Photochemical evolution of submicron aerosol chemical composition ...

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D14304, doi:10.1029/2007JD009493, 2008

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Photochemical evolution of submicron aerosol chemical composition in the Tokyo megacity region in summer T. Miyakawa,1 N. Takegawa,1 and Y. Kondo1 Received 11 October 2007; revised 29 January 2008; accepted 14 February 2008; published 18 July 2008.

[1] We investigate the chemical transformation of submicron aerosol in the Tokyo

megacity region in summer. An Aerodyne quadrupole aerosol mass spectrometer (AMS) was deployed both at an urban site in Tokyo (35°390N, 139°400E) and a suburban site (downwind site) in Saitama (36°050N, 139°330E) in the summer of 2004. The temporal evolution of size-resolved chemical compositions of submicron (PM1) aerosols during photochemical smog episodes are investigated using the photochemical age derived from the combination of alkyl nitrate-to-hydrocarbon ratio and NOz/NOy ratio (where NOz is defined as the total reactive nitrogen oxides (NOy) excluding nitrogen oxides (NOx)). The photochemical age observed at the downwind site was about 12 h in most aged air. Organic aerosols (OA) and sulfate (SO2 4 ) were major constituents of PM1 aerosols (40–50% and 20–30%, respectively) at both sites during the observation period and their fractions showed no large variation with the NOz/NOy ratio. Mass ratios of OA and SO2 4 to black carbon (BC) largely increased with the NOz/NOy ratio (by factors of 3 and 2, respectively), indicating the significance of secondary formation of these compounds in controlling PM1 mass concentrations. We also investigate the photochemical evolution of OA mass spectra observed by the AMS. The mass-to-charge ratio (m/z) peaks of organic compounds relative to BC mass generally showed an increase with the NOz/NOy ratio. These increasing trends vary significantly for different m/z peaks, suggesting the complexity of the temporal evolution of organic functional groups. The m/z 44 and 45 peaks, which are good markers of carboxylic groups in organic particles, showed larger increases than any other m/z peaks, suggesting an efficient formation of carboxylic functional groups on a timescale of hours during the measurement period. Citation: Miyakawa, T., N. Takegawa, and Y. Kondo (2008), Photochemical evolution of submicron aerosol chemical composition in the Tokyo megacity region in summer, J. Geophys. Res., 113, D14304, doi:10.1029/2007JD009493.

1. Introduction [2] Urban areas are significant sources of aerosols, ozone (O3), nitrogen oxides (NOx), sulfur dioxide (SO2), and volatile organic compounds (VOCs). Extremely high levels of these compounds are found in megacities in developing countries [Molina and Molina, 2004]. While the Asian megacities contribute only 2% of the land area of Asia in total, they produce 10% of the anthropogenic emissions of trace gases and aerosols in this region [Guttikunda et al., 2005]. Secondary formation of aerosols (e.g., sulfate aerosol and organic aerosols (OA)) as well as O3 is an active area of research both in air quality (health effects and visibility reduction [Dockery et al.,1993; White, 1976]) and climate change issues [e.g., Chung and Seinfeld, 2002]. OA consist of hundreds to thousands of compounds in many categories that differ widely in molecular functional groups [Saxena and Hildemann, 1996] and account for a significant fraction 1 Research Center for Advanced Science and Technology, University of Tokyo, Japan.

Copyright 2008 by the American Geophysical Union. 0148-0227/08/2007JD009493$09.00

of total particulate mass [Duce et al., 1983]. A more detailed characterization of OA (composition and changes with air mass aging) is needed to understand the physical and chemical properties of aerosol particles (hygroscopicity and refractive index). [3] Photochemical smog over the Kanto area (Tokyo metropolis and surrounding prefectures) (Figure 1) during summer is one of the most serious environmental issues in this region. High photochemical activity combined with the export of polluted air, which is impacted by large emissions of NOx and VOCs over the Tokyo metropolitan area (TMA), causes significant photochemical smog over the surrounding region in the Kanto area as well as TMA [Wakamatsu et al., 1999, and references therein]. It has been found that OA is the dominant aerosol component (50% of total nonrefractory aerosol mass), and major emission source of primary organic aerosol (POA) is motor vehicle emissions in Tokyo [Takegawa et al., 2006a]. In summer, the high-O3 episodes have been observed simultaneously with high OA formed secondarily in the atmosphere over the Kanto area [e.g., Satsumabayashi et al., 1990; Takegawa et al., 2006a]. However, our understanding of the photochemical evolution of chemical compositions of

