Photochemical aging of organic and inorganic ambient aerosol from 1 the Potential Aerosol Mass ( PAM ) reactor experiment in East Asia

1Department of Earth and Environmental Sciences, Korea University, Republic of Korea 6 2Department of Urban and Environmental studies, Suwon Research Institute, 7 Republic of Korea 8 3Department of Meteorology, Pennsylvania State University, USA 9 4Department of Environmental Science, Hankuk University of Foreign Studies, 10 Republic of Korea 11 5National Institute of Environmental Research, Republic of Korea 12 13 14


Introduction
In East Asia, atmospheric aerosols are a cause of public concern because of the frequent occurrence of haze in mega cities and industrial areas and dust storms in deserts and extremely dry regions, and their transboundary transport (Takami et al., 2007;Wu et al., 2009;Kim et al., 2009;Ramana et al., 2010;Kang et al., 2013).These occurrences impact the regional air quality and climate (Li et al., 2011;Huang et al., 2014).Aerosol plumes are able to remain in the atmosphere for up to 10 days and can be transported across the Pacific Ocean.During transport, air masses become photochemically aged, leading to the generation of secondary aerosols and subsequent modification of the optical and microphysical properties of aerosols (Dunlea et al., 2009;Lim et al., 2014;Lee et al., 2015).
This transformation process has been studied by collecting ambient air across the Pacific Ocean or by tracking the Asian plumes onboard aircrafts (Brock et al., 2004;Aggarwal and Kawamura, 2009;Dunlea et al., 2009;Peltier et al., 2008).
Secondary aerosols (SA) comprise inorganics such as sulfate and nitrate as well as organics.Of these, secondary organic aerosols (SOA) are of more interest because they are produced in the atmosphere from numerous organic species and are aged through complex mechanisms, during which their physicochemical properties such as volatility, hygroscopicity, and optical properties are altered.The absorption and scattering properties of aerosols in northeast Asia was reported to be intimately linked with their chemical composition (Lim et al., 2014).As aerosols are oxidized, the hygroscopicity of organic aerosols (OAs) increases, suggesting photochemically driven CCN activation of SOA (Massoli et al., 2010;Lambe et al., 2011;King et al., 2010;Morgan et al., 2010).
To understand SOA formation and aging processes, experiments have been conducted using environmental chambers (Kroll and Seinfeld, 2008;Hallquist et al., 2009).
In these large environmental chambers, atmospheric simulations are limited to the equivalent of only about a day, which is much shorter than the nominal lifetime of aerosols, which is about a week.In addition, ambient air masses are under influence of various emissions and mixing processes, which are not properly represented in these wellmixed and long-residence time chambers (Jimenez et al., 2009;Ng et al., 2010).
Thus, we introduced the potential aerosol mass (PAM) chamber, a continuous flow reactor under high levels of OH and O 3 , which is applicable for both controlled lab studies and ambient air (Cubison et al., 2011;Kang et al., 2007Kang et al., , 2011b;;Lambe et al., 2012;Massoli et al., 2010).The highly oxidizing conditions of the PAM reactor are suitable for examining SOA formation and oxidation processes for the equivalent of a week or more (Jimenez et al., 2009;George and Abbatt, 2010).In particular, the PAM reactor has less wall loss mainly due to much shorter residence time compared to conventional chambers.Thus, PAM reactor is able to reasonably simulate aging processes of SOA after formation (Kang et al., 2011a).In the first field deployment of PAM in northeast Asia, Kang et al. (2011a) reported PAM simulation results for different air masses and demonstrated that oxidation processes occurring in the natural atmosphere were plausibly integrated in the PAM reactor.Recently, PAM reactor has been used to examine secondary aerosol formation and evolution from ambient air masses (Hu et al., 2016;Ortega et al., 2016;Palm et al., 2016) and emission sources (Ortega et al., 2013;Link et al., 2017;Timonen et al., 2017).The wall losses of aerosols and condensable gases and photochemistry of the PAM reactors have been studied to quantitatively understand the experimental results of PAM reactor (Lamb et al., 2015;Palm et al., 2016;2017;Peng et al., 2015;2016).In this study, we deployed a PAM reactor at an island site in the Yellow Sea to investigate the photochemical aging processes of ambient aerosols such as secondary aerosol formation and evolution of preexisting aerosol over the northeast Asia.Their size, mass, chemical, and transformation characteristics were thoroughly examined with a particular emphasis on SOA formation and transformation.

Experimental methods
Experiments were conducted at a measurement station on Baengnyeong Island in the Yellow Sea (37.967°N, 124.630°E, 100 m asl) from August 4 to August 12, 2011 (Fig. 1a).
As the northernmost and westernmost part of South Korea, Baengnyeong Island is located 740 km west of Beijing and 211 km east of Seoul.The measurement station was established by the National Institute of Environmental Research (NIER) as a core background site of the National Monitoring Network to observe dust and pollution plume transported from China.In previous studies conducted at the same site, sulfate and organic aerosols were enhanced under the influence of pollution plumes transported from eastern China and the Korean Peninsula (Choi et al., 2016;Lee et al., 2015).Ambient air sampled using a PM 1.0 cyclone and was pulled through a tubing (1 cm diameter) into the PAM reactor for 6 minutes, during which time the ambient aerosols were oxidized (hereafter referred to as "PAM aerosols").For another 6 minutes, the sampled ambient air was directly pumped into the analytical instruments, bypassing the PAM reactor.The ambient and PAM aerosols were alternately measured every 6 minutes, thereby producing pseudo-simultaneous measurements.
The PAM reactor employed in this study is the same version as that described in Lambe et al. (2011), which was also used for laboratory studies of SOA aging (Lambe et al. 2012;2015).The ambient air was introduced into the PAM reactor through an inlet plate and endcap and then rapidly dispersed before entering the reactor through a Silconertcoated (Silcotech, Inc.) stainless steel screen.For sampling, copper or stainless steel tubing was used to minimize the particle loss on the tubing walls.The residence time of air in the PAM reactor was 100 seconds based on flow rate and the volume of the reactor.During the experiment, the loss associated with PAM reactor and inlet was determined.The SO 2 loss through a cyclone and inlet plate was 11 ± 7 % and aerosol loss in the PAM reactor was about 12 %.The loss of aerosol and SO 2 gas was measured as difference in the concentration between ambient air and air pulled through PAM reactor with UV lights off.
The PAM reactor is equipped with long Hg lamps emitting 185 nm and 254 nm light (82-9304-03, BHK Inc.) in order to produce large amounts of OH and O 3 , creating a highly oxidizing environment.The UV lamps were housed in Teflon sleeves being purged by nitrogen to prevent heat and O 3 from building up.The results of Ortega et al. (2016) using the PAM reactor same as that used in the present study, demonstrated that temperate was increased by about 2 °C inside the PAM reactor and its effect on evaporation of OA or nitrate was insignificant.
The OH exposure of PAM reactor was estimated to be 7 x 10 11 molecules cm −3 s against sulfur dioxide decay that was conducted by Kang et al., 2011b.It is equivalent to an integrated OH concentration over 4.6 days at a typical noon-time concentration of 1.5 x 10 6 molecules cm −3 (Mao et al., 2010), which is lower by 25 % than was expected due to the external OH suppression.The OH suppression from VOCs and other OH-reactive gases were calculated using the chemistry model with 30 s -1 of external OH reactivity, representing rural areas (Feiner et al., 2016;Lee et al., 2008;Peng et al., 2016;Yoshino et al., 2006).In the present study, therefore, we examined how the air masses reaching Baegnyeong Island were further oxidized during the effective aging time of 4.6 days, which is consistent with transport time from China but slightly longer than the time of typical maximum SOA production from aging (Ortega et al., 2016) under OH diel-mean of 1.5 x 10 6 molecules cm −3 .The chemical composition of aerosol was measured by a highresolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, hereafter referred to as "AMS") and their number concentration was determined in the mobility diameter range of 10.4-469.8nm with a scanning mobility particle sizer (SMPS 3034, TSI) (Jayne et al., 2000;Jimenez et al., 2003;DeCarlo et al., 2006;Drewnick et al., 2006).The aerosol mass concentration was obtained from the volume concentration multiplied by a fixed particle density of 1.2 g cm −3 . Detailed descriptions of the HR-ToF-AMS and the sampling site can be found elsewhere (Lee et al., 2015).For concentrations measured by AMS, composition dependent collection efficiency was applied by adopting the result of Middlebrook et al. (2012).Ozone (O 3 ), elemental carbon (EC), and organic carbon (OC) were simultaneously measured, along with meteorological parameters (Table 1).The HYSPLIT backward trajectory model, which was developed by the National Oceanic and Atmospheric Administration (NOAA), was used to examine the history of the sampled air masses (Wang et al., 2009).

Measurement overview of ambient and PAM aerosols
PM 1.0 aerosol mass concentrations varied from 0.5 to 38 μg m −3 for both the PAM aerosols and the ambient aerosols (Figure 2a) during the entire experiment period.Choi et al. (2016) reported the AMS measurements (PM 1.0 ) made at the same site during March-April, 2012 and November-December, 2013, where PM 1.0 varied from the detection limit to ~100 μg m -3 .These PM 1.0 concentrations were lower than those measured in Changdao that is located in Bohai Sea (Hu et al., 2013;Choi et al., 2016;Lee et al., 2015).The PAM aerosol masses were generally greater than the ambient aerosol masses, but not all the time.The difference in mass concentrations between the PAM aerosols and the ambient aerosols either a gain or loss of particle mass in the range of -3 ~ 7 μg m −3 , indicating that photo-oxidation.
Particle mass distributions of the ambient and PAM aerosols were averaged for the entire experiment and their difference is presented in Fig. 3.In the PAM chamber, nucleimode particles were formed (average dN/dlogDp = 2 x 10 5 cm −3 ) but their contribution to the total aerosol mass was relatively insignificant due to their small sizes of less than 50 nm in diameter (Dp).In comparison, the mass of PAM aerosol was distinctively increased at sizes larger than 200 nm.Particles between 50 and 200 nm in diameter were either lost or produced in the PAM reactor, depending on the history of the air masses.The formation of nuclei mode particles in PAM reactor were also observed in previous studies (Ortega et al., 2016;Palm et al., 2016).The enhancement of accumulation mode particles was dependent on the equivalent ages of PAM reactor and condensation sink (CS) by preexisting aerosols.Major constituents including sulfate, organics, ammonium, and nitrate for both ambient and PAM aerosols are presented in Fig 3 .Sulfate and ammonium concentrations in the PAM reactor were mostly higher or similar to those in the ambient air.In contrast, total organics and nitrate were mostly lower in the PAM aerosols than ambient aerosols.
The measurement results of PAM experiment include uncertainty associated with losses of condensable gases and aerosols in PAM reactor and sampling system.The condensable gases fate model for PAM reactors (Palm et al., 2016) predicts 57~66% and 62~81% of organic gases and sulfates to be condensed on the existing aerosols, respectively at 4.6 days of equivalent aging time.We used the same constants as those in Palm et al. (2016) for our PAM condition (residence time of 100 s and aging time of 4.6 days).The fraction of low-volatility gases that were not condensed in the PAM reactor was higher for organic-dominated case (33%) than sulfate-dominated case (27%) because of greater CS in latter than former.Previous studies showed that the SOA formation from VOCs including Semi/Intermediate Volatile Compounds (S/IVOCs) could be enhanced up to a few times greater than SOAs from VOCs only (Hayes et al., 2015;Palm et al., 2016).In our experiment, air masses that were transported from the Korean Peninsula or from east China for at least 1 day were sampled in the PAM reactor, thus it is likely that S/IVOCs were already partitioned into the aerosol phase, if existed and the loss of S/IVOCs or their contribution to SOA formation would be much less significant than was reported in previous studies.Nonetheless, S/IVOCs could be lost to the reactor inlet plate as well as reactor wall due to their low saturation vapor pressures, leading to the underestimation of their contribution to SOA formation in the PAM reactor of this study.

