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Secondary organic aerosols (SOAs) account for a large fraction of atmospheric aerosol mass and play significant roles in visibility impairment by scattering solar radiation. However, comprehensive evaluations of SOA scattering abilities under ambient relative humidity (RH) conditions on the basis of field measurements are still lacking due to the difficulty of simultaneously direct quantifications of SOA scattering efficiency in dry state and SOA water uptake abilities. In this study, field measurements of aerosol chemical and physical properties were conducted in winter in Guangzhou (lasting about 3 months) using a humidified nephelometer system and aerosol chemical speciation monitor. A modified multilinear regression model was proposed to retrieve dry-state mass scattering efficiencies (MSEs, defined as scattering coefficient per unit aerosol mass) of aerosol components. The more oxidized oxygenated organic aerosol (MOOA) with an O/C ratio of 1.17 was identified as the most efficient light scattering aerosol component. On average, 34 % mass contribution of MOOA to total submicron organic aerosol mass contributed 51 % of dry-state organic aerosol scattering. The overall organic aerosol hygroscopicity parameter κOA was quantified directly through hygroscopicity closure, and hygroscopicity parameters of SOA components were further retrieved using a multilinear regression model by assuming hydrophobic properties of primary organic aerosols. The highest water uptake ability of MOOA among organic aerosol factors was revealed with κMOOA reaching 0.23, thus further enhancing the fractional contribution of MOOA in ambient organic aerosol scattering. In particular, the scattering abilities of MOOA were found to be even higher than those of ammonium nitrate under RH of <70% , which was identified as the most efficient inorganic scattering aerosol component, demonstrating that MOOA had the strongest scattering abilities in ambient air (average RH of 57 %) during winter in Guangzhou. During the observation period, secondary aerosols contributed dominantly to visibility degradation (∼70%), with substantial contributions from MOOA (16 % on average), demonstrating significant impacts of MOOA on visibility degradation. The findings of this study demonstrate that more attention needs to be paid to SOA property changes in future visibility improvement investigations. Also, more comprehensive studies on MOOA physical properties and chemical formation are needed to better parameterize its radiative effects in models and implement targeted control strategies on MOOA precursors for visibility improvement. [ABSTRACT FROM AUTHOR]

Many studies have recently been done on understanding the sources and formation mechanisms of atmospheric aerosols at ground level. However, vertical profiles and sources of size-resolved particulate matter within the urban boundary layer are still lacking. In this study, vertical distribution characteristics of size-segregated particles were investigated at three observation platforms (ground level, 118 m, and 488 m) on the 610 m high Canton Tower in Guangzhou, China. Size-segregated aerosol samples were simultaneously collected at the three levels in autumn and winter. Major aerosol components, including water-soluble ions, organic carbon, and elemental carbon, were measured. The results showed that daily average fine-particle concentrations generally decreased with height. Concentrations of sulfate and ammonium in fine particles displayed shallow vertical gradients, and nitrate concentrations increased with height in autumn, while the chemical components showed greater variations in winter than in autumn. The size distributions of sulfate and ammonium in both seasons were characterized by a dominant unimodal mode with peaks in the size range of 0.44–1.0 µ m. In autumn, the nitrate size distribution was bimodal, peaking at 0.44–1.0 and 2.5–10 µ m, while in winter it was unimodal, implying that the formation mechanisms for nitrate particles were different in the two seasons. Our results suggest that the majority of the sulfate and nitrate is formed from aqueous-phase reactions, and we attribute coarse-mode nitrate formation at the measurement site to the heterogeneous reactions of gaseous nitric acid on existing sea-derived coarse particles in autumn. Case studies further showed that atmospheric aqueous-phase and heterogeneous reactions could be important mechanisms for sulfate and nitrate formation, which, in combination with adverse weather conditions such as temperature inversion and calm wind, led to haze formation during autumn and winter in the Pearl River Delta (PRD) region. [ABSTRACT FROM AUTHOR]

To grasp the key factors affecting particle mass scattering efficiency (MSE), particle mass and number size distribution, PM 2.5 and PM 10 and their major chemical compositions, and the particle scattering coefficient (bsp) under dry conditions were measured at an urban site in Guangzhou, southern China, during 2015–2016. On an annual average, 10±2 %, 48±7 % and 42±8 % of PM 10 mass were in the condensation, droplet and coarse modes, respectively, with mass mean aerodynamic diameters (MMADs) of 0.78±0.07 in the droplet mode and 4.57±0.42 µm in the coarse mode. The identified chemical species mass concentrations can explain 79±3 %, 82±6 % and 57±6 % of the total particle mass in the condensation, droplet and coarse mode, respectively. Organic matter (OM) and elemental carbon (EC) in the condensation mode, OM, (NH4)2SO4 , NH4NO3 , and crustal element oxides in the droplet mode, and crustal element oxides, OM, and CaSO4 in the coarse mode, were the dominant chemical species in their respective modes. The measured bsp can be reconstructed to the level of 91±10 % using Mie theory with input of the estimated chemically resolved number concentrations of NaCl, NaNO3 , Na2SO4 , NH4NO3 , (NH4)2SO4 , K2SO4 , CaSO4 , Ca(NO3)2 , OM, EC, crustal element oxides and unidentified fraction. MSEs of particle and individual chemical species were underestimated by less than 13 % in any season based on the estimated bsp and chemical species mass concentrations. Seasonal average MSEs varied in the range of 3.5±0.1 to 3.9±0.2 m 2 g -1 for fine particles (aerodynamic diameter smaller than 2.1 µm), which was mainly caused by seasonal variations in the mass fractions and MSEs of the dominant chemical species (OM, NH4NO3 , (NH4)2SO4) in the droplet mode. MSEs of the dominant chemical species were determined by their lognormal size-distribution parameters, including MMADs and standard deviation (σ) in the droplet mode. [ABSTRACT FROM AUTHOR]

This study investigates seasonal variations of mass absorption efficiency of elemental carbon (MAE EC) and possible influencing factors in urban Guangzhou of South China. Mass concentrations of elemental carbon (EC) and organic carbon (OC) in PM 2.5 and aerosol absorption coefficient (b ap) at multi-wavelengths were simultaneously measured in four seasons of 2018–2019 at hourly resolution by a semi-continuous carbon analyzer and an aethalometer. Seasonal average mass concentrations of EC were in the range of 1.36–1.70 µgC/m3 with a lower value in summer than in the other seasons, while those of OC were in the range of 4.70–6.49 µgC/m3 with the lowest value in summer and the highest in autumn. Vehicle exhaust from local traffic was identified to be the predominant source of carbonaceous aerosols. The average aerosol absorption Ångström exponents (AAE) were lower than 1.2 in four seasons, indicating EC and b ap were closely related with vehicle exhaust. Seasonal MAE EC at 550 nm was 11.0, 8.5, 10.4 and 11.3 m2/g in spring, summer, autumn, and winter, respectively. High MAE EC was related with the high mass ratio of non-carbonaceous aerosols to EC and high ambient relative humidity. [ABSTRACT FROM AUTHOR]

To evaluate the impact of fine particulate matter (PM 2.5 ) size distribution on aerosol chemical and optical properties, dominant chemical components including water-soluble inorganic ions (WSII), organic carbon (OC) and elemental carbon (EC) in PM 1 and PM 2.5 , aerosol scattering coefficient (b sp ), and aerosol absorption coefficient (b ap ) were collected synchronously at an urban site in Guangzhou, south China during a typical summer month in 2009 and a winter month in 2010. PM 1 (sizes smaller than 1 μm) constituted 77% and 63% of PM 2.5 in summer and winter, respectively. From the reconstructed mass concentrations, the sum of SO 4 2 − , NO 3 − and NH 4 + (SNA) distributed more in PM 1 than in PM 1–2.5 (PM 2.5 minus PM 1 ) in summer and the opposite was found in winter, while carbonaceous aerosols distributed more in PM 1 in both summer and winter. With the aggravation of PM 2.5 pollution, the mass fraction of PM 1 /PM 2.5 increased for (NH 4 ) 2 SO 4 (AS), NH 4 NO 3 (AN) and EC but decreased for organic matter (OM) in summer, and the opposite was found in winter. B sp of PM 1 and PM 1–2.5 was estimated from the mass extinction efficiencies (MSEs) of the dominant chemical components, which showed good correlations ( R 2 = 0.99) with measured ones and those estimated using the IMPROVE formula. The fractional contributions of dominant chemical components to extinction coefficient (b ext ) were consistent with their respective mass size distributions, indicating the importance of chemically-resolved aerosol size distributions on aerosol optical properties and haze formation. [ABSTRACT FROM AUTHOR]

To identify potential formation mechanisms of water-soluble organic carbon (WSOC) and quantify their contributions to WSOC in urban Guangzhou of south China, a comprehensive campaign was carried out in winter of 2019–2020. During the campaign, WSOC, total carbon (TC), black carbon (BC), water-soluble inorganic ions (WSIIs) and fourteen elements in PM 2.5 were collected using inline instruments. Bulk PM 2.5 and size-segregated particle samples were also synchronously collected using offline instruments for analyzing the dominant chemical components including WSOC, organic carbon (OC), elemental carbon (EC) and WSIIs. In addition, gaseous pollutants (e.g., NH 3 , SO 2 , HNO 3 , NO 2 , O 3) and meteorological parameters were also measured during the same period. PM 2.5 pollution episodes during the campaign period were mainly driven by increased nitrate concentrations. The mass concentration of WSOC increased from 3.9 ± 1.1 μg m−3 on non-episode days to 6.8 ± 0.6 μg m−3 on episode days, although the mass ratio of WSOC to OC in PM 2.5 changed little (<4%). Photochemical processes dominated WSOC formation in the afternoon and aqueous phase chemical processes played the dominant role in the night, from which newly formed WSOC distributed in the condensation mode and the droplet mode, respectively. Source apportionment analysis using positive matrix factorization (PMF) model suggested that on average 35% and 65% of WSOC mass in PM 2.5 were related with the photochemical processes and aqueous phase chemical processes, respectively. Aqueous phase chemical processes were highly affected by nitrate pollution, which was closely related with O 3 pollution. [Display omitted] • Aqueous process of nitrate was the main formation mechanism of WSOC. • Nitrate pollution was mainly related to the hydrolysis of N 2 O 5. • The noon peaks of nitrate and WSOC were related with regional transport. • O 3 has an impact on both aqueous and photochemical processes forming WSOC. [ABSTRACT FROM AUTHOR]

Though significant associations between particulate matter (PM) air pollution and cardiovascular diseases have been widely reported, it remains unclear what characteristics, such as particle size and chemical constituents, may be responsible for the effects. A time-series model was applied to examine the cardiovascular effects of particle size (for the period of 2009–2011) and chemical constituents (2007–2010) in Guangzhou, we controlled for potential confounders in the model, such as time trends, day of the week, public holidays, meteorological factors and influenza epidemic. We found significant associations of cardiovascular mortality with PM 10 , PM 2.5 and PM 1 ; the excess risk (ER) was 6.10% (95% CI: 1.76%, 10.64%), 6.11% (95% CI: 1.76%, 10.64%) and 6.48% (95% CI: 2.10%, 11.06%) for per IQR increase in PM 10 , PM 2.5 and PM 1 at moving averages for the current day and the previous 3 days (lag 03 ), respectively. We did not find significant effects of PM 2.5-10 and PM 1-2.5 . For PM 2.5 constituents, we found that organic carbon, elemental carbon, sulfate, nitrate and ammonium were significantly associated with cardiovascular mortality, the corresponding ER for an IQR concentration increase at lag 03 was 1.13% (95% CI: 0.10%, 2.17%), 2.77% (95% CI: 0.72%, 4.86%), 2.21% (95% CI: 1.05%, 3.38%), 1.98% (95% CI: 0.54%, 3.44%), and 3.38% (95% CI: 1.56%, 5.23%), respectively. These results were robust to adjustment of other air pollutants and they remained consistent in various sensitivity analyses by changing model parameters. Our study suggests that PM 1 and constituents from combustion and secondary aerosols might be important characteristics of PM pollution associated with cardiovascular mortality in Guangzhou. [ABSTRACT FROM AUTHOR]

To evaluate the effectiveness of the integrated control measures for reducing PM 2.5 (aerosol particles with an aerodynamic diameter of less than 2.5 μm) and hazy weather, day- and night-time PM 2.5 samples were collected at an urban site in Guangzhou during the 16th Asian Games period in November 2010. PM 2.5 samples were subject to chemical analysis for major water-soluble ions, organic carbon (OC), element carbon (EC), and biomass burning tracers–anhydrosugar levoglucosan (LG). In addition, aerosol scattering coefficient (b sp ) at dry condition and aerosol absorption coefficient (b ap ) and visibility at ambient condition were measured. The seven major control measures were effective for reducing PM 2.5 mass concentration and improving visibility during the Asian Games period. All monitored air pollutants except PM 2.5 satisfied the National Ambient Air Quality Standards (NAAQS). However, daily PM 2.5 concentrations still exceeded the NAAQS on 47% of the days and hazy weather also occurred on 80% of the days during this period. One factor causing the high frequency of hazy weather occurrence was the increased relative humidity during the Asian Games period. To avoid hazy weather occurrence, new PM 2.5 standard was recommended based on visibility calculations using three available aerosol hygroscopic curves previously obtained for this city. The recommended PM 2.5 standard was 63 μg m − 3 under dry condition and lower than 42 μg m − 3 under humid condition (RH ≥ 70%). These recommended values were much stricter than the NAAQS value of 75 μg m − 3 . To reach the new standard, more rigorous control measures for coal industries should be established in the Pearl River Delta (PRD) region. [ABSTRACT FROM AUTHOR]

Daily PM2.5 samples were collected in Guangzhou — the largest megacity in South China, for a period of one month in each season during 2009–2010. Mass concentrations of water-soluble inorganic ions, organic carbon (OC) and elemental carbon (EC) in PM2.5 were determined, and aerosol scattering coefficient (bsp) was synchronously measured. The daily PM2.5 mass concentrations ranged from 21.0 to 213.6μgm−3 with an annual average of 76.8±41.5μgm−3. The highest seasonal average PM2.5 was observed in winter (103.3±50.1μgm−3) and the lowest in summer (38.6±15.7μgm−3). Annual average PM2.5 mass scattering efficiency (MSE) was 3.5±0.9m2 g−1, with obvious seasonal variations in sequence of autumn (4.5±0.2m2 g−1)>winter (3.9±0.5m2 g−1)>spring (3.0±0.4m2 g−1)>summer (2.3±0.3m2 g−1). To determine the relationship between bsp and the chemical components of PM2.5, bsp was reconstructed in each season using the original IMPROVE formula with a modification of including sea salt aerosols. The estimated bsp using this method was 22±28% smaller on annual average compared to the measurements. Multiple linear regression of measured bsp against (NH4)2SO4, NH4NO3, OM (Organic Mass), SS (Sea Salt), FS (Fine Soil), and CM (Coarse Mass) were also performed in all the four seasons. The estimated bsp from using the regression equation was 4±12% larger than the measured values. On average, (NH4)2SO4, NH4NO3, OM, SS, FS and CM accounted for 50±11%, 18±10%, 19±5%, 5±4%, 3±2% and 5±6%, respectively, of the estimated bsp. [ABSTRACT FROM AUTHOR]

