Secondary organic aerosols: formation potential and ambient data

The Scienceof the Total Environment 205(1997) 167-178

ELSEVIER

Secondary organic aerosols: formation potential and ambient data R.J. Barthelmie”, Climate

and Meteorology

Program,

Department

of Geography,

S.C. Pryor Indiana

University,

Bloomington,

IN 47405,

USA

Received 15 March 1997;accepted27 June 1997

Abstract Organic aerosols comprise a significant fraction of the total atmospheric particle loading and are associated with radiative forcing and health impacts. Ambient organic aerosol concentrations contain both a primary and secondary component. Herein, fractional aerosol coefficients (FAC) are used in conjunction with measurements of volatile organic compounds (VOC) to predict the formation potential of secondary organic aerosols (SOA) in the Lower Fraser Valley (LFV) of British Columbia. The predicted concentrations of SOA show reasonable accord with ambient aerosol measurements and indicate considerable seasonal variability in SOA potential. Particulate carbon contributes only approx. 3% of total carbon concentrations in the LFV, and it is shown that variability in total carbon concentrations is significantly larger than variability in gas/particle partitioning. 0 1997 Elsevier Science B.V. Keywords: pounds

Organic aerosols; Atmospheric

particle loading; Fractional aerosol coefficients; Volatile organic com-

1. Introduction Organic compounds comprise a significant fraction of measured atmospheric aerosol mass. Analysis of aerosol data collected during IMPROVE

suggested that organic tributor to fine aerosol non-urban monitoring 1994a). Heintzenberg surements from urban

*Corresponding author. Also affiliated to: Wind, Energy and Atmospheric Physics, Risoe National Laboratory, Denmark.

that an average of 31% of fine aerosol mass is attributable to organic species. Elemental and organic carbon fractions typically account for approx. 40% of fine mass in Los Angeles (Gray et al., 1986) and approx. 60% of fine mass in two

0048-9697/97/$17.00 0 1997Elsevier ScienceB.V. All rights reserved. PIZ

SOO48-9697(97)00200-3

carbon is the largest conmass at nearly half of the locations (Maim et al., (1994) summarizes meaenvironments and reports

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northwestern National Parks (Malm et al., 1994b). Organic compounds are important atmospheric components because: 1. They contain many species which are known to be toxic (e.g. polyaromatic hydrocarbons). 2. They play an important role in photochemical reactions leading to ozone formation (Bowman et al., 1995). The formation of organic aerosols forms part of the removal process of volatile organic compounds (VOC) (and hence terminates their role in the ozone formation cycle) and may compete with inorganic compounds for oxidizing species (hydroxyl and nitrate radicals, ozone). 3. A large fraction (up to 95%) of organics in urban aerosols are associated with small particles (diameters less than 3 pm) which can be drawn into the respiratory tract (Isidorov, 1990). 4. Organic aerosols are optically active and therefore contribute to visibility degradation and direct climate forcing (IPCC, 1995). 5. Organic aerosols can form cloud condensation nuclei if they occur in the submicron range and have been implicated in indirect climate forcing (Noakov and Penner, 1993). 6. They may alter the chemical, optical and hygroscopic behavior of inorganic aerosols (Saxena et al., 1995). The research described herein focuses on a comparison of the formation potential and in-situ measurements of secondary organic aerosols (SOA). SOA formation potential is calculated based on ambient measurements of volatile organic compounds WOC) conducted in the Lower Fraser Valley (LFV), British Columbia (Dann et al., 1994) (Fig. 1) and fractional aerosol coefficients (FAC) given in Grosjean (1992). SOA concentrations obtained are compared with measured concentrations of organic aerosols from two field campaigns conducted in the LFV during the summer of 1993 (REVEAL) and from April 1994 to April 1995 (REVEAL II) (Pryor et al., 1997; Pryor and Barthelmie, 1996a) and used to assess; 0

the seasonal variability this environment,

of SOA formation

in

iA

I)

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0 -REVEAL / REVEAL II monitoring l - VOC gas monitoring IocatiorTs

locations

fl

.