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Figure 1. Map of the Kanto area. The locations of two sites, RCAST (square) and CESS (cross), where intensive measurements were conducted, and air quality monitoring stations (circles) maintained by the Atmospheric Environmental Regional Observation System (AEROS) are shown in this figure. Area bounded by shaded lines depicts the approximate urban boundary. These monitoring sites are colored by the mixing ratio of O3 obtained by AEROS (13 September 2004 15:00LT). PM1 aerosols, especially OA, exported from TMA under photochemical smog events is still limited. [4] The purpose of this study is to characterize the sizeresolved chemical composition of submicron aerosols at a downwind site and the aerosol transport process from the source region to the surrounding area. For the purpose of this study, we deployed two Aerodyne quadrupole aerosol mass spectrometers (Q-AMS) at an urban site in Tokyo and a suburban site in the outflow region. The Q-AMS used in this study is hereafter referred to simply as the AMS. The AMS measures size-resolved mass loadings of nonrefractory submicron (NR-PM1) aerosols (vaporized at 600°C under vacuum) with a time resolution of the order of minutes [Jayne et al., 2000; Jimenez et al., 2003; Allan et al., 2003]. The use of two AMSes provides useful insights into the chemical evolution of aerosols during photochemical smog episodes.

2. Experimental Description 2.1. Measurements [5] In situ measurements of aerosols and gases were conducted at an urban and a suburban site in Japan in summer 2004 (from 26 July to 15 August). The observation sites were located at the Research Center for Advanced Science and Technology (RCAST), Komaba, Tokyo metropolis (35°390N, 139°400E) and the Center for Environmental Science in Saitama (CESS), Kisai, Saitama prefecture (36°050N, 139°330E), which is located about 50 km north of TMA (Figure 1). When sea breeze dominates in the summer season, the Kisai site is downwind of TMA. The Kisai site is hereafter referred to as the downwind site. The Komaba site is near the center of TMA and referred to as the urban site. Figure 1 shows locations of routine environmental monitoring stations (http://soramame. taiki.go.jp/) colored by O3 mixing ratio at 15:00LT on 13 August 2004, as a typical example of photochemical

smog events over the Kanto area. As indicated, the two observation sites were strongly impacted by high O3 concentration (>100 ppbv), which spread out over the Kanto area. As discussed in previous studies [Kondo et al., 2006; Morino et al., 2006; Miyakawa et al., 2007], large emission sources of pollutants such as carbon monoxide (CO) are distributed in a coastal area inside the urban boundary (Figure 1). Transport of polluted air masses from TMA to the surrounding region could not be largely impacted by emission sources near the downwind site. [6] The measurements of aerosol and trace gases at both sites, which are used in this analysis, are summarized in Table 1. Details of the measurements at the urban site were described by Takegawa et al. [2006a] and Kondo et al. [2006]. A part of the experimental description of the downwind site has already been given by Takegawa et al. [2006b]. Some key details of the measurements at both sites in the present study are described as follows. [7] The mass loadings and size distributions of NR-PM1 2 aerosols (nitrate (NO 3 ), sulfate (SO4 ), chloride (Cl ), + ammonium (NH4 ), and OA) at both sites were measured using the AMS with an integration time of 10 min. The performance of the AMS deployed at the urban site is described by Takegawa et al. [2005]. The operational procedure of the AMS (calibration) at the downwind site was the same as that at the urban site. We focus on the performance of the AMS at the downwind site. The fluctuation in the ratio of the ionization efficiency of pure ammonium nitrate (IENO3) to the AMS signal at m/z = 28 (air beam, AB), IENO3/AB, is an indicator of the variability in the sensitivity of the AMS [Takegawa et al., 2005]. The average and standard deviation of IENO3/AB are 0.81 1012 Hz1 and 0.07 1012 Hz1, suggesting 9% variability in the sensitivity of the AMS at the downwind site. The temperature of the AMS inlet was controlled at 35– 40°C to dry particles in the sampling air, as was done by

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Table 1. Summary of the Measurements of Aerosols and Trace Gases at the Urban and Downwind Sites Species

Measurement Techniques

Time Resolution

Reference Takegawa et al. [2006a] Takegawa et al. [2006a]

Urban site CO NOx NOy NR-PM1 aerosols BC

NDIR chemiluminescence w/photolytic /catalytic converter aerodyne AMS aethalometer

1 min 1 min 1 min 10 min 5 min

O3 CO NOx NOy SO2 HNO3 NMHCs NR-PM1 aerosols BC

Downwind site UV-absorption NDIR chemiluminescence w/photolytic /catalytic converter UV-fluorescence diffusion scrubber-IC whole air sampling aerodyne AMS aethalometer

1 min 1 min 1 min 1 min 1 min 30 min 2 h 10 min 5 min

Takegawa et al. [2005]. It is possible that evaporative loss of highly volatile compounds may have occurred in the inlet. Takegawa et al. [2005] have shown that this effect does not significantly contribute to evaporative loss of ammonium nitrate in the sampling system used in this study. The mass concentrations of all compositions (inorganic and organic aerosols) reported here were calculated assuming that the mass-based particle collection efficiency of the AMS (CEAMS) was 50% at both sites. We assume a CEAMS of 50% at the urban site, as has been validated for inorganic and organic aerosols in previous studies [Takegawa et al., 2005; Kondo et al., 2007]. In this study, ionic components measured by independent measurements showed good agreement with those measured by the AMS, assuming that the CEAMS of 50% at the downwind site. The validity of the assumed CEAMS at the downwind site was tested based on intercomparison of the AMS with a filter sampling method, as discussed in section 2.2. In the AMS data standard analysis procedure, the m/z 28 signals are not assigned for organic compounds because molecular nitrogen in the air largely dominates the m/z 28 signal (mostly N+2 ). In the present study, we estimated the contribution to the m/z 28 signals from organic compounds, and then included them in the OA mass spectra. The method for estimating the m/z 28 signals from organic particles is discussed in Appendix A1. [8] The mass concentration of BC at both sites was measured by an aethalometer (model AE31; MAGEE Scientific, Berkeley, USA) with a time resolution of 5 min. The model AE31 aethalometer measures light attenuation with seven colored light sources (l = 370 nm, 470 nm, 520 nm, 590 nm, 660 nm, 880 nm, and 950 nm). We used the l = 880 nm data in the analysis because the light attenuation in the wavelength range near the ultraviolet is significantly enhanced by some organics such as aromatics [Jacobson, 1999]. The instrumental response of the aethalometer to nonabsorbing particles such as inorganic aerosols has been tested in previous studies [e.g., Weingartner et al., 2003]. It is suggested that the nonabsorbing particles collected on the filter scatter more light to the coloaded absorbing particles such as BC resulting in an apparent enhancement of light absorption. In order to reduce this effect, the inlet heated at 400°C was located in front of the aethalometer. The

Takegawa et al. [2005] Takegawa et al. [2006b] Takegawa et al. [2006b] Takegawa et al. [2006b] Miyakawa et al. [2007] Komazaki et al. [1999] Simpson et al. [2003] Takegawa et al. [2006b] -

absorption coefficients measured by the aethalometer at both sites were well correlated with the mass concentrations of elemental carbon measured by the thermal optical method (r2 0.9). This indicates that the aethalometer measurements were not largely affected by light absorbing organic compounds such as high molecular weight OA, which tend to be refractory compounds upon 400°C heating [e.g., Baltensperger et al., 2005]. The limits of detection (LODs) of these aerosol measurements during the observation period are summarized in Table 2. The LOD of BC concentrations at the urban site was not measured. However, we used the same instrument setups for measuring BC concentrations at both sites. Thus the LOD of BC concentrations at the downwind site is assumed to be applicable to that at the urban site. In order to verify this assumption, we calculated three times the standard deviation of low-BC concentration data at the urban site to be 0.11 mg m3 for a 5-min integration time. This indicates that the assumption for using the same LOD value at both sites is reasonable. [9] We also conducted a filter sampling of PM1 aerosols followed by off-line analysis at the downwind site. A low volume sampler (FRM-2000, Rupprecht & Patashnick, Co., Inc., USA) was used to collect aerosol samples. A PM1 cyclone (PM-1 SCC, Rupprecht & Patashnick, Co., Inc., USA) was placed upstream of the inlet of the sampler. The sampling flow rate was 16.7 l min1. Sampling periods were 26– 31 July 2004, and 9 – 14 August 2004, and the sampling intervals were 3 h in most cases (76 samples) and 6 h for five samples. Aerosol samples were collected using a 47-mm precombusted (800°C for 1 h) quartz filter (2500QAT, Pall Co., German). Elemental carbon (EC) and organic carbon (OC) were analyzed using a Thermal-Optical-Transmittance method carbon analyzer (Sunset Laboratory, Inc.). The temperature profile used to determine the split point of EC or OC was based on the National Institute for Occupational Safety and Health (NIOSH) protocol. An activated carbon denuder (VOC denuder, Rupprecht & Patashnick, Co., Inc., USA) was placed in front of the sampler in order to reduce adsorption artifacts from semivolatile organic vapors. Data for daily PM2.5 ionic species (SO2 4 ) measured by off-line analysis and meteorological

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Table 2. Limits of Detection (LODs) of Observed PM1 Aerosols at the Urban and Downwind Sites LODa, mg m3 Species

Best Estimate

Nitrate Sulfate Chloride Ammonium Organics

0.02 0.01 0.02 0.2 0.5

Nitrate Sulfate Chloride Ammonium Organics BC

0.01 0.01 0.01 0.06 0.05 0.12

95% Confidence Interval Urban siteb 0.01 – 0.04 0.008 – 0.03 0.01 – 0.04 0.2 – 0.6 0.3 – 0.1 Downwind site 0.01 – 0.05 0.01 – 0.01 0.00 – 0.02 0.03 – 0.08 0.02 – 0.06 0.08 – 0.24

Integration Time, min 10 10 10 10 10 10 10 10 10 10 5

a LODs are defined as three times the standard deviation of the mass concentration measured by placing a particle filter in front of the AMS and aethalometer. b LODs of NR-PM1 aerosols at the urban site were previously estimated by Takegawa et al. [2005].

parameters at the downwind site were provided by CESS [Yonemochi et al., 2001]. 2.2. Comparisons Between the AMS and Other Methods at the Downwind Site [10] First, we show comparisons between PM1 SO2 4 measured by the AMS and PM2.5 SO2 4 measured by filter sampling at the downwind site. PM1 and PM2.5 SO2 4 are 2 referred to as SO2 4 AMS and SO4 Filter, respectively. There 2 was good agreement between SO2 4 AMS and SO4 Filter in terms of temporal variation and mass concentration. A 2 correlation plot between SO2 4 AMS and SO4 Filter is shown in Figure 2a. The linear regression slope of the correlation is 0.98 ± 0.05 (r2 = 0.96), suggesting that SO2 4 AMS was in good agreement with SO2 4 Filter, although the size cut of the AMS was different than that of filter sampling. Previous studies suggest that a majority of SO2 4 mass is in the fine mode particles [e.g., Cabada et al., 2004]. Assuming that SO2 4 mass in the coarse particles (>PM1) was negligible at

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the urban site during the observation period, we can at the conclude that the assumed CEAMS (50%) of SO2 4 downwind site is valid. [11] Second, we show an intercomparison of AMS OA (OAAMS) and OC obtained by filter sampling (OCFilter) at the downwind site. Figure 2b depicts the correlation of OA AMS versus OCFilter. The linear regression slope assuming zero-offset of OAAMS-OCFilter correlation was 1.95 ± 0.17 mg mgC1 (r2 = 0.64). We have found a significant offset of OCFilter (2.5 mg m3) without the assumption of zero-offset. Similar result has been described by Offenberg et al. [2007]. Linear regression slope without the assumption of zero-offset was 2.75 ± 0.33 mg mgC1, which was 40% higher than that derived assuming zerooffset. Furthermore, the value of r2 was somewhat low. This may be due to the uncertainties of CEAMS, the assumed relative ionization efficiency of OA (RIEOA), and the uncertainty in the filter analysis in addition to natural variability in OA compositions (i.e., an increase in noncarbon elements in OA). The RIEOA is defined as the ratio of the ionization efficiency of OA to that of NO3-. The mass concentration of OAAMS is inversely proportion to the product of CEAMS and RIEOA, as shown in the equation (7) of Allan et al. [2004]. RIEOA using the calculation of OCAMS was an assumed constant value. The RIEOA value is dependent upon the functional groups of OA [Jimenez et al., 2003]. CEAMS can vary from 0.5 to 1 depending on particle shape and particle phase (i.e., liquid or solid). Both factors may have contributed somewhat to low correlation between OAAMS and OCFilter. The organic mass-to-organic carbon (OM/OC) ratio derived here is consistent with 1.6 (urban) to 2.2 (nonurban) mg mgC1 reported by Turpin and Lim [2001], suggesting that the assumed CEAMS (i.e., 0.5) of OA is valid in spite of the above uncertainties.

3. General Characteristics of PM1 Aerosols at the Urban and Downwind Sites 3.1. Temporal Variations of Atmospheric Pollutants at the Downwind Site [12] Figure 3a shows time series plots of PM1 aerosols observed at the downwind site. Temporal variations of SO2

Figure 2. (a) Correlation between SO42AMS (y axis) and SO42Filter (x axis). The dashed line is included as a guide for y = x. (b) Correlation between OAAMS (y axis) and OCFilter (x axis). The dashed line is included as a guide for y = 2x. 4 of 18

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Figure 3. (a) Temporal variations of mass loadings of NO3 (blue), SO42 (red), Cl (pink), NH4+ (yellow), OA (light green), and BC (black) at the downwind site. (b) Temporal variations of mixing ratios of ozone (black, left axis) and SO2 (gray, right axis) at the downwind site.

and O3 are shown in Figure 3b. In this study, we focus on the temporal variation of aerosol and trace gases at the downwind site. From 26 July to 30 July 2004, it was cloudy or raining in most cases. Heavy precipitation was observed on 29 July. Mass loadings of inorganic aerosols were relatively low on 29 July. On the other hand, the weather was mostly sunny from 31 July to 15 August. The observation period from 25 July to 30 July is not analyzed in the following section, because the purpose in the present study is to characterize PM1 aerosols in the urban plumes exported from TMA under conditions with high photochemical activity. We classified the observation period from 31 July to 15 August into three parts (phase 1, phase 2, and episodes N) according to the wind flow pattern and chemical composition of PM1 aerosols. [13] The phase one was from 31 July to 9 August (except for 1, 2, 5, and 8 August). In the phase1, continuous southerly wind winds (>2 m s1) dominated (Figure 4a), carrying clean maritime air from Tokyo Bay. Therefore the concentrations of PM1 aerosols and O3 were relatively low (Figure 3). The phase 2 was from 10 August to 15 August. In the phase 2, sea-land circulation was dominant. The wind pattern for this period is also shown in Figure 4b. In the afternoon the wind blew from south or southeast. On the other hand, northerly or northeasterly winds were observed mostly during the night and early morning. On 10, 12, and 13 August, the concentration of O3 was higher than 100 ppbv during daytime. Frequency distributions of the observed concentrations of CO, O3 during daytime (10:00 – 16:00LT), SO2 4 , and OA at the downwind site during the phases 1 and 2 are shown in Figures 5a– 5d.

but also CO showed higher conNot only O3 and SO2 4 centrations during the phase 2 than the phase 1, suggesting that accumulation and photochemical production were important processes affecting the high concentrations of atmospheric pollutants during the observation period. OA, which contains primary and secondary components, also shows consistent results. The mass concentration of NH+4 predicted from observed anions, Predicted NH+4 , was calculated in order to investigate the ion balance of PM1 ionic species measured by the AMS. During the phases 1 and 2, observed 2 were almost fully neutralized by NO 3 , SO4 , and Cl + observed NH4 . Note that mass concentrations (PM2.5) of other cations (e.g., Na+) measured by the filter analysis were relatively low ( 0.8), the SO2 4 /BC and OA/BC increased by a factor of 2 and 3, respectively. NR-PM1/BC also increased by a factor of 2. This indicates that significant amounts of major constituents of PM1 aerosols were formed secondarily in the atmosphere with air mass aging on a timescale of 0.5 d. Characteristic timescale of the SO2 4 formation via reaction of SO2 + OH in daytime can be calculated using the rate coefficients of the SO2 + OH reaction (8.8 1013 cm3 s1) [DeMore et al., 1997] and average OH concentration (4 106 molecules cm3) in daytime. This is estimated to be about 3 d. Local in situ

Figure 9. Changes in the mass ratio of SO42 (red), OA (light green), and inorganic compounds other than SO42 (shaded) to BC as a function of NOz/NOy ratio at the downwind site. The data for X/BC (X; each NR-PM1 constituent) on the left panel are shown as a reference of the freshest air masses near the source area (see text for details). This figure is a cumulative plot. 9 of 18


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Table 3. Major m/z Values in the AMS Mass Spectra of OA at the Downwind Site m/z 15 27 29 41 42 43 44 45 50 51 53 55 57 58 65 67 69 77 83 85 91 105 115

Da 2 0 2 0 1 2 3 4 5 4 2 0 2 3 4 2 0 6 0 2 6 6 4

Fraction [%]b

Possible Ion Compositionsc

REm/z,

d Aged

Typee

2.7 4.6 6.2 4.1 2.6 7.7 8.3 1.4 0.7 0.9 1.3 3.3 1.5 0.6 0.9 1.1 1.4 0.8 0.6 0.5 0.8 0.4 0.9

CH3+ C2H3+ C2H5+, CHO+ C3H5+, C2HO+ C3H6+, C2H2O+ C3H7+, C2H3O+ C3H8+, CO2+, C2H4O+ CHO+2 , C2H5O+ C4H2+ C4H3+ C4H5+, C3HO+ C4H7+, C3H3O+ C4H9+, C3H5O+ C4H9+, C2H2O2+, C3H6O+ C5H5+ C5H7+, C4H3O+ C5H9+, C4H5O+ C6H5+, C2H5O3+ + , C5H7O+ C6H11 + , C5H9O+ C6H13 C7H7+, C3H7O3+ C8H9+, C7H5O+ C6H11O2+, C7H15O+, C5H7O3+

2.4 2.3 2.6 1.8 2.6 2.4 3.2 3.4 2.4 2.2 2.3 1.9 1.6 2.3 2.0 1.9 1.8 1.9 1.7 2.2 1.7 1.6 1.9

0.4 0.4 0.4 0.3 0.4 0.4 0.5 0.6 0.4 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.4 0.3 0.3 0.3

3 3 3 2 3 3 1 1 3 3 3 2 2 3 2 2 2 2 2 3 2 2 2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

a

D = (m/z) – 14n + 1 (n; carbon number). Relative contribution (%) of each m/z to the average OA mass spectrum at Kisai. c Possible ion composition at each m/z is assigned according to previous studies [e.g., McLafferty and Turec´ek, 1993]. d Values of REm/z in the aged air (see text for detail) are listed. e Signals were categorized by the relative enhancement of its m/z signal/BC ratio. b

gas-phase formation does not fully account for the observed levels, suggesting that aqueous-phase formation in SO2 4 aerosol or cloud water [Seinfeld and Pandis, 1998] could play a significant role in the observed increase of SO2 4 /BC ratio. [22] Previous studies reported the relative increase of OA or water soluble organic carbon (WSOC) transported from urban area with air mass aging [de Gouw et al., 2005; Sullivan et al., 2006; Takegawa et al., 2006b; Kleinman et al., 2007a, 2007b]. A common feature found in this study and previous studies is that the OA or WSOC exported from anthropogenic sources linearly increased by a factor of 45 within 0.5 d. Furthermore, previous studies showed a common feature that the relative rate of increase of OA or WSOC appeared to level off after about a day possibly because SOA precursors are largely consumed. It is indicated that SOA precursors were not fully consumed in the aged plumes (after 0.5 d of processing) observed at the downwind site. Assuming that SOA concentration in the freshest air masses is zero and loss of POA is negligible, SOA fraction in the air masses with NOz/NOy ratios > 0.8 is estimated to be about 70%. This indicates the strong SOA formation near the source area with the timescale of about a day. Dominance of SOA in processed air presented here is also found in previous studies near megacities [Volkamer et al., 2006] and over outflow regions [Crosier et al., 2007]. 4.3. Interpretation of OA Mass Spectra Obtained at the Downwind Site [23] In addition to the bulk concentration of OA, we discuss the chemical composition of OA using the OA mass spectra obtained by the AMS. Some m/z peaks of electron impact ionization mass spectra are closely related to specific

