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An online coupled regional chemistry-aerosol-climate model was developed and applied to explore the distribution, evolution and indirect effects of marine primary organic aerosols (MPOA) over East Asia during the period from 12 March to 22 April 2014. Model results were compared with a variety of observations from ground measurement, cruise experiment and satellite retrievals, which demonstrated the model was capable of reproducing reasonably well the major features in meteorological variables, gas species, aerosol components and cloud properties in the western Pacific Ocean in springtime. Active bloom events were observed in the western Pacific Ocean during the study period, with mean surface seawater Chlorophyll a (Chl-a) concentrations up to 10 mg m−3 in the Yellow Sea, East China Sea, Sea of Japan, as well as the ocean northeast of Japan. The mean surface MPOA concentration was simulated to be up to 2.5 μg m−3 in the East China Sea, followed by that in the ocean northeast of Japan, and about 0.5 μg m−3 in the coastal areas of east China. MPOA generally caused increases in the cloud condensation nuclei (CCN), cloud droplet number concentration (CDNC), cloud optical depth (COD) and cloud liquid water path (CLWP), but decrease in the cloud droplet effective radius (CDER). The above changes in cloud properties induced a negative indirect radiative effect (IRE), with the mean values being −5.3 Wm-2, −8.2 Wm-2 and -12.2 Wm-2 over the ocean, East China Sea and the north western Pacific Ocean, which accounted for about 40%, 35% and 51% of the IRE due to all aerosols in these regions, respectively. It was noteworthy that the MPOA induced IRE was −3.6 Wm-2 in east China, accounting for 20% of the IRE by all aerosols and the percentage contribution was about 32% for the whole domain, suggesting its important influence on cloud and radiation during the study period. The sensitivity of MPOA activation to the factors affecting hygroscopicity and surface tension was examined by sensitivity simulations. The indirect effects of MPOA tended to suppress precipitation in most of the domain, with the maximum decrease in the accumulated precipitation up to 50 mm in parts of south China and the East China Sea. In terms of domain average, MPOA accounted for 16%, 22%, 18% of the precipitation reduction due to all aerosols over the land, ocean and the whole domain, respectively, indicating its nonnegligible influence on precipitation over East Asia in springtime 2014.

A regional air quality model system (RAQMS) was developed by coupling the treatment of heterogeneous reactions between sea salt aerosols (SSAs) and trace gases and applied to the investigation of aerosol properties and evolutionary features in the western Pacific Ocean in the spring of 2014. Model results for meteorological variables, PM concentrations, and size-resolved water soluble inorganic aerosol (WSIA) concentrations were compared and analyzed with a variety of observations from in situ measurements and the research cruise Dongfanghong II. Model validation demonstrated that the model can simulate the spatial-temporal distribution and size distribution of aerosol inorganic components in the marine atmosphere over East Asia, and the inclusion of heterogeneous reactions on SSAs apparently improved the model simulation for WSIA concentration, especially for aerosol size distribution. In the western Pacific Ocean, the non-sea salt SO42− and NO3− formed on SSAs accounted for up to 30% and 90% of surface SO42− and NO3− concentrations on average, respectively. The atmospheric depositions of total inorganic sulfur and nitrogen were estimated to be 13184 × 103 kgS/d and 10728 × 103 kgN/d, respectively. Wet deposition was the dominant removal pathway, which accounted for 75% and 68% of sulfur and nitrogen depositions, respectively. The deposition of fine-mode SO42− exceeded that of coarse-mode SO42−, whereas the deposition of coarse-mode NO3− was comparable to that of fine-mode NO3−. The non-sea salt SO42− and NO3− formed on SSAs contributed 16% and 9% of total sulfur and nitrogen depositions on average, respectively. The above results revealed the important role of SSAs in both atmospheric chemistry and deposition in the western Pacific Ocean.

In literature, particulate oxalate has been widely studied in the total suspended particles (TSP), particles 10 and PM 2.5 ) and size-segregated particles >100 nm. In this article, we measured oxalate's concentrations in size-segregated atmospheric particles down to 10 nm or 56 nm during eight campaigns performed at a semi-urban coastal site, over the marginal seas of China and from the marginal seas to the northwest Pacific Ocean (NWPO) in 2012–2015. When the sum of the oxalate's concentration in particles −3 were observed during the two campaigns performed at NWPO. The highest average value of 0.38 μg m −3 was observed at the coastal site during a heavy pollution event. Mode analysis results of particulate oxalate and the correlation between oxalate and sulfate suggested that the elevated concentrations of oxalate in PM 10 were mainly related to enhanced in-cloud formation of oxalate via anthropogenic precursors. Size distribution data in the total of 136 sets of samples also showed approximately 80% of particulate oxalate's mass existing in atmospheric particles >100 nm. Consistent with previous studies, particulate oxalate in particles >100 nm was a negligible ionic component when comparing to particulate SO 4 2− in the same size range. However, the mole ratios of oxalate/sulfate in particles 100 nm atmospheric particles such as PM 2.5 , PM 10 , TSP, etc.

This study aims to simulate depositions of size-segregated particles in human airway in Beijing, China during seasons when fine particulate matter concentrations are high (December 2011 and April 2012). Particle size distributions (5.6–560 nm, electrical mobility diameter) near a major road in Beijing were measured by the TSI Fast Mobility Particle Sizer (FMPS). The information of size distributions provided by FMPS was applied in the Multiple-Path Particle Dosimetry model (MPPD) to quantify number and mass depositions of particles in human airway including extrathoracic (ET), tracheobronchial (TB), and pulmonary (PUL) regions of exposed Chinese in Beijing. Our results show that under ambient conditions, particle number concentration (NC) deposition in PUL is the highest in the three major regions of human airway. The total particle NC deposition in human airway in winter is higher than that in spring, especially for ultrafine particles (1.8 times higher) while particle mass concentration (MC) deposition is higher in spring. Although particle MC in clean days are much lower than that in heavily polluted days, total particle NC deposition in human airway in clean days is comparable to that in heavily polluted days. NC deposition for nucleation mode particles (10–20 nm, aerodynamic diameter) in clean days is higher than that in heavily polluted days. MC deposition for accumulation mode particles (100–641 nm, aerodynamic diameter) in heavily polluted days is much higher than that in clean days, while that of nucleation mode is negligible. The temporal variation shows that the arithmetic mean and the median values of particle NC and MC depositions in the evening are both the highest, followed by morning and noon, and it is most likely due to increased contribution from traffic emissions.

Regional haze-fog events over the North China Plain (NCP) have attracted much attention in recent years. Their increase has been attributed to anthropogenic emissions of air pollutants and synoptic weather conditions. We investigated the influence of local meteorological conditions and large-scale circulation on the haze-fog events over the NCP during 2001–2012, and found a significant interannual increase in the number of summer regional haze-fog days. Analysis indicated that local meteorological conditions could partly explain the increase but failed to explain the spatial variation; meanwhile, regional circulation change induced by large-scale circulation and orographic forcing unveiled a possible spatiotemporal variation mechanism. In summer, the prevalent southerly winds over the NCP were obstructed by the Taihang and Yanshan mountains, steadying the outflow direction to the southeast, while different inflow direction controlled by large-scale circulation had different effects on regional circulation. In weak (strong) East Asian summer monsoon years, an intensified eastward (westward) zonal inflow wind component reinforced (weakened) the negative vorticity and formed an anomalous anticyclone (cyclone), which strengthened (weakened) the downward motion, so the dissipation capability was weakened (strengthened) and the wind speed decreased (increased), ultimately resulting in the increased (decreased) occurrence of haze-fog. We also found that the circulation anomaly had a good relationship with strong El Nino and La Nina events. There was more haze-fog over the NCP in the summers that followed a La Nina event, and less in summers that followed an El Nino event. This suggested the possibility that summer haze-fog phenomena could be predicted based on the phase of ENSO.

Nitrogen and phosphorus in aerosols is an important source of nutrients in the coastal waters. To study all forms of composition and potential source of nitrogen and phosphorus in atmospheric particulates in the coastal waters of Qinhuangdao, 135 atmospheric aerosol samples were collected from August 2015 to August 2016. The result showed that the nitrogen in aerosols in the Qinhuangdao coastal area was more soluble (accounting for 91.7%), while phosphorus was more insoluble (62.3%). The same seasonal variation of nitrogen and phosphorus components was discovered, with the highest concentration in winter and the lowest in summer. Frequent dust and haze weather processes have a significant impact on the nitrogen and phosphorus species in aerosols. The concentrations of soluble nitrogen and phosphorus increased more significantly in haze days, while insoluble nitrogen and phosphorus increased more significantly in dust days. Both weather processes may aggravate the phosphorus limitation in the coastal waters of Qinhuangdao. Quantitative analysis of potential sources area indicated that Beijing–Tianjin–Hebei, Mongolia, Inner Mongolia contributed the most to the atmospheric nitrogen and phosphorus in Qinhuangdao coastal area. The total contribution of these three regions to atmospheric nitrogen and phosphorus pollution of Qinhuangdao were about 70%, which indicated that most of the atmospheric nitrogen and phosphorus in Qinhuangdao offshore come from the atmospheric pollutant transport channel in the northwest.

In this paper, we studied concentrations of cloud condensation nuclei (CCN) and CCN activity measured at a suburb site of Qingdao in the North China during a non-heating period from 19 May to 3 June 2013 and a heating period from 6 November to 6 December 2013. Concentrations of CCN during the non-heating period were 3.1 ± 1.9 (mean ± standard deviation), 4.9 ± 3.4, 5.6 ± 3.9, 6.1 ± 4.3 and 6.5 ± 4.6 in unit of ×103 cm−3 at supersaturation (SS) of 0.2%, 0.4%, 0.6%, 0.8% and 1.0%, respectively. The corresponding CCN activities of atmospheric particles were 0.28 ± 0.17, 0.43 ± 0.24, 0.48 ± 0.26, 0.52 ± 0.27 and 0.54 ± 0.28, respectively. Concentrations of CCN during the heating period were 3.1 ± 1.3, 6.4 ± 2.3, 8.5 ± 2.9, 9.6 ± 3.5 and 10 ± 3.9 in unit of ×103 cm−3 at SS of 0.2%, 0.4%, 0.6%, 0.8% and 1.0%, respectively. The corresponding CCN activities were 0.11 ± 0.06, 0.24 ± 0.11, 0.31 ± 0.14, 0.35 ± 0.15 and 0.37 ± 0.15, respectively. At SS ≥ 0.4%, concentrations of CCN were significantly larger during the heating period than during the non-heating period with 95% confidence because of larger particle number concentrations associated. However, CCN activities were significantly lower during the heating period than during the non-heating period at each SS. The lower CCN activities during the heating period were analyzed in terms of origins, median mobility diameter and possible chemical composition of atmospheric particles. However, when new particle formation events during the heating and the non-heating periods were considered alone, CCN activities of grown new particles at the same size range sometimes appeared to be constant regardless of the heating or non-heating periods.

This study applies land-use regression (LUR) to characterize the spatial distribution of ultrafine particles (UFP) in a large city. Particle number (PN) concentrations were measured in residential areas around Toronto, Canada, between June and August 2008. A combination of fixed and mobile monitoring was used to assess spatial gradients between and within communities. The fixed monitoring locations included a central site, two downtown sites, and four residential sites located 6–15 km from the downtown core. The mobile data included average PN concentrations collected on 112 road segments from 10 study routes that were repeated on three separate days. The mobile data was used to create the land-use regression model while the fixed sites were used for validation purposes. The predictor variables that best described the spatial variation of PN concentration (R2 = 0.72, validated R2 = 0.68) included population density within 300 m, total resource and industrial area within 1000 m, total residential area within 3000 m, and major roadway and highway length within 3000 m. The LUR model successfully predicted the afternoon peak PN concentration (slope = 0.96, R2 = 0.86) but over-predicted the 24-h average PN concentration (slope = 1.28, R2 = 0.72) measured at seven fixed monitoring sites.

In this study, we investigated new particle formation (NPF) in the vertical direction using high time-resolution (1 s) measurements made by Fast Mobility Particle Sizers at ground level and at sea level. The coefficient of variation (CV), i.e., the ratio of standard deviation to mean value for

In this paper, we conducted a comparative study of new particle formation (NPF) events occurring between Qingdao and Toronto during spring. The extent of air pollution in Qingdao was much severer than that in Toronto, but the occurrence frequency of NPF events in Qingdao (41%) was almost same as that (42%) in Toronto. The geometric median diameter of new particles (Dpg,1) increased up to >40 nm in 15 days out of the total 16 NPF days in Qingdao, the Dpg,1 at least in eight days increased up to >60 nm and even reached >80 nm in two days. Two-stage growth was generally observed in these eight NPF events. The first-stage growth occurred in daytime and it was likely associated with formation of secondary organic aerosol (SOA) on basis of the modeling results. The second-stage growth was generally observed at nighttime when the modeling results showed increases of NH4+ and NO3− in concentration together with SOA, implying that NH4NO3 possibly played a role in the growth. In Toronto, the maximum Dpg,1 of the observed new particles in all 13 NPF events was less than 50 nm. A slight second-stage growth of new particles was observed only in four days when either the increase of NH4+ and NO3− in concentration or the increase of relative humidity occurred. The NPF events in Toronto less likely had a significant contribution to cloud condensation nuclei due to the small size of the observed new particles.

The variability of atmospheric ammonia (NH 3 ) in downtown Toronto was investigated through the analysis of 24 hr integrated denuder NH 3 samples collected at a site above a street canyon every third day from July 2003 to September 2011 (Dataset 1). The measured NH 3 mixing ratios ranged from below the detection limit to 14.7 ppb during the eight-year period. Distinctive seasonal variations were observed with summer averages (±standard deviation) of 3.9 ± 1.6 ppb and winter averages of 1.1 ± 0.6 ppb. Two other datasets, weekly/biweekly passive samples of NH 3 monitored at 74 agricultural and remote sites across southern Ontario during the period of June 2006 to March 2007 (Dataset 2) and semi-continuous measurements of NH 3 and ammonium (pNH 4 + ) in PM 2.5 collected at a site inside a street canyon approximately 170 m away from the downtown site in December 2008 and February, March and May 2009 (Dataset 3), were further used to evaluate the potential NH 3 sources in downtown Toronto. The NH 3 mixing ratios at the downtown location were higher than those at the surrounding agricultural sites, and the mixing ratios within the street canyon were almost double those measured above it. These results suggested that the observed NH 3 at the downtown location was mainly from local sources within the street canyon. Analysis of Dataset 3 showed that the diurnal cycles of NH 3 at the downtown location cannot be simply explained by traffic and water consumption patterns. A green space inside the street canyon was identified as a potential important source contributing to the observed NH 3 when T > 0 °C. However, the major NH 3 emission sources at T ≤ 0 °C are yet to be investigated.

This study explores the application of wavelet decomposition as a means to distinguish between local and regional sources of ultrafine particles (UFP). Particle number concentrations were measured at a central site, two downtown sites, and four residential sites located across Toronto, Canada. Using a wavelet decomposition algorithm, particle concentration time series were separated into two signals: high frequency local-to-neighbourhood scale sources and low frequency urban-to-regional scale sources and processes. At the field sites, local–neighbourhood sources contributed between 13 and 32% of the total particle concentration. The urban–regional signal at each field site exhibited stronger correlation and greater homogeneity with respect to the central site than the original concentration time series. In contrast, the high frequency local–neighbourhood source signals exhibited limited correlation and high heterogeneity with respect to the central site. Traffic volume within a 2.5 km buffer explained 87% of the variability in the local–neighbourhood level signal observed between field sites while no significant association with traffic was found for the original particle number concentration data. This study has demonstrated that wavelet decomposition can be a useful tool for estimating exposure to UFP from local–neighbourhood and urban–regional scale sources and processes.

To investigate emissions of NH3 from on-road vehicles and their contribution to atmospheric NH3, we analyzed a suite of measurements including NH3, NO, SO2, black carbon (BC), NH4+ in PM2.5, etc., collected in summer 2007 at a site near a highway with the highest traffic density in Canada. From 26 to 28 August 2007, large variations of NO mixing ratio (from 110 ppb) and BC concentration (from 6 μg m−3) were simultaneously observed and were found to be associated with varying contributions of air pollutants emitted by vehicles on the highway. Using NO as a tracer of vehicular plumes, the estimated emission factor of NH3 relative to NO from on-road vehicles was less than 0.5–0.9 ppb NH3 per 50 ppb NO. The average mixing ratios of NH3 and NO during the three days were 4.0 ± 2.4 ppb and 23 ± 33 ppb, respectively. The NH3 derived from on-road vehicular emissions was estimated to be less than 0.4 ppb on average during the measurement period, accounting for ∼10% of the mixing ratio of NH3 measured at the sampling site. Several spikes of NH3 were observed on these days with the maximum mixing ratio of NH3 reaching 16.9 ppb. The observed spikes of NH3 were probably ascribed to non-traffic emissions of NH3 near the sampling site. We conclude that emissions of NH3 from on-road vehicles only accounted for a much small fraction of urban atmospheric ammonia, and more important sources are yet to be identified. However, it is still unknown whether negligible emissions of NH3 from vehicles in Toronto are just the result of its local traffic composition or they are generally true world wide. In addition, the negligible emissions could also be due to unknown loss in the GP-IC, although it is not found so far.

This study reports the diurnal, seasonal, and annual variation of ultrafine particle size distributions in downtown Toronto. The k-means clustering algorithm was applied to five years of size-resolved data for particles with diameters less than 100 nm. Continuous particle number concentrations were measured 16 m from a major arterial roadway between March 2006 and May 2011 using a Fast Mobility Particle Sizer. Eight particle size distribution (PSD) types were identified. The PSD types exhibited distinct weekday–weekend and diurnal patterns. The relative frequency that each PSD occurred varied with season and wind direction and was correlated with other pollutants. These temporal patterns and correlation helped in elucidating the sources and processes that each of the eight PSD represent. Finally, similar PSD types were observed in residential areas located 6 and 15 km away from the central monitoring site suggesting that these PSD types may be generalizable to other sites. Identification of PSD types was found to be a valuable tool to support the interpretation of PSD data so as to elucidate the sources and processes contributing to ultrafine particle concentrations.

This study reports the diurnal, seasonal and annual variation of ultrafine particles (UFP) in a large city. Particle number (PN) concentrations were measured in Toronto, Canada on a major arterial roadway between March 2006 and May 2011 using a Fast Mobility Particle Sizer. The PN in the size range of 8–300 nm decreased during the measurement period primarily due to changes in the vehicle fleet. PN 50 (particles with diameters less than 50 nm) decreased by 21% between 2006 and 2010. Notably, the majority of the reduction occurred during the winter months. PN 50 exhibited the strongest seasonality and diurnal trend. PN 50–100 (particles between 50 and 100 nm) and PN 100–300 (particles between 100 and 300 nm) decreased by 17% and 24%, respectively. Correlation analysis between gas phase criteria pollutants showed good correlation between PN 50–100 and NO 2 , SO 2 , and PM 2.5 . In contrast, PN 50 exhibited the highest correlation with temperature, NO and NO 2 . A multiple linear regression model was developed for each size fraction. The model adequately explained the annual, seasonal and day-to-day variability of PN 50–100 ( R 2 = 0.64) and PN 100–300 ( R 2 = 0.83). The model captured the annual and seasonal variability of PN 50 but only partially explained the day-to-day variability ( R 2 = 0.52). The long-term reductions in PN 50 indicate that policy interventions are having some success in slowly decreasing UFP concentrations in Toronto.

Chloride depletion in sea-salt aerosols was studied using size-segregated inorganic ions data collected at a Canadian rural coastal site during a summer (June 29–July 15, 2002) and a fall campaign (October 25 – November 15, 2002). Three samples collected in the fall campaign had high concentrations of sea-salt and ammoniated sulfate and nitrate aerosols and were used to study the relative importance of different chemical reactions contributing to the depletion. The percentage depletion (Cl−depletion(%)) increased substantially with decreasing particle size (up to 86% for particles in the size range of 1.0–3.1 μm). For particles >6.2 μm, the observed NO3− was responsible for all the depleted Cl−; but less than a quarter of the depletion was explained by the HCl-released reaction between NaCl and HNO3; the rest of the depletion was likely due to the non-HCl-released reactions, e.g., between NaCl and N2O5. For particles in the size range of 3.1–6.2 μm, the NO3− was responsible for nearly 80–90% of all the depleted Cl− via HCl-released and/or non-HCl-released reactions; the remaining depletion was likely due to the reactions releasing Cl2, HOCl, etc. Particles >3.1 μm were mostly neutral acidity while a portion of particles at 1.0–3.1 μm was acidic. SO42− was only responsible for Cl−depletion(%) in acidic particles at 1.0–3.1 μm and the highest Cl−depletion(%) was observed in acidic particles of this size range.

Extent of neutralization (EoN) of atmospheric aerosol is an important parameter in understanding related nucleation mechanisms, acid-catalyzed reactions and gas–aerosol partitioning. Ion m/z −195 ( HSO 4 H 2 SO 4 − ) detected by the Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) has been used as an indicator of incompletely-neutralized sulphate aerosol, but there are no laboratory data to support this assumption. In this study, experiments using artificially generated sulphuric acid nucleated aerosol and metal sulphate aerosol across a range of EoN found that the peak area ratio and hit ratio of ion m/z −195 ( HSO 4 H 2 SO 4 − ) to ion m/z −97 ( HSO 4 − ) detected by the ATOFMS increased with decreasing EoN. Area ratio and hit ratio are sensitive to EoN at the low and high value zones, respectively. In ambient air measured by the ATOFMS and a Gas Particle Ion Chromatograph (GPIC) in Toronto, Canada, ion m/z −195 was always detected in ammonium sulphate aerosol, and its hit number and peak area varied widely, regardless of EoN indicated by the equivalent ratio of NH 4 + to ( SO 4 2 − + NO 3 − ) . Thus, ion m/z −195 alone is not an indicator of acidic sulphate aerosol. The combined approach using the ATOFMS and the GPIC found that cloud-processing formed incompletely-neutralized acidic sulphate aerosol in 2 out of 35 days sampled in winter in Toronto, Canada. It is interesting that the two episodes both occurred at night. Formation of incompletely-neutralized acidic sulphate aerosol caused a decrease in the concentration of particulate nitrate. This can be explained by acidic sulphate aerosol reacting with ammonium nitrate, leading to the release of HNO3 to the gas phase. It was also found that the GPIC results occasionally suffered a positive artifact of NH 4 + concentration caused by the clogging-induced high back-pressure in the instrument.

