Seasonality of ultraﬁne and sub-micron aerosols and the inferences on particle formation processes

The aim of this study is to investigate the seasonal variations in the physicochemical properties of atmospheric ultraﬁne particles (UFPs, d ≤ 100 nm) and submicron particles (PM 1 , d ≤ 1 µm) in an East-Asian urban area, which are hypothesized to be a ﬀ ected by the interchange of summer and winter monsoons. An observation exper- 5 iment was conducted at the TARO, an urban aerosol station in Taipei, Taiwan, from October 2012 to August 2013. The measurements included the mass concentration and chemical composition of UFPs and PM 1 , as well as the particle number concentration (PNC) and size distribution (PSD) with size range of 4–736 nm. The results indicate that the mass concentration of PM 1 was elevated during cold seasons with peak level 10 of 18.5 µgm − 3 in spring, whereas the highest UFPs concentration was measured in summertime with a seasonal mean of 1.62 µgm − 3 . Moreover, chemical analysis revealed that the UFPs and PM 1 were characterized by distinct composition; UFPs were composed mostly of organics, whereas ammonium and sulfate were the major constituents in PM 1 . The seasonal median of total PNCs ranged from 13.9 × 10 3 cm − 3 in 15 autumn to 19.4 × 10 3 cm − 3 in spring. The PSD information retrieved from the corresponding PNC measurements indicates


Introduction
Due to the significant impact of particulate matter on human health and climate change, it is vital to understand the formation process of atmospheric particles (Charlson et al., 1992;Donaldson et al., 1998). A number of mechanisms have been proposed by which 10 atmospheric particles are formed, including binary nucleation, ternary nucleation and ion-induced nucleation for charged particles, under different environment conditions (Kulmala, 2003;Kulmala et al., 2004). Numerous studies have been conducted in different locations to elucidate particle formation processes under various environmental settings in the free troposphere, boreal forest and coastal areas, where new particles 15 formation processes are observed frequently O'Dowd et al., 1999;Weber et al., 2001;Vehkamäki 2004). Recently, investigations were also carried out on new particle formation within urban boundary layer (e.g., Cheung et al., 2013 and references therein), where particle formation was suggested to be mainly influenced by the photo-oxidation of SO 2 . Furthermore, formation of particulate matter by heterogeneous reactions of gases on dust particles was reported (Hsu et al., 2014;Nie et al., 2012). Previous investigations have indicated that the air pollutants, both in gaseous and particulate form, associated with the continental outflows of air masses could have affected a wide region in East Asia and caused severe regional air pollution (e.g., Lin et al., 2004;Wang et al., 2003). However, the formation processes of ultrafine particles 25 (UFPs, d ≤ 100 nm) and sub-micron particles ( In urban environment, major contributing sources of aerosol particles include the vehicular exhausts (e.g., Pey et al., 2008;Pérez et al., 2010), industrial emissions (Gao et al., 2009) and new particle formation by photochemical reactions (e.g., Pey et al., 2009). Approximately 55-69 % of the total particle number concentrations (PNCs) were attributed to secondary aerosols during midday in several European cities (Reche et al., 5 2011). In a subtropical urban area, Cheung et al. (2013) observed that there have been a ten-fold increase in nucleation mode PNCs (N 9−25 , with size 9 < d < 25 nm) compared to that contributed by the vehicle emission in Taipei, Taiwan. Besides the local sources, air quality of the East Asian countries is also strongly affected by the transport of air pollutants from mainland China during periods of winter monsoons (Cheung et al.,10 2005; Lin et al., 2004;Matsumoto et al., 2003). Lin et al. (2004) reported that the mass concentration of particulate matter (PM 10 ) due to the long-range transport associated with winter monsoons was 85 µg m −3 , about 79 % higher than that due to local pollution (∼ 47.4 µg m −3 ) in urban Taipei. Chemical composition of fine and coarse particles was measured during a winter monsoon period at Rishiri Island, near the northern tip 15 of Japan, to study the transport of continental aerosols (Matsumoto et al., 2003). The results showed that higher levels of particle mass concentrations were associated with the outbreaks of continental polluted air masses. In addition, Cheung et al. (2005) found deterioration in visibility around the southern China during wintertime as indicated by a two-fold increase in aerosol light scattering coefficient under the influences of winter 20 monsoons. All these studies were limited to measurements in terms of PM 10 or PM 2.5 for a particular period, and the seasonality of particles in either ultrafine or sub-micron range has not been well illustrated yet.