CLBR

I\

I

Fig. 1. Map of the Lower Fraser Valley showing GVRD VOC gas monitoring locations and REVEAL/REVEAL II sites. l

the relative contributions of organic groups to SOA (and the error in calculated SOA due to omissions in the sampling protocol),

and to speculate as to the variability ing in total carbon.

of partition-

2. Organic gases and aerosols SOA are formed from both biogenic and anthropogenic gaseous precursors. Biogenic emissions are estimated to dominate organic aerosol formation in some urban areas (e.g. Atlanta) and to contribute significantly (up to 15%) in others such as Los Angeles (Pandis et al., 1992). The major biogenic compounds involved in aerosol formation are thought to be monoterpenes which comprise up to 80% of VOC emitted from conifers (Isidorov, 1990). Mobile sources are estimated to contribute approx. half of anthropogenic VOC emissions in urban areas of the US (Atlas et al., 1992), while industrial sources are typically the second largest source type (Atlas et al., 1992). Ambient gas phase concentrations of VOC depend largely on source region and reactivity of the compound. A broad range of compounds exist in measurable quantities at most urban and rural sites. In US cities, the alkane group is typically present in the highest concentrations, followed by aromatic and olefin compounds (Altshuller, 1991; Atlas et al., 1992).

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Ambient organic aerosol concentrations average between 2 and 30 pg mm3 in US cities and between 0.6 and 5 pg rnp3 for non-urban areas (Isidorov, 1990). Organic aerosol formation is typically dominated by C5-Cl0 species because species with high molecular weights (C > 10) tend to be present only at low concentrations while those with low molecular weights (C < 5) have high saturation vapor pressures (Isidorov, 1990). In Los Angeles, the contribution of VOC groups to aerosol concentrations were estimated to be approx. 62% from aromatics, 13% from biogenic monoterpenes, 18% from alkanes and 7% from olefins (Grosjean and Seinfeld, 1989; Pandis et al., 1992). The formation of SOA is complex, many chemical pathways and reaction products remain unknown. However, methods have been developed to approximate SOA formation based on smog chamber measurements where the approximate aerosol yield from a particular compound or group is given as a fractional aerosol coefficient (FAC) which is defined as the ratio of the aerosol yield from a specific compound to its initial concentration (Grosjean and Seinfeld, 1989; Grosjean, 1992). Organic species can condense to form new aerosols or react heterogeneously on pre-existing aerosols (Pandis et al., 1992). Pankow (1994a,b) suggests that, even if products are present at less than their saturation vapor pressure, they may still condense onto existing aerosols. This has been disregarded in the current analysis although it is recognized that aerosol mass resulting from this process may be incorporated within FAC yields. 3. Ambient VOC / organic aerosol measurements in the LFV 3.1. GaseousVOC in the Greater Vancouver Regional District (GVRD)

Speciated VOC concentrations are measured at a number of sites within the GVRD (Fig. 1). The longest and most complete data set is from site T9 where concentrations of 165 species are cd lected as 24-h samples on a 6-day sampling regime (see Dann et al. (1994) for more details of the

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VOC sampling methods and data analysis). VOC data are also collected at the remaining GVRD sites shown in Fig. 1, but data sampling is less frequent. Mean total gaseous carbon concentration at T9 over the period 1989-1995 was 375 ppb C. The median, 25th and 75th percentiles of 112 nonhalogenated compounds for the year 1993 are shown in Fig. 2. The hydrocarbon profile at T9 appears to be consistent with data from US cities reported in Atlas et al. (1992) and Altshuller (1991) with similar concentrations of alkanes such as butane, isopentane and propane and of aromatics such as toluene and xylene’. In the US cities reported by Altshuller (1991) saturated hydrocarbons typically contributed the largest proportion of hydrocarbons (47-69%) compared with 56% at T9, aromatics were the next largest group comprising 21-29% compared with 33% at T9 and finally olefins provided between 4 and 10% (excluding ethene) compared with 10% at T9. Despite the fact that neither database is comprehensive and there are clearly differences in the compounds monitored and techniques used, there is broad agreement between the T9 data set and data from the US cities. 3.2. Aerosol measurements

Fine aerosol measurements (diameter < 2.5 pm) were performed in the LFV during two field campaigns: REVEAL (July and August 1993) and REVEAL II (April 1994-June 1995) (Pryor et al., 1997; Pryor and Barthelmie, 1996a). During REVEAL, IMPROVE aerosol samplers (Eldred et al., 1990) collected 24-h average samples (7.00-7.00 h PST) on a daily basis with modules A, B and C at two sites (PIME, CHIL) in the LFV. Pryor et al. (1997) estimated organic carbon (00 mass at a third site (CLBR) based on concentrations of hydrogen and sulfur collected on module A filters (using the IMPROVE protocols (Malm et al., 1994a)). During REVEAL II, samples were made on a twice weekly basis (Wednes-

‘Concentrations reported in Altshuller (1991) were for samples collected between 6.00 and 9.00 h.

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days and Saturdays) at two sites, CHIL and CLBR (using A, B and C modules). The site locations are shown in Fig. 1. The IMPROVE Module C filters are analyzed for both OC and elemental carbon (EC) using thermal optical reflectance (Chow et al., 1993a). Note that Chow et al. (1993a) indicate that during this analysis EC is assumed to consist of EC plus any OC which absorbs light, hence concentrations of OC reported herein are likely to be underestimates of ambient OC concentrations. The observations indicate that organic aerosols contributed between 34 and 44% of fine aerosol mass in July/August 1993 (Pryor et al., 1997) and between 25 and 39% during April-August 1994 (Pryor and Barthelmie, 1996a). Although inter-sample variability is high, the concentration of organic aerosol appears to be higher in summer than spring.

16

1

27.0

4. Organic aerosol formation 4.1. Useof FAC

Using FAC (Grosjean, 1992) and measured 24-h concentrations of each gaseous organic compound, an estimate of potential SOA formation associated with each organic compound can be calculated. Median, 25th and 75th percentile SOA concentrations for three sites within the LFV (T9, T22 and T27) which have relatively long records and frequent sampling are given in Table 1. The solid line in Fig. 3a shows the potential SOA calculated using FAC for the year 1993 at T9 and indicates considerable sample-to-sample variability. Highest SOA concentrations are predicted to occur in the winter months due in part to higher concentrations of the precursor gases. Compounds from the aromatic group contribute

21 .o

I

TT

potential

<-75th p~entileri

,I6

- T

13 1-Bulyne 1 Ethane 2 Ethylene 3 Acet@ne 4 Propylene 5 Propane 6 1 -Propyne 7 lkobutane 8 1-Butenekobulene 9 1.SButadiena 10 Butane 11 bans-2.Butene 12 2.2-Dimethylpropane 23 2-Methyl-2-btiene 14 cis-2-FWtene 15 Ikopentane 16 1 -Pentene 17 2MethyCI-Pentene 18 2.Methyl-l-butene 19 Pentane 2U Isoprene 21 bans-2-Pen&e 22 cis-2-Pe&ne 24 2.2-Dimethyibutane 25 Cycbpentene 26 4-Methyl-I-pentene 27 1MethyLI-pentene 26 Cyctopentane 29 2,5Dimethyitutane 30 bans4Methyl-2-pentene 31 Wethylpantane 37 Zmethki-2-Pentene 36 2-Ethel-I-Butene 39 trdns-3-Melhvl-2-wntene 40 clo-2-Hewene 32 ci&-Methvl-2-wntene 33 3-Methvlcenbne 34 I-Hexene 35 Hexane 36 bans-2.Heuene . 41 cis&Methyi-2-pentene 42 2,ZDimethytpentane 43 Whyicycbpentane 44 2.4-Dimethytpentane 45 2.2.3Trimethytbutana 46 1-Methyicyctopentena 47 Benrene 48 Cyctohexane 49 P-Whythexane 50 2.3-Dimeth~pantane 51 Cyclohsxene 52 3-Meth)ihex.ana 53 1-Heptene 54 2.2.4Trimethylpentan 55 tared-Heptene 56 cis-%t~e~W 57 Heplane 56 bane2-Heptene 59 cis-2Heptene 60 2.2-Dimethylhexane 61 Methylcycbhexane 62 2,BDimethylhexsn 63 P,+Dimethyihexane 64 2.3.4Tdmathytpentane 65 Toluene 68 2-Mathytheptane 67 1 -Methyicyclohexene 66 CMethyiheptane 69 3Methylheptane 70 ci&l.3-DimethylnFtohexan 71 bans-1 ,I-Dimethykwcbhauane 72 2,2,STrimethylherana 73 1-Octene 74 Octane 75 trancl,2-Dimethyicyclohexane 76 bans-2-Octens 77 ck-l.&-1 .SDtt~bhexsne 78 cia-2-C&ne 79 d&l ,2-Dinwth@@Ohe 80 Ethylbacuene 61 2,5-DimethyVleptane 82 m and pXykne 83 4Methylcctans 64 bhkthylatane 85 Styrene 86 eX)iene 87 1-Nonera 66 Nonarm 69 loo-PmMbeNene 90 S.SDii 91 II-Pmpytbenzerea 92 SEthy#duene 93 CEthyitotwne 94 1.3.5.Trtmethylbenzene 95 ZEthflotwna 96 1-Decene 97 ted-Butytbenzena Q9 1.2.4-Tdmethyk1enz~e QQ Decane 100 Iso-Blltyasmens 101 WButylbcnrene 108 1,2-Dtethylbenzena 109 Undecane 110 Naphthakne 102 1,2,3-Tdmethyltenzene 103 pCymena 104 Indana 105 1.BDiethyitemma 106 l+D~benzene 107 n-BuW+lWnE 111 Dodecane 112 Hexylbenzene

,*

Fig. 2. Median measured concentrations of non-halogenated hydrocarbons at site T9 in 1993 (in pg mW3). Error bars show the 75th percentile (above) and 25th percentile (below). If the 75th percentile > 16pg rnd3 it isindicated by an arrow and the value.

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Table 1 Median VOC concentrations from T9 (ppb C) and predicted SOA formation potential at GVRD sites, T9, T22 and T27 based on: (i) measured VOC concentrations; and FAC (ii) backcast VOC concentrations and FAC from Grosjean (1992) ( pg rnm3) Site

Annual

Spring m‘4M)

Summer (JJA)

Autumn (SON)

Winter (DJF)

Median VOC cones. at T9 (ppb C) (non-halogenated)

375

292

340

416

506

(i) SOA from measured VOC concentrations and FAC ( pg mm31 T9 (Jan 1989-Jan 1996)

0.87 1.44 2.26 n = 417

0.68 1.18 1.70 n = 105

0.73 1.28 1.87 n = 137

1.25 1.83 2.44 n = 83

1.13 1.94 3.45 n = 92

T22 (Mar 1990-Jan 1996)

0.44 0.67 1.11 n = 131

0.38 0.53 0.84 n = 34

0.32 0.51 0.81 n = 22

0.51 0.77 1.43 n = 39

0.52 1.08 1.31 n = 36

T27 (Jul 1993-Jan 1996)

0.16 0.25 0.40 n = 26

0.19 0.23 0.26 n=5

0.26 0.28 0.62 n=9

0.17 0.32 0.51 n=6

0.14 0.16 0.17 n=6

(ii) SOA from backcast VOC concentrations and FAC ( pg rnm3) T9 (Jan 1989-Jan 1996)

1.28 2.11 3.19

0.95 1.58 2.48

1.73 1.01 2.55

1.78 2.59 3.72

1.79 3.15 5.31

T22 (Mar 1990-Jan 1996)

0.65 1.00 1.55

0.52 0.75 1.16

0.35 0.65 0.90

0.72 1.17 2.05

0.82 1.53 2.11

T27 (Jan 1993-Jan 1996)

0.23 0.35 0.57

0.28 0.33 0.40

0.35 0.37 0.84

0.24 0.52 0.85

0.21 0.23 0.25

Notes: Median concentrations are given in regular font, 25th percentile concentrations are given above in italics and 75th percentile concentrations are given below in italics. n = the number of observed values from which summary concentrations were calculated.

more than 80% of potential SOA except in one sample on 23 August when olefins were predicted to produce 14%. 4.2. Useof FAC and ‘backcast’ concentrations

Using FAC to predict total SOA neglects; l l

l

the presence of primary organic aerosols, aerosol formation prior to the VOC measurement, gaseous concentrations of reactive compounds which have previously reacted leaving lower (measured) concentrations.

It may therefore be more reasonable to ‘backcast’ initial concentrations using information regarding the reacted fraction (Grosjean, 1992). Assuming measurements denote final concentrations, initial concentrations can be estimated using the fraction of VOC reacted (given for each compound in Grosjean (1992)): RC=ICxRF FC=IC-RC.

Thus; IC = FC/(l

-RF)

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(4 6 1 Measurement

of cyclic alefins began nr

0

Back-cast Concentration

+Measured concentration

20

0

Fig. 3. (a) Time series of potential secondaly organic aersosol concentrations (SOA) (in pg m -3) predicted using fractional aerosol coefficients and measured concentrations (solid line) and backcast concentrations (bars). (b) Time series showing contribution of organic groups to potential secondary organic aerosol concentrations (as percentages) using fractional aerosol coefficients and backcast concentration.

where: RC, reacted concentrations; ZC, initial or ‘backcast’ concentrations; RF, reacted fraction; and FC, final concentrations. Using the backcast concentrations (ZC) potential SOA formation was calculated using the same procedure as in Section 4.1. Table 1 gives median, 25th and 75th percentile SOA concentrations for sites T9, T22 and T27. Using backcast VOC concentrations increases SOA by approx. one-third at each site. Fig. 3a shows a time series of potential SOA formation predicted using IC (open bars) at T9. The inter-sample variability remains, as does the seasonal variability. The percentage of poten-

tial SOA formation attributable to each organic group in each sample is shown in Fig. 3b. The contribution from the various groups is strongly affected by the use of backcast concentrations since cyclic olefins, in particular, are very reactive. The average contribution from cyclic olefins and olefins increased to 5% for each group. (Note, these groups were not measured prior to September 1993.) 4.3. Uncertainties

in the FAC analysis

Use of FAC with measured concentrations

con-

RJ. Barthelmie,

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/ The Science of the Total Environment

including:

1. A lack of measurements of organic species potentially important in aerosol formation (e.g. pinene concentrations are not measured within the GVRD and reactive species may have short atmospheric lifetimes and so are not represented in the samples at measurable levels). The potential error introduced by these omissions can be estimated by comparing the proportion of potential SOA yield from each group measured with data given in Grosjean (1992) (Table 2). Assuming that the ratios of groups should be similar in Grosjeans data and at T9, comparing the fractions involved in potential SOA formation, it appears that alkenes and paraffins are underrepresented in the T9 measurements. At both T22 and T27, aromatics contribute approx. 87% of SOA; this relatively high fraction appears to be partly because olefins which could contribute significantly to secondary aerosol formation are frequently not recorded or recorded with zero concentrations, possibly due to their high reactivity. This also implies that SOA concentrations may be under-predicted. Particular VOC which contribute significantly to SOA formation potential in Grosjean (1992) but are not measured at the GVRD sites are: C9 and Cl1 terminal alkenes (6%) and branched ClO, Cl1 and Cl2 alkanes (3%). Neglecting pinene concentrations may

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cause SOA formation potential to be underpredicted by approx. 12%. 2. Differences in aerosol yields under differing environmental conditions. Allocation of FAC is based on smog chamber experiments. Reactions leading to aerosol formation are dependent on temperature and concentrations of the precursors. Typically smog chamber conditions represent highly concentrated mixtures (e.g. high NO, and oxidant concentrations) relative to the real atmosphere and are conducted under conditions of high radiative flux and high temperatures. Hence, FAC may represent optimal aerosol yields. 3. Where specific smog chamber data do not exist, FAC have been allocated by assuming similar values to isomers or from structural considerations in comparison with other similar compounds. Comparison of FAC derived SOA and measured OC aerosol concentrations may also differ due to uncertainties associated with measurement practice including filter artifacts (Turpin and Hering, 1994). 4.4. Seasonalvariability

Seasonal variations of atmospheric concentrations are the product of a number of non-linear processes including:

Table 2 Potential secondary organic aerosol attributed to organic groups (%)

Aromatics Alkenes Cyclic olefins Paraffins Cycloalkanes Oxygenated aliphatics Miscellaneous

Grosjean (1992)

T9 (SOA predicted from measured cont.) 1993

T9 (SOA predicted from backcast cont.) 1993

60.8 12.1 2.8 7.3 0.2 0.1 14.98

89.5 2.2 2.9 3.1 2.6 not measured 0.6

83.6 5.3 5.1 1.7 3.4

a Including pinenes 12% which are not measured at T9.

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0.8

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1. Changes in emission patterns reflecting both anthropogenic and biogenic contributions to VOC emissions. variability which may be man2. Meteorological ifest as lower mixed layer depths in winter and decreased diffusion and/or increase in the importance of meso- and micro-scale circulation in summer, and may also influence the relative importance of different source regions. 3. Changes in the importance of chemical processes in response to decreased actinic flux in winter (e.g. lower oxidant concentrations) and/or changes in relative humidity and temperature. Note that while VOC concentrations, and hence predicted SOA concentrations, are affected by emissions changes and some meteorological variables (e.g. diffusion, mixing depth), FAC do not reflect seasonal changes in chemical processing and/or gas/particle partitioning. Fig. 4 shows considerable month-to-month variability in VOC concentrations. Mean concentrations are typically lowest in June and highest in January and February. Concentrations of the three major groups of interest (paraffins (PAR), aromatics (AR) and olefins COLE)) are minimum in spring while concentrations are highest in winter. Consequently, predicted SOA concentrations follow the same pattern (the high concentration in September may reflect of the relatively low number of samples - there are 23 in September while the average is 38). Samples have been grouped by climatological seasons (where winter is December, January, February; spring is March, April, May etc.) even though this grouping may not be an optimal representation of climatologitally similar months in this region (e.g. June is a transition month in the LFV with a relatively high number of cyclone passages (Pryor et al., 1995)). Table 1 shows SOA concentrations calculated by season using measured and backcast VOC concentrations at sites T9, T22 and T27. SOA predicted from measured concentrations are lowest in spring/summer and highest in autumn/

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1

I 600 DSOA

(Measured

OSOA

(backcast

3 wa 2.5 3

500 G 400 g

6

2

IE

1.5

1

’

200

0.5

100

s 300%

0

3 5g 0”

0 1

2

3

4

5

6 7 Month

8

9

10

11

12

Fig. 4. Monthly median concentrations of VOC (ppb C) and SOA from measured VOC and backcast VOC concentrations at T9 (in pg mm3).

winter at T9 and T22. The low number of samples at T27 means that seasonal changes cannot be discussed with confidence. 4.5. Comparison with measuredorganic aerosol concentrations 4.51. REVEAL

OC concentrations (in the fine fraction of total aerosol mass) observed at (a) PIME and (c) CHIL and those derived for (b) CLBR, during REVEAL are shown in Fig. 5. Concentrations at PIME (the site located closest to T9) are typically higher than those at CLBR and CHIL. These differences may reflect the influence of local sources or the distance from the metropolitan Vancouver region (GVRD) - the location of maximum VOC emissions. Some aerosol forming reactions may be comparatively rapid and thus quickly reduce the gas-phase concentrations of the reactive species while other aerosol forming reactions may occur slowly or as part of a chain of reactions. Neither FAC nor RF can fully represent these processes although the use of RF is an attempt to backcast initial concentrations according to the reactivity of different species. As expected, there is considerable day-to-day variability in the OC concentrations depending on a number of factors such as differing source regions, temperature, humidity, mixing depth etc. The most noticeable feature of the time series is the peak in concentrations which occurred on 5

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August 1993 during a visibility episode (Pryor and Barthehnie, 1996b). This peak is also evident in SOA calculations from VOC concentrations at T9 (Fig. 3). The bars in Fig. 5 show estimates of primary OC calculated using EC concentrations. The concentration of primary OC is assumed to be approx. twice the concentration of EC, following Chow et al. (1993b) and Turpin and Hering (1994). Potential SOA formation calculated from the VOC measurements at T9 during REVEAL is shown as the crosshatched bar area. The additional SOA calculated using RF is marked with shading. Thus the total SOA produced using backcast VOC concentrations is the sum of shaded and crosshatched bar area (Fig. 5) and the total bar area is total predicted OC aerosol from the sum of primary OC aerosol and SOA. It can be compared with total measured OC concentrations which are shown as the squares. With so few measurements it is not possible to evaluate the FAC method in detail. However, there is broad agreement between observed and predicted concentrations and the method does capture the dominant features of temporal variability. 4.5.2. REVEAL II Data from the second field campaign, REVEAL II, are available for April to August 1994

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inclusive, from CLBR and CHIL. The twice weekly samples were not collected on the same days as VOC measurements at the GVRD sites and hence there are only a few days of coincident sampling. Measurements have been grouped into two seasons April-June and July and August based on Fig. 4. Table 3 shows measured OC and EC from CLBR and CHIL. Particle carbon concentrations are slightly higher in the second period (July and August). SOA concentrations from VOC measurements at T9, T22 and T27 are also shown. At T9 and T22, SOA concentrations are also higher in the second period (July and August). 4.6. Total carbon concentrations

Total carbon is the sum of measured carbon in both the gas and aerosol phases. To assess total carbon ( pg C me31 variability vs. differences in carbon partitioning, total carbon concentrations were calculated for T9 and PIME during REVEAL (Table 4). VOC measurements from T9 have been assigned to the major organic groups (AR, PAR, OLE). PAR are present at the highest concentrations and show the greatest variability. OC and EC make a comparatively small contribution to total carbon, OC showing greater variabil-

Table 3 Particulate carbon concentrations measured at CLBR and CHIL during REVEAL II where OC is the concentration of organic carbon, EC is the concentration of elemental carbon ( pg rnm3) and RF is the predicted concentration of SOA at T9, T22 and T27 calculated from VOC concentrations and FAC ( pg mm3 1 CLBR

April-June

1994

July, August 1994

CHIL

T9

T22

T27

oc

EC

oc

EC

RF

RF

RF

1.4

0.5

1.4

2.0 2.4 n = 20

0.7 0.9 n = 20

1.6 2.4 n = 21

0.7 1.0

1.0

0.4

0.4

1.1 n = 21

n = 10

n=6

n=3

1.3 1.8

0.7 0.8

1.5

0.8 1.1

2.6

1.0

0.3

2.4 n = 17

1.0

1.3 n = 18

n=5

n=4

n=l

n = 17

2.1 3.2 n = 18

Note: Median concentrations are given in regular font, 25th percentile concentrations are given above in italics and 75th percentile concentrations are given below in italics. n = the number of observed values from which summary concentrations were calculated.

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aerosol contributes approx. 3% of total carbon and that variability of total carbon exceeds variability in partitioning between gaseous and aerosol phases. Analysis of a 2-day smog episode in Los Angeles in September 1993 (Fraser et al., 1996) indicates that particulate carbon contributed 12.5% at the site farthest downwind of major emissions compared with 2.6-5.4% at the coast.

PIME

10

5. Summary and future work

Fig. 5. Time series of observed organic aerosol concentrations (in pg mm31 at PIME, CLBR and CHIL during the REVEAL campaign ( W). Open bars are estimates of primary organic carbon, crosshatched bars are concentrations of SOA using measured VOC concentrations and FAC and shaded bars are the additional concentration of SOA due to RF.

The aim of this analysis is to assesswhether the use of FAC and RF (Grosjean, 1992) combined with observed VOC concentrations could reproduce the concentrations of measured organic aerosols in the LFV. Predicted concentrations of SOA are in reasonable accord with observed OC values. Seasonal variability in predicted SOA with highest concentrations in winter shows reasonable accord with observed OC, and although this may be due to a combination of changes in emissions and meteorology, because use of FAC and RF does not take into account possible changes in chemistry (e.g. in photochemical reactions or gas/particle partitioning) in different seasons, this interpretation is speculative. Aromatic species appear to dominate SOA, although their contribution is probably exaggerated and SOA is likely to be under-predicted since a number of VOC which are potentially important contributors to aerosol mass are not measured within the GVRD. It is estimated that these compounds (pinenes, higher molecular weight alkenes and branched alkanesj2 would contribute an additional 20% to SOA. Comparison of VOC and particulate carbon measurements during REVEAL suggests that particulate carbon is a comparatively small contributor to total carbon and that variability in total carbon concentrations is greater than variability in gas/particle partitioning. This research was undertaken to develop an understanding of the sources of organic aerosols and the conditions which are conducive to the

ity than EC. Although the small sample size prevents detailed interpretation it appears that OC

2Measurements of cyclic olefins (which have high FAC and high RF) did not begin until September 1993.

b) CLBR 10

0

6

c) CHIL 10

6

.

. n

R

. n

I

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Table 4 Contributions to total carbon concentrations during REVEAL calculated from VOC concentrations at T9 and particulate carbon at PIME ( pg C rnw3) Conc.(pgCm-3)

18 July 23 July 30 July 5 Aug 12 Aug

Particulate from PIME

VOC from T9 Ar Par

Ole

Other

Total

OC

EC

Total

20.1 18.9 17.2 48.3 32.7

13.7 11.6 11.0 25.8 26.0

1.6 10.9 1.3 2.9 1.5

79.2 85.7 67.5 243.5 145.0

2.3 1.3 0.7 6.1 1.6

0.6 0.7 0.3 1.8 1.1

2.9 2.0 1.0 7.9 2.7

43.3 44.3 38.1 166.9 84.9

of secondary organic aerosols in this environment. Because of the large uncertainties in the formation mechanisms of SOA (and hence lack of explicit chemical mechanisms), future research will utilize the findings presented herein to parameterize the chemistry of the most important components of organic aerosol concentrations within a numerical model. formation

Acknowledgements The authors would like to acknowledge Tom Dann of the Environmental Protection Center for supplying VOC measurements and Steve Sakiyama and Patrice Rother for supplying aerosol measurements. We are also grateful to Bruce Thomson and Keith Puckett of Environment Canada for providing a grant under which this research was conducted. References Altshuller AP. Chemical reactions and transport of alkanes and their products in the troposphere. J Atmos Chem 1991;12:19-61. Atlas EL, Li SM, Standley I-J, Hites, RA. Natural and anthropogenic organic compounds in the global atmosphere. In: Radojevic M, Harrison RA, editors. Atmospheric Acidity. Elsevier Applied Science, 1992313-381. Bowman FM, Pilinis C, Seinfeld JH. Ozone and aerosol productivity of reactive organics. Atmos Environ 1995; 29579389. Chow JC, Watson JG, Lowenthal D, Solomon PA, Magliano K, Ziman SD, Richards LW. PM10 and PM2.5 compositions in California’s San Joaquin Valley. Aerosol Sci Techno1 1993a;43:74-84. Chow JC, Watson JG, Pritchett LC, Frazier CA, Purcell RG. The DRI thermal/optical reflectance carbon analysis sys-

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