Organics-and sulfate-dominated episodes
Throughout the experiment, ambient aerosols were highly enhanced during two separate periods (shaded in Fig. 2a), with distinct differences in chemical composition between the two.While the ambient air was enriched in organics during the first episode (August 6, 11 AM to August 7, 9 AM), sulfate was dominant in the second episode (August 9, 1 AM to August 9, 6 PM).During the two episodes, the levels of gaseous precursors including NOx, SO 2 , and CO were higher than in the remaining periods (Fig. 4).However, the ratios of both SO 2 /NOx and OC/EC were higher for the first case than the second case.
It was opposite for O 3 /CO ratios.These two cases were distinguished by the air masses backward trajectories (Fig. 1b).Higher concentrations of organics than sulfate during the first episode resulted from air that had passed through the Korean Peninsula.The sulfatedominated air in the second episode had been transported from Southeast China.In addition, the air mass trajectories imply that sulfate-dominated aerosols lingered over the Yellow Sea and were aged more than the organic-dominated aerosols.
In addition, the aerosol masses differed in terms of size distributions between the two episodes (Fig. 5).The mass difference between the ambient and PAM aerosols (gain and loss) was greater in all size ranges for the organic-dominant episode than the sulfatedominant episode.In the PAM reactor, the mass of particles smaller than 50 nm and larger than 200 nm increased, but it decreased in the size range of 100-200 nm.The particles masses of separate size range are summarized for ambient and PAM processed aerosols in Table 1.For the two cases, aerosol masses increased at nuclei (10-50 nm) and condensation (200-500 nm) mode in PAM reactor.At size between 50 and 200 nm, however, the mass of PAM-processed aerosol was not evidently increased during sulfatedominated episode and even decreased during organic-dominated episode.The measurement results of size-separated chemical compositions provide detailed information on transformation processes in the PAM reactor.In general, sulfate increased but total organics and nitrate were reduced in the PAM reactor compared to the ambient aerosols (Fig. 6).The contribution of ammonium ions to the total mass was also greatest when aerosols were enriched in sulfate.The organic m/z 43 and m/z 44 components exhibited different behavior in the PAM reactor between the two episodes (Fig. 5).While m/z 43 decreased in the PAM reactor in both episodes, m/z 44 only increased during the sulfate-dominant episode.
Therefore, the following discussion is focused on these two distinct aerosols episodes, for which the size-separated chemical compositions were thoroughly examined and compared in order to elaborate on the formation of secondary aerosols and the evolution of ambient aerosols upon photo-oxidation in the PAM reactor.

Formation of nuclei-mode particles
In the current study, the formation of nuclei-mode particles (Dp <50 nm) was always observed in the PAM reactor.SO 2 is primarily responsible for the formation of new nuclei mode particles.In previous field studies, increases in the amounts of PAM aerosols were dependent on the ambient SO 2 concentrations (Kang et al., 2013).Palm et al. (2016) also observed the nuclei mode particles formed, competing for the role of condensation sink (CS) with preexisting accumulation mode particles.For the two episodes in this study, the number concentrations of nuclei-mode particles differed by less than an order of magnitude and SO 2 concentrations were similar.Chemical compositions are not available for nuclei-mode particles due to an AMS cut-off size of 50 nm in the present study.VOC concentrations for ambient air were not determined, either.In the previous laboratory experiment, Kang et al. (2011) examined the transient peaks of OA mass concentration at nuclei mode and smaller stable peaks at larger size range under high OH exposure.These transient peaks were supposed to be related to the non-linear oscillatory nucleation and growth of OA, which was referred by McGraw and Saunders (1984).An initial burst of particles followed by the oscillatory mode with a lower particle number density was explained by the competition between the rapid formation of nuclei mode clusters and the condensation onto new particle surfaces of condensable species.

