A closure study of sub-micrometer aerosol particle hygroscopic behaviour

Atmospheric Research 50 Ž1999. 205–240

A closure study of sub-micrometer aerosol particle hygroscopic behaviour Erik Swietlicki a,) , Jingchuan Zhou a , Olle H. Berg a , Bengt G. Martinsson a , Goran ¨ Frank a, Sven-Inge Cederfelt a, Ulrike Dusek b, Axel Berner b, Wolfram Birmili c , Alfred Wiedensohler c , Brett Yuskiewicz c , Keith N. Bower d b

a DiÕision of Nuclear Physics, Lund UniÕersity, P.O. Box 118, S-221 00 Lund, Sweden Institut fur ¨ Experimentalphysik, UniÕersitat ¨ Wien, Strudlhofgasse 4, A-1090 Wien, Austria c Institute for Tropospheric Research, Permoserstrasse 15, D-04318 Leipzig, Germany d Physics Department, UMIST, P.O. Box 88, Manchester M60 1QD, UK

Abstract The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser ŽTDMA. for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11–1.15, 1.38–1.69 and 2.08–2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74–87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer ŽDMPS.. The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected

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Corresponding author. Fax: q46-46-2224709; e-mail: [email protected]

0169-8095r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 0 9 5 Ž 9 8 . 0 0 1 0 5 - 7

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particles with aerodynamic diameters between 0.17–0.53 mm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Aerosol particles; Hygroscopic growth; Tandem differential mobility analyser ŽTDMA.; Closure study

1. Introduction 1.1. EnÕironmental importance of the hygroscopic properties of aerosol particles The hygroscopic material present on aerosol particles is continuously interacting with the ambient water vapour of the surrounding air. The amount and nature of the hygroscopically active material controls how much water condenses on the individual aerosol particles at a given relative humidity ŽRH., and thereby also determines the actual particle size. The water content influences the aqueous phase chemistry since many reactions are dependent on the ionic strength of the aqueous solution Že.g., Pandis et al., 1995; Lagrange et al., 1996.. The optical properties of the aerosol also depend strongly on the amount of condensed water, both through the effect on the size distribution ŽSloane and Wolfe, 1985. and on the refractive index ŽTang and Munkelwitz, 1994.. The amount of hygroscopic particle material, or rather the number of ions in the aqueous particle solution, determine which particles act as cloud condensation nuclei ŽCCN. at water vapour super-saturation or remain as an interstitial aerosol. Lately, it has been recognised that anthropogenic aerosol particles might have a discernible effect on global climate ŽIPCC, 1995.. Aerosol particles can scatter incoming short-wave solar radiation back into space which causes a cooling of the Earth’s surface. This is the so called direct effect. The micro-physical structure and radiative properties of clouds depend on the CCN number concentration. Increased particle number concentrations cause the number of cloud droplets to increase and their effective radius to decrease ŽMartinsson et al., 1999.. This in turn increases the cloud albedo ŽTwomey, 1977. and might also have consequences for the cloud life cycle. This is the indirect effect of aerosols on climate. Human activities have caused large perturbations of the natural background atmospheric aerosol, most notably regarding the sulphate loading of the lower atmosphere. Even in remote locations on the Northern Hemisphere, the average sulphate column burden is estimated to have roughly doubled compared to pre-industrial times ŽLangner and Rodhe, 1991.. The tropospheric aerosol originating from anthropogenic sources, and the sulphate aerosol in particular, contribute to the direct and indirect effects and thus counteract the global warming caused by the greenhouse gases. Several model calculations indicate that the cooling due to the direct climate forcing effect of anthropogenic sulphate aerosols is in the range from y0.3 to y1.3 Wrm2

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ŽCharlson et al., 1991; Kiehl and Briglieb, 1993; Kiehl and Rodhe, 1995., while the direct effect of the long-lived greenhouse gases and CFC’s is estimated to be q2.5 Wrm2 ŽIPCC, 1995.. Plinis et al. Ž1995. considered the sensitivity of direct climate forcing by a ‘global mean’ aerosol to several aerosol properties Ždry particle size distribution, state of mixture, relative humidity, deliquescencercrystallisation hysteresis, particle refractive index. as well as solar zenith angle. They concluded that ‘‘the single most important parameter in determining direct aerosol forcing is relative humidity, and the most important process is the increase of aerosol mass as a result of water uptake’’. The climate forcing effects of other anthropogenic aerosol components Žcarbonaceous particles, bio-mass burning, soil dust resuspended by human activities. as well as the indirect sulphate aerosol effect, are even more uncertain than the direct climate forcing effect of anthropogenic sulphate aerosol. The estimation of the combined aerosol climate forcing effect is at present the largest source of uncertainty in predictions of the human influence on future global climate, and is currently an area of intense research. The estimates of the human impact on global and regional climate are determined by the use of Global Climate Models ŽGCM.. Any detailed description of the global aerosol system is precluded by the lack of reliable data needed to characterise the aerosol and the sheer complexity of the processes involved. The GCM’s therefore rely heavily on field experiments for aerosol characterisation as well as selection and parametrisation of the vital processes. Penner et al. Ž1994. recommend ways to quantify and minimize the uncertainties of climate forcing by anthropogenic aerosols. The proposed research strategy includes the need for surface-based observations of aerosol chemical and physical properties and modelling studies to demonstrate consistency between the observations as well as to provide guidance for determination of the most important parameters. Regarding the hygroscopic behaviour, Penner et al. Ž1994. address the need to determine to what extent the RH-dependent size of ambient aerosol particles can be described according to the equilibrium thermodynamic properties of the bulk composition. It is the one of the objectives of this study to contribute to the future development of model parametrisations by proposing a model for the hygroscopic behaviour of tropospheric continental sub-micrometer aerosol particles at water vapour sub-saturation, and test the model assumptions against actual measurements. A parsimonious, albeit satisfactory model description of the hygroscopic properties of tropospheric aerosol particles is important, not only for predictions of future global climate, but can also be used for model descriptions of the atmospheric cycling of harmful substances, visibility degradation and the effect that aerosol particles have on the human respiratory system through inhalation. 1.2. PreÕious obserÕations Numerous measurements in continental air masses have shown that tropospheric aerosol particles exhibit a modal external mixture with respect to their hygroscopic behaviour, implying an external mixture also from a chemical point of view. This is clearly manifested in measurements with H-TDMA instruments ŽHygroscopic Tandem

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Differential Mobility Analyser; see below for a description. by a handful of groups around the world Že.g., McMurry and Stolzenburg, 1989; Zhang et al., 1993; Covert and Heintzenberg, 1993; Svenningsson et al., 1997; Berg et al., 1998a,b; Swietlicki et al., 1997b,c.. However, the particles cannot be described simply as being either hydrophobic or hygroscopic. When taken from a dry state to a fixed humidified state Že.g., from - 10% to 90% RH., a group of Žsub-micrometer. particles exhibits significant diameter growth. These particles are denoted more-hygroscopic particles. In most continentally polluted air masses, the observed hygroscopic growth is smaller or even considerably smaller than what could be expected from an entirely soluble particle composed of a mixture of the major inorganic ions found in the bulk aerosol. This growth discrepancy can be accounted for by attributing a hygroscopically inactive volume fraction of typically 50% to each individual particle in this group. The inactive volume fraction can be water-insoluble, but it may also consist of water-soluble compounds which do not contribute significantly to the hygroscopic growth of the particles Že.g., heavy organic acids such as humic acids.. On the other hand, a large number of particles show even less diameter growth when analysed in the same way with the H-TDMA instrument. These particles are denoted less-hygroscopic particles. Typically 10% of the volume of the individual particles in this group can be accounted for by inorganic salts. Only a small number fraction of the particles are truly hydrophobic. In continental air masses, the submicrometer aerosol particles thus exhibit a bimodal hygroscopic behaviour. Instead of displaying a continuum of hygroscopic growth factors, the particles separate into the less- and the more-hygroscopic groups discussed above, thus implying an external mixture also in chemical composition. A third Žsub-micrometer. hygroscopic group of particles can occasionally be observed in marine environments influenced by production of sea spray particles through bubble bursting ŽBlanchard, 1983.. The diameter growth factors for these sea spray particles are even higher than for the so called more-hygroscopic particles and have been observed to be in the order of 1.9–2.3 when taken from a dry state to 90% RH ŽSwietlicki et al., 1997b.. Over the Pacific and Southern Oceans, externally mixed sea spray particles were observed with a TDMA as far down in size as 50 nm ŽBerg et al., 1998b.. 1.3. Possible influence of organic compounds on the hygroscopic properties The hygroscopic behaviour of atmospheric aerosol particles is often assumed to be adequately described by the inorganic material, more specifically the major inorganic ions. In other words, the inorganic particle fraction is considered hygroscopically active, while the organic fraction is assumed to be inactive and does not interact either with the water condensed on the particles nor the surrounding water vapour. The are several reasons for making this assumption: Ž1. A few inorganic ions make up a large mass fraction of tropospheric aerosol particles ŽHeintzenberg, 1989.; Ž2. Thermodynamic data exist for the inorganic compounds, while similar data is largely lacking for the majority of relevant organic species ŽSaxena and Hildemann, 1996.; Ž3. Analysis of the major inorganic ions is rather straight-forward Že.g., using ion chro-

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matography. while analysis of the numerous organic compounds present in tropospheric aerosol particles is far more complex; Ž4. The relevant organic species which might be of importance for the hygroscopic behaviour are yet to be identified. Some recent studies have raised the question whether it is necessary to take into account also the organic volume fraction when describing the hygroscopic properties of aerosol particles. Saxena et al. Ž1995. used H-TDMA and impactor data from Grand Canyon and Los Angeles in USA to indicate that organic material can affect the hygroscopic behaviour of aerosol particles at water vapour sub-saturation. From their data evaluation, they drew the conclusion that the organic fraction of freshly produced aerosol particles hinders the hygroscopic growth, while organic species in particles found in aged air masses seemed to increase the growth. Model calculations by Schulman et al. Ž1996. based on new as well as existing thermodynamic data for some organic compounds with limited solubility in water resulted in considerable modification of the shape of the classical Kohler curve which ¨ describes the activation of aerosol particles to cloud droplets. They suggested that a gradual dissolution of organic compounds at relative humidities close to water vapour super-saturation might cause a decrease in the super-saturation ratio required for activation and thus result in more particles being activated in a cloud with given updraft velocity. However, in their examples, large organic particle mass fractions were required for the effect to become important. Saxena and Hildemann Ž1996. reviewed the existing thermodynamic data and observations of particle-bound water-soluble organic compounds in order to establish which compounds might be of importance for the hygroscopic behaviour. These organic species should be both water-soluble and liable to condense on existing aerosol particles under atmospheric conditions. The review highlights the lack of observations of water-soluble compounds, which is partly due to analytical difficulties such as handling of polar substances in GC-MS analysis. The data on hygroscopic growth obtained with the TDMA during the field experiment described here were used in conjunction with measurements of aerosol particle size distributions and impactor measurements in a closure study. The purpose of this study was to investigate whether the inorganic particle fraction can fully explain the hygroscopic behaviour of the sub-micrometer aerosol particles or whether other hygroscopically active chemical species Že.g., soluble organic species. are needed in the models of hygroscopic behaviour to account for the measured growth.

2. Experimental The hygroscopic properties of sub-micrometer tropospheric aerosol particles were measured with a H-TDMA ŽHygroscopic Tandem Differential Mobility Analyser. instrument during the Great Dun Fell ŽGDF. hill cap cloud experiment in March–April 1995 ŽChoularton et al., 1999.. The H-TDMA ŽRader and McMurry, 1986; Stolzenburg and McMurry, 1988. measures the hygroscopic diameter growth of individual aerosol particles when taken from a dry state Ž- 10% RH. to a controlled humidified state ŽRH ranging between 15 and 93%.. The H-TDMA consists mainly of three parts; Ž1. a

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Differential Mobility Analyser ŽDMA. which selects a narrow size range of the atmospheric aerosol, Ž2. humidifiers which condition the air to a well-defined RH and Ž3. a second DMA which determines the change in diameter caused by the humidification. During normal operation, the humidity in the second DMA is kept constant Že.g., 90% RH., but it can also be varied so that atmospheric aerosol particles showing RH-hysteresis behaviour Ždeliquescence and crystallisation. can be studied. The H-TDMA observations during the Great Dun Fell 1995 cloud experiment were made at the up-wind site at Fell Gate Ž430 m.a.s.l... Similar measurements were performed at the same site during the Great Dun Fell 1993 cloud experiment ŽSvenningsson et al., 1997., as part of the EUROTRAC sub-project GCE ŽGround-based Cloud Experiment.. The H-TDMA measurements in 1995 were made for the dry particle sizes 35, 50, 75, 110, 165 and 265 nm. Apart from the study presented here, the H-TDMA observations provided vital input data for models describing the evolution of the hill cap cloud micro-physics ŽMartinsson et al., 1999., the modification of the aerosol size distribution from a single cloud passage ŽBradbury et al., 1999. and the implications of this modification on the aerosol radiative properties ŽYuskiewicz et al., 1999.. The sheath air and the aerosol entering DMA-2 were humidified separately. During normal operation, the sheath air humidification was set to control at 90% RH while the aerosol was set to 85% RH. This ensured that no accidental deliquescence occurred in the aerosol line before entering DMA-2. The performance of the H-TDMA instrument was checked at regular intervals. This included tests of the TDMA transfer function using dry sheath air in both DMA’s, and tests using pure NaCl particles at relative humidities around the deliquescence point Ž75% RH. and the growth of these salt particles at 90% RH. The H-TDMA instrument was computer-controlled and was normally set to operate uninterrupted for 24 h. Apart from the data on hygroscopic growth, several parameters needed for the subsequent quality assurance were logged continuously. All H-TDMA data presented was quality assured and fulfilled a number of criteria based mainly on temperature and relative humidity stability in DMA-2 and lack of influence from local pollution. The raw data obtained with the H-TDMA instrument was evaluated off-line using the TDMAFIT programme ŽStolzenburg and McMurry, 1988.. This first evaluation step yielded the following primary parameters: 1. Diameter growth factor Žfrom - 10% to 90% RH. of individual particles; 2. Diameter growth dispersion factor; 3. Relative fraction of particles in each hygroscopic mode. Sub-micrometer Ž3–840 nm. aerosol particle size distributions were measured with DMPS ŽDifferential Mobility Particle Sizer. instruments at four sites, including Fell Gate, during the Great Dun Fell experiment 1995 ŽBirmili et al., 1999.. The Fell Gate aerosol inlet used for both the H-TDMA and the DMPS consisted of a slit impactor with a 2 mm aerodynamic cut-off diameter at ambient RH conditions. During the official cloud events, Berner-type low-pressure cascade impactors were operated at the up-wind ŽFell Gate. site ŽChoularton et al., 1999.. The impactors were placed outdoors next to the aerosol container housing the H-TDMA and DMPS

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instruments. The four-stage impactors Žstage 0: - 0.17 mm, stage 1: 0.17–0.53 mm, stage 2: 0.53–1.6 mm, stage 3: 1.6–5.1 mm in aerodynamic particle diameter at ambient RH. collected the aerosol particles on Tedlar foils for subsequent ion chromatography ŽIC. analysis.

3. Observations of hygroscopic growth during the experiment The H-TDMA was set to operate more or less continuously from 10 March to 4 April 1995, with an interruption between 16–19 March due to power failure at Fell Gate and bad weather Žsnow fall.. Table 1 summarises the TDMA data observed during the course of the experiment. Not all data taken were evaluated. Priority was given to the cloud events ŽChoularton et al., 1999. and adjoining time periods. Furthermore, only data from time periods during which the Fell Gate site was out of cloud ŽRH - 99%. was included in Table 1, since the appearance of cloud at the site clearly affected the measurements. A general observation when the site was in cloud was that the larger-sized particles Žmostly in the accumulation mode; dry particle geometric mean diameters between ca.

Table 1 Summary of the TDMA observations made at the Fell Gate site Ž430 m.a.s.l.. during the Great Dun Fell 1995 experiment Out of cloud

Dry particle diameter 35 nm

50 nm

75 nm

110 nm

165 nm

265 nm

Total number of observations 215

216

220

217

217

214

Less-hygroscopic particles Growth factor Žaverage"1 s.d.. Soluble fraction Žaverage"1 s.d.. Frequency of occurrence

83%

More-hygroscopic particles Growth factor Žaverage"1 s.d.. Soluble fraction Žaverage"1 s.d.. Frequency of occurrence

1.11"0.05 1.12"0.05 1.11"0.06 1.12"0.08 1.14"0.11 1.15"0.12 0.11"0.05 0.11"0.05 0.09"0.05 0.09"0.07 0.11"0.11 0.12"0.11 88%

79%

87%

83%

70%

1.38"0.09 1.44"0.09 1.52"0.08 1.58"0.07 1.64"0.06 1.69"0.07 0.47"0.14 0.50"0.13 0.59"0.13 0.66"0.11 0.73"0.11 0.81"0.13 99%

96%

100%

100%

100%

99%

Fraction less-hygroscopic particles

0.36"0.23 0.37"0.26 0.39"0.28 0.31"0.20 0.24"0.18 0.20"0.19

Frequency of bimodal growth behaviour

87%

81%

79%

86%

83%

74%

Data was only included for time periods when the site was clearly out of cloud ŽRH -99%.. Average hygroscopic growth factors for the more- and less-hygroscopic particle groups, as well as the fraction of particles belonging to the less-hygroscopic group, are given. The average soluble fractions were calculated under the assumption that only ammonium sulphate contributed to the hygroscopic growth.

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80–400 nm. and the particles belonging to the more-hygroscopic group were preferentially removed in the aerosol inlet, while the smaller-sized particles Žmostly in the Aitken mode; 20–80 nm. and the particles belonging to the less-hygroscopic group remained in the sampled air stream. This behaviour is hardly surprising since the particle size as well as the soluble volume fraction determine the amount of soluble material present on the individual particles and thus also the potential for growth at elevated humidities. As mentioned previously, the ambient aerodynamic cut-off diameter of the inlet was 2 mm. Particles which had grown to be large enough at ambient RH to be removed by the inlet were not necessarily thermodynamically activated. The average hygroscopic behaviour observed out of cloud during the 1995 Great Dun Fell experiment displayed some of the characteristic features which are generally seen also at other sites with continental anthropogenic influence ŽSwietlicki et al., 1997b.. These features are: Ž1. A bimodal hygroscopic behaviour; Ž2. An increase in the growth factors for the more-hygroscopic particle group with dry particle size; Ž3. Nearly constant or decreasing growth factors for the less-hygroscopic particle group with dry particle size; Ž4. A slight decrease in the number fraction of less-hygroscopic particles with dry particle size. H-TDMA measurements were made at the same site ŽFell Gate. during a similar experiment in April–May 1993 ŽSvenningsson et al., 1997.. Even though the two experiments shared the overall characteristic features in hygroscopic behaviour as described above, the growth factors observed in 1993 were significantly lower for both the less- and more-hygroscopic particle groups than during the 1995 experiment. Furthermore, the fraction of less-hygroscopic particles seen in 1993 was about half of the 1995 values. The discrepancy in hygroscopic behaviour can at least partly be explained by the fact that the meteorological conditions were quite different during the two experiments. In 1993, Fell Gate was a down-wind site during most of the experiment, i.e., the winds approached Great Dun Fell from the north-east ŽColvile et al., 1997.. The 1993 H-TDMA data were mostly taken in air masses which had originated over Eastern Europe before being transported over the North Sea and across Northern England to Great Dun Fell. These air masses showed a substantial anthropogenic influence ŽSwietlicki et al., 1997a., which might account for the lower growth factors observed in 1993. In 1995, the more commonly experienced wind pattern with a south-westerly air flow prevailed, and continental Europe had only a slight influence on the air masses reaching Great Dun Fell ŽChoularton et al., 1999.. It should be mentioned that a nearly identical H-TDMA instrument was used during both Great Dun Fell experiments and that the presented data were subjected to the same analytical procedure and quality assurance criteria. The different hygroscopic behaviour observed in 1993 and 1995 is therefore not likely to be caused by instrumental and analytical differences. The H-TDMA data described below in more detail are from the cloud events a 2b, 3, 4, 7 and 8 Žthe official cloud events were defined in Choularton et al., 1999. and some adjoining time periods. This data was chosen to be presented here Žin Fig. 1a–g. since it was either used for the present closure study or for other cloud-related studies ŽMartinsson et al., 1999; Bradbury et al., 1999; Yuskiewicz et al., 1999. or since the data exemplify interesting features in the hygroscopic behaviour of aerosol particles.

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Fig. 1a–g show the hygroscopic growth data obtained with the H-TDMA averaged over the time periods given in Table 2. 3.1. Run a2b; start of Run a3 The period around Run a2b ŽFig. 1a. was used by Martinsson et al. Ž1999. to study the evolution of cloud micro-physical properties Žthere denoted period A., while the start of Run a3 was used in the hygroscopic closure study Žoccasions 3a–d.. The unique features of Run a2b are the unimodal hygroscopic behaviour of the accumulation mode particles Ždown to about 75 nm in dry particle diameter. and the growth factors which remain nearly constant for almost 9 h. The particle size distribution also varied very little over this period ŽMartinsson et al., 1999.. The air mass back trajectories at the 1000 hPa level showed that clean Arctic air had passed close to GDF and had then made a loop over the London Metropolitan area and the English Channel only to return 2.5 days later in the early morning hours of 23 March. The influence from sources in the Liverpool region seen on the evening of 22 March faded as the returning air masses took a slightly more westerly path and passed over Wales instead of Liverpool. Satellite Žvisible and infrared. images of cloud cover obtained from the NOAA ŽNational Oceanic and Atmospheric Administration, United States Department of Commerce. satellites F11 and F12, show that the air mass looping over southern England had been out of cloud for the entire 2.5 day period before returning to GDF. On the satellite images, clouds appear only over the Pennine ridge, where indeed a hill cap cloud was observed at GDF summit from midnight to around 05:00 on 23 March. The Fell Gate site, where the H-TDMA observations were made, was located upstream of this cap cloud. The combination of anticyclonic conditions with several days of clear skies and the influence from large anthropogenic emission sources in southern England made the conditions extremely favourable for production of photochemical smog. Concentrations of peroxyacetyl nitrate ŽPAN. and ozone measured at Fell Gate peaked at 1.5 ppb and 50 ppb respectively around 22:00 on 22 March and again at 1.6 ppb and 55 ppb around 17:00 on 23 March. These PAN concentrations are among the highest values ever to have been recorded in Great Britain ŽMcFadyen et al., 1999.. The PAN concentrations exceeded 1 ppb from the evening of 22 March through to the evening of 23 March. Ozone levels were around 40–55 ppb, while SO 2 and NO 2 concentrations varied between 1.5–3 and 4–5 ppb respectively. The hygroscopic behaviour observed during Run a2b was most likely the result of high photochemical activity taking place in an aged polluted air mass. Secondary organic aerosol mass can be formed from oxidation Žvia, e.g., O 3 , OH and NO 3 . of gaseous hydrocarbons for which the oxidation products have vapour pressures that are sufficiently low to enable them to partition into the particulate phase. The exact chemical reactions involved are often too numerous and complex to be described in detail at present. Nevertheless, experiments with sunlight-irradiated smog chambers containing mixtures of gaseous hydrocarbons, nitrogen oxides and ozone have been able to relate the production of secondary organic aerosol mass to the loss of the precursor

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hydrocarbons Že.g., Odum et al., 1997 and references therein.. Weingartner et al. Ž1997. exposed freshly generated diesel particles to ozone and UV irradiation. The amount of hygroscopic material found on the particles increased substantially after only short periods of ozone exposure or UV irradiation. Weingartner et al. Ž1997. did not determine the organic compounds responsible for the water uptake but attributed it to ozone adsorption on diesel soot particles leading to a degradation of particle bound polyaromatic hydrocarbons. Even the surface of dry soot particles from a carbon spark

Fig. 1. Average hygroscopic growth factors Žvalues)1., defined as the particle diameter at 90% RH divided by the diameter at -10% RH, for the more-hygroscopic ŽM-H GF. and the less-hygroscopic ŽL-H GF. groups of particles and number fraction of particles found in the less-hygroscopic particle group Žvalues-1. during Run a2b Ža., Start of Run a3 Žb., End of Run a3 Žc., Run a4 Žd., After Run a4 Že., Run a7 Žf. and Run a8 Žg.. The averaging times are given in Table 2. The growth factor and number fraction intervals are "one standard deviation around the mean. The percentages are the frequencies of occurrence and are only given if -100%. In Žd. ŽAfter Run a4., a separate sea spray group was also present ŽSS GF..

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Fig. 1 Žcontinued..

generator can be modified by ozone exposure thus preparing the otherwise hydrophobic particles for further heterogeneous reactions Že.g., Mohler et al., 1996.. Saxena and ¨ Hildemann Ž1996. identified the oxygenated organic compounds most likely to be incorporated in aerosol particles to be water-soluble C2–C7 multifunctional compounds such as dicarboxylic and ketoacids. It is interesting to note that cloud-processing is obviously not a prerequisite for removing particles from the less-hygroscopic particle group. Also photochemical oxidation processes seem capable of doing so, thus adding soluble material to aerosol particles and moving the particles to the more-hygroscopic group. As seen in the closure study Žoccasions 3a–c., the soluble material added was evidently not inorganic.

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Table 2 Time periods over which the H-TDMA data was averaged for presentation in Fig. 1a–g Official cloud events

TDMA data presented

Run a

Duration Ž1995.

Start

Stop

2b 3 3 4 Ž4. 7 8

03r22–23 23–06 03r23–24 11–05 03r23–24 11–05 03r24 10–18 – 04r01 12–19 04r03 00–18

03r22 21:50 03r23 10:00 03r24 00:20 03r24 11:30 03r24 23:10 04r01 09:45 04r02 21:00

03r23 06:40 03r23 20:50 03r24 07:30 03r24 23:10 03r25 05:00 04r01 23:20 04r03 19:50

Comment Start of Run a3 End of Run a3 After Run a4

The pollution episode lasted well into the beginning of Run a3, for which the hygroscopic behaviour is displayed in Fig. 1b Žno H-TDMA data is available for the time period 06:40–10:00 on 23 March, i.e., between Runs a2b and a3.. The H-TDMA observations made during the start of Run a3 Ž23 March 10:00–20:50. show a much less stable behaviour than for Run a2b, due to the gradual shift of air masses taking place over the afternoon hours. 3.2. End of Run a3 The end of Run a3 Ž24 March 00:20–07:30, Fig. 1c. was used in the hygroscopic closure study Žoccasions 3e–g. and by Martinsson et al., 1999 Žthere denoted period BX .. The photochemical pollution episode came to an end as cleaner and more moist-laden air was brought to GDF after about 19:00 on 23 March. The new air mass had low particle number and volume concentrations as well as low trace gas concentrations ŽSO 2 and NO - 0.5 ppb, NO 2 - 5 ppb.. The Fell Gate site had been above cloud base between about 21:00–24:00 on 23 March, i.e., in the middle of Run a3. The larger and more hygroscopic particles were then lost in the aerosol inlet, since they had grown in size at ambient RH to become larger than the inlet cut-off diameter at 2 mm. Fig. 1c shows the hygroscopic behaviour for the end of Run a3, when the H-TDMA site had emerged from cloud again. The hygroscopic behaviour for the end of Run a3 is characterised by high stability and a bimodal distribution of growth factors, implying that the aerosol is to some extent externally mixed. As mentioned previously, a bimodal hygroscopic behaviour is what is normally observed in continental aerosols. 3.3. Run a4; after Run a4 H-TDMA data from Run a4 on 24 March was used for the hygroscopic closure study Žoccasions 4a–c.. The hygroscopic behaviour was bimodal and quite stable during the whole day until midnight Ždisplayed in Fig. 1d.. The air masses reaching GDF were fairly clean ŽChoularton et al., 1999. with some influence from sources on Ireland. The growth factors for the less-hygroscopic group of particles were rather low, and in some

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cases hardly any growth was observed, especially for the 110 and 165 nm particles around 19:00–23:00. This was immediately after a heavy rain fall at GDF Summit between 17:00–18:00 in connection with the passage of an occluded front. The Summit site came out of cloud by 18:00 on 24 March. The observed polarisation in hygroscopic growth factors between the more- and less-hygroscopic particle groups might have been the result of cloud processing taking place in the cloud deck associated with the frontal and postfrontal regions. Nearly hydrophobic particles are then likely to have remained in the interstitial aerosol ŽMartinsson et al., 1999. while particles in the less-hygroscopic group containing somewhat larger amounts of soluble material were thermodynamically activated. Once activated, additional soluble material could be fixed on the particles which made them even more hygroscopic ŽHoppel et al., 1994; Laj et al., 1997., and thus added to the observed polarisation of hygroscopic behaviour. The number size distribution data support this. The Aitken mode particle concentration was highly variable during the frontal passage Ž17:00–18:00. but later stabilised in the postfrontal air. The geometric mean diameter was around 60 nm and the Aitken mode particle number concentrations showed a steady decrease by a factor of three between 18:00–23:00. The accumulation mode was remarkably stable both during the frontal passage and thereafter, with a geometric mean diameter around 150 nm. This mode also decreased, but only by about a factor of two between 17:00–23:00. While the Aitken mode dominated shortly after the frontal passage, the accumulation and Aitken mode reached about equal number concentrations around 20:15. Particles in the Aitken mode size range might have subsided from above the lowest cloud deck during the passage of the occluded front, but not to any significant degree in the postfrontal air. Whatever the origin of the Aitken mode particles, the stability of the accumulation mode particles suggests that these were transported within the boundary layer and thus subjected to cloud processing. Shortly before midnight between 24–25 March, the hygroscopic behaviour changed rather abruptly to include also an externally mixed sea spray derived group of aerosol particles ŽFig. 1e.. These particles had diameter growth factors which were considerably higher Žaround 2.1. than what is normally observed for particles in the more-hygroscopic group. For comparison, pure NaCl particles have diameter growth factors of around 2.4 when taken from a dry state to 90% RH. During the 6-h period shown in Fig. 1e ŽAfter Run a4., between 30–60% of the 265 nm dry size particles in each scan were found to be externally mixed sea spray particles. Such particles were only observed once for the 165 nm particles Žlast scan.. Local wind speeds were around 8 m sy1 at the Fell Gate site with air mass back trajectories coming in from over the Irish Sea. Externally mixed sea spray particles are rarely found in continental air masses, but have previously been observed in remote marine environments in the Southern, Pacific and Arctic Oceans down to particle diameters as small as 50 nm ŽBerg et al., 1998b; Swietlicki et al., 1997c.. These particles are believed to have been produced through bubble-bursting during conditions with high wind speeds ŽBlanchard, 1983.. The externally mixed sea spray particles were probably also fresh enough to have escaped processing which would have reduced their growth factors to resemble those of the more-hygroscopic particle group or removed them from the atmosphere completely.

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3.4. Run a7 H-TDMA data from Run a7 on 1 April ŽFig. 1f. was used for the hygroscopic closure study Žoccasions 7a–e.. An anticyclone with its centre west of France brought air masses from Ireland across the Irish Sea to GDF. Conditions were mostly sunny except for the cap cloud forming at GDF Summit between 10:00–15:35. The typical bimodal hygroscopic behaviour was seen for all dry sizes Ž35–265 nm.. Somewhat surprisingly, the nearly ten-fold increase in particle number concentrations from around 2000 to almost 20 000 particles cmy3 observed during the time period 12:00 to 16:00 did not have any significant impact on the hygroscopic behaviour, with a possible exception for the 50 nm particles for which the growth factor for particles in the more-hygroscopic group decreased to values as low as 1.3 between 17:00–18:10. The increase in number concentration was also most evident in this ŽAitken mode. size range. The sources responsible for the increase in particle concentrations are most likely found on Ireland ŽChoularton et al., 1999.. 3.5. Run a8 Hygroscopic growth data from Run a8 on 2–3 April ŽFig. 1g. were used for the hygroscopic closure study Žoccasions 8a–n. and for modelling of cloud chemistry and micro-physics ŽBradbury et al., 1999.. The hygroscopic behaviour was bimodal and stable during the whole time period except for the 165 and 265 nm particles. Between 02:00–04:00 on 3 April, the less-hygroscopic particles were almost entirely absent and when they reappeared at 04:00 their growth factors had increased from 1.2 to 1.45. A similar rise in hygroscopic growth factors was observed also for the 265 nm particles. Gas phase SO 2 concentrations, which were otherwise around 0.1 ppb, showed a concurrent rise between 02:00–04:00, peaking at 0.5 ppb around 03:00 ŽCape et al., 1999.. The observed hygroscopic behaviour might have been caused by in-cloud gas-to-particle conversion taking place on less-hygroscopic particles which had activated down to at least 165 nm in size to form droplets in clouds up-wind of the Fell Gate site. 4. Model calculations for the hygroscopic closure experiment 4.1. Model hypothesis and assumptions Closure experiments produce an over-determined set of observations such that the measured value of an aerosol property can be compared to a value calculated with an appropriate model based on independent measurements ŽQuinn et al., 1996.. The purpose of the closure study presented here was to investigate whether only the inorganic particle material need to be taken into account when modelling the hygroscopic behaviour of the sub-micrometer aerosol particles or if other components, such as organics, also play a significant role. This is done by comparing the number of ions in various particle size intervals derived from the dry aerosol particle size distribution ŽDMPS. and a model of hygroscopic growth Žbased on H-TDMA data. with the observed number of inorganic ions Žfrom IC analysis on cascade impactor samples.. In short, the closure study was performed as follows.

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The observed diameter growth factors for the less- and more-hygroscopic group of particles were used to derive the soluble volume fraction of individual particles as a function of particle dry size. Combined with the dry number size distribution, this yielded the dehydrated soluble volume size distribution. Once the cascade impactor aerodynamic cut-off diameters had been converted to corresponding electrical mobility diameters, these in turn were shifted down in size from ambient RH ŽRH at which the impactors were operating. to dry conditions Žat which the number size distributions were determined. using a model of hygroscopic growth. The derived dehydrated soluble volume size distributions were integrated over the various RH-corrected impactor intervals, and the resulting volumes were used to calculate the number of ions in each interval. This number could then be compared to the actual measurements obtained from IC analysis of the impactor samples. The basic model hypothesis is: The hygroscopic growth of the aerosol particles can be adequately described using a model which assumes that only the inorganic substances interact with the ambient water vapour. Ammonium sulphate was chosen to model the hygroscopic growth of the inorganic substances for several reasons: Ž1. The aim is to propose a fairly simple model of aerosol particle hygroscopic growth in continental air masses; Ž2. Neutralised sulphate aerosols constitute a large mass fraction of the sub-micrometer aerosol mass in background continental air masses ŽHeintzenberg, 1989.; Ž3. Water activity and density data are available also for super-saturated ammonium sulphate solutions; Ž4. Particles consisting of both sub- and super-saturated solutions of ammonium sulphate and ammonium nitrate show quite similar hygroscopic growth Že.g., Chan et al., 1992.. Results for the hygroscopic closure study will be presented here for impactor stage 1 Ž0.17–0.53 mm in aerodynamic particle diameter at ambient RH. and the last stage Žstage 0: - 0.17 mm., for which all pertinent TDMA and size distribution data were also available. For the 29 impactor samples included in the hygroscopic closure study, 69–95% Žmedian 91%. of the inorganic ion equivalent molar concentrations on impactor stage 1 consisted of ammonium Žcation., sulphate and nitrate Žanions.. Fig. 2 depicts the result of the ion chromatography analysis of particles collected on stage 1, and additional information regarding the ionic composition is given in Table 3. On this stage, the ammonium to sulphate equivalent molar ratios ranged between 0.79–3.61 Žmedian 1.08.. This ratio only deviated significantly from unity for samples 3a–3e, mostly due to a high nitrate content. The ammonium to sulphate plus nitrate Žequivalent molar. ratios ranged between 0.74–1.27 Žmedian 0.98.. Table 3 also gives the ion equivalence imbalance for stage 1. The ion equivalence imbalance was calculated as the sum of all positive ions minus the sum of all negative ions divided by the total ion concentration, all in equivalent concentrations. The range here was from y1.9% to 16.2% Žmedian 5.7%., i.e., the positive inorganic ions were slightly more abundant than the negative. This imbalance is well within the measurement uncertainties of the IC analysis.

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Fig. 2. The result of the ion chromatography analysis of aerosol particles collected on stage 1 Ž0.17–0.53 mm in aerodynamic particle diameter at ambient RH. given in equivalent concentrations. Inorganic ion balance probably existed as the deviations were all within the analytical errors.

Zhang and McMurry Ž1992. studied the evaporative losses of fine particulate nitrates during sampling in both impactors and filters. Based on their Eq. Ž2., the losses of ammonium nitrate from the nitrate-rich samples 3a–e were estimated to have been - 2.5% of the total ion concentration for stage 1 and - 17% for stage 0. Zhang and McMurry Ž1992. verified their theoretical calculations of nitrate sampling efficiency for a Berner impactor with laboratory as well as field data Že.g., Wang and John, 1988.. They concluded that ‘‘ volatilization losses from the ŽBerner. impactor for ammonium nitrate sampling are small and in reasonable agreement with theoretical expectations’’. Considering the small ion losses for the Fell Gate impactor samples estimated from their Eq. Ž2., it seems reasonable to assume that there were no severe ion volatilisation losses in the impactor. Unless there were substantial amounts of negative ions not detected in the IC analysis, the ion balance therefore indicates that the aerosol was largely neutralised with ammonium and therefore not very acidic. Losses of nitrate have previously been observed in DMA-1 of the TDMA for aerosols containing large mass fractions of nitrates ŽBerg et al., 1998a., but then only for highly acidic aerosols. The losses were believed to have occurred when the particles were dried out in DMA-1 causing evaporation of nitric acid, which was then probably removed via

Table 3 Results of the hygroscopic closure study Žimpactor stage 1. and some characteristics of the sampled air. All concentrations are given at STP RH Ž%.

3a 3b 3c 3d 3e 3f 3g 4a 4b 4c 7a 7b 7c 7d 7e 8a 8b 8c 8d 8e 8f 8g 8h 8i 8j 8k 8l 8m 8n

03r23 12:00–15:00 57–64 03r23 13:30–16:30 53–64 03r23 15:00–18:00 53–85 03r23 16:30–19:30 58–94 03r24 01:30–04:30 93–96 03r24 03:00–06:00 93–95 03r24 04:30–07:30 86–94 03r24 11:30–14:30 86–90 03r24 13:00–16:00 88–90 03r24 14:30–17:30 88–91 04r01 11:00–14:00 81–92 04r01 12:30–15:30 80–84 04r01 14:00–17:00 77–84 04r01 15:30–18:30 77–89 04r01 17:00–20:00 77–91 04r02 21:00–24:00 86–92 04r02–03 22:30–01:30 86–93 04r03 00:00–03:00 88–93 04r03 01:30–04:30 88–93 04r03 03:00–06:00 87–90 04r03 04:30–07:30 86–89 04r03 06:00–09:00 86–90 04r03 07:30–10:30 86–91 04r03 09:00–12:00 87–91 04r03 10:30–13:30 87–93 04r03 12:00–15:00 88–93 04r03 13:30–16:30 92–93 04r03 15:00–18:00 90–93 04r03 16:30–19:30 90–97

Number conc. Ž3–840 nm. Žcmy3 .

All stages Ion mass Žmgrm3 .

Stage 1 Ion equiv. imbalance Ž%.

Stage 1 wNHq x 4 eq r wSO42y xeq

Stage 1 wNHq x 4 eq r ŽwSO42y xeq x . qwNOy 3 eq

Stage 1 d NDMPSrTDMA r d log d p Žnmolesrm3 .

Stage 1 d NIC r d log d p Žnmolesrm3 .