Abstract: Daily samples of ambient sub-micrometer particles (PM1, particles with an aerodynamic diameter≤1.0μm) were collected from July 2009 to April 2010 at an urban site over Guangzhou in southern China. Mass concentrations of water-soluble inorganic ions, organic carbon (OC) and elemental carbon (EC) were determined to characterize the chemical composition of PM1. The mass concentration of PM1 ranged from 14.6μgm−3 to 143.3μgm−3, with an annual mean value of 52.4±27.3μgm−3. Seasonally-averaged PM1 concentrations decreased in the order winter>autumn>spring>summer. The annual mean concentrations of OC and EC were 6.2±3.5 and 5.0±2.9μgm−3, respectively. The OC and EC concentrations were measured following the IMPROVE_A thermal/optical reflectance (TOR) protocol. Total carbonaceous aerosol (the sum of organic matter and elemental carbon) accounted for 23.0±4.4% of PM1 mass. Clear seasonal variations in OC and EC suggested sources of these two constituents were remarkable difference among the four seasons. Seasonally averaged OC/EC ratios were 1.2, 1.7, 1.4, and 1.5, from spring to winter respectively. Low OC/EC ratios in comparison with other cities in China revealed that vehicle emissions play an important role in carbonaceous aerosol levels in Guangzhou. SO4 2−, NO3 − and NH4 + were the three major inorganic ions in PM1, collectively contributing 30.0%±6.3% of the PM1 mass. SO4 2− and NH4 + were both the highest in autumn and the lowest in summer. In contrast, NO3 − was the highest in winter. Sulfur oxidation ratio was positively correlated with solar radiation and O3, but negatively correlated with SO2. Nitrogen oxidation ratio was positively correlated with NO2, NH4 + and Cl−, but showed a negative correlation with temperature. By applying the IMPROVE equation, PM1 mass was reconstructed and showed that (NH4)2SO4, NH4NO3, OM and EC accounted for (30.7±11.4) %, (9.7±5.2) %, (22.6±5.0) % and (9.7±2.3) % of PM1, respectively. Finally, source apportionment by positive matrix factorization revealed that (1) secondary aerosol and biomass burning, (2) diesel emissions, (3) gasoline emissions and sea salt, and (4) coal combustion were the greatest contributors to PM1. [Copyright &y& Elsevier]

The objective of this study was to reconstruct light extinction coefficients ( b) according to chemical composition components of particulate matter up to 2.5 µm in size (PM). PM samples were collected at the monitoring station of the South China of Institute of Environmental Science (SCIES, Guangzhou, China) during January 2010, and the online absorbing and scattering coefficients were obtained using an aethalometer and a nephelometer. The measured values of light absorption coefficient by particle ( b) and light scattering coefficient by particle ( b) significantly correlated ( R > 0.95) with values of b and b that were reconstructed using the Interagency Monitoring of Protected Visual Environments (IMPROVE) formula when RH was <70%. The measured bext had a good correlation ( R > 0.83) with the calculated bext under ambient RH conditions. The result of source apportionment of b showed that ammonium sulfate [(NH)SO] was the largest contributor (35.0%) to b, followed by ammonium nitrate (NHNO, 22.9%), organic matter (16.1%), elemental carbon (11.8%), sea salt (4.7%), and nitrogen dioxide (NO, 9.6%). To improve visibility in Guangzhou, the effective control of secondary particles like sulfates, nitrates, and ammonia should be given more attention in urban environmental management. [ABSTRACT FROM AUTHOR]