In this study, the Daily Air Pollution Index (DAPI) of 81 cities in China from June 2004 to June 2007 was used to study the air pollution of city clusters. The cluster analysis results show that the DAPI of 71 cities in central and eastern China is divided into two regimes, the north regime and the south regime along the Qinling Mountain and its eastern extension to Huaihe River. The demarcation line between the two DAPI regimes duplicates the demarcation line of the watersheds of the northern and southern climate regions and the DAPI were statistically negatively correlated to the amount of rainfall, suggesting that precipitation could play an important role in determining the two regimes. The two DAPI regimes are further divided into seven city clusters mainly associated with emissions of air pollutants and geographical distributions, supported by remote sensing data of aerosol optical depth (AOD), NO2 and emission inventories of air pollutants in China in 2006 reported in the literature. DAPI of three city clusters in the north regime exhibited a similar seasonal trend, which was different from the four city clusters in the south regime. Outbreak of dust storms in spring and an increase in emissions due to space-heating impacted the DAPI of the northern city clusters. Although the consumption of electricity decreased in the southern city clusters in the fall and winter, the reported high DAPI were the result of less rainfall and poor dispersion. Seven cities in western China were classified into two groups associated with their unique nature environments. Three cities were not allocated to any of the city clusters.

Accurate measurements of SO2 at low ambient concentrations are needed in order to investigate the role of SO2 in particle nucleation events and the long-term impact of reductions in sulfur emissions in recent decades. In this study, artifacts in SO2 concentration measurements were investigated using two identical ion chromatography-based instruments (the Gas Particle Ion Chromatograph, GP-IC, Dionex Corporation) and two identical UV fluorescence-based SO2 analyzers (the TECO 43CTL, an industry standard). The SO2 concentration values measured with the GP-IC at roadside sites were compared with simultaneous side-by-side measurements made with the fluorescence analyzers. The SO2 concentration measured with the GP-IC had an ∼30% negative calibration artifact. When the GP-ICs were calibrated using an improved procedure developed in the course of this study, only a ±5% difference from the TECO analyzers remained, except under high NO concentration conditions. The fluorescence analyzers exhibited a positive artifact under elevated NO concentration conditions. Sulfur oxidation ratios were calculated based on the GP-IC-measured SO2 and SO42- concentrations and used to help identify potential emission sources. The SO2 concentrations measured with the GP-IC were also compared to data obtained from a National Air Pollution Surveillance (NAPS) speciation sampler equipped with a Na2CO3-coated denuder. Good correlation between SO2 data from the two methods was seen during five months of measurement, but the GP-IC SO2 data were ∼30% lower than the NAPS data. Deposition of SO2 within an urban street canyon is discussed as a possible explanation for this difference.

As the world gets ready to begin the second decade of the twenty-first century, global climate change has been recognized as a real threat to civilization as we know it. The rapid and successful economic growth of developing nations, particularly China and India, is contributing to climate change. The route to initial economic success in China followed that of the developed nations through the development of industries. Unfortunately, China's environmental protection efforts have not been the same as in developed countries because China is vastly different culturally, socially, economically and, especially, politically from developed nations. When China started to deal with environmental concerns in the late 1970s, it took advantage of the experiences of other countries in establishing environmental standards and regulations, but it did not have a model to follow when it came to implementing these standards and regulations because of the abovementioned differences. Economically, China is transitioning from an agricultural base into an industrial base; however, even now, 60% of the population remains farmers. China has been and still is heavily dependent upon coal for energy, resulting in serious atmospheric particulate pollution. While growing efforts have been expended on the environment, at this juncture of its economic development, China would be well served to revisit the traditional “develop first and clean up later” approach and to find a balance between development and protecting the environment. Against this backdrop, a reflective look of the effort to manage air quality from 1949–2008 (with an emphasis on the past 30 years) in China is presented in this paper. The environmental component of the 2008 Olympic Games is examined as a special example to illustrate the current measures being used to improve air quality in China.

Due to its rapidly expanding economic and industrial developments, China is currently considered to be the engine of the world's economic growth. China's economic growth has been accompanied by an expansion of the urban area population and the emergence of a number of mega cities since the 1990. This expansion has resulted in tremendous increases in energy consumption, emissions of air pollutants and the number of poor air quality days in mega cities and their immediate vicinities. Air pollution has become one of the top environmental concerns in China. Currently, Beijing, Shanghai, and the Pearl River Delta region including Guangzhou, Shenzhen and Hong Kong, and their immediate vicinities are the most economically vibrant regions in China. They accounted for about 20% of the total GDP in China in 2005. These are also areas where many air pollution studies have been conducted, especially over the last 6 years. Based on these previous studies, this review presents the current state of understanding of the air pollution problems in China's mega cities and identifies the immediate challenges to understanding and controlling air pollution in these densely populated areas.

Volume concentration data of submicron particles ( 0.1 – 1.0 μ m ) in on-road vehicle plumes ( NO x > 400 ppb ) gathered by the Mobile Real-time Air Monitoring Platform (MAP) on city streets, highways and in tunnels in Hong Kong are used to study the size distributions and growth of vehicular submicron particles due to gas condensation and, particularly, its dependency on ambient temperature. Three particle volume size distributions are observed: a uni-modal distribution with an accumulation mode at 0.2 ± 0.1 μ m , and two bi-modal distributions with a minor mode at 0.2 ± 0.1 μ m and the dominant mode at either 0.5 ± 0.1 or 0.7 ± 0.1 μ m . In tunnels, the ratio of particle volume concentration to black carbon (BC) mass concentration correlates negatively with ambient temperature ( r 2 = 0.57 ) ; the dominant mode tends to be at the large particle size when the temperature is low, and when the temperature is high, the uni-mode appears at the small particle size. Thus temperature seems to exert a significant influence on the condensation growth of fresh vehicular particles. On the other hand, this ratio correlates positively with ambient particle concentrations ( r 2 = 0.35 ) . Ambient particles measured in this study are mostly > 0.3 μ m while BC in vehicle plumes is generally believed to be mainly in the 0.3 μ m size range in the literature. Temperature-dependent gas-condensation competition between fresh BC and ambient particles is suggested to play a role in the bi-modal formation.

Some studies have reported that small submicron atmospheric particles are more acidic than large submicron particles; other studies demonstrated a reversed trend. In this study, the size dependence of in situ pH in submicron particles in Hong Kong was investigated. The equivalent ratios of [NH4+]measured to [SO42−]measured in submicron particles were found to be generally less than unity and size dependent, suggesting the possibility of incomplete gas–aerosol equilibrium. The Aerosol Inorganic Model-II (AIM-II) model using measured ionic compositions with the gas–aerosol partitioning disabled was used to estimate the in situ pH in different sized particles. The estimated in situ pH of different sized submicron atmospheric particles was between −2.5 and 1.5 and it generally decreases with increasing submicron particle size. At such low in situ pH, the estimated HSO4− equivalent concentrations were 3.2 times (on average) of those of H+ in different sized particles. The trends of the size dependence of the [NH4+]measured to [SO42−]measured ratio and pH under different regimes of relative humidity are discussed.

In situ aerosol pH is important in the study of atmospheric chemistry because many heterogeneous atmospheric chemical processes are pH dependent, but it is not easy to measure the in situ pH of PM2.5 due to the low liquid water content of aerosol particles. In situ aerosol pH is usually estimated by thermodynamic modeling. This study compared the in situ pH of PM2.5 in Hong Kong estimated by different thermodynamic methods such as AIM-II (Aerosol Inorganics Model-II) with the gas–aerosol partitioning calculation disabled, HCl/Cl−, HNO3/NO3− and NH3/NH4+ gas–aerosol equilibrium equations, and two thermodynamic gas–aerosol equilibrium models (ISORROPIA and SCAPE2). In the first method, actual measured particulate composition was used as input while only the total (gas+aerosol) composition was used in the other methods. AIM-II with the gas–aerosol partitioning calculation disabled gives the lowest in situ aerosol pH, 3.5, 2.8, 3.1, 2.7 and 4.3 unit (on average) less than that estimated by HCl/Cl−, HNO3/NO3− and NH3/NH4+ gas–aerosol equilibrium equations, and by SCAPE2 and ISORROPIA, respectively. The differences in estimated in situ pH are discussed in terms of the extent of the gas–aerosol equilibrium achieved, especially the NH3/NH4+ equilibrium. Although the estimated equilibrium concentrations deviated from the measured NH4+ in PM2.5 by only 20–35%, the estimated in situ pH is very sensitive to the deviation. It is possible that approaches based on the assumption of the gas–aerosol equilibrium may not be suitable for estimating in situ aerosol pH in the atmosphere.

Dicarboxylic acids in atmospheric aerosols have received much attention because of their potential roles in affecting the global climate. The composition and the sources of dicarboxylic acids in PM2.5 were studied at one remote and two urban sites in Hong Kong in the winter of 2000 and in the summer of 2001. Oxalate was the dominant dicarboxylic acid in all samples. The winter oxalate concentrations were high and spatially uniform, with an average value of 0.36 μg m −3 , but the summer oxalate concentrations were low and had a large spatial variation. The influence of meteorological factors on the concentrations of dicarboxylic acids was also studied. The ratio of malonate to succinate was used to distinguish primary sources from secondary sources of these acids. This ratio at all three sites was close to that from direct vehicular exhaust in the winter, but it was close to that of secondary reactions in the summer. Hence, the acids were attributed to vehicular emissions in the winter and secondary sources in the summer. This hypothesis is also supported by a good correlation of oxalate with sulfate in the summer but a poor one in the winter. The correlations of oxalate with malonate, succinate, sulfate and K + were also studied in terms of the routes of secondary formation of these dicarboxylic acids.