To attain a better understanding of the seasonal variations of ultrafine and submicron particles and the factors affecting particle formation, particularly under the influ-25 ences of Asian monsoon circulations, we conducted a 1 year aerosol characterization experiment in Taipei, Taiwan, a typical subtropical urban area in East Asia. In this study, we analyzed number concentration and size distribution of aerosol particles, together with the mass concentration and chemical composition of UFPs and PM 1 measured ACPD 15,2015 -15 November 2012, 4-24 January, 17 March-11 April, and1-14 August 2013). The results of this study will contribute to the management strategies of the severe air pollution over the East Asia region.  Table 1 for measurement details). The aerosol observatory is on the top floor of the Building-B of the Department of Atmospheric Sciences, National 10 Taiwan University (ASNTU), which is ∼ 20 m a.g.l. (Cheung et al., 2013). Particle size distribution (PSD) in the range of 4-736 nm was measured by two scanning mobility particle sizer (SMPS) systems. One was equipped with a long-differential mobility analyzer (long-DMA, Model: TSI 3081, TSI Inc.) and a condensation particle counter (CPC) (Model: TSI 3022A, TSI Inc.) to measure the particles from 10-736 nm, 15 which was named long-SMPS. Another one was equipped with a nano-DMA (Model: TSI 3085, TSI Inc.) and an ultrafine water-based CPC (UWCPC, Model: TSI 3786, TSI Inc.) for measuring the particles from 4-110 nm, which was named nano-SMPS. The poly-disperse particles were classified into selected mono-disperse particles according to their electrostatic mobility by the DMAs. The number concentration of the 20 mono-disperse particles was then counted by the CPCs. Ambient air was drawn into the SMPS systems from outside the building through a 0.635 cm (inner diameter) conductive tube, and a sampling duration of 5 min was adopted for each PSD measurement. The SMPS systems' flow rates were checked weekly during the sampling period and the accuracy of the particle sizing of the DMAs was checked using polystyrene Introduction  Cheung et al. (2013). Size segregated aerosol samples were collected by a pair of Micro-Orifice Uniform Deposition Impactors (MOUDI,Model: 110,MSP Corp.). Taking the advantage that the cut diameter of the 10th MOUDI impaction stage is exactly 100 nm, the 11th im-5 paction stage (cut diameter = 56 nm) of each MOUDI was removed to allow the after filter function as a collector of UFPs. Besides, a pair of PM 1 samplers, each consisted of a standard aerosol sampler (PQ-200, BGI Inc.) and a PM 1 sharp cut cyclone, were deployed to collect PM 1 samples. For both UFPs and PM 1 sampling arrangements, one of the paired samplers was equipped with Teflon filters, whereas another was equipped 10 with quartz fiber filters. The Teflon filter samples were used for gravimetric measurement. The quartz filter samples were deployed for analysis of soluble ions (Na + , NH + 4 , 4 ) using ion chromatograph (IC), and carbonaceous components (i.e. OC and EC) in the aerosols using a DRI-2001A carbonaceous aerosol analyzer with IMPROVE-A protocol. Details of the in-lab analysis are as de- 15 scribed previously (Salvador and Chou, 2014). Both the PM 1 and UFPs were collected with double-layered quartz filters (i.e. QBQ setup) and the artifacts due to adsorption of gaseous components were corrected as suggested by Subramanian et al. (2004). The sampling duration of each sample set was from 14:00-12:00 LT (22 h), and a total of 69/75 sets of UFPs/PM 1 samples were collected during the entire investigation period 20 (Autumn 20/21 sets, Winter 15/16 sets, Spring 25/25 sets and Summer 9/13 sets).
Moreover, to assist the data interpretation, the hourly averaged mass concentration of PM 10 , the mixing ratio of trace gases (i.e. NO x , SO 2 and O 3 ) and the meteorology parameters (i.e. wind direction/speed and UVB) from the Guting air quality station of Taiwan Environmental Protection Agency, which is about 1 km from the TARO, were 25 analyzed in this study.

Data processing and analysis
The PSD of 4-736 nm presented in this study was combined from two sets of SMPS data, where the nano-SMPS corresponded to the size range of 4-49.6 nm, and the long-SMPS corresponded to the size > 49.6 nm. The diffusion loss of the particles during the sample transport in the tubing was corrected according to the algorithm pro-5 posed by Holman (1972). Particle number concentrations for different size ranges were then calculated from the SMPS measurements. The 5 min PSD data were synchronized into hourly averages, and fitted by the DO-FIT model developed by Hussein et al. (2005) according to the multiple lognormal distribution algorithms. Based on the fitted PSD data, the PNCs were classi- , 100 ≤ d < 736 nm (N 100−736 ) and 4 ≤ d ≤ 736 nm (N 4−736 ), for nucleation mode, Aitken mode, ultrafine, accumulation mode and total particles, respectively. Pearson correlation coefficient, r, was calculated by PASW Statistics ver. 18 (SPSS Inc.) to determine the correlation between the respective parameters.

Back-trajectory analysis
Backward trajectories were calculated using the HYSPLIT model (Hybrid Single Particle Lagrangian Integrated Trajectory, Version 4.9) of NOAA (National Oceanic and Atmospheric Administration) (Draxler, 1999) for TARO during the sampling period, in order to trace the origins of the air masses. Note that the grid resolution of the mete-20 orological data used for back-trajectories calculation is 1 • × 1 • , which is not enough to trace the detailed air mass passage over the scale of the study region and, therefore, the trajectories only provide an indication of the region from which the air mass was originated.
As mentioned above, the air quality of urban Taipei is significantly affected by both the local vehicular exhausts and long-range transport of pollution, where the later one is dominated by meteorological factors. The information on the meteorological condi-ACPD 15,2015 Seasonality of ultrafine and sub-micron aerosols tions, particularly the wind patterns, is important and thus presented here. The backtrajectories of the air masses for the TARO are illustrated in Fig. 1 (left panel). The results showed that northeasterly winds were dominating in autumn and winter seasons, passing through the Asian continent before reaching Taiwan, whereas southerly winds were prevailing in summer that passed through the Taiwan Island. The air masses 5 observed in spring period were found to be mainly associated with Asian continental outflows and occasionally with the southerly flows. This observation agreed with the surface wind direction measured in urban Taipei area (see Fig. 1, right panel), where northeasterly winds were dominating during the period from November 2012 to May 2013, and southerly winds were prevailing from May 2013 to August 2013. 10 3 Results and discussions

PNCs and PSDs in respective seasons
The PNCs of various size ranges during each season are summarized in Table 1. Relatively higher N 4−736 were observed in spring and winter with median concentrations of 17.4 × 10 3 and 19.4 × 10 3 cm −3 , respectively, followed by summer (16.6 × 10 3 cm −3 ) 15 and minimized in autumn (13.9 × 10 3 cm −3 ). This result is comparable to the previous measurements conducted in urban Taipei where the seasonal means of PNCs (10 < d < 560 nm) ranged from 11 × 10 3 to 17 × 10 3 cm −3 (Cheng et al., 2014). Figure 2 illustrates the number, surface and volume size distributions of the aerosol particles. The geometric mean diameter (GMD) of each PSD mode was retrieved from the data 20 of number concentrations. It was found that the GMDs of the nucleation, Aitken and accumulation modes were 10.4-12.8 nm, 26.5-38.4 nm, and 91.8-159.0 nm, respectively.
