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Ammonia (NH 3 ) is a major contributor to secondary aerosol in the atmosphere and can alter the kinetics of their formation. However, systematic studies related to the role of NH 3 in aerosol nucleation processes and further particle size growth under complex scenarios are lacking. In this study, we conducted 16 experiments in the CSIRO smog chamber under dry conditions using aromatic hydrocarbons (toluene, o -/ m -/ p -xylene) and different concentrations of NH 3 . The presence of NH 3 did not change the gas-phase chemistry or nucleation onset time, but slowed the nucleation rate (5%–94%) once it began. From the response of nitrogen oxides (NOx) measurement and mechanism modeling results, we hypothesised that the surface reaction between NH 3 and nitric acid played a central role in aerosol nucleation and further growth. After nucleation, the subsequently formed ammonium nitrate and organic condensation vapours may partition together into the initial growth process of new particles, thus increasing the aerosol initial growth rate (8%–90%) and size growth potentials (7%–108%), and leading to high aerosol mass formation. Further investigation implied that the initial growth and further growth rate determine the aerosol mass concentration, rather than the nucleation rate. We conclude that both the initial NOx concentration and volatile organic compound (VOC)/NOx ratio are crucial for the initial and further growth, and aerosol mass of new particles, when NH 3 levels are high. Our results provide crucial insights into the complex chemistry of VOC/NOx/NH 3 in the atmosphere, and highlight the importance of NH 3 reduction for particulate matter control. [ABSTRACT FROM AUTHOR]

Because ambient ozone (O 3) has fine spatial scale variability in addition to a large scale regional distribution, accurate exposure predictions for population health studies need to also capture fine spatial scale differences in exposure. To address these needs, we developed a 3-year average land use regression (LUR) and combined LUR and Bayesian maximum entropy (BME) by incorporating a national area variability LUR model for China from 2015 to 2017 along with data that take into account incompleteness of O 3 monitoring data into a BME framework. Spatio-temporal kriging models that either included or did not include "soft" data were used for comparison. The final LUR model included five predictor variables: road length within a 1000 m buffer, temperature, wind speed, industrial land area within a 3000 m buffer and altitude. The 1-year predicted O 3 concentrations based on the ratio method moderately agreed with the measured concentration, and the regression R2 values were 0.53, 0.57 and 0.59 in the year of 2015, 2016 and 2017, respectively. The LUR/BME model performed better (R2 = 0.80, root mean squared error [RMSE] = 23.5 μg/m3) than the ordinary spatio-temporal kriging model that either included "soft" data (R2 = 0.57, RMSE = 49.2 μg/m3) or did not include the "soft" data (R2 = 0.52, RMSE = 58.5 μg/m3). We have demonstrated that a hybrid LUR/BME model can provide accurate predictions of O 3 concentrations with high spatio-temporal resolution at the national scale in mainland China. Unlabelled Image • A 3-year average LUR model was developed to predict the annual O 3 concentration. • A combined LUR/BