 2001; Zhuang et al., 1999). Prior to chemical analysis, the aerosol
particles were generally collected using filter packs or impactor samplers. However, owing
to the semi-volatile character of ammonium nitrate and the reactivity of strong acid and
gaseous species, such as sulphuric acid, nitric acid and ammonia, sampling artefacts are a
well-known problem with atmospheric particles collected for the determination of nitrate
and ammonium salts. Evaporative loss of ammonium nitrate during filter sampling has
been the subject of many studies (e.g., Appel et al., 1980; Wang and John, 1988; Cheng
and Tsai, 1997; Hering and Cass, 1999), and previous results have shown that the losses of
ammonium nitrate were variable and dependent on many factors, such as sampling period,
properties of the substrate, ambient temperature, relative humidity, and even the particle
concentration in the air (loaded mass). From previous experiments, two major mechanisms
have been identified for the negative sampling artefacts, i.e., pressure drop across the
particle-collecting substrates, and changes in solid– gas equilibrium between particulate
ammonium nitrate and gas-phase nitric acid and ammonia during sampling. It has however
been reported variously that evaporative losses of ammonium nitrate from impactor
samplers are much less than those from filter samplers (Wang and John, 1988), that
losses of semi-volatiles in impactors are low (Hering et al., 1997), and that nitrate,
ammonium and sulphate agreed well (within 10% or better) between filter pack and
MOUDI impactors (Hughes et al., 2000). In particular, field intercomparisons made in the
highly polluted atmosphere of California have shown considerably greater loss of nitrate in
Teflon filter sampling than from collection in a Berner impactor (Wang and John, 1988;
Hering and Cass, 1999). On the other hand, sampling ambient air in California, Chang et
al. (2000) found broadly comparable efficiencies for nitrate collection for Teflon filters and
MOUDI impactors. Efficiencies for nitrate with both samplers increased at higher
concentrations, a finding also reported by Tsai and Perng (1998) using high volume and
dichotomous samplers.
Much of the previous systematic work on sampling artefacts was carried out in the
laboratory, and much has concentrated primarily on negative sampling artefacts (i.e.,
losses of ammonium nitrate from filter substrates). As a result, the denuder-filter pack
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
217
technique has been widely recommended for aerosol nitrate collection (Allegrini, et al.
1987; Harrison and Kitto, 1990). However, field intercomparison tests provide the best
insights into sampler behaviour in practical situations of use.
In the past few years, the ACSOE Eastern Atlantic Experiment (EAE) and the Pollution
in the Urban Midlands Atmosphere (PUMA) projects have been conducted in the northeastern Atlantic Ocean (during the summers of 1996 – 1997) and in the West Midlands area
of the UK (during the summer of 1999 and winter of 2000), respectively. During the field
campaigns, both traditional filter packs and a Micro Orifice Uniform Deposit Impactor
(MOUDI) sampler were employed for collecting aerosol particles from ambient air. In this
paper, a performance comparison of filter pack and MOUDI as samplers for aerosol
nitrate, ammonium, and sulphate is described.
2. Field measurements
2.1. Sampling sites and campaigns summary
During the summers of 1996 and 1997, two coastal field campaigns were conducted at
Mace Head Research Station (Galway, Ireland). This atmospheric sampling station is an
isolated site situated on the headland of a peninsula in western Ireland, and is exposed to
predominately westerly air streams from the Atlantic Ocean. Aerosol particles and gaseous
species were collected continuously from 9 June to 5 August 1996, and from 29 April to
29 May 1997. Another coastal sampling site was located at Tenerife, Canary Islands, and
atmospheric aerosol samples were collected here continuously from 28 June to 23 July
1997, during the ACE-2 experiment. In the PUMA programme, the field campaigns were
carried out at the University of Birmingham in the West Midlands of the United Kingdom,
during the summer and winter of 1999 – 2000. During field campaigns, the aerosol
particles were collected simultaneously with both filter pack and impactor samplers.
The sampling dates, the temperature ranges and the ranges of relative humidity are listed in
Table 1. The Birmingham location is a polluted inland urban site.
2.2. Collection of aerosol particles and gaseous species with filter packs
Filter packs comprised four 47-mm diameter sequential filters. Coarse particles (>2.5
Am in aerodynamic diameter) were collected on a 12-Am pore size Nuclepore polycarTable 1
Summary of field sampling campaigns
Campaign
Sampling date
Mace Head 1996
Mace Head 1997
Tenerife 1997
PUMA 1999
PUMA 2000
June 9 – August 5
April 29 – May 29
June 28 – July 23
June 14 – July 11
January 20 – February 17
Number of samples
T (jC)
RH (%)
Filter pack
MOUDI
Min – Max
Min – Max
146
74
184
116
71
7
8
24
24
11
9 – 22
7 – 16
18 – 30
8 – 27
0 – 13
54 – 99
68 – 99
52 – 100
28 – 95
45 – 99
218
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
bonate filter, fine particles ( < 2.5 Am in aerodynamic diameter), on 1-Am pore size Teflon
filter, acidic gases and ammonia gas on cellulose filters impregnated with 1% Na2CO3 and
5% ascorbic acid, respectively. Note that no size selective inlet was used, and therefore, the
coarse particle fraction has an undefined upper limit and does not correspond to the
commonly measured 2.5 – 10-Am size fraction. The use of sequential filtration by
Nuclepore filters as a means of aerosol size fractionation has been reviewed by Heidam
(1981).
At the coastal sites, the open-face filter packs were mounted in a vertical orientation,
with inlet facing downwards, about 3 m above the ground on a specially assembled
sampling platform (approximately 20 m from the high tide mark), and were connected to a
diaphragm pump (Charles Austen, UK), which was regulated to a flow rate of 30 l min 1
during the sampling period. An automated eight-port timer was employed to control the air
flow through eight separate filter packs to perform 24 h of sample collection in a day (each
filter pack sampling for 3 h). The total air volume sampled with the filter pack was
measured by using a gas meter, and the air flow rate was checked by rotameters. After
sample collection, all the filters were placed in polypropylene bottles and stored in a
freezer before taking back to Birmingham for laboratory analysis. At the inland PUMA
site, daily samples were collected onto 47-mm diameter Whatman PTFE filters (1-Am pore
size) using an R&P Partisol-Plus Model 2025 sequential air sampler at a flow of 16.7
l min 1 fitted with a PM10 inlet.
2.3. Collection of particulate matter by MOUDI and gaseous species by denuder
The size-segregated sampler used in this project was a MOUDI 110 (MSP, Minneapolis, MN, USA), which provides 10-stage fractionation of aerosol particles. The 50% cutpoints for the particle aerodynamic diameter (Am) are as follows: 18, 9.9, 6.2, 3.1, 1.8, 1.0,
0.55, 0.325, 0.175, 0.099 and 0.054. With the inlet and after-filter, 12 fractions of aerosol
particles can be obtained for the sampled air masses. The pump used to supply the
sampling flow rate (30 l min 1) was a Gast twin piston pump, and the airflow was
measured by a rotameter. The fractionated particles at every stage in the MOUDI were
collected on a Teflon filter (the same material as that used for fine particles collection in
the filter pack). The MOUDI inlet (2.3-cm diameter) was positioned under a simple plastic
shelter to protect from rain while allowing free ventilation. Particle size distributions were
inverted using the procedure described by Winklmayr et al. (1990) and modified for the
MOUDI by Keywood et al. (1999).
During the PUMA campaigns, gaseous nitric acid in the atmosphere was determined by
ion chromatography after collection with a glass denuder coated with sodium carbonate.
Ammonia was determined using an AMANDA continuous ammonia denuder gas analyser
(Wyers et al., 1993).
2.4. Filter extraction and sample analysis
Samples collected on filters were extracted directly with 10.0 ml of distilled deionized
water in polypropylene bottles (shaken for 30 min in a mechanical shaker), Teflon filters
being previously wetted with 0.5 ml of isopropanol before water extraction (total volume
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
219
10.0 ml). The filter extraction processes were all performed in a laminar flow cabinet, and
the sample solutions were stored at 4 jC or below while waiting for chemical analysis.
The anion concentrations (NO3, and SO42 ) in the extract solution were analysed by
using ion chromatography (Dionex DX-500 IC, equipped with AS40 automated sampler,
AG11-AS11 columns and CD20 conductivity detector). A non-sea-salt sulphate (nssSO42 ) concentration was calculated by subtraction of marine sulphate based on the
chloride to sulphate ratio in seawater of 0.14 from [nss-SO42 ]=[SO42 ] 0.14[Cl]
(Millero, 1974). The NH4+ concentration in the extract solutions was measured by using
a flow injection fluorescence detector (JASCO model 821-FP Intelligent Spectrometer),
based on the reaction of ammonium and phthalic dicarboxaldehyde (OPA) and sodium
sulphite at pH 11 (Rapsomanikis et al., 1988).
3. Results and discussion
3.1. Mass concentrations and particle size distributions of the analytes
Mass concentrations of soluble particulate and gaseous species were measured
simultaneously by using filter packs, impactor sampler and denuder-based gas samplers,
as described above. Sampling times with the MOUDI varied from 24 to 93 h, while all
filter pack samples were collected for 3 h. At the coastal sites, clean oceanic air masses
were sampled for much of the time when westerly air flow was encountered, but at the
inland site, where local anthropogenic sources made a significant contribution, more
polluted air masses were sampled during most sampling periods. During the 1997
campaign, however, polluted air masses containing high levels of secondary particulate
species were sampled at the Mace Head sampling site. Summary statistics for the analytes
of interest are listed in Table 2 and have been calculated using the original sampling
intervals, rather than aggregating to a common length. However, a cut size of 2.5 Am is not
directly available in the MOUDI, and for the purpose of mode comparison between the
two samplers, the mass concentration of particulate species at the stage of 3.1 – 1.8 Am was
separated into the coarse and fine fractions by plotting a cumulative distribution curve and
determining the masses above and below 2.5 Am.
Sampling efficiency can be affected by a multitude of factors. These include the inlet
efficiency and cut-off of the air sampler, filter efficiency (generally assumed to be very
high), chemical reactions leading to collection or loss of the analyte of interest from the
sample and the evaporative loss of semi-volatile substances. Where possible, this study has
sought to interpret differences between the efficiencies of filter and impactor samplers in
terms of these processes.
In Table 2, the mean concentration ratios of filter pack to MOUDI (FP/MO) for nonsea-salt particulate sulphate (nss-SO42 , FP/MO = 1.02– 1.05) show that the two samplers
provide almost the same mean concentrations for nss-sulphate in each field campaign,
independent of both sampling site and season. In the cases of nitrate and ammonium,
however, the filter pack results are usually higher than those obtained with the MOUDI,
with the only one exception of ammonium determined in the campaign of Tenerife 1997
where the mean ammonium mass concentration ratio of the two samplers is close to one.
220
Table 2
Summary statistics of aerosol and gas species concentrations (Ag m 3)
Campaign
NO3
Filter pack
(1)
(3)
(4)
(5)
MOUDI
Filter pack
MOUDI
Filter pack
MOUDI
NH3
(Gas)
HNO3
(Gas)
[NH3]
[HNO3]
(ppbv2)
TM
CM
FM
TM
CM
FM
TM
CM
FM
TM
CM
FM
TM
CM
FM
TM
CM
FM
1.02
1.81
0.07
5.09
0.60
1.94
2.52
1.99
0.34
6.38
0.53
1.88
1.26
0.86
0.51
3.41
0.85
1.19
1.93
1.54
0.22
6.43
0.62
1.03
0.05
2.95
0.41
0.78
0.02
2.14
0.53
0.74
0.08
2.17
0.67
0.36
0.60
0.03
1.71
0.17
0.14
0.04
0.46
0.06
0.13
0.002
0.35
0.59
1.00
0.08
2.83
0.39
0.60
0.02
1.70
0.02
0.006
0.007
0.001
0.02
0.38
0.59
0.02
1.68
1.55
2.47
0.25
7.09
1.63
2.12
0.30
6.30
0.05
0.09
0.06
0.02
0.17
1.54
2.07
0.28
6.13
0.29
0.41
0.04
1.10
0.35
0.61
0.03
1.73
0.14
0.33
0.001
0.89
1.18
1.34
0.13
4.30
1.34
1.05
0.21
3.13
0.64
0.86
0.65
0.08
1.70
0.48
0.50
0.13
1.63
0.32
0.35
0.003
0.95
1.48
0.90
0.10
3.08
1.39
0.83
0.13
2.50
0.03
0.04
0.03
0.004
0.08
1.36
0.81
0.13
2.42
0.57
0.24
0.21
0.86
3.06
1.54
0.42
4.65
3.54
1.94
0.43
6.25
0.05
0.16
0.11
0.04
0.39
3.38
1.85
0.38
5.86
0.50
0.23
0.11
0.74
1.05
0.68
0.05
2.01
0.26
0.18
0.003
0.53
1.07
0.77
0.38
3.00
0.19
0.09
0.09
0.41
1.06
0.77
0.36
3.22
0.66
0.71
0.56
0.18
2.19
0.36
0.23
0.17
1.03
0.02
0.01
0.004
0.047
0.48
0.34
0.18
1.39
0.51
0.40
0.16
1.74
0.03
0.02
0.007
0.007
0.03
0.49
0.40
0.15
1.72
0.43
0.36
0.05
1.25
1.80
1.52
0.47
5.49
2.16
1.64
0.58
6.26
0.11
0.24
0.16
0.003
0.63
1.92
1.53
0.47
5.85
0.16
0.09
0.06
0.42
0.15
0.07
0.06
0.35
0.01
0.007
0.004
0.03
1.01
0.69
0.19
2.85
0.60
0.61
0.53
0.10
1.99
0.40
0.27
0.07
0.92
1.11
0.57
0.33
2.39
0.03
0.04
0.05
0.002
0.15
1.07
0.55
0.33
2.39
1.71
2.57
0.31
7.47
0.09
1.05
3.63
1.75
0.63
5.51
0.16
1.02
2.25
1.79
0.63
6.34
0.19
1.04
3.26
1.99
0.97
8.71
0.16
0.11
0.03
0.38
1.33
0.84
0.21
2.35
0.65
1.13
0.08
3.18
0.10
1.68
1.80
1.17
0.10
3.85
0.18
1.29
0.50
0.36
0.18
1.43
0.04
0.99
1.26
0.83
0.34
3.84
3.16
1.64
1.00
6.81
0.06
0.18
0.10
0.07
0.51
2.98
1.64
0.84
6.61
2.06
0.87
0.55
3.63
1.49
1.31
0.21
4.47
1.50
1.40
0.09
4.97
1.32
1.42
0.31
4.80
0.29
0.38
0.46
0.05
1.44
0.94
0.98
0.26
3.36
1.14
1.51
1.77
0.17
3.75
0.97
0.82
0.24
2.36
0.02
0.02
0.01
0.005
0.04
0.95
0.82
0.20
2.33
1.03
2.57
1.72
1.05
5.61
2.53
1.80
0.89
5.99
0.08
0.20
0.11
0.10
0.40
2.32
1.77
0.77
5.78
2.25
1.17
0.77
5.19
0.27
0.13
0.06
0.58
0.25
0.13
0.07
0.50
1.91
3.14
2.86
0.57
8.74
2.38
1.55
1.02
Campaigns: (1), Mace Head 1996; (2), Mace Head 1997; (3), Tenerife 1997; (4), PUMA 1999; (5), PUMA 2000.
TM, CM and FM represent total mass, coarse and fine mode concentrations, respectively; FP/MO represents the mean TM ratio of filter pack to MOUDI; nss-SO24 represents the non-sea-salt
SO24 (i.e., [nss-SO24 ]=[SO24 ] 0.14 [Cl]).
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
(2)
Mean
S.D.
Min
Max
CM/TM
FP/MO
Mean
S.D.
Min
Max
CM/TM
FP/MO
Mean
S.D.
Min
Max
CM/TM
FP/MO
Mean
S.D.
Min
Max
CM/TM
FP/MO
Mean
S.D.
Min
Max
CM/TM
FP/MO
nss-SO24 NH+4
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
221
Obviously, during the field collection of particulate nitrate and ammonium sampling
artefacts must have been encountered with at least with one of the samplers. In addition,
the wide range of the mean concentration ratios (FP/MO, 1.19 –2.38 for nitrate, 0.99 –1.68
for ammonium) indicates that the sampling artefact is strongly dependent on the characteristics of the sampled air mass (e.g., the concentrations of gaseous nitric acid and ammonia,
temperature, relative humidity).
From the concentration ratios of the coarse mode fraction to total particulate species
(CM/TM) listed in Table 2, it can be concluded that ammonium and nss-sulphate are
mainly present in the fine particles, independent of both the sampling site and season. In
the case of particulate nitrate, however, the partition pattern between coarse and fine
modes is more complicated, especially in the measurements with the filter pack. In the
summer campaigns, however, a relatively constant partition ratio was obtained with the
MOUDI for particulate nitrate (roughly 64% of particulate nitrate was present in the coarse
mode). On the other hand, it is interesting to note that just the reverse partition pattern was
obtained with the MOUDI for particulate nitrate in the winter campaign carried out at the
inland site, where about 29% of particulate nitrate was present in the coarse mode. Similar
results have been reported in recent literature for the partition of particulate nitrate in field
campaigns. In a coastal field campaign carried out with a MOUDI in Hong Kong (12 –25
jC, 27– 87% RH), Zhuang et al. (1999) found that about 74% of PM10 nitrate was present
in the size range of 1.8– 10 Am. During an intensive field campaign carried out in Mexico
City in the spring of 1997, Moya et al. (2001) found that about 28% of PM10 nitrate was
present in the coarse mode. These measured field results suggest that the partition pattern
of particulate nitrate is season dependent (sensitive to meteorological change, especially
change in ambient temperatures). This is because high temperatures inhibit the thermodynamic formation of ammonium nitrate which mainly resides in fine particles, whereas
lower temperatures favour its formation process, although the nitrate present as sodium
nitrate mainly in the coarse fraction is not affected by the change in ambient temperature.
The measurements by MOUDI provide information on the particle size distribution of
aerosol species. Fig. 1 shows the average size distribution patterns for nitrate, ammonium,
and nss-sulphate measured by MOUDI in the five field campaigns. Ammonium and nsssulphate are found to co-exist mainly in the fine mode with a peak at about 0.2– 0.7 Am.
Nevertheless, the distribution of nss-sulphate is rather broad in size range, ranging from
0.03 to 18 Am. The size distribution of nitrate is more complicated and dependent on the
sampling season and the sampling site, although it consistently shows a peak in the coarse
mode. For the Mace Head campaigns of 1996 and 1997, the distribution of nitrate shows
a peak at about 1 –8 Am, although the nitrate measured in Mace Head 1997 has a slightly
broader distribution of sizes than that in the 1996 campaign. In the campaigns of Tenerife
1997 and PUMA 1999, the distributions of nitrate are bimodal in the size range of 0.03 –
18 Am, with one main mode peaking at about 1 –10 Am and another smaller mode at
about 0.2 – 0.6 Am. In the samples from PUMA 2000, however, almost 30% of particle
nitrate is distributed in the mode at about 0.2– 0.7 Am, co-existing with ammonium and
nss-sulphate.
The Tenerife measurements were at times influenced by the presence of air masses
likely to have contained continental dust. The prevailing meteorology was controlled by
the Azores High over the North Atlantic. This brought northerly air masses to Tenerife
222
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
Fig. 1. Particle size distributions of nitrate, ammonium and nss-sulphate measured with the MOUDI. Campaigns:
(a) Mace Head 1996; (b) Mace Head 1997; (c) Tenerife 1997; (d) PUMA 1999; (e), PUMA 2000.
with periodic entrainment of continental air due to the passage of mid-latitude cyclones,
resulting in alternate periods of clean and polluted air. A cyclone over western Europe
resulted in clean polar air reaching the site from 25 June to 3 July. High pressure from 4 to
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
223
10 July was associated with continentally polluted air. Cyclonic weather bringing clean
maritime air prevailed between 11 and 13 July, followed by a large high pressure with
continental pollution from 14 July to the end of the campaign. The high CM/TM and low
FP/MO ratios for nitrate (Table 2) observed in this campaign appears to imply the presence
of involatile forms, perhaps associated with the continental dusts. The low amounts of
coarse particle sulphate in all campaigns appear to imply relatively young air masses in
which sulphate has not displaced nitrate from the aerosol.
The winter PUMA 2000 Campaign shows low concentrations of NH4NO3 and gaseous
HNO3 (Table 2). The FP/MO ratio for NO3 is the highest of all periods at this time (FP/
MO = 2.38), suggesting that during sample collection much of the NH4NO3 in fine
particles was lost from the MOUDI by evaporation.
3.2. Regression analysis and comparison of sampling protocols
Paired data for particulate species collected simultaneously with both filter packs and
MOUDI were selected for comparative analysis. For this purpose, the filter pack data for
any given time interval were averaged according to the corresponding sampling period of
the MOUDI. As far as possible, inlet cut points of samplers were matched. Therefore, for
comparison with the open-face filter packs operated at Mace Head, a MOUDI inlet cut point
of 18 Am was used although it is likely that inlet efficiencies of both samplers had reduced
substantially by this size. For the data collected at the Birmingham site, the MOUDI cut
point of 9.9 Am was used to match the 10-Am size selective inlet of the Partisol.
Fig. 2 shows scatter plots of nss-sulphate mass concentration data pairs from all the five
field campaigns (the data pairs for mode comparison are available only from the coastal
sites). For total particle mass concentrations, the agreement between filter pack and
MOUDI is very good with a gradient of 1.06 and a correlation coefficient (R2) of 0.92,
indicating that nss-sulphate is measured accurately by the two samplers. Nevertheless, the
filter pack collected more coarse sulphate, and this is probably due to the difference
between the inlet configurations of the two samplers (open-face filter pack and MOUDI).
Fine mode sulphate collection is very similar.
Mass concentration comparisons of the nitrate in total particles and size fractions for
filter packs and MOUDI in the coastal campaigns (Mace Head 1996 and 1997, Tenerife
1997) are presented in Fig. 3. The data shown in Fig. 3(a, b, and c) represent the cases
where the concentrations of nitric acid and ammonia in the atmosphere were low (with
measured concentration products [NH3][HNO3] < 0.1 ppbv2), while Fig. 3(d, e, and f)
represents the measurements where the concentrations of nitric acid and ammonia were
relatively higher (more polluted air masses, with measured concentration products
[NH3][HNO3]>0.1 ppbv2). In the coastal campaigns, there are seven pairs of data
associated with polluted air masses, with one from the campaign of Mace Head 1996,
and the others from Mace Head 1997.
During sampling of clean air masses, as shown in Fig. 3(a), the data pairs for total
particulate nitrate from the two samplers relate almost linearly with each other, but the
filter pack collects 15% more nitrate than the MOUDI. A mechanism that may explain a
higher nitrate measurement from the filter pack is enhanced coarse nitrate collection
efficiency for the open-face filter pack over that of the MOUDI. Such an effect may be
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Fig. 2. Comparison of filter pack and MOUDI nss-sulphate mass concentrations (Ag m 3): (a) total nss-sulphate;
(b) coarse nss-sulphate (>2.5 Am); (c) fine nss-sulphate ( < 2.5 Am). The data for coarse and fine mode
comparisons are from the coastal campaigns, while the total nss-sulphate data are from all five campaigns.
greater for nitrate than for sulphate (see Fig. 2) because of the greater relative abundance of
nitrate in coarse particles. On the other hand, however, the fact that the filter pack has more
coarse nitrate and less fine nitrate suggests the existence of a size cut difference between
the two samplers. The over-collection of coarse relative to fine particles by the filter pack
could be a function of the shape of the cut-off curve of the Nuclepore filter (Heidam,
1981). As shown in Fig. 1(a and b), the mode in the nitrate size distribution at Mace Head
was typically close to 2.5 Am, and therefore, small differences in cut off curves could make
a significant difference to the coarse-fine split. While the MOUDI has very sharp cut
points, the 12-Am Nuclepore filter is a much less sharp discriminator (Heidam, 1981). The
tendency of the sequential filters to underestimate fine nitrate relative to the MOUDI is
however difficult to reconcile with the shape of the collection efficiency curve which
might be expected to allow significant penetration of coarse particles to the fine particle
filter. An alternative explanation is that particle bounce in the MOUDI led to an over-
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
225
Fig. 3. Comparison of filter pack and MOUDI nitrate mass concentrations (Ag m 3) measured at coastal sites: (a)
total nitrate, (b) coarse nitrate (>2.5 Am), and (c) fine nitrate ( < 2.5 Am) represent the data for clean air masses; (d)
total nitrate, (e) coarse nitrate (>2.5 Am) and (f) fine nitrate ( < 2.5 Am) represent polluted air masses.
estimation of the fine fraction concentration. During more polluted periods when the
concentrations of nitric acid and ammonia in the gas phase are relatively higher, the filter
packs provide almost twofold higher total particle nitrate mass concentrations than the
MOUDI, as shown in Fig. 3(d). In contrast with Fig. 3(c) (for clean air masses), where the
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filter pack has less fine nitrate than the MOUDI, Fig. 3(f) shows that from polluted air
masses the filter pack system has collected much more fine nitrate than the MOUDI. The
relationship between filter pack and MOUDI particle nitrate mass concentrations measured
at the inland site is presented in Fig. 4. It is observed that the samples from the filter pack
show generally higher nitrate mass concentrations than those from the MOUDI, particularly during PUMA 2000.
Fig. 5 shows scatter plots of particle ammonium mass concentrations data pairs from
the filter pack and the MOUDI at both the coastal and inland sites. For total and fine
particle ammonium, there is no significant systematic difference between the two methods
during the periods when clean air masses are sampled, as shown in Fig. 5(a and c),
indicating the two samplers collect the same particle ammonium in these cases. The coarse
fraction has low ammonium concentrations and shows a fair amount of scatter (Fig. 5(b)).
In the cases of polluted air masses, the filter pack system generally collects more particle
ammonium than the MOUDI, as shown in Fig. 5(d, e, and f).
3.3. Chemical balance of nitrate, sulphate and ammonium in collected particles
As shown in Fig. 1, most of particulate ammonium and nss-sulphate are present in fine
particles, while nitrate is mainly present in the coarse particles. However, these ionic
species also co-exist at a range of particle sizes, especially in the fine mode. In coarse
particles, nitrate is mainly present as sodium nitrate, formed from the reaction of nitric acid
and sea salt (Harrison and Pio, 1983), but in the fine particles, it may be present as
ammonium nitrate. As the main alkaline species in the atmosphere, ammonia reacts with
sulphuric acid and nitric acid to form both ammonium sulphate and ammonium nitrate,
which are mainly present in the fine particles. Therefore, analysis of the chemical balance
between ammonium, nss-sulphate and nitrate is a useful approach to check if the nitrate
resides as NH4NO3 in the collected particles, especially in the fine particles.
Fig. 4. Comparison of filter pack and MOUDI nitrate mass concentrations (Ag m 3) measured at the inland site:
(a) total nitrate (PM10) for PUMA 1999; (b) total nitrate (PM10) for PUMA 2000.
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
227
Fig. 5. Comparison of filter pack and MOUDI ammonium mass concentrations (Ag m 3). Coastal sites: (a) total
ammonium, (b) coarse ammonium (>2.5 Am) and (c) fine ammonium ( < 2.5 Am) represent the data for clean air
masses, while (d) total ammonium represents polluted air masses. Inland site: (e) and (f) total ammonium (PM10)
for PUMA 1999 and PUMA 2000, respectively.
The relationships between the concentrations of the ionic species in the fine particles
are shown in Fig. 6. The concentrations of ionic species are calculated for the individual
stage in the MOUDI or for the individual 3-h sampling sample in the filter pack. In the
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3
Fig. 6. Relationship between [NH+4] and (2 [nss-SO24 ]+[NO
). Campaigns: (a)
3 ]) in fine particles (nMol m
and (b) represent the MOUDI and filter pack data for Mace Head 1997, respectively; (c), (d), (e) and (f) represent
the MOUDI data for PUMA 1999, PUMA 2000, Mace Head 1996 and Tenerife 1997, respectively.
cases of polluted air masses (Mace Head 1997, PUMA 1999 and 2000), the one-to-one
relationship between [NH4+] and (2 [nss-SO42 ]+[NO3]) indeed demonstrates that
NH4NO3 is the probable chemical form of the nitrate present in the fine particles collected
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
229
both by filter pack and MOUDI. This is an important finding, as it indicates that both filter
pack and MOUDI had collected significant NH4NO3 in the field campaigns. In the cases
of clean air masses (Mace Head 1996 and Tenerife 1997), however, the concentrations of
[NH4+] in the fine particles are not high enough to match with ([2 [nss-SO42 ]+[NO3]),
as shown in Fig. 6(e and f), and the concentrations of fine particle nitrate are low. A
missing cation is indicated, which may be Na+, H+, or both (Kitto and Harrison, 1992;
Ottley and Harrison, 1992). In the < 2.5-Am size range, the latter appears more probable.
3.4. Equilibrium relationship
In the atmosphere, gaseous NH3 and HNO3 may be in equilibrium with NH4NO3 (solid
or aqueous), and this equilibrium has been the subject of many studies (Stelson and
Seinfeld, 1982; Allen and Harrison, 1989).
NH4 NO3
ðsÞ or ðaqÞ
¼ NH3
ðgÞ
þ HNO3
ðgÞ
ð1Þ
K ¼ ½NH3 ½HNO3 The relationship between the measured concentration products [NH3][HNO3] in units
of ppbv2, and the calculated dissociation constant for NH4NO3 is presented in Fig. 7.
Concentration products are plotted on a logarithmic scale against reciprocal mean absolute
temperatures. The lines represent the dissociation constant calculated as a function of
temperature for solid and aqueous ammonium nitrate at 95%RH (Stelson and Seinfeld,
1982; Allen and Harrison, 1989). During polluted periods, the measured concentration
products are inside or above the zone defined by the calculated lines, while during clean
periods, the measured concentration products are below the line for 95%RH. In the cases
of polluted air masses, the concentrations of gaseous NH3 and HNO3 were high enough to
form NH4NO3, and this supports the suggestion that the nitrate collected in fine particles is
present as NH4NO3, as discussed above. Therefore, it is reasonable to believe that during
sampling polluted air masses, the negative sampling artefacts in the MOUDI for both
nitrate and ammonium are related to the evaporative loss of fine particle ammonium
nitrate. Evaporative losses are likely to occur both in impactors due to surface losses of
nitric acid within the impactor and the reduced pressures, especially on the lower stages, as
well as in the filter pack where a reduced pressure exists between the sequential filters, and
particles within the filter are subject to some reduction in pressure. The effect of a pressure
reduction is to reduce the partial pressures of the nitric acid and ammonia thereby shifting
the ammonium nitrate dissociation (Eq. (1)) in the sense of evaporation. The magnitude of
evaporative losses in a filter and impaction system has been compared by Babich et al.
(2000) who corrected mass measurements for nitrate by collection of evaporative losses.
Positive nitrate artefacts can also be formed due to reaction of particles with nitric acid
vapour. While such processes are known to occur in sampling systems, the results of our
work give no clear indication that they are causing a significant artefact in our samples.
The work of Zhang and McMurry (1997) and Cheng and Tsai (1997) indicates that
losses of semi-volatile ammonium nitrate from filter substrates are enhanced due to the
disperse nature of the particle deposit and its consequent high surface area. In contrast,
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Fig. 7. Measured concentration products [NH3][HNO3] (ppbv2) and calculated dissociation constant of NH4NO3.
The solid and 95%RH lines are derived from Stelson and Seinfeld (1982), Allen and Harrison (1989). Campaigns:
(a), Mace Head 1996; (b), Mace Head 1997; (c), Tenerife 1997; (d), PUMA 1999; (e), PUMA 2000.
most impactors deposit particles in small piles beneath the impactor jets where their
surface area is far smaller. However, the MOUDI impactor achieves uniform deposits by
rotating the impaction substrates, thus increasing the surface area greatly relative to a
conventional impactor. It appears likely that this effect leads to much enhanced ammonium
Z. Huang et al. / Atmospheric Research 71 (2004) 215–232
231
nitrate evaporation in the MOUDI, which in our work exceeded losses from Teflon filters.
This idea is supported by the fact that on four sampling occasions during the Mace Head
1997 campaign, our MOUDI operated over the same periods as the Berner impactor of
Lancaster University. On average, the sum of nitrate concentrations from the MOUDI was
only 61% of those collected by the Berner impactor.
4. Conclusions
For particle sulphate, there is no substantial difference between the measurements by
the filter pack system and the MOUDI, indicating that sulphate being of low volatility may
be accurately determined by the two samplers, independent of sampling season or site.
The filter pack system collects 15% more particle nitrate than the MOUDI at coastal
sites, and this is probably due to enhanced coarse particle inlet efficiency for the open-face
filter pack over that of the MOUDI. It is particularly influenced by a high sea-salt or dust
associated nitrate presence. No other sampling artefacts have been found in this study for
nitrate in both the filter pack and the MOUDI during the periods when clean air masses
were sampled. During more polluted periods, however, when the sampled air masses
contain relatively higher levels of gaseous ammonia and nitric acid which have the
potential to form ammonium nitrate, the filter pack system provides higher fine nitrate
mass concentrations than the MOUDI. This appears to be due to greater evaporation of
NH4NO3 from the MOUDI. The magnitude of evaporative losses from the filter pack is
not known. The two samplers collect the same particulate ammonium from clean air
masses, but for more polluted air masses, the filter pack system provides higher
ammonium mass concentrations than the MOUDI, supporting the concept of loss of
NH4NO3 from the impactor.
Our work suggests that there may be an important difference between the Berner
impactor, which samples ammonium nitrate (at least in California) with high efficiency,
and the MOUDI impactor which underperforms in nitrate sampling relative to Teflon
filters at our sites.
Acknowledgements
The authors are grateful to the Natural Environment Research Council for funding the
measurements as part of the ACSOE and URGENT PUMA Consortium Programmes. We
also acknowledge NERC support of studentships (JDJ and RMT) and the China
Scholarship Council for a study visit to the UK (for ZH). The Berner impactor data were
provided by Professor Nick Hewitt and Dr Brian Davison of Lancaster University.
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