To study the formation of atmospheric sulfate, nitrate and chloride particulates in Beijing, China, 14 sets of measurements of the concentrations of SO 2 , HNO 3 , NH 3 and size-segregated particles were made in the summer of 2001 and the spring of 2002. Because of the very different humidity levels in Beijing in the summer and the spring, significant differences in the size characteristics and in the formation mechanisms of these ions were found. In the summer, the major fraction of sulfate was present in the fine mode with a mass median aerodynamic diameter (MMAD) of 0.7±0.1 μm. Its formation was attributed to in-cloud processing, which was supported by the observation of a large mole ratio of the sulfate at 0.54–1.0 μm to SO 2 , with a maximum value of 3.0. The major fraction of nitrate sometimes appeared in the fine mode with an MMAD of 0.7±0.1 μm and sometimes in the coarse mode with an MMAD of 6.0±1.5 μm. The coarse mode nitrate was associated with Ca 2+ . The major peak of chloride, in most cases, appeared in the coarse mode. Chloride was expected to have formation mechanisms similar to nitrate formation in the coarse mode. In the spring, the major fraction of sulfate was present in the fine mode with an MMAD of 0.45±0.05 μm and its formation was ascribed to non-cloud heterogeneous processes. A small mole ratio of the fine mode sulfate to SO 2 , with a maximum value of only 0.06, was observed. Nitrate and chloride generally have a similar MMAD as sulfate in the fine mode, suggesting that both may originate from atmospheric processes similar to sulfate formation. Alternatively, they could be formed by gas condensation onto existing sulfate particles. Overall, much more efficient sulfate formation by cloud processing led to a higher sulfate concentration in the summer than in the spring, although the SO 2 concentration followed the reverse trend in Beijing.

Sparse data on the size distributions of ionic species, especially organic ions, in atmospheric particles are available in the literature. To study the size distributions and the formation of atmospheric ionic species, 14 sets of measurements of the concentrations of the gases and size-segregated atmospheric particles were made in Beijing in the summer of 2001 and the spring of 2002. Significant differences of the size distribution characteristics and the formation mechanism of the ions in these two periods were found. Using the size distributions of SO 4 2− , K + and Ca 2+ as references, the size distributions of oxalic, malonic and succinic acids were examined to investigate the formation of these acids in the atmospheric particles in Beijing. In the summer, oxalate was present mostly in the fine mode with an MMAD of 0.7±0.1 μm, coincident with the droplet mode of sulfate. The formation of fine mode oxalate, like sulfate, was attributed to in-cloud processes. Malonate was also present mostly in the fine mode with an MMAD of 0.8±0.2 μm and its formation was also attributed to in-cloud processes. The MMAD of fine mode malonate sometimes shifted to values larger than that of sulfate, which can be explained by a minor evaporation of malonate during water evaporation from cloud droplets. The size distribution and the formation of succinate are similar to those of oxalate. In the spring, oxalate sometimes dominated in the fine mode, where oxalate was highly correlated with sulfate ( r =0.86) and had an MMAD (0.45±0.05 μm) similar to sulfate, but it sometimes dominated in the coarse mode, in association with Ca 2+ . The fine mode oxalate was attributed to heterogeneous formation but the coarse mode oxalate was probably related to biological formation.

The strong acidity and the concentrations of ammonium, sodium, sulfate, nitrate and chloride in PM2.5 in Hong Kong were measured using a Harvard honeycomb denuder/filter-pack system. The role of atmospheric mixing height in the variation in concentrations of the species in PM2.5 was investigated and the contribution of long-range transport to the ambient aerosols in Hong Kong was quantitatively determined. The average measured strong acidity in the spring, summer, autumn and winter of 2000, and the spring of 2001 was 49(23–92), 27(11–52), 97(71–145), 45(26–57), and 103(78–167) nmol/m3, respectively. Simulations using the aerosol inorganic model (AIM2) revealed that the in situ acidity of PM2.5 was only a minor fraction (23–39%) of the measured strong acidity because of the presence of bisulfate ions. The in situ acidity was correlated with a large difference between the ambient relative humidity and the estimated deliquescence relative humidity of the aerosols. In equivalent concentrations, acidity, ammonium and sulfate accounted for 16% (9–21%), 31% (26–39%) and 42% (40–45%) of the water-soluble inorganic species, respectively. Analysis of air mass trajectories showed that long-range transport of aerosols through the East/South China Sea (marine) and mainland China (continental) was possible. When Hong Kong was under the influence marine air masses, the measured concentrations of sulfate, ammonium and nitrate showed a strong correlation with the atmospheric mixing height. Continental long-range transport of aerosols was found to increase the concentrations of sulfate and ammonium by 49–383% and 33–302%, respectively. In total, about 40% of the sulfate and ammonium of PM2.5 in Hong Kong measured in this study was from continental air masses. However, the influence of the continental air masses on nitrate concentration and acidity was not obvious.

Weekly PM 2.5 samples were collected in Shanghai, China at two sites, Tongji University and Hainan Road. Sampling started in March 1999 and was conducted for 1 year. The ambient mass concentration and chemical composition of the PM 2.5 were determined. Chemical analyses included elemental composition, water-soluble ions, and organic and elemental carbon. Weekly PM 2.5 mass concentrations ranged from 21 to 147 μg/m 3 , with annual average concentrations of 57.9 and 61.4 μg/m 3 at the two sites, respectively. Seasonal variation of PM 2.5 concentrations was significant, with the highest concentrations observed from mid-November through December and the lowest from June through September. Ammonium sulfate and nitrate accounted for 41.6% of the PM 2.5 mass with sulfate alone accounting for 23.4% of the PM 2.5 mass. Carbonaceous material accounted for 41.4% of the PM 2.5 mass, with 73% of that mass being organic, as defined by the TOR analysis method. Crustal components averaged 9.6% of the PM 2.5 mass. Potassium, which was 95% water soluble, accounted for 2.7% of the PM 2.5 mass.