7400–9940 6910–9940 6640–9600 5500–7830 1740–2970 2240–3350 2460–4830 1760–8630 2250–8630 3430–6860 1360–5540 1430–16 040 5540–18 710 7540–18 710 7080–14 050 2360–6060 2150–4030 2150–2880 2150–2880 1820–2750 1820–6400 1975–6400 2350–6400 2350–3660 2350–6000 2480–6000 4290–7320 4570–7630 3440–9290

32.4 25.1 18.5 22.7 4.78 5.08 6.21 2.95 2.91 4.31 6.08 7.56 8.06 9.30 9.54 9.47 10.9 11.6 12.0 12.8 10.5 8.48 9.54 8.09 9.46 12.2 14.7 11.7 9.77

7.8 10 11 8.8 y1.9 4.5 y0.47 4.1 16 5.4 2.6 10 1.4 14 9.2 5.7 14 1.4 1.8 2.6 4.6 0.74 8.0 0.33 9.4 6.1 7.7 6.1 y0.43

3.43 3.61 3.33 3.05 0.93 0.97 1.00 0.79 0.86 1.13 0.99 1.00 0.97 1.05 1.25 1.08 1.06 1.09 1.11 1.13 1.05 1.01 1.05 0.97 1.09 1.25 1.37 1.27 1.11

1.18 1.27 1.19 1.26 0.77 0.84 0.91 0.74 0.77 1.02 0.95 0.96 0.91 0.94 1.10 1.00 0.96 0.98 1.01 1.04 0.98 0.97 0.99 0.91 1.00 1.10 1.17 1.08 0.93

1030 736 502 364 27.3 38.4 47.6 30.3 31.3 42.6 95.5 109 120 116 110 80.5 102 144 191 179 139 130 122 120 129 149 160 162 126

840 593 257 360 43.1 49.0 66.5 38.1 41.2 54.5 96.0 138 125 121 112 110 132 147 185 198 140 116 134 109 146 173 237 200 139

221

Impactor sampling interval

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the DMA-1 excess air or lost to the walls. Laboratory TDMA measurements with artificial Žneutral. ammonium nitrate particles showed good agreement with calculated growth factors at RH s 90% based on water activity data from Potukuchi and Wexler Ž1995a.. Since the aerosol particles collected at Fell Gate were probably not very acidic, no significant losses of nitrates are expected to have occurred in either the TDMA or DMPS for the occasions included in the closure study. In conclusion, no substantial sampling losses of inorganic ions are likely to have occurred, and the ammonium, sulphate and nitrate ions constituted the major part of the ions. The fact that ammonium sulphate and ammonium nitrate show similar hygroscopic behaviour further supports the assumption that the hygroscopic behaviour can be modelled as being that of ammonium sulphate. The air drawn through the low-pressure stages of the cascade impactor expands and cools which leads to an increase in RH on the low-pressure stages. Fortunately, it has been verified experimentally that the time spent by the particles in the jets of the low-pressure region is too short to result in any significant particle size changes ŽHochrainer and Zebel, 1981.. It will therefore be assumed here that the particles retain their ambient sizes through all stages of the cascade impactor. It was further assumed that the particles collected in the impactor were all spherical with a particle density of 1.15 grcm3. The density and shape factor at ambient conditions are needed for the conversion of aerodynamic diameters to mobility diameters Žsee Section 4.2.. The RH of deliquescence is around 80% for ammonium sulphate particles ŽWexler and Seinfeld, 1991.. Once the particles deliquesce to form aqueous solution particles, they have the ability to remain as solution particles ŽRood et al., 1989; Shaw and Rood, 1990. even when the RH drops below the mutual RH of deliquescence for the solution mixture in question ŽPotukuchi and Wexler, 1995a,b.. This means that the particles are most likely spherical down to the point of crystallisation, which for ammonium sulphate particles occurs at an RH of around 37% ŽTang and Munkelwitz, 1994.. This phenomenon is known as RH-hysteresis. For the 29 occasions studied, the RH varied between 53–97% Žsee Table 3.. It was therefore assumed that none of the particles had crystallised, but that they all still contained condensed water and remained at supersaturated aqueous solution particles even at RH’s below the mutual RH of deliquescence. It may be noted here that inorganic aerosol equilibrium models ŽPlinis and Seinfeld, 1987; Wexler and Seinfeld, 1991; Kim et al., 1993. do not acknowledge the presence of super-saturated salt solution particles, since this is in contradiction with the assumption of thermodynamic equilibrium between the gaseous, aqueous and solid phases. The density of both sub- and super-saturated ammonium sulphate aqueous solution particles is given in Tang and Munkelwitz Ž1994. as a polynomial Žsee below.. Assuming that the insoluble particle core has a density of 1.0 grcm3 , the ambient particle density range from 1.07 to 1.33 grcm3, when the RH is varied between 60–97% and the soluble volume fraction between 0–1. The assumed ambient density of 1.15 grcm3 is a compromise choice considering the range of soluble volume fractions observed with the TDMA and the RH interval in question. The details of the model calculations are given below.

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4.2. Hygroscopic growth model The water equilibrium between an ideal aqueous solution containing n w moles of water molecules and n s moles of solute molecules, and an air parcel with water vapour pressure e over the plane solution surface is described by Raoult’s law: e nw s Ž 1. es Ž T . ns q n w where esŽT . is the saturation water vapour pressure at the prevailing temperature T. The relative humidity RH of the air Žgiven in percent. is simply RH s 100 P eresŽT .. Deviations from Raoult’s law get increasingly larger once the ambient RH falls a few percent below 100%, and the aqueous solution can then no longer be considered to be ideal. Also, the increase in equilibrium water vapour pressure over the curved surface of aerosol particles has to be taken into account for particles less than a few hundreds of nanometers in diameter. Non-ideal solutions can be described by replacing the molar ratio in the right term of Eq. Ž1. with the water activity, a w , while the curvature effect is described by the Kelvin curvature correction factor C k . Thus, e Ck aw s Ž 2. es Ž T . is the extension of Raoult’s law to non-ideal solutions and sub-micrometer aerosol particles. The Kelvin factor is given by C k s exp

ž

4s M w

r w RTd a

/

Ž 3.

where s , M w and r w are the surface tension, the mole weight and the density of the solution Žtaken here to be those of pure water., R is the standard gas constant, T the temperature and d a the ambient diameter of the aerosol particle. Several investigators have used electrodynamic balances to determine the relationship between the water activity of an ammonium sulphate solution and the solute concentration ŽCohen et al., 1987; Chan et al., 1992; Tang and Munkelwitz, 1994; Potukuchi and Wexler, 1995a.. At water vapour sub-saturation Ž e - esŽT .., the hygroscopic response of ammonium sulphate to changes in RH, can be described by the model given by Potukuchi and Wexler Ž1995a., which is based on data from Cohen et al. Ž1987.. Potukuchi and Wexler Ž1995a. give the molality Žh ; moles of solute per kg of water. as a function of the water activity, a w :

h Ž a w . s 135.91 y 464.03a w q 492.36 a 2w q 94.33a3w y 459.29 a4w q 200.7a5w

Ž 4. for 0.48 - a w - 1 or h - 17.9 molrkg. Dry ammonium sulphate particles deliquesce at a RH of 80% and form saturated solution particles. When decreasing the RH below the point of deliquescence, the particle solution will be super-saturated with respect to the salt. Particles of ammonium sulphate show RH-hysteresis and will only crystallise again at a much lower RH of about 37%, as observed by Tang and Munkelwitz Ž1994..

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The quantity measured with the TDMA instrument is the diameter growth factor, g, defined as da gs Ž 5. d0 where d 0 and d a are the dry and the humidified diameters of the aerosol particle. For a completely soluble ammonium sulphate particle, the diameter growth factor, g sol , can be calculated according to

rs

3

g sol s

(

1q

Ž 6.

r w h Ž a w . Ms

where rs is the density Ž1.77 grcm3 . and Ms the mole weight of dehydrated ammonium sulphate. Eq. Ž4. can be used to describe the molality as a function of water activity and thereby RH. In deriving Eq. Ž6., it was assumed that the density of the aqueous solution, r w , is the added masses of the salt and the condensed water divided by the added volumes of the dry salt and pure water. Tang and Munkelwitz Ž1994. give the density, ra , of ammonium sulphate aqueous solutions between 0.37 - a w - 1, i.e., also for super-saturated salt particles as

ra s 0.9971 q 5.92 = 10y3 x y 5.036 = 10y6 x 2 q 1.024 = 10y8 x 3 Ž 7. Ž . where x is the weight fraction of solute in percent and the resulting density is in units of grcm3. Knowing the molality of the solution from Eq. Ž4., the weight fraction of solute is given by

ž ž

x s 100 1 q

y1

1 Ms h Ž a w .

//

Ž in % .

Ž 8.

If expression Ž7. is used for the density of the solution, then the growth factor of totally soluble ammonium sulphate particles is given by 3

g sol s

(

rs ra Ž h .

ž

1q

1

h Ž a w . Ms

/

Ž 9.

instead of Eq. Ž6.. The growth factors calculated according to Eq. Ž9. differ only about 1% from those calculated according to Eq. Ž6.. At constant ambient RH, the growth factors for fully soluble ammonium sulphate particles decrease for decreasing dry particle size owing to the Kelvin factor, as described by Eq. Ž2.. Particles observed with the TDMA to have growth factors less than those of totally soluble particles of identical dry size and ionic composition are assumed to include a water-insoluble and hygroscopically inactive volume. As already noted by Pitchford and McMurry Ž1994., the soluble volume fraction, ´ , of the dry particle is given by

´s

g´3 y 1 3

g sol Ž d a . y 1

Ž 10 .

where g´ is the observed growth factor. The water activity, and thereby the molality of the aqueous solution, is affected not only by the ambient RH, but also by the curvature

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of the particle surface ŽEq. Ž2... If the Kelvin effect on ´ is to be taken into account properly, the growth factor of the fully soluble particle, g solŽ d a ., should be calculated at the same ambient size, d a , as that of the observed particle. This results in a fictive dry size of the fully soluble particle which is lower than the actual dry size, d 0 , of the observed particle. If the Kelvin effect is not considered, the calculated soluble volume fractions become too low, especially around ´ s 0.5 and for particles with d 0 - 100 nm. The Kelvin effect on ´ was not considered in Pitchford and McMurry Ž1994.. If one wishes to calculate the diameter growth factor, g Ž a w , ´ ., as a function of the water activity, a w , and the soluble volume fraction, ´ , for a given dry size, d 0 , then Žusing the same assumption regarding the density of the aqueous solution, r w , as in Eq. Ž6..: g Ž aw , ´ . s

da

rs

3

s

d0

(

1q´

Ž 11 .

r w h Ž a w . Ms

If the density, raŽh ., of the aqueous solution is known as a function of molality, h Ž a w ., then g Ž aw , ´ . s

da d0

3

s

( ž 1q´

rs ra Ž h .

ž

1q

1

h Ž a w . Ms

/ / y1

Ž 12 .

Once the growth factor and thus also the ambient diameter, d a , has been calculated by means of either Eq. Ž11. or Eq. Ž12., then the ambient RH corresponding to the water activity in question is given by Eq. Ž2.. Note that the dry particle size, d 0 , does not enter explicitly in Eq. Ž11. or Eq. Ž12., but only through its effect on the Kelvin factor in Eq. Ž2.. Growth factors calculated according to Eq. Ž12. are about 0.5% lower than those given by Eq. Ž11.. 4.3. Treatment of impactor data The cascade impactors collected the aerosol particles at ambient RH. This RH was assumed to prevail through all impactor stages. The cut-off sizes of the impactor stages are defined in aerodynamic diameters, while the differential mobility analysers measure electrical mobility diameters. The aerodynamic diameters d ae therefore need to be converted to the corresponding electrical mobility diameters d p assuming that the particles are spherical Žshape factor x s 1. and that each particle has a particle density rp s 1.15 grcm3 at ambient RH ŽHinds, 1982.. Note that the Cunningham slip correction factor Cc Ž d . is defined for the diameter in question Ž d ae or d p .. For impactor stage i, the upper and lower aerodynamic cut-off diameters are thus converted to the corresponding upper and lower equivalent mobility cut-off diameters d p, i,u and d p,i,l . An example is given in Fig. 3. 4.4. Treatment of DMPS size distribution data Sub-micrometer Ž3–840 nm. aerosol number size distributions were measured at Fell Gate with a DMPS instruments at dry conditions ŽRH - 10%.. One entire size distribu-

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Fig. 3. The sub-micrometer particle volume size distributions obtained with the DMPS instrument were integrated over the various cascade impactor size intervals, once these had been converted from aerodynamic diameter to electrical mobility diameter and corrected for hygroscopic growth at the ambient RH at which the impactor sampling took place for both the less- and more-hygroscopic group of particles Žindicated here for the upper stage 1 cut-off at 0.53 mm in aerodynamic particle diameter at ambient RH.. The shaded area shows this integral for the total volume of less-hygroscopic particles collected in the cascade impactor stage 1 Ž0.17–0.53 mm.. The same procedure is repeated for the soluble Žactive. volume from the less-hygroscopic particles and correspondingly for the more-hygroscopic particles.

tion was obtained every 15 min. Let n j Ž d p .d d p denote the number of particles in a small size interval between d p and d p q d d p during DMPS occasion j. In order to reduce the amount of data handled in the calculations, three or four log-normal modes were fitted to the measured size distributions ŽBirmili et al., 1999.. Each log-normal mode was characterised by three parameters; Ž1. the number of particles, Ž2. the geometric mean diameter and Ž3. the geometric standard deviation. Integrated particle volumes derived from the original and fitted distributions respectively agree within 10% when the integration is carried out over the entire DMPS size range. The agreement is better when the integration is taken over specific size ranges, as, e.g., 100 nm - d p - 300 nm, and the volumes then normally differ with maximum a few percent only. 4.5. Treatment of TDMA data For each TDMA dry size, the active volume fractions of individual particles in the less-hygroscopic and the more-hygroscopic particle groups, ´ LH Ž d p . and ´ MH Ž d p ., were estimated from Eq. Ž10. assuming that the soluble volume fraction was ammonium sulphate. Laboratory RH-hysteresis tests with the TDMA on sub-micrometer particles of

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pure ammonium sulphate resulted in a growth factor of 1.8 at 90% RH, which is somewhat higher than the value of 1.75 derived from Eq. Ž9.. The reason for the discrepancy is not known. The particle soluble volume fractions were calculated assuming a diameter growth factor of 1.8 at 90% RH, since the volume fractions were all derived from the TDMA field measurements carried out at this relative humidity. These calculations were done for the dry sizes at which the TDMA measured Ž35, 50, 75, 110, 165 and 265 nm dry particle diameter.. The full functions, ´ LH Ž d p . and ´ MH Ž d p ., of soluble volume fractions as a function of particle size was estimated through linear interpolation between the diameters for which measurements were actually made. The active volume fraction for diameters larger than the maximum dry size measured Ž d p,max . was set to ´ LH Ž d p,max . and ´ MH Ž d p,max . respectively. In the same way, the active volume fraction for diameters smaller than the minimum dry size measured Ž d p,min . was set to ´ LH Ž d p,min . and ´ MH Ž d p,min . respectively. The same procedure was applied to the number fractions of particles f MH Ž d p . and f LH Ž d p .. The lower and upper Žmobility. cut-off diameters for impactor stage i at ambient RH, d p, i,l and d p,i,u , were used to calculate the corresponding lower and upper Žmobility. cut-off diameters, d LH,i, j,l ), d LH,i, j,u ), d MH,i, j,l ) and d MH,i, j,u ), for impactor stage i, DMPS occasion j, and the less- and more-hygroscopic particle groups at dry conditions Žhere RH - 10%.. These corrections for hygroscopic growth were based on the soluble volume fractions as a function of dry particle size, ´ LH Ž d p . and ´ MH Ž d p ., and the hygroscopic diameter growths, g ŽRH j ,d p , ´ LH Ž d p .. and g ŽRH j ,d p , ´ MH Ž d p .. derived from Eq. Ž12.. The RH j used in the calculations were taken from the Fell Gate automatic weather station continuous ambient RH measurements and were averaged over each DMPS sampling period. The procedure is iterative since the soluble volume fractions, and thereby also the growth factors, vary with dry particle size. The shift is larger for the more-hygroscopic group of particles than for the less-hygroscopic ŽFig. 3.. 4.6. Comparison between measured and calculated number of ions As mentioned above, closure experiments compare measured values of an aerosol property with the corresponding values calculated by means of an appropriate model based on data from independent measurements. In the present hygroscopic closure study, the model is the one described in Section 4.2. There are several possibilities for the aerosol property with which to compare measured and predicted values. Previous hygroscopic closure experiments have concentrated on the amount of water taken up by the aerosol particles or the active volume fraction ŽSaxena et al., 1995; Berg et al., 1998a.. Closure studies involving these aerosol properties require either accurate measurements of size-resolved total Žactive plus inactive. aerosol mass, or independent measurements of the actual ambient particle size as a function of dry size, neither of which were available in this study. An aerosol property which is more closely linked to the hygroscopic behaviour at both sub- and super-saturated water vapour conditions than the active volume fraction is the number of molecules other than water molecules dissolved in the aqueous solution. This is clearly seen from Raoult’s law ŽEq. Ž1... In the present study, only inorganic ions will be taken into account, even though other ionic as well as non-ionic species can also

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contribute to the reduction of the equilibrium water vapour pressure. At an RH close to 100%, the solution can be considered as ideal and Raoult’s law is applicable. The Kohler equation then describes the behaviour at RH’s above 100%, and is obtained by ¨ simply replacing the water activity in Eq. Ž2. with Raoult’s law. The number of ions in solution is therefore an aerosol property of fundamental importance for the overall hygroscopic behaviour and will be used here for the closure study. The number of ions can be derived from the DMPS and TDMA data in the following way. The soluble particle volume fractions derived from Eq. Ž10. were assumed to be the only hygroscopically active material present on the particles. The volumes constituted by the active volume fraction of the dry particle size distributions for each DMPS occasion j were integrated from the lower to the upper dry impactor mobility cut-off sizes and summed over all DMPS occasions j which fell within the corresponding sampling period of the impactor. Vi sol , the volume of water-soluble material which according to the hypothesis should be sampled on impactor stage i was thus calculated as: Vi sol s Ý j

p

d LH ,i , j,u )

H 6 d

p qÝ j

6

f LH Ž d p . ´ LH Ž d p . d p3 n j Ž d p . d d p

LH ,i , j,l )

d MH ,i , j,u )

Hd

f MH Ž d p . ´ MH Ž d p . d p3 n j Ž d p . d d p

Ž 13 .

MH ,i , j,l )

Since most of the impactor samples had a duration of 3 h, j is normally 12. The number of ions, NDM PSrTDMA , on impactor stage i derived from the combined DMPSrTDMA data was estimated as NDM PS r TDMA s 3

rs Vi sol Ms

Ž 14 .

since each ammonium sulphate molecule dissociates into 3 ions in dilute aqueous solutions such as those prepared for the IC analyses of the impactor samples. The density, rs , is that of dehydrated ammonium sulphate Ž1.77 grcm3 .. This ion concentration, NDM PSrTDMA , is then compared with the total number of inorganic ions, NIC , detected in the IC analyses of the corresponding impactor stage: NIC s Ý k

mk Mk

Ž 15 .

Here m k and M k are the measured mass and mole weight of inorganic ion species k respectively. Rather than expressing the results directly as number of ions Žin moles., the comparison will be presented for the ion number concentration Žd NDM PSrTDMA rd log d p and d NIC rd log d p . in units of nmolesrm3. Here, d log d p was calculated from the aerodynamic size intervals of impactor stage 1 Ž0.17–0.53 mm. and stage 0 Žhere taken to be 0.055–0.17 mm. respectively which do not vary from one occasion to the other.

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All volumes refer to conditions at standard temperature and pressure ŽSTP; 08C and 1013.25 hPa..

5. Sensitivity study In closure studies, a measured quantity is compared with an estimate of the same dependent quantity which has been derived from a set of independent measurements by means of a suitable model. In the present study, the quantity to be compared is the ion number concentration in selected particle size intervals and the model is that describing the aerosol particle hygroscopic behaviour. Any closure study is incomplete without an estimation of the errors caused by inaccurate or imprecise measurements. Another type of error arises when some of the independent parameters entering the model cannot be measured directly, but have to be given assumed values. The sensitivity of the model results to variations in the assumed parameter values should then be evaluated. The model approach is not considered a source of error here, since the model itself constitutes the hypothesis which is to be tested in the closure study. Naturally, several models can be tested for closure, so that the optimum model can be chosen to serve a specific purpose. Closure is achieved when the measured quantity and the same quantity, derived from the model based on the set of independent measurements, agree within the estimated errors, provided the errors are small enough to allow a sensible comparison to be made. In principle, all measured or assumed parameters entering the model calculations should be used in the sensitivity study. These parameters should be identified and their errors estimated. In practice, parameters can often be grouped so that their effects on the final model result can be studied by varying only a selection of parameters. The impactor aerodynamic cut-off diameters at ambient RH were varied "10% for the cut-off at 0.53 mm and higher, and "15% for the cut-off at 0.17 mm. This reflects uncertainties in the impactor calibration, especially for the low-pressure stages ŽWang and John, 1988; Hillamo and Kauppinen, 1991., and variations between the various impactors used. Variations in impactor cut-off diameters also include particle sizing errors in the DMPS, deviations from the assumed ambient density and ambient shape factor and the fact that the collection efficiency of the impactor stages were considered to be step functions in the model calculations. The sensitivity of the model results to variations in all these parameters can all be simulated by varying the impactor cut-off diameters. The soluble volume fractions of individual particles were estimated at 90% RH, the RH at which the second DMA of the H-TDMA instrument was operated. The RH in the second DMA can deviate somewhat from this value due to temperature changes in the H-TDMA instrument. Uncertainties in the estimation of the diameter growth factor by the TDMAFIT fitting programme, inaccuracies in RH sensor calibrations and particle sizing errors in both DMA’s of the H-TDMA are other parameters affecting the calculation of soluble volume fractions. Furthermore, the spread in the diameter growth factor around the mean value, and thereby the also the spread in soluble volume fraction, was not taken into account in the model calculations.

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To incorporate all these effects, the RH in the second DMA was varied from 89 to 91%. This was simulated by allowing the soluble volume fractions of the individual particles to vary "ca. 6–8%, the exact value of which depends on particle size. It should be kept in mind that data for which the DMA-2 temperature varied extensively were excluded from the data set altogether. The values of ambient RH, which were used to shift the impactor cut-off diameters from wet to dry sizes, were measured with a psychrometer determining the wet and dry bulb temperatures. The uncertainty in the ambient RH values were estimated to be "1% in the sensitivity analysis. Fluctuations in the aerosol flow in the DMPS and discrepancies between the log-normal representations of the aerosol particle number size distributions and the original distributions were simulated in the aerosol size distribution by varying the number concentrations "10%. The fraction of less-hygroscopic particles is one of the primary parameters given by the TDMAFIT fitting programme and was estimated to vary within a factor of 0.9–1.1 around the fitted value. Table 4 summarises the parameters varied in the sensitivity study which were used in the hygroscopic model to calculate ion number concentrations Žd NDM PSrTDMA rd log d p .. These concentrations were then compared with the corresponding value obtained from the cascade impactor measurements Žd NIC rd log d p .. The error for the latter concentrations was caused by uncertainties in the ion chromatography analysis and in the volume flow rate through the impactors as well as displacement of particle mass between stages due to particle bounce-off from the impaction substrates and internal losses in the impactor. The total error for the sum of all inorganic ions was estimated to

Table 4 Parameters varied in the sensitivity study to calculate the errors in the ion number concentrations Žd NDM PSrTDMA rd log d p . Parameter

Symbol

Range

Simulates errors in

Impactor cut-off diameters

d ae

"10% at 0.53 mm or higher, "15% at 0.17 mm

RH in second DMA of the TDMA

–

RH s89–91% Žsimulated by varying ´ LH and ´ MH "6–8%.

Fraction of lesshygroscopic particles Ambient RH

f LH

Factor of 0.9–1.1 around estimated value

Impactor cut-off sizes and characteristics, variation between impactors, DMPS sizing errors, ambient particle density and shape factor DMA2 temperature stability, DMA2 RH sensor calibration, estimation of diameter growth, spread in growth factor Fitting of TDMA growth spectra

RH j

Measured RH"1%

n jŽ dp .

"10%

Particle number concentration

Psychrometer RH sensor calibration DMPS flow adjustment, log-normal modal fitting

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be "20% for stage 1 samples and 50% for stage 0 samples with measured ion number concentrations - 15 nmolesrm3 Žall cases except 3a–d.. The impactor sampling occasions 3e–g and 7a had stage 0 ion number concentrations - 5 nmolesrm3 and were not considered in the hygroscopic closure study since the analytical uncertainties were too high to allow a sensible comparison to be made.

6. Results of the hygroscopic closure and sensitivity studies The results of the hygroscopic closure study for the 29 selected impactor sampling occasions are given in Table 3 for stage 1 Ž0.17–0.53 mm in aerodynamic particle diameter at ambient RH. and for stages 1 and 0 Ž- 0.17 mm. in Figs. 4 and 5. The striking feature of the comparison shown in Figs. 4 and 5 is the good agreement which could be achieved between the modelled and measured ion number concentrations. The uncertainty range indicated in these figures should be considered as "3 s . In other words, it almost covers the entire range of accumulated ‘worst case’ errors. The total uncertainty for the modelled ion number concentrations fell in the approximate

Fig. 4. Results of the hygroscopic closure study showing the number of ions in two particle size intervals for the full range of data Žlogarithmic scales.. The d NDM PSrTDMA rd log d p values were calculated by means of the hygroscopic model while the d NIC rd log d p values were derived from cascade impactor measurements Žstage 0: - 0.17 mm, stage 1: 0.17–0.53 mm in aerodynamic particle diameter at ambient RH.. The error bars resulting from the sensitivity study are also shown for a selection of cases. The two cases for which the modelled values are significantly higher than the measured Žstage 1, case 3c and stage 0, case 3a. were collected during an episode of intense photochemical activity.

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Fig. 5. Results of the closure study showing only the lower 0–400 nmolesrm3 range Žlinear scales..

non-symmetrical range of 0.6–1.4 times the value of best estimate, although the exact range varied from case to case. The imposed variations in impactor aerodynamic cut-off diameters were responsible for almost half of this uncertainty range, while the other factors listed above contributed nearly equally to add up to the total error. One immediate conclusion of the sensitivity analysis is that the errors involved are adequately small to enable a closure study to be performed. When the cases with modelled ion number concentrations higher than 400 nmolesrm3 Ž3a, 3b and 3c. are excluded, the ratio between modelled and observed ion number concentrations on impactor stage 1 has an arithmetic average of 0.88 and a geometric average of 0.87. A linear regression with origo intercept using d NDM PSrTDMA rd log d p as dependent variable and d NIC rd log d p as independent variable yields a slope of 0.910 Žerror in slope s "0.024, r 2 s 0.93.. A similar linear regression but with the ordinate intercept as a free parameter results in a slope of 0.949 Žconstants y6.64 nmolesrm3, error in slope s "0.053, r 2 s 0.93.. All these estimates of the relationship between modelled and measured ion number concentrations indicate that the concentrations derived from the DMPSrTDMA data, based on the model of hygroscopic growth and the assumptions described in Section 4, were approximately 10% lower than the corresponding measured impactor concentrations. As mentioned in Section 4.5, the particle soluble volume fractions were calculated assuming a diameter growth factor for pure ammonium sulphate particles of 1.8 at 90% RH as measured in the laboratory, and not the value of 1.75 derived from Eq. Ž9.. Had the modelled growth factor value been used to calculate the soluble volume fractions

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instead, then these would have been about 10% higher. This would, in turn, result in an even better agreement with the ion number concentrations measured in the impactors. The sensitivity analysis shows that, except for case 3c, the discrepancies between stage 1 modelled and measured values are not significant considering the accumulated errors. The model hypothesis can therefore be said to have been validated on the basis of the existing data. During none of the occasions included in the closure study were an externally mixed sea spray group of particles present. Only cases for which the ambient RH was less than 97% are presented, since particles were lost in the aerosol inlet leading to the TDMA and DMPS at RH’s around 100%. The impactor had no such inlet and consequently did not suffer the same losses. Model calculations were performed also for several occasions when the Fell Gate site was in cloud, but the ion number concentrations derived from the DMPSrTDMA data were then always considerably less than the corresponding impactor concentrations. To incorporate these occasions in the closure study would therefore be misleading. The cases included in the hygroscopic closure study were characterised by a neutralised aerosol with a good inorganic ion balance and had Žexcept for cases 3a–d. ammonium-to-sulphate equivalent ratios close to unity Žsee Table 3.. The only stage 1 case for which the modelled ion number concentrations significantly overestimated the corresponding measured values Ž3c., was during a pollution episode with intense photochemical activity when nitrate constituted a significant part of the inorganic anions found on the sub-micrometer aerosol particles. The same was true for cases 3a–d, which all had stage 1 ion number concentrations higher than 300 nmolesrm3. The ratio between modelled and measured concentrations Žd NDM PSrTDMA rd log d p divided by d NIC rd log d p . were 1.23, 1.24, 1.95 and 1.01 for stage 1 cases 3a, 3b, 3c and 3d. Another feature distinguishing cases 3a–d from the other samples was the rather low ambient RH, reaching below 60% ŽTable 3.. The model of hygroscopic growth assumes that all particles were above their point of deliquescence Ž80% RH for ammonium sulphate. or remained on the upper loop of the RH-hysteresis curve and therefore retained some condensed water. The lowest ambient RH Ž53% for case 3b and 3c. is still higher than the RH of crystallisation of about 37% for particles consisting of pure ammonium sulphate. Partly neutralised sulphates such as ammonium bisulphate ŽNH 4 HSO4 . is however difficult to dry out completely. The adiabatic cooling of the air parcels rising to the Fell Gate site elevation of 430 m.a.s.l. should in general have caused the RH to increase as the air moved up the hill. In other words, the upstream RH was probably less than what was measured at the site. Therefore, it cannot be completely ruled out that the sampled particles experienced a low enough RH to cause the particles to crystallise upstream of Fell Gate, and that they never reached their point of deliquescence again before being sampled by the impactor. In order to test whether this scenario might in fact be the cause for the observed deviation between the modelled and measured ion number concentrations for stage 1 cases 3a–c, the model calculations were repeated with the hygroscopic growth factors set to unity. This would simulate the behaviour of particles following the lower loop of the RH-hysteresis curve, where no particle growth is taking place. The modelled ion number concentrations were however

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calculated to be even higher Žby 23–27%. than the original values. The conclusion is that crystallisation cannot account for the high modelled ion number concentrations found for stage 1 cases 3a–c. All stage 0 data, except cases 3a–d and 7a, have measured ion number concentrations - 15 nmolesrm3. As the stage 0 ion concentrations get increasingly smaller, the number of inorganic ions found in the impactor deviate more and more from the modelled values. In some of the stage 0 cases Ž7b, 8a, 8j–l and 8n. the points fall just below the 1:2 line. There is no reason why the DMPS and the H-TDMA should have performed less accurately in the stage 0 size range compared to stage 1. Instead, the stage 0 discrepancies might at least partly be due to systematic errors in the ion chromatography analysis. These errors are difficult to estimate and were therefore not included in the sensitivity analysis. Erroneous blank subtraction is such a systematic error which increases in importance as the concentrations become lower, and is a possible cause of the observed deviations.

7. Discussion As shown in the previous sections, the ion number concentrations derived from the DMPSrTDMA data were in agreement with the measured impactor concentrations within the accumulated measurement errors. The basic model hypothesis can therefore be said to have been validated by the closure study. More precisely, for the aerosol studied during the Great Dun Fell 1995 experiment, the hygroscopic growth of the aerosol particles can be adequately described using a model which assumes that only the inorganic substances interact with the ambient water vapour. Ammonium sulphate was chosen to represent the hygroscopic growth of all inorganic substances. This does not mean that ammonium sulphate actually is the only hygroscopically active substance, but rather that the model simplification is valid. The result of the closure study has implications relating to the modelling of the response of aerosol particles to changes in the ambient water vapour saturation ratio. For particles with dry sizes in the sub-micrometer size range, the humidified aerosol particle number size distribution can be calculated as a function of RH Ž- 97%. by means of the hygroscopic model given above. The required model input data is a combination of DMPS dry aerosol number size distributions and H-TDMA data on individual particle soluble volume fractions. Alternatively, the soluble volume fractions can be derived from size-resolved Žcascade impactor. data on the aerosol inorganic ion composition, since the DMPSrH-TDMA ion number concentrations were shown to agree with the corresponding impactor concentrations. However, the latter approach does not include any information of the soluble volume fractions of individual aerosol particles. It also requires two sets of impactor measurements to be made in parallel; one cascade impactor for determination of ions and another for size-resolved Žgravimetric. mass. Knowledge of the ambient aerosol size distribution is essential for assessing the direct climate forcing effect of aerosols and visibility degradation in general. Not only ambient particle size is important for such calculations. Additional assumptions have to be made

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regarding the refractive indices of the individual aerosol particles ŽTang and Munkelwitz, 1994; Lacis and Mishchenko, 1995; Yuskiewicz et al., 1999. so that the optical properties of the entire aerosol can be derived from Mie theory ŽVan de Hulst, 1981; Bohren and Huffman, 1983.. The light scattered by an aerosol can be measured with a nephelometer ŽHeintzenberg and Charlson, 1996. in forward as well as backward angles. The RH in the nephelometer can also be controlled so that light scatttering at elevated RH’s can be determined. A closure study can thus be performed in which the measured aerosol light scattering properties can be compared with predictions obtained by means of a model of hygroscopic growth, such as the one described here, combined with Mie theory ŽQuinn et al., 1995, Anderson et al., 1996.. The amount of water condensed on individual aerosol particles is another quantity which can be determined once the ambient aerosol size distribution is known. From this, solute concentrations and ionic strengths can be estimated to give a better picture of aqueous phase reaction rates as a function of particle Ždry or ambient. size. The model assumption that ammonium sulphate can be used to represent all soluble particle material can be extrapolated to water vapor super-saturation ŽRH ) 100%. with increased confidence once the model approach has been validated at sub-saturated conditions. During the Great Dun Fell experiment, a Droplet Aerosol Analyser ŽDAA; Martinsson et al., 1999. was operated at the GDF Summit site. The DAA measured the ambient droplet size as a function of the dry residual particle size when Summit was in cloud. Martinsson et al. Ž1999. showed that the cloud droplet spectra could be adequately explained by an adiabatic cloud parcel model based on the Fell Gate DMPS dry particle size distributions, the soluble volume fractions of individual aerosol particles derived from H-TDMA observations of hygroscopic behaviour and the assumption that ammonium sulphate could be used to represent the hygroscopic behaviour of all soluble particle material. The water uptake at water vapour super-saturation was described by the Kohler equation using Raoult’s law, since the relationship between water activity ¨ and solute concentration ŽEq. Ž4.. is not applicable at these saturation ratios. The bimodal hygroscopic behaviour observed with the H-TDMA was shown to have a large influence for the selection of CCN from a given particle size distribution and the resulting droplet spectrum. As discussed already, the modelled ion number concentrations exceeded the measured concentrations for most of the cases collected during the photochemical pollution episode Žstage 1, cases 3a–c and stage 0, cases 3a, 3b and 3d., and for two of these Žstage 1, case 3c and stage 0, case 3a., the discrepancy was clearly significant. This implies that there were more substances contributing to the hygroscopic growth other than the inorganic ions. Although no direct observations were made, it is likely that the intense photochemical activity had produced a range of oxidised and nitrated organic compounds which at least to some extent partitioned to the aerosol particle phase. Saxena et al. Ž1995. re-examined H-TDMA and cascade impactor observations made at Grand Canyon National Park, USA in January–March 1990 and July–August 1992 ŽZhang et al., 1993; Pitchford and McMurry, 1994. and concluded that organic material in particles found in aged, non-urban air masses tended to increase the hygroscopic growth. As a contrast to this observation, data taken at an urban location in the outskirts of Los Angeles in June–September 1987, ŽMcMurry and Stolzenburg, 1989. implied

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that freshly emitted organic compounds decreased the uptake of water by aerosol particles. Saxena et al. Ž1995. speculated that the diminished hygroscopic growth observed at the urban location might have been caused by either hydrophobic organics forming a surface film which hindered transport of water to the particles, or that the presence of dissolved organics changed the water activity of the aqueous phase in the direction of decreased water uptake. Previous studies give a somewhat unclear picture of whether or not surface active compounds are actually able to significantly hinder the transport of water between the liquid and gas phase ŽGill et al., 1983; Daumer et al., ¨ 1992; Hameri et al., 1992; Andrews and Larson, 1993.. ¨ Yet another study was based on data collected at a site in the Po Valley, Italy in November 1994 ŽBerg et al., 1998a.. The Po Valley site was influenced by an aged, polluted air mass but conditions were mostly hazy, i.e., the photochemical activity was low. For that site and for the conditions prevailing during that experiment, the hygroscopic behaviour could be adequately explained by the inorganic ions present, although a considerable fraction of the sub-micrometer aerosol particle mass probably consisted of water-soluble organic compounds. The hygroscopic closure studies described above were all performed in a way similar to the one presented here, since they were all based on H-TDMA and size-resolved ion chemistry data. The study by Saxena et al. Ž1995. relied on size-resolved impactor observations to construct continuous distributions of dry particle volume, while the Po Valley study ŽBerg et al., 1998a. used DMPS size distribution data. The quantity for which closure was tested were different from one study to the other. Saxena et al. Ž1995. calculated aerosol particle water content Žin mgrm3 . while ŽBerg et al., 1998a. tested for active volume fractions. The existing data sets give no conclusive evidence as to the nature of the organic compounds which seem to enhance the hygroscopic growth. According to the review by Saxena and Hildemann Ž1996., the water-soluble organic compounds most likely to appear in significant quantities in the particulate phase are those who are not only soluble in water but also condensable under atmospheric conditions. These include polar as well as C2–C7 multifunctional compounds such as dicarboxylic acids.

8. Conclusions The hygroscopic properties of the sub-micrometer tropospheric aerosol particles were studied during the Great Dun Fell hill cap cloud experiment March–April 1995 using a H-TDMA ŽHygroscopic Tandem Differential Mobility Analyser. instrument. A hygroscopic closure study was performed to test the consistency between the number of ions present in various sub-micrometer particle size intervals as measured by means of ion chromatography analysis of cascade impactor samples, and the same quantity calculated with a model of hygroscopic behaviour based on the combined dry size distribution ŽDMPS. and hygroscopic growth ŽH-TDMA. data. The closure study showed that the hygroscopic growth of the aerosol particles can be adequately described using the model which assumes that only the inorganic substances interact with the ambient water vapour.

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A sensitivity analysis was also performed to test whether the observed model-measurement discrepancies were significant or not. This analysis clearly showed that the errors involved were adequately small to enable the closure study to be performed. Furthermore, the outcome of the closure study was not dependent on the various model assumptions made. The single largest source of error was the uncertainty in the cascade impactor cut-off characteristics. Therefore, the uncertainties in future hygroscopic closure studies can be greatly reduced by careful impactor calibrations, including cut-off size and shape, bounce-off and losses of particles to the walls as well as losses of volatile species to the air stream. The few cases for which the modelled ion number concentrations were significantly higher than the measured, were all sampled during a pollution episode with intense photochemical activity. The excess hygroscopically active material was obviously not inorganic. Although not directly measured, this material is instead believed to have consisted of oxygenated organic compounds, produced from organic gas- as well as particle-phase precursors during several days of transport in a cloud-free and photochemically active air mass. The aerosol particle ionic composition was characterised by a neutralised aerosol with good inorganic ion balance. Substantial amounts of particle nitrate were only present during the photochemical pollution episode. The ammonium-to-sulphate equivalent ratios were otherwise close to unity. Although not entirely conclusive, the outcome of the present hygroscopic closure study combined with those reported in literature seem to indicate the following: Ž1. The water uptake by aerosol particles found in aged, continental air masses having undergone little or no photochemical activity can be adequately described by the inorganic ions; Ž2. Photochemical oxidation processes produce additional, and most likely organic, hygroscopic material which increases the water uptake and; Ž3. Freshly emitted organic material restricts the hygroscopic growth of aerosol particles. Acknowledgements This work was carried out with financial support from the European Commission ŽEnvironment and Climate project EV5V-CT94-0450.. E. Swietlicki also wishes to acknowledge the support from the Swedish Natural Science Research Council under contract G-AArGU 06788-306. The groups from Lund, Vienna and Leipzig are greatly indebted to the people at UMIST, whose organising skills and hard preparatory field work made this field experiment possible. References Anderson, T.L., Covert, D.S., Marshall, S.F., Laucks, M.L., Charlson, R.J., Waggoner, A.P., Ogren, J.A., Caldow, R., Holm, R.L., Quant, F.R., Sem, G.J., Wiedensohler, A., Ahlquist, N.A., Bates, T.S., 1996. Performance characteristics of a high-sensitivity, three-wavelength, total scatterrbackscatter nephelometer. J. of Atmos. and Oceanic Technol. 13, 967–986. Andrews, E., Larson, S.M., 1993. Effect of surfactant layers on the size changes of aerosol particles as a function of relative humidity. Environ. Sci. Technol. 27, 857–865. Berg, O.H., Swietlicki, E., Frank, G., Martinsson, B.G., Cederfelt, S.-I., Laj, P., Ricci, L., Berner, A., Dusek,

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