New particle formation (NPF) and its subsequent growth plays a key role in air quality and climate change at regional and global scales. Especially under complex air pollution in China, nucleation and growth can be highly efficient, claimed to be a main source of cloud condensation nuclei (CCN) and an important cause of secondary aerosol pollution. Currently, the mechanism of particle formation and growth as well as its environmental effects are still poorly understood. Thereby, fully understanding of the atmospheric nucleation and subsequent growth still presents a big challenge to atmospheric chemistry researches. This study reviews the current results from studies on mechanisms and environmental effects of atmospheric nucleation and growth. We summarize that traditional nucleation theories such as binary nucleation of H2SO4-H2O, ternary nucleation of H2SO4-NH3-H2O, ion-induced nucleation are not capable in explaining new particle formation under complex air pollution, while newly proposed mechanisms such as organic acids and amine induced nucleation were not verified because of technique limitation. We propose that the future researches should focus on identifying the key chemical precursor response for driving nucleation and initial and subsequent growth, and understand the physical and chemical processing of new particle formation and growth. In particularly, application and development of novel techniques, such as APi-TOF-CIMS, PSM, Nano-HTDMA in new particle formation study is very important. Also, future researches should establish whole process tracking on new particle formation, from precursor, nucleation, growth till the environmental effects, by integrating field observation, chamber simulation, and modelling. Currently, the mechanism of highly efficient nucleation and rapid growth taking place under complex air pollution in China is urgently needed to be in-depth studied in order to improve our understanding of regional haze formation. This could be helpful to understand the similarity and difference in the nucleation mechanism between clean and polluted atmospheric environments.

Severe air pollution and its associated health impacts have become one of the major concerns in China. A detailed analysis of PM2.5 chemical compositions is critical for optimizing pollution control measures. In this study, daily 24-h bulk filter samples were collected and analyzed for totally 21 field campaigns at 17 sites in China between 2008 and 2013. The 17 sites were classified into four groups including six urban sites, seven regional sites, two coastal sites in four fast developing regions of China (i.e. Beijing-Tianjin-Hebei region, Yangtze River Delta, Pearl River Delta and Sichuan Basin), and two ship cruise measurements covered the East China Sea and Yellow Sea of China. The high average concentrations of PM2.5 and the occurrences of extreme cases at most sites imply the widespread air pollution in China. Fine particles were largely composed of organic matter and secondary inorganic species at most sites. High correlation between the temporal trends of PM2.5 and secondary species of urban and regional sites highlights the uniformly distributed air pollutants within one region. Secondary inorganic species were the dominant contributors to the high PM2.5 concentration in Northern China. However in Southern China, the relative contributions of different chemical species kept constant as PM2.5 increased. This study provides us a better understanding of the current state of air pollution in diversified Chinese cities. Analysis of chemical signatures of PM2.5 could be a strong support for model validation and emission control strategy. (C) 2016 Elsevier Ltd. All rights reserved.

Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO2 by NO2 is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH3 neutralization or under cloud conditions. Under polluted environments, this SO2 oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH3 and NO2 control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.

Chemical composition, morphology, size and mixture of fine particles were measured in a heavy haze and the post-haze air in Beijing in January 2012. With the occurrence of haze, the concentrations of gaseous and particulate pollutants including organics, sulfate, nitrate, and ammonium grew gradually. The hourly averaged PM2.5 concentration increased from 118 mu g m(-3) to 402 mu g m(-3) within 12 h. In contrast, it was less than 10 mu g m(-3) in the post-haze air. Occupying approximately 46% in mass, organics were the major component of PM1 in both the haze and post-haze air.Analysis of individual particles in the size range of 0.2-1.1 mu m revealed that secondary-like particles and soot particles were always the majority, and most soot particles had a core-shell structure. The number ratio of secondary-like particles to soot particles in accumulation mode in the haze air was about 2:1, and that in the post-haze air was 8:1. These results indicate both secondary particle formation and primary emission contributed substantially to the haze. The mode size of the haze particles was about 0.7 mu m, and the mode size of the post-haze particles was 0.4 mu m, indicating the remarkable growth of particles in haze. However, the ratios of the core size to shell size of core-shell structure soot particles in the haze were similar to those in the post-haze air, suggesting a quick aging of soot particles in either the haze air or the post-haze air. (C) 2016 Elsevier B.V. All rights reserved.

Understanding the evolution of aerosols in the atmosphere is of great importance for improving air quality and reducing aerosol-related uncertainties in global climate simulations. Here, a unique haze episode at a regional receptor site near the East China Sea was examined as a case study of the aging process of atmospheric aerosols during transport. An increase in photochemical age from 5 h. to more than 25 h and a progressive increase in the fitted mean particle diameter from 70 nm to approximately 300 nm were observed. According to the pollution features and meteorology conditions involved, pollution accumulation (PA), sea breeze (SB), and land breeze (LB) periods were identified. Concentrations of black carbon (BC), hydrocarbon-like organic aerosols (HOA), semi-volatile oxidized organic aerosols (SV-OOA), and nitrate increased by 7-fold up to 39-fold when the air masses passed through Taizhou, a nearby city. In addition, nitrate and SV-OOA dominated the aerosol composition in the urban outflow plumes (52% and 18%, respectively), yet they gradually decreased in concentration during transport. In contrast, sulfate and the low-volatile oxidized organic aerosols (LV-OOA) exhibited more regional footprints and potentially have similar formation mechanisms. The atomic oxygen-to-carbon (O/C) ratio also increased from 0.45 to 0.9, thereby suggesting that rapid formation of highly oxidized secondary organic aerosols (SOA) occurred during transport. Overall, these results provide valuable insight into the evolution of the chemical and physical features of aerosol pollution during transport and also highlight the need for regulatory controls of nitrogen oxides, sulfur dioxide, and VOCs to improve air quality on different scales. (C) 2016 Published by Elsevier Ltd.

Black carbon (BC) exerts profound impacts on air quality and climate because of its high absorption cross-section over a broad range of electromagnetic spectra, but the current results on absorption enhancement of BC particles during atmospheric aging remain conflicting. Here, we quantified the aging and variation in the optical properties of BC particles under ambient conditions in Beijing, China, and Houston, United States, using a novel environmental chamber approach. BC aging exhibits two distinct stages, i.e., initial transformation from a fractal to spherical morphology with little absorption variation and subsequent growth of fully compact particles with a large absorption enhancement. The timescales to achieve complete morphology modification and an absorption amplification factor of 2.4 for BC particles are estimated to be 2.3 h and 4.6 h, respectively, in Beijing, compared with 9 h and 18 h, respectively, in Houston. Our findings indicate that BC under polluted urban environments could play an essential role in pollution development and contribute importantly to large positive radiative forcing. The variation in direct radiative forcing is dependent on the rate and timescale of BC aging, with a clear distinction between urban cities in developed and developing countries, i.e., a higher climatic impact inmore polluted environments. We suggest that mediation in BC emissions achieves a cobenefit in simultaneously controlling air pollution and protecting climate, especially for developing countries.

New particle formation (NPF) and its subsequent growth plays a key role in air quality and climate change at regional and global scales. Especially under complex air pollution in China, nucleation and growth can be highly efficient, claimed to be a main source of cloud condensation nuclei (CCN) and an important cause of secondary aerosol pollution. Currently, the mechanism of particle formation and growth as well as its environmental effects are still poorly understood. Thereby, fully understanding of the atmospheric nucleation and subsequent growth still presents a big challenge to atmospheric chemistry researches. This study reviews the current results from studies on mechanisms and environmental effects of atmospheric nucleation and growth. We summarize that traditional nucleation theories such as binary nucleation of H2SO4-H2O, ternary nucleation of H2SO4-NH3-H2O, ion-induced nucleation are not capable in explaining new particle formation under complex air pollution, while newly proposed mechanisms such as organic acids and amine induced nucleation were not verified because of technique limitation. We propose that the future researches should focus on identifying the key chemical precursor response for driving nucleation and initial and subsequent growth, and understand the physical and chemical processing of new particle formation and growth. In particularly, application and development of novel techniques, such as APi-TOF-CIMS, PSM, Nano-HTDMA in new particle formation study is very important. Also, future researches should establish whole process tracking on new particle formation, from precursor, nucleation, growth till the environmental effects, by integrating field observation, chamber simulation, and modelling. Currently, the mechanism of highly efficient nucleation and rapid growth taking place under complex air pollution in China is urgently needed to be in-depth studied in order to improve our understanding of regional haze formation. This could be helpful to understand the similarity and difference in the nucleation mechanism between clean and polluted atmospheric environments.

Laboratory experiments are conducted to investigate aging of size-classified black carbon (BC) particles from OH-initiated oxidation of m-xylene. The variations in the particle size, mass, effective density, morphology, optical properties, hygroscopicity, and activation as cloud condensation nuclei (CCN) are simultaneously measured by a suite of aerosol instruments, when BC particles are exposed to the oxidation products of the OH-m-xylene reactions. The BC aging is governed by the coating thickness (Delta r(ve)), which is correlated to the reaction time and initial concentrations of m-xylene and NOx. For an initial diameter of 100 nm and Delta r(ve) = 44 nm, the particle size and mass increase by a factor of 1.5 and 10.4, respectively, and the effective density increases from 0.43 to 1.45 g cm(-3) due to organic coating and collapsing of the BC core. The BC particles are fully converted from a highly fractal to nearly spherical morphology for Delta r(ve) = 30 nm. The scattering, absorption, and single scattering albedo of BC particles are enhanced accordingly with organic coating. The critical supersaturation for CCN activation is reduced to 0.1% with Delta r(ve) = 44 nm. The results imply that the oxidation of m-xylene exhibits larger impacts in modifying the BC particle properties than those for the OH-initiated oxidation of isoprene and toluene.