It was revealed that the nucleation mode particles were predominant in the PNCs during autumn, winter and spring in this study area, whereas a distinct size distri- 25 bution pattern was observed in summertime where the fraction of nucleation mode ACPD 15,2015 Seasonality of ultrafine and sub-micron aerosols (N 4−25 /N 4−736 ) decreased to 0.44 and the Aitken mode PNCs increased to be comparable to that of the nucleation mode in summer. Observation from another aspect is that the PNC of nucleation mode (N 4−25 ) peaked in winter and minimized in summer, whereas the PNCs of Aitken mode (N 25−100 ) and accumulation mode (N 100−736 ) reached their respective maxima in summertime. Apparently, a large number of nu-5 cleation mode particles could have been shifted into the Aitken and/or accumulation modes in summer. The changes in the size distribution in summer season are most likely due to the seasonally enhanced photochemical production of condensable vapors that, in turn, will contribute to the growth of aerosol particles in the atmosphere. This seasonality agrees with our previous findings that the growth rate of newly formed par-10 ticles was dominated by the photolysis of ozone, an indicator of photochemical activity (Cheung et al., 2013). The causes responsible for the observed seasonal variations in PNCs will be discussed in more details in following sections.  fraction of mass was contributed by the group of "others", which consisted of mineral (K + , Ca 2+ , PO 3− 4 and Mg 2+ ), sea-salt (Na + and Cl − ), and unidentified species. The results showed that, in average, mineral and sea salt components attributed only 3.5 % (ranging from 2.0-6.0 %) to UFPs mass concentration. Thus a substantial amount of UFPs remained unidentified, which was most likely relevant to the hydrogen and oxygen associated with organic carbon (OC). The conversion factors used to estimate the average molecular weight per carbon in particulate organic matter varied depending on the characteristic of aerosols. A lower factor value, 1.2, was usually suggested for saturated organic molecules, while higher value, 1.6, was adopted for water-soluble compounds which consist of multifunctional oxygenated groups, and even higher factor 10 value was used for aged aerosols which contain higher portion of low and semi-volatile products of photochemical reactions (Turpin and Lim et al., 2001). The high fraction of the "others" group found in UFPs suggested that the photochemical production of secondary organic aerosols was a significant process responsible for the elevated UFPs levels observed in this study. 15 As shown in Fig. 3b, annual average of PM 1 was estimated to be 14.7 µg m −3 (11.6-18.5 µg m −3 ), which is similar to the results of a previous study in urban Taipei (average:

Mass concentration and chemical composition
14.0 µg m −3 , Li et al., 2010). The measured PM 1 level was relatively higher than that of the urban areas of Phoenix, United States (5.9 µg m −3 , Lundgren et al., 1996) and Helsinki, Finland (6.1 µg m −3 , Vallius et al., 2000). For chemical composition, sulfate 20 was the major mass contributor of PM 1 (average: 39.0 %, ranging from 33.8 to 46.8 %), followed by ammonium (average: 12.7 %, 12.0-13.2 %) and OC (average: 11.5 %, ranging from 9.2 to 14.3 %). The results presented above indicate that UPFs exhibited distinct seasonality and composition from PM 1 in the study area. The highest UFPs concentration was ob- was noteworthy that the mass concentration of sulfate in UFPs also peaked in summer (64 ng m −3 ), suggesting enhancement in photo-oxidation of SO 2 . Cheung et al. (2013) found that photo-oxidation of SO 2 was the major mechanism for the formation of new particles in Taipei, Taiwan and the production of condensable vapors was also dominated by photo-oxidation. The co-variations in sulfate and OC revealed in this study 5 further suggest that secondary organic compounds are the major condensable matter contributing to the growth of newly formed particles. While the organics predominated in the mass concentration of UFPs, which included nucleation mode and Aitken mode particles, the measurements of PM 1 in this study suggest that sulfate was the major constituent of accumulation mode aerosols. In con-10 trast to the seasonality of UFPs, the mass concentration