To explore the controlling factors for mass absorption efficiency (MAE) of elemental carbon (EC) in fine particles (PM 2.5), major chemical compositions in size-segregated aerosol samples and bulk PM 2.5 samples and light absorption coefficient (b ap) of PM 2.5 samples under dry condition were measured during four seasons in an urban environment in Guangzhou of south China. On seasonal average, absorption Ångström exponent (AAE) in the wavelength range of 370–880 nm measured by a transmissometer or in the range of 370–950 nm measured by an aethalometer ranged from 0.95 to 1.02 and from 1.01 to 1.14, respectively. The estimated EC MAE at 550 nm in PM 2.5 were 10.1 ± 1.0, 8.9 ± 0.7, 9.1 ± 1.0 and 9.1 ± 0.7 m2 g−1 in spring, summer, autumn and winter, respectively, which were evidently higher than the value of 7.5 m2 g−1 for "pure EC". More than 61% of EC mass in PM 2.1 was distributed in the droplet mode (0.43–2.1 μm), which should be related to hygroscopic growth of aged EC particle as well as external mixing with coating materials. No significant correlations were found between EC MAE and mass fraction of EC or mass ratio of (SO 4 2-+NO 3 −+OC)/EC in the droplet mode. The deliquescence of aerosol was primarily determined by NH 4 NO 3 , and the high EC MAE was mainly related to high nitrate mass concentration and positive ΔRH (ambient relative humidity minus deliquescence relative humidity of NH 4 NO 3). [Display omitted] • Elemental carbon was mainly originated from vehicle emissions in urban Guangzhou. • EC was mainly distributed in the droplet mode largely due to external mixing. • The estimated EC MAE at 550 nm was evidently higher than that for pure EC. • Enhanced EC MAE was mainly caused by aerosol deliquescence by nitrate. [ABSTRACT FROM AUTHOR]

To assess the uncertainties in particle scattering coefficient (b sp) estimated from using the original and revised IMPROVE formulas, particle mass size distribution, bulk PM 2.5 and PM 10 and their major chemical compositions, and b sp under dry condition were synchronously measured in urban Guangzhou during 2015–2016. The estimated b sp , mass concentrations (C m), and mass scattering efficiencies (MSEs) of the dominant chemical species in the fine, condensation and droplet modes were compared between those calculated using the original and revised IMPROVE formulas and the Mie model. On annual average, the reconstructed b sp using the original IMPROVE formula, the revised IMPROVE formula, and the Mie model explained 81%, 91%, and 98% of the measured b sp at 550 nm, respectively. The better performance of the Mie model than the original and revised IMPROVE formula in the reconstructed b sp was mainly due to the inclusion of the unidentified chemical species which explained 14% of the measured b sp at 550 nm. The estimated percentage contributions of the dominant chemical species to the measured b sp differed by < 3% in most cases, except for (NH 4) 2 SO 4 between the Mie model and the revised IMPROVE formula (6%). By including the unidentified chemical species in the revised IMPROVE formula can best reconstructed b sp to within 5% of the measured value. Image 1 • b sp was reconstructed using three models with similar input data. • Model-estimated b sp were biased low by 2–19% on average than measurements. • Mie model performed better than the original and revised IMPROVE formulas. [ABSTRACT FROM AUTHOR]

Concentrations and compositions of PAHs and oxygenated PAHs (OPAHs) in four size ranges of ambient particles (< 1.1, 1.1–3.3, 3.3–9.0 and > 9.0 μm) collected in Xi'an and Guangzhou, two megacities of China, during the winter and summer of 2013 were measured and compared with those in 2003. The TSP-equivalent concentrations of Σ14PAHs in Xi'an and Guangzhou are 57 ± 20 and 18 ± 23 ng m − 3 in winter, 5–10 times higher than those in summer. PAHs in both cities are dominated by 5- and 6-ring congeners in summer. In contrast, they are dominated by 4- and 5-ring congeners in winter, probably due to enhanced gas-to-particle phase partitioning of the semi-volatile PAHs. TSP-equivalent Σ7OPAHs during winter are 54 ± 15 and 23 ± 32 ng m − 3 in Xi'an and Guangzhou and dominated by 5-ring OPAHs. Size distribution results showed that the fine modes (< 3.3 μm) of PAHs and OPAHs in both cities are dominated by 4- and 5-ring congeners in winter and 5- and 6-ring congeners in summer. Relative abundances of 3-ring PAHs and OPAHs increased along with an increase in particle sizes, accounting for from about 1% of the total PAHs or OPAHs in the smallest particles (< 1.1 μm) to > 90% of the total in the largest particles (> 9.0 μm). The toxicity of PAH assessment indicated that atmospheric particles in Xi'an and Guangzhou during winter are much more toxic than those during summer and fine particles are more toxic than coarse particles. Compared to those in 2003, fine particulate PAHs and OPAHs in both cities during winter decreased by 50–90%, most likely due to the replacement of coal by natural gas in the country. [ABSTRACT FROM AUTHOR]