The size dependence of the percentage of chloride depletion (%Cl dep ) in sea-salt aerosols was investigated using 16 sets of 48–96 h size distribution measurements of atmospheric aerosols, collected from June 2000 to May 2001 in Hong Kong. Chloride and sodium in HK are predominantly from sea-salt aerosols because the prevailing wind is from the sea. In the particles larger than 1.8 μm (coarse particles), the %Cl dep generally decreased with increasing particle size, consistent with the literature. In the particles smaller than 1.8 μm (fine particles), the mode diameter of %Cl dep was associated with the mode diameter of sulfate in the droplet mode. When the sulfate peak appeared at 0.32–0.54 μm, the %Cl dep peaked at 0.54–1.0 μm. Heterogeneous reactions between sea-salt particles and acidic gases are the major routes of the sulfate formation and chloride depletion, leading to a decrease in the %Cl dep with increasing particle size. When the sulfate peak appeared at 0.54–1.0 μm, the maximum %Cl dep appeared at 1.0–1.8 μm and the %Cl dep increased with increasing particle size in the fine mode. Both heterogeneous reactions and in-cloud processes occurred to form sulfate and to deplete chloride. However, heterogeneous reactions, sampling artifacts, and anthropogenic emissions of chloride cannot explain this observed size dependence. Cloud processing including the activation of sea-salt particles with subsequent SO 4 2− formation, the neutralization by NH 3 and the evaporation of HCl in conjunction with NH 3 during water evaporation from cloud droplets can cause chloride depletion in the droplet mode. The smaller amount of evaporation of NH 3 and HCl from cloud droplets in forming the 0.54–1.0 μm particles than the 1.0–1.8 μm particles can account for the observed size dependence of %Cl dep although direct evidence based on cloud measurements is not available.

Aerosol samples for PM10 and PM2.5 were collected in wintertime from November 2000 to February 2001 at three different sampling locations in Hong Kong. PM10 and PM2.5 were collected by high-volume (hi-vol.) samplers and the concentrations of major elements, ions, organic and elemental carbons were quantified. The ratios of PM2.5/PM10 were 0.61 and 0.78 at the PolyU campus and Kwun Tong (KT), respectively. These results indicated that the concentrations of PM2.5 contribute the majority of the PM10 fraction. The concentrations of anthropogenic species (e.g. Pb and Cu) in PM10 and PM2.5measured at urban areas were generally higher than at an urban background site (Hok Tsui, HT). The major fractions of sulfate at three monitoring sites are non-sea-salts (nss) sulfates. Although HT is located in coastal areas, the contribution of sea salts to sulfate in fine particles was small, indicating a substantial anthropogenic origin. The OC/EC ratios were less than 2 in PolyU and KT monitoring stations for PM10 and PM2.5. However, the OC/EC ratios were over 3 at HT for both PM10 and PM2.5. This indicates the presence of secondary organic aerosols. Correlations between OC and EC as well as OC and sulfate in HT during both seasons were used to prove that atmospheric transport and transformation of anthropogenic organic species from northeastern area was the dominant source in winter. The chemical composition of the samples was reconstructed from the observed elemental composition. The contribution of the seven components, namely crustal matter, sea salt, ammonium, sulfate, nitrate, elemental carbon and organic matter accounted for 77–84% of the PM10 and PM2.5 mass in the urban area (PolyU and KT) and 74% for Hok Tsui (HT). Sulfate, organic matter and elemental carbon are the major constituents in particles especially in PM2.5 collected at PolyU and KT. The major constituents of PM10 in HT are sea salt and sulfate.

A year-long field study to characterize the ionic species in PM2.5 was carried out in Shanghai and Beijing, China, in 1999–2000. Weekly samples of PM2.5 were collected using a special low flow rate (0.4 l min−1) sampler. In Shanghai, SO42− NO3− and NH4+ were the dominant ionic species, which accounted for 46%, 18% and 17% of the total mass of ions, respectively. Local SO2 emissions were an important source of SO42− in PM2.5 because the SO42− concentration was correlated with the SO2 concentration (r=0.66). The relatively stable SO42−/SO2 mass ratio over a large range of temperatures suggests that gas-phase oxidation of SO2 played a minor role in the formation of SO42−. The sum of SO42− and NO3− was highly correlated with NH4+ (r=0.96), but insufficient ammonium was present to totally neutralize the aerosol. In Beijing, SO42−, NO3− and NH4+ were also the dominant ionic species, constituting 44%, 25% and 16% of the total mass of water-soluble ions, respectively. Local SO2 emissions were an important source of SO42− in the winter since SO42− was correlated with SO2 (r=0.83). The low-mass SO42−/SO2 ratio (0.27) during winter, which had low humidity, suggests that gas-phase oxidation of SO2 was a major route of sulfate formation. In the summer, however, much higher mass ratios of SO42−/SO2 (5.6) were observed and were ascribed to in-cloud sulfate formation. The annual average ratio of NO3−/SO42− was 0.4 and 0.6 in Shanghai and in Beijing, respectively, suggesting that stationary emissions were still a dominant source in these two cities.

Weekly PM2.5 samples were simultaneously collected at a residential (Tsinghua University) and a downtown (Chegongzhuang) site in Beijing from July 1999 through September 2000. The ambient mass concentration and chemical composition of the PM2.5 were determined. Analyses included elemental composition, water-soluble ions, and organic and elemental carbon. Weekly PM2.5 mass concentrations ranged from 37 to 357 μg/m3, with little difference found between the two sites. Seasonal variation of PM2.5 concentrations was significant, with the highest concentration in the winter and the lowest in the summer. Spring dust storms had a strong impact on the PM2.5. Overall, organic carbon was the most abundant species, constituting no less than 30% of the total PM2.5 mass at both sites. Concentrations of organic and elemental carbon were 35% and 16% higher at Tsinghua University than at Chegongzhuang. Ammonium, nitrate and sulfate were comparable at the sites, accounting for 25–30% of the PM2.5 mass.