Formation and evolution of organic aerosols
The SMPS mass size distributions highlight the size range of 100~200 nm, where PAM aerosol was reduced in mass only for organic-dominated episode (Fig. 5).Mohr et al.
(2012) observed that the ambient Semi-Volatile Oxygenated Organic Aerosols (SV-OOAs) and Low-Volatile Oxygenated Organic Aerosols (LV-OOAs) were mostly found in the range of 100-200 nm and greater than 200 nm, respectively.In addition, the concentration of organic m/z 43 was higher in SV-OOAs than LV-OOAs.In the present study, the contribution of m/z 43 to total organics was greater in organics-dominated than sulfatedominated episode.So was the loss of organics in PAM reactor.Volatility of organics with m/z 43 and 44 were previously described in Ng et al., (2011).These results suggest that there were less oxidized OAs (e.g., SV-OOAs) in the organics-dominated than the sulfatedominated episode.The ratios of O:C were lower for organic-dominated aerosols than those of sulfate-dominated aerosols (Fig. 7).In conjunction with O:C ratio, the air mass trajectories (Fig. 1b) imply organics-dominated air masses were relatively less aged thereby, including more less oxidized OAs than those of the sulfate-dominated episode (Jimenez et al., 2009;Ng et al., 2011).
The AMS measurement results indicate that total organics and the organic m/z 43 component were consistently reduced in the PAM reactor.Possible loss mechanisms are the deposition of aerosols on the chamber wall (McMurry and Grosjean, 1985;La et al., 2016) and fragmentation reactions from further photo-oxidation to form products with higher vapor pressure (Lamb et al., 2012).As described in the section 3, the condensable organics loss by wall surface and by heterogeneous oxidation were calculated to be about 30~40%.For the entire experiment, the O:C ratios of PAM aerosols were greater than those of ambient aerosols, with O:C ratios corresponding to SV-OOAs and LV-OOAs (Jimenez et al., 2009).Thus, a chemical transformation from low O:C to high O:C is more likely to explain the organic mass loss.
Organics are known to be oxidized by OH undergoing functionalization and fragmentation.The pathway by which this occurs is determined by the oxidation state of the existing organic aerosols.Functionalization dominates in the early stage of oxidation, which increases total organics and organic m/z 43, while fragmentation dominates in the later stage of oxidation, reducing OA mass (Jimenez et al., 2009;Kroll et al., 2009;Chacon-Madrid et al., 2010;Henry and Donahue, 2012;Lambe et al., 2012).For highly oxidized OAs with O:C ratios greater than 0.4, fragmentation becomes especially dominant, resulting in OA mass loss.In this study, the measured O:C ratios of the ambient aerosols were greater than 0.4 for both episodes (Fig. 7), which indicates that the observed ambient organic aerosols were aged enough to be fragmented.molecules cm -3 s.At OH exposure of 7 x 10 11 molecules cm -3 s, they observed 20 % of OA mass loss by volatilization followed by fragmentation of heterogeneous reaction products on the particles.In the present study, the OH-induced OA mass loss in PAM reactor was about 22 % and 37 % for organic-dominated and sulfate-dominated episode, respectively at our OH exposure of 7 x 10 11 molecules cm -3 s (Table 1).
In comparison, the organic m/z 44 mass increased in PAM aerosols for the organics-dominated episode but not for the sulfate-dominant episode.In particular, the increase in organic m/z 44 mass was associated with larger sizes than the organic m/z 43 mass loss (Fig. 6).As mentioned above, organic m/z 43 loss was significant for sizes less than 200 nm in AMS diameter, but most of the increase in organic m/z 44 mass was observed in the size greater than 200 nm.If particles grew in size by heterogeneous oxidation of carbonyls to carboxylic acids on pre-existing particle surfaces, the mass decrease in m/z 43 should also have been associated with an increase in the m/z 44 mass by the addition of oxygen in the sulfate-dominated episode.During the sulfate-dominated episode, however, there was no difference in the organic m/z 44 mass between the ambient and PAM aerosols, implying that a gas-phase reaction in the photo-chemical oxidation of organic aerosols was involved.Thus, the mass increase of the m/z 44 component in PAM aerosols was considered in terms of gas-to-particle partitioning.
Upon being aged, OAs are not only formed from precursor gaseous phases but also evaporated by partitioning between gas and aerosol phases.The evaporated OAs possibly undergo chemical oxidation, being partitioned into aerosol phase again.Therefore, SOAs can form from the oxidation of evaporated primary OAs as well as VOCs and Intermediate VOCs (Donahue et al., 2009).The organic-dominated episode of this study was characterized by higher organic concentrations and higher OC/EC ratios than the sulfate-dominated episode, which implies the availability of primary OAs and relatively less loss by aging or greater SOA formation, compared to photo-chemically inert EC.
The oxidation of organics in the atmosphere can occur both in the gas phase and through heterogeneous reactions.The gas-phase reaction is tens of times faster than the heterogeneous reaction, being limited by diffusion to the particle surface (Lambe et al., 2012).In our experiment, it was not feasible to distinguish gas-phase oxidation of semivolatile organics in equilibrium with the particle phase from heterogeneous oxidation of organics on the particle surface.Nonetheless, the main result of this study demonstrates that a distinct loss in m/z 43 was accompanied by little change in m/z 44, which supports the possibility that gas-phase oxidation was involved in SOA formation.The distributions of m/z 43-like compounds such as carbonyl groups with a semi-volatile nature in gaseous and particulate phases are controlled by the partitioning equilibrium between the two phases.In contrast, m/z 44-like compounds such as organic acid groups with low volatility tend to preferentially remain in the particle phase (Ng et al., 2011).It is, therefore, quite likely to occur in PAM reactor that the gas-phase concentration of m/z 43-like compounds was decreased by further oxidation, leading to evaporation of organic m/z 43 in particle phase to be re-equilibrated with the decreased concentration in gas phase.On the other hand, m/z 44-like compounds were sufficiently less volatile that they underwent little evaporation to the gas phase.During the ~100 s of the residence time in the PAM reactor, the gas-phase reactions would be more efficient than relatively slower heterogeneous oxidation (Lambe et al., 2012).It was also found in a previous study that much less OA mass loss occurred for highly oxidized OAs with low volatility than for less oxidized OAs due to heterogeneous oxidation (Kessler et al., 2012).In addition to the loss of less oxidized organics (m/z 43), the AMS measurements indicated that highly oxidized OAs (m/z 44) were produced in the PAM reactor.In particular, the m/z 44 peak was found to occur in the same size range as that of sulfate.These results suggest that SOAs formed by gas-phase oxidation and subsequent condensation on the surface of existing sulfate particles.Indeed, robust evidence for this can be found in detailed laboratory studies of SOA formation on acidic seed particles (Jang et al., 2002;Jang et al., 2006;Kang et al., 2007) In the present study, the overall mass spectra of organics indicate significant loss of less oxidized OAs (e.g., m/z 41, 42, 43, …) in the PAM reactor for both episodes.In addition, CO + and COO + groups increased and decreased in the PAM aerosols for the organicdominant and sulfate-dominant episodes, respectively (Fig. 5).Therefore, the discussion on single mass of organic m/z 43 and m/z 44 will also be valid for the entire organic classes.