of PM 1 reached the maximal at 18.5 µg m −3 in spring and exhibited the minimal at 11.6 µg m −3 in summer. The PM 1 differences between spring and summer were mostly due to declined ambient levels of sulfate, nitrate, and ammonium ions. As a result, the mass contribution of the three inorganic ions in PM 1 reduced from 55.7 to 46.2 % and, on the contrary, the mass 15 fraction of OC increased from 10.2 to 14.3 %. The seasonal characteristics of PM 1 concentration and composition are attributed mostly to the changes in the origin areas of background air mass, which shifts from the Asia Continent to the western Pacific Ocean during summertime. Our previous studies reported that the fine particulate matter (PM 2.5 ) transported on the Asian outflows to northern Taiwan maximized in spring-20 time and were enriched in sulfate, nitrate, and ammonium (Chou et al., 2008(Chou et al., , 2010. The seasonality of PM 1 found in this study is consistent with the previous observations for PM 2.5 and thereby suggests the significance of Asian outflow aerosols to the PM 1 budget in the downwind areas of the Asia Continent. 25 In order to study the influences of photochemical production of particles, the measurements of PNC and PSD were analyzed per daytime (07:00-17:00 LT) and nighttime (17:00-07:00 LT), respectively (see Fig. 4). Since the particles in nighttime are mainly 21813 Introduction

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Interactive Discussion
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | emitted from the vehicular exhausts and the elevated PNCs in daytime are due to both the primary and secondary sources of the particles in the study area (Cheung et al., 2013), a larger difference between the PNCs observed in daytime and nighttime indicates stronger influence of photochemical production on the PNCs. The most striking seasonal features shown in Fig. 4 is the large difference between daytime against 5 nighttime PSDs in summer as indicated by the low N 4−736 (nighttime)/N 4−736 (daytime) ratios, whereas higher ratios were observed in other seasons. This result is as expected because the photochemical production of nucleation mode particles was more intense during warm season in subtropical areas (Cheung et al., 2011). Moreover, as discussed in previous section, the photochemical reactions could produce condensable 10 organics that allows the newly formed nucleation mode particles to grow into the Aitken mode. The relatively smaller differences between the daytime and nighttime N 4−736 in autumn and winter show that the photochemical contribution in PNCs was declined as compared to that in summertime.

15
Vehicle emission is known as the major source of the particulate matter in urban environment, particularly during the nighttime. In order to investigate the relationship between the vehicular exhausts and PNCs, the scatter plots of NO x (as an indicator of vehicle emission) against N 4−25 , N 25−100 and N 100−736 during the nighttime were examined for winter and summer periods (see Fig. 5). The values of the Pearson correlation 20 coefficient (r) and the slope of linear regression between NO x and PNCs are summarized in Table 2. The highest r values were found in both the plots of NO x against N 25−100 for winter (r = 0.88) and summer (r = 0.87). This result suggests that a strong linear correlation between the vehicle emission and the N 25−100 , coincided with the results from previ- 25 ous studies (e.g., Morawska et al., 2008). During wintertime, stronger correlation was found between NO x against N 4−25 (r = 0.84) and N 25−100 (r = 0.88) compared to that between NO x and N 100−736 (r = 0.38). In contrast, high r values were obtained be-21814 Introduction tween NO x and all particle modes in summer (r = 0.70-0.87). The robust correlation between NO x and N 4−25 /N 25−100 suggests that local vehicle emission was the predominant source of UFPs throughout a year. However, in the winter case the PNCs of accumulation mode particles (N 100−736 ) were dominated by NO x -independent sources, which was most likely relevant to the pollution outbreaks from the Asian continent. Lin 5 et al. (2004) indicated that the long range transported air mass was characterized by high level of PM 10 and low mixing ratio of NO x due to its short atmospheric lifetime.
The slope values shown in Fig. 6 can serve as a relative emission factor of particles per NO x in the study area, which indicates the degree of the influences of vehicle emission on the PNCs (Cheung et al., 2013). Relatively higher slope values found in summertime compared to winter period evidence a greater influence of the vehicle emission on particle concentration. Furthermore, the lower emission factor for nucleation mode and higher one for the Aitken mode demonstrate the size shift effects of particle growth in summer evening. The seasonal effects on the emission ratio of PNCs and NO x are rather difficult to address since the complexity of different controlling fac- 15 tors, such as formation mechanisms and meteorological conditions. For example, Nam et al. (2010) reported negatively exponential correlation between the PM/NO x ratio in vehicle emission and ambient temperature, and suggested that the impact of ambient temperature on particulate matter was larger than that on NO x . Nevertheless, the observed differences in the PNCs/NO x ratios for winter and summer periods of this study 20 necessitate further investigations on the formation mechanisms of aerosol particles in urban areas, in particular the nucleation and the Aitken modes.

Influence of long-range transport (LRT)
During the seasons of winter monsoons, i.e. from autumn to spring, the continental outflows have been frequently observed in urban Taipei, which is indicated by the stable 25 northeasterly wind and increase of O 3 level . Previous studies of longrange transport (LRT) of air pollutants on air quality of northern Taiwan showed that an elevated PM 10 was observed under the influence of continental outflows (Lin et al., 21815 Introduction  Chou et al., 2004). Figure 6 demonstrates an LRT pollution event observed at the TARO during this study. The wind direction changed from westerly/northwesterly to northeasterly at 21:00, 24 March and which continued until 06:00, 26 March. During this period, the O 3 mixing ratio remained at moderate level (∼ 30-55 ppb) and PM 10 increased from 10 to 98 µg m −3 . In this section, we attempt to analyze the PSDs/PNCs 5 under the influences of continental pollution outbreaks. The periods of the respective LRT events are listed in Table S2 in the Supplement. As shown in Fig. 6, the diurnal variations of PSD during the LRT event exhibited two N 4−25 peaks associated to the morning and afternoon traffic rush hours, whereas the PNCs of the Aitken mode particles kept at a low level. This result suggests that the influences of local vehicle emission on PNCs were still in place, whereas growth of particles due to secondary production of condensable vapors could have been suppressed. The averaged PSDs for LRT and non-LRT cases are shown in Fig. 7. The geometric mean diameters of the nucleation, Aitken, and accumulation modes in PSDs were found to be at 10.6, 37.2 and 156.8 nm for LRT and 11.3, 30.0 and 113.4 nm for non-LRT cases, re-15 spectively. The median N 4−25 (11.1 × 10 3 cm −3 ), N 25−100 (7.3 × 10 3 cm −3 ) and N 100−736 (1.8×10 3 cm −3 ) observed in non-LRT events are significantly higher than those for LRT events (N 4−25 : 9.2 × 10 3 cm −3 , N 25−100 : 3.8 × 10 3 cm −3 , N 100−736 : 1.3 × 10 3 cm −3 ). This could be attributed to the lower wind speed (and hence poor dispersion) during non-LRT events (1.5±0.8 m s −1 ) than that for LRT events (3.0±0.8 m s −1 ). In contrast to the 20 increases in PM 10 observed usually during LRT episodes (e.g., Lin et al., 2012), the relatively lower PNCs suggest that the number concentration of submicron particles, in particular UFPs, are still dominated by local emissions during the episodes of continental pollution outbreaks. This is consistent with the observation of seasonal UFPs mass concentration that peaked in summertime when Taiwan was isolated from the

Factors affecting new particle formation (NPF)
As shown in previous study, the NPF events were frequently observed in summer, which subsequently induced a notable increase in N 4−25 in urban Taipei (Cheung et al., 2013). Figure 8a-d shows the scatter plots of N 4−25 against NO x for daytimes in each season. During the NPF events, a non-linear relationship between these two pa-5 rameters was usually observed during the daytime (Cheung et al., 2013). The results show that remarkable NPF events were observed often in summer and occasionally in spring, but rarely in autumn and winter in the study area. The frequency of remarkable NPF events was found to be 8 out of 84 measurement days and the events were observed only in spring (3 out of 26 days) and summer (5 out of 14 days) seasons.

10
The averaged particle growth and formation rates were found to be 4.3 ± 0.8 nm h −1 and 1.6 ± 0.8 cm −3 s −1 , which were comparable to those measured in previous urban studies (e.g., Cheung et al., 2013). The particle growth and formation rates of each case are listed in Table S3 in the Supplement. Table 3 Table S1), as well as the results of previous urban studies (Woo et al., 2001;Cheung et al., 2013). This strongly supports that the new particle formation was mainly driven by the photochemical oxidation of SO 2 under low condensation sink conditions, where the SO 2 could be transported from the upwind area on the summer monsoons (see Fig. 1d). On the contrary, the absence of particle formation events in 25 wintertime could be attributed to the suppression of NPF by particles transported from the Asian continent . The results of this work evidenced that low PM 10 concentration and high sulfuric acid production favor the particle formation process in urban areas.

Conclusions
The mass concentration and chemical composition of UFPs and submicron particles (i.e. PM 1 ) as well as the PNCs and PSDs with size ranged from 4 to 736 nm were mea-5 sured during four seasonal campaigns in the period from October 2012 to August 2013 at the TARO, a subtropical urban aerosol station in East Asia. Significant differences in the seasonality and chemical composition of UFPs and PM 1 were revealed. The UFPs were composed mostly of organic matter and reached the maximal in summer, whereas the PM 1 composition was dominated by ammonium and sulfate and exhibited a seasonal peak in the spring. It was found that the total PNCs in Taipei, Taiwan were elevated significantly during cold seasons, which were caused mostly by the high levels of nucleation mode particles (N 4−25 ). On the contrary, both the Aitken mode (N 25−100 ) and accumulation mode (N 100−736 ) PNCs reached their respective maxima in summertime. Consistent 15 correlation without significant seasonal differences was found between the UFPs (i.e. nucleation and Aitken mode particles) and NO x , suggesting that local vehicle emission is the major source of UFPs in the study area throughout a year. The local vehicle emission is also dominating the accumulation mode PNC in summer, but not in wintertime. The declined correlation between NO x and N 100−736 in winter (r = 0.38) is likely 20 due to the influences of air pollution associated with the Asian outflows.
The elevated UFPs level in summer is attributed to the increases in the concentration of Aitken mode particles (N 25−100 ). It was revealed from the measurements of PSD that a large number of nucleation mode particles could have shifted into the Aitken mode during summertime, which is most likely relevant to the photochemical production of Introduction of the condensed materials in UFPs are mostly organic matter, underlining the significance of secondary organic aerosols in the ambient UFPs. A total of 8 new particle formation (NPF) events occurred out of 84 measurement days in this study, which were observed in spring (3 events out of 26 days) and summer (5 events out of 14 days). The prevalence of NPF in summer agrees with the highest H 2 SO 4 proxy and lowest PM 10 observed in this study, which provided favorable atmospheric conditions for new particle formation. The averaged particle growth and formation rates for the NPF events are 4.3 ± 0.8 nm h −1 and 1.6 ± 0.8 cm −3 s −1 , respectively, which are comparable to those measured in previous urban studies.