Ambient fine particulate matter (PM 2.5) levels in South China have been decreasing in the past decade, but the decreasing rates differed between its major chemical components, e.g., with much small rates for carbonaceous aerosols than for secondary inorganic aerosols. To investigate the sources of carbonaceous aerosols in this region, a comprehensive campaign was carried out in urban Guangzhou in the winter of 2019–2020 using a combination of various instruments. Data generated from this campaign include hourly total carbon (TC), black carbon (BC), criteria air pollutants and meteorological parameters, 4-hourly particle-bound elements, and chemically-resolved daily PM 2.5. Similar diurnal patterns were observed for TC, CO and NO 2 , suggesting TC was very likely related to vehicle exhaust emission. Secondary organic carbon (SOC) estimated using the Minimum R squared (MRS) method accounted for 35 ± 17% of OC, indicating strong atmospheric oxidation capacity. Four major source factors for carbonaceous aerosols were identified by positive matrix factorization (PMF) model, including coal combustion, traffic emissions, soil dust and ship emissions, which accounted for 37 ± 23%, 39 ± 23%, 14 ± 10% and 10 ± 13%, respectively, of TC mass concentration, 38 ± 24%, 38 ± 23%, 14 ± 10% and 10 ± 12%, respectively, of OC mass concentration, and 29 ± 21%, 43 ± 22%, 14 ± 11% and 14 ± 15%, respectively, of EC mass concentration. Among these sources, traffic emission was the most important one, suggesting the necessity for promoting clean energy vehicles and relieving urban traffic congestion. • Total carbon accounted for nearly 30% of PM 2.5 during the campaign. • High SOC/OC ratio was related with high ozone concentration. • Traffic emission is the most important source of total carbon. [ABSTRACT FROM AUTHOR]

Atmospheric pollutions have an important impact on aerosol, condensation nuclei (CN) and cloud condensation nuclei (CCN) loadings near the ground through disturbing particle size, number, chemical composition and reactions, mixing state, hygroscopicity, and so on. Aerosols and CCN were measured in urban Guangzhou during pollution and post-rain periods to examine effects of particulate pollutants on aerosol CCN activity and compare their mechanisms between summer and winter. In contrast with different levels of pollutions, particle matter (PM 2.5 ) and number (CN) and CCN almost showed an opposite trend to aerosol activity (CCN/CN). In summer, new particle formation (NPF) events triggered by photochemical reactions (e.g. O 3 ) always occurred in no-pollution daytime, and increased significantly CN and CCN as a dominant contributor to secondary aerosols. Under pollution conditions, the gas-to-particle transition driven by photochemical reactions guided the formation and aging processes of particles in daytime, especially in changing soluble species, whereas atmospheric oxidation and heterogeneous reactions dominated at night. In winter, stagnant weather conditions, high pollutant levels and relatively high RH were in favor of particle growing and aging through enhancing secondary particle formation and heterogeneous reactions. The wet scavenging of precipitation reduced greatly CCN amount by scouring pre-existing particles in winter, and during post-rain period the photochemical reactions did not promote the burst of secondary particle formation in the absence of ozone, compared with summer. The results may provide insights into the relationship between aerosol moisture absorption and pollution that may be useful for improving air quality. [ABSTRACT FROM AUTHOR]