functional groups [e.g., McLafferty and Turec´ek, 1993]. It is generally difficult to quantitatively estimate the amount of parent molecules and/or functional groups from the AMS mass spectra. Temporal evolution of those m/z peaks would be useful to investigate the chemical transformation of organic compounds with air mass aging. The major fragments in the OA mass spectra observed at the downwind site are summarized in Table 3. The sum of signals from all fragments in Table 3 accounted for about 50% of OA mass spectra observed at the downwind site. Figure 10a depicts the photochemical evolution of the concentration ratio of the signals at m/z 43, 44, and 57 to BC (m43/BC, m44/BC, and m57/BC) as a function of the NOz/NOy ratio during phases 1 and 2. As the NOz/NOy ratio increased, m43/BC, m44/BC, and m57/BC all increased significantly. [24] The m/z 43 signal (C3H+7 and C2H3O+) is a predominant mass peak in the AMS mass spectrum of SOA from the photooxidation of 1,3,5-trimethylbenzene (135TMB) [Alfarra et al., 2006]. Jang et al. [2004] reported the production of low-molecular weight (LMW) dicarbonyls such as methyl glyoxal from the ring opening reactions through the photooxidation of 135TMB, indicating that these carbonyl (aldehydes and ketones) compounds can significantly contribute to the m/z 43 signal of the SOA in the chamber studies. The m/z 44 signal (likely CO+2 ) was highly correlated with LMW dicarboxylic acids such as oxalic acid in summer 2003 in Tokyo, and about 10% of the m/z 44 signal on average originates from oxalic acid in the summer in Tokyo [Takegawa et al., 2007]. Organic peroxides can be formed from the reaction of alkyl peroxy radical with hydroperoxy radical, as suggested by some laboratory studies [e.g., Docherty et al., 2005]. However, it is unlikely that organic peroxides contributed to the

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ionizer system [George et al., 2007]. The increase of m43/ BC and m44/BC can be attributed to the secondary formation of OA, which have carbonyl and carboxylic groups in organic particles. The m/z 57 signal (C4H+9 ) was previously used as a good marker for HOA, because the major possible elemental composition of m/z 57 is considered to be C4H+9 from long-chain aliphatic hydrocarbons. DeCarlo et al. [2006] have recently reported a 5 min-averaged mass spectrum measured using a high-resolution time-of-flight aerosol mass spectrometer (HR-T-of-AMS) during observation at Riverside, California, and that the m/z 57 signal in the mass spectrum was composed of both C4H+9 (70%) and C 3 H 5 O + (30%). It is suggested that the AMS observed the increase of the m/z 57 signal of C3H5O+ from oxygenated compounds and/or C4H+9 from aliphatic structures in SOA particles in the summer over the Kanto area. Assuming that the ion composition of the m/z 57 peak at NOz/NOy ratios 0.8. [25] The relative enhancements for each m/z signal/BC ratio (REm/z) were calculated by normalizing the average value in the first bin of the NOz/NOy ratio (i.e., 0 – 0.2) to 1 (equation (4)). REm=z ¼

Figure 10. (a) Changes in the mass ratio of m/z 44 (black lines between closed circles), 57 (black lines between open circles), and 43 (shaded lines between closed circles) signals to BC as a function of NOz/NOy ratio at the downwind site. Each marker and bar corresponds to the average and standard deviation in each 0.2-NOz/NOy bin, respectively. (b) Changes in the REm/z of Type1 peaks (m/z 44 (closed circles) and 45 (open circles) on the top panel), Type2 peaks (m/z 41 (black squares), 57 (open squares), and 65 (shaded squares) on the middle panel), and Type3 peaks (m/z 15 (black triangles), 29 (open triangles), and 43 (shaded triangles) on the bottom panel) as a function of NOz/NOy ratio at the downwind site. Each marker corresponds to the average in each 0.2-NOz/NOy bin. observed increase of the m/z 44 signal because organic peroxides are highly unstable and may not yield peroxide functional group at m/z 44 (COO+) in the AMS vaporizer/

Mm=z MBC ; Avg Mm=z MBC NOz =NOy