Formation and evolution of inorganic aerosols
In the PAM aerosols, sulfate concentrations were always greater than or similar to those of the ambient aerosols for the entire experiment period.This indicates a significant contribution of sulfate to secondary aerosols in the PAM reactor, in which sulfuric acid was produced through photo-oxidation of SO 2 under high OH exposure and then nucleated or was deposited on pre-existing particles (Kang et al., 2007).For the two selected cases especially, sulfate mass was noticeably increased in condensation mode where the condensation of gas on particle surfaces would be favored, particularly under highly oxidative conditions.Although nuclei-mode particles increased in number to a great extent, their mass contribution was insignificant at the ambient level of gaseous precursors.In this study, the variation in ammonium concentrations was similar to that of sulfate (Fig. 2b).In addition, the equivalent ratios of sulfate and nitrate to ammonium indicated that the ambient particles were mostly acidic In the organic-dominated episode, the increase of the PAM aerosol mass in particles larger than 200 nm resulted from the formation of sulfate and organic m/z 44 as described earlier (Figs. 5,6), in which sulfate exhibited a broad peak in 200-500 nm particles, as in ambient air.In comparison, the sulfate increase shifted toward smaller sizes in the 200-400 nm range during the sulfate-dominant episode, leading to a sharp peak at 200 nm.Unlike the organic-dominated episode, the loss of organic m/z 43 was not accompanied by an increase in organic m/z 44 during the sulfate-dominated episode.The loss of organic m/z 43 was observed in smaller size than the increase in organic m/z 44 was observed.These features resulted in the difference in overall mass distributions between the two episodes shown in Fig. 5.
For organic dominated episode, the aerosol mass was decreased at 100-200 nm in PAM reactor, of which particles seemed to grow in size into the condensation mode by the addition of sulfuric acid formed from the oxidation of SO 2 .In figure 6, m/z 44 of PAM aerosol was enhanced in the condensation mode (AMS size >200nm) where the sulfate of PAM aerosol was enhanced.It suggests that further oxidized OAs were formed together with sulfate at condensation mode in the PAM reactor.This then implies that photooxidation efficiently activates organic particles to become cloud condensation-mode particles under SO 2 -sufficient conditions.
In addition, an increase in sulfate mass was noticeable between 200-400 nm.A major inorganic constituent, nitrate was lost in the PAM reactor during both episodes, with an ambient nitrate concentration that was comparable to the levels of sulfate and organics (Fig. 2b).The nitrate loss is rather explicit in the PAM reactor because of efficient conversion of SO 2 to sulfate, causing the aerosols to become acidic and causing particulate nitrate (HNO 3 (p)) to evaporate.A plausible source of HNO 3 (p) in the PAM reactor is the deposition of gaseous HNO 3 (g) or heterogeneous reaction of NO 2 on the particle surfaces (Underwood et al., 2001),even though the latter is not clearly understood.
If a particle is acidic in the presence of sulfuric acids, nitrate easily evaporates back to the gas phase.As stated in section 2, the loss of nitrate by temperature-induced evaporation would be insignificant.
In organic-dominated episode, the balance of ammonium with sulfate and nitrate [(sulfate+nitrate)(μN)]/[ammonium(μN)] was 1.34 in ambient aerosol, which was reduced to 1.22 in PAM aerosol with enhanced sulfate and ammonium but with nitrate being lost.In sulfate-dominated episode, the ammonium balance remained unchanged in PAM aerosol due to an equivalent loss of nitrate over the condensation mode with its mode being shifted toward larger size.These results illustrate the role of sulfate in determining chemical compositions and mass loadings of aerosols in northeast Asia.

Atmospheric implications
The ambient OAs in the present study were moderately to well aged, as indicated The O:C ratios of OAs from this study were plotted against aging time and compared with those observed in East Asia (Fig. 8), where the O:C ratios were found to increase with transport time across the Pacific Ocean (Takegawa et al., 2006;Takami et al., 2007;Dunlea et al., 2009).The O:C ratios of the bulk OAs depend on the concentrations of organic constituents because the saturation vapor pressure varies with the molecular weight of the organics (Donahue et al., 2006).Thus, the O:C ratios from different studies are not directly comparable if their OAs concentrations vary over a wide range.In Figure 8, OA concentrations ranged up to 10 μg m 3 and thus a comparison among different sets of measurements is suitable.In the real atmosphere, the fate and evolution of secondary aerosols could be affected by scavenging of oxidized OAs and inorganic aerosols on the cloud droplets by aqueous aerosol surface reaction (Dunlea et al., 2009), nucleation of new particles due to the entrainment of free tropospheric air (Song et al., 2010), or dry deposition on the dust particles (Dunlea at al., 2009).It is noteworthy that the increase in O:C ratios with photochemical aging was slightly higher in our results than in those of previous studies, which is possibly due to the omission of these scavenging processes in PAM reactor .
The results of this study imply that SO 2 plays a key role in increasing secondary aerosol concentrations in East Asia because the lifetime of SO 2 is longer than those of VOCs and because sulfate is relatively stable in the particle phase once formed, contrary to nitrate and organics.While SOA formation is more important near sources or in fresh air masses, OAs oxidation occurs continuously during the transport of air masses.The formation yield of sulfate from SO 2 is greater than that of organic aerosols during 3~4 days of aging in the Asian pollution plume because of fast depletion of SOA precursors (Dunlea et al., 2009), which is consistent to our results.In particular, this study indicates that relatively less aged OAs were in equilibrium with the gas phase, through which oxidation of less oxidized OAs was carried out, leading to increased OA mass in the CCN size range (200-400 nm).The increased O:C ratios rendered particles more hygroscopic, thereby facilitating their activation as CCNs (Massoli et al., 2010).Thus, climate effect of OA aging should be considered along with deceases in OA mass loading when they are transported across long distances.

Conclusions
A During the experiment, two periods of noticeably enhanced aerosol concentrations with different chemical compositions and degrees of air mass aging were observed.While organic concentrations were highest during August 6-7 and sulfate was more elevated on August 9, the ratio of O:C was higher for the latter than the former.In addition, the sizedependent mass distributions of major constituents were clearly distinguished between the two episodes, which were used to understand the photochemical and volatile properties of the aerosols.
In the PAM reactor, sulfate formed in condensation mode in particle sizes between 200 and 400 nm and presumably in nucleation mode for particles smaller than 50 nm, which suggests that SO 2 was not limited in the study region for generating secondary aerosols, even during summer.In contrast, nitrate was lost in particles of all size ranges due to evaporation by the addition of sulfate.The total mass of organics was reduced in particles between 100 and 400 nm, where the loss in the m/z 43 component was evident for both episodes.In contrast, the more oxidized organic m/z 44 component was produced at larger sizes of 200-400 nm only during the organic-dominated episode.These results suggest partitioning of less oxidized OAs into a gas-phase that was in equilibrium with the particle phases.As the concentration of m/z 43 and total organics decreased upon oxidation, less oxidized OAs were likely evaporated away from the particle phase in the process of re-equilibration.
As the air mass aged, the loss was apparent for photochemically and physically unstable component such as organics and nitrate, whereas sulfate was stable in the aerosol phase.Therefore, organics and nitrate are likely to be relatively more important in near-source regions while sulfate is probably dominant in areas far from the source.Note  & Aerosol mass concentrations were obtained from SMPS measurements with an aerosol density of 1.2 μg cm -3 and sulfate, nitrate, ammonium and organics were from HR-ToF-AMS measurements.additions (Heald et al., 2010).Shaded gray areas represent the H:C and O:C ranges observed in ambient OAs (Ng et al., 2011).
Figure 7 also shows PAM OAs have higher O:C ratio and lower H:C ratio than ambient aerosols do and the Van Krevelen slope (Δ(H:C)/ Δ(O:C)) of both episodes were about -0.6.In a laboratory PAM experiment, Lambe et al. (2012) observed a similar tendency and explained that as SOA oxidized, the Van Krevelen slope changed from minor fragmentation of carbonyl and acids/alcohol to major fragmentation of acids.In addition,Hu et al. (2016) demonstrated that fragmentation became an important pathway of OAs oxidation at OH exposure greater than 10 11 by their O:C ratios greater than 0.4.They were chemically and physically transformed in the PAM reactor, resulting in increased O:C ratios and decreased OA mass concentrations by photochemical oxidation and fragmentation processes.Although the oxidant levels of OH and O 3 in the PAM reactor far exceeded the ambient levels, the H:C and O:C ratios of the ambient and PAM OAs were in close agreement with those observed in the atmosphere(Ng et al., 2011) (Fig 7).These results provide good evidence for the ability of the PAM reactor to accelerate oxidation processes in ambient air under high O 3 and OH conditions and to represent atmospheric aging of approximately 5 days without physical removal processes such as dry/wet deposition.It further confirms that the PAM reactor is applicable for field studies to observe aging processes of various types of precursors and aerosols including emission sources and long-range transported air masses.
PAM reactor was used to analyze ambient air at Baengnyeong Island in the northern part of the Yellow Sea during August 4-12, 2011.The chemical compositions and number concentrations of ambient and PAM aerosols were alternately determined with ToF-AMS and SMPS, respectively.The integrated OH exposure was equivalent to 4.6 days of atmospheric photo-oxidation.The results of the present study demonstrate that the high levels of OH and O 3 in the PAM reactor can expedite slow atmospheric reactions and that chemical aging processes of the natural atmosphere are plausibly represented.
photochemical aging, OAs were transformed from less oxidized OAs to further oxidized OAs, as demonstrated by an increase in the organic m/z 44 component at sizes of 200-400 nm where sulfate was consistently increased.In conjunction with the increased O:C ratio of organics, this underscores the potential of organics to act as cloud condensation nuclei under SO 2 -sufficient conditions.

Figure 1 .
Figure 1.(a) The location of the measurement site on Baengnyeong Island, the northernmost island in South Korea.The red circle indicates the measurement station location.(b) 72-hour backward trajectory for the two episodes.Green represents the organics-dominated episode during August 6, 11 AM to August 7, 9 AM, 2011, while red represents the sulfate-dominated episode during August 9, 1 AM to August 9, 2 PM, 2011.

Figure 2
Figure 2 (a) Aerosol mass concentrations from SMPS measurements for ambient and PAM aerosols.(b) Mass concentrations of major components measured by HR-ToF-AMS including organics, nitrate, sulfate, ammonium, chloride, and organic m/z 43 and m/z 44.Solid lines and lines with markers represent ambient aerosols and PAM aerosols, respectively.Shaded periods represent the organics-dominated episode (August 6, 11 AM to August, 7 9 AM) and the sulfate-dominated episode (August 9, 1 AM to August 9, 2 PM).The lowest mass concentration observed on August 8 was due to rain..

Figure 3 .
Figure 3. SMPS mass difference between PAM and ambient aerosols averaged for the entire sampling period.

Figure 4 .
Figure 4. Hourly measurements of SO 2 , NOx, CO, O 3 , and meteorological parameters for the entire sampling period.

Figure 5 .
Figure 5. (a) AMS mass spectra of organics and SMPS mass size distribution averaged for organics-dominated episode and (b) AMS mass spectra of organics and SMPS mass size distribution averaged for sulfate-dominated episode.

Figure 6 .
Figure 6.AMS p-ToF size distributions of PAM and ambient aerosol components averaged for each case.

Figure 7 .
Figure 7. Van Krevelen diagram for two episodes.Dashed lines represent the Van Krevelen

Figure 8 .FiguresFigure
Figure 8.Comparison of O:C ratios in this study and other studies with respect to photochemical age.The photochemical ages in our measurement were obtained by the transport time calculated from a back trajectory analysis and photochemical aging times in the PAM chamber.Other study data were obtained from Takegawa et al. (2006), Takami et al. (2007), and Dunlea et al. (2009).

Table 1 .
Meteorological parameters and measurement summary for organic-dominated # Units are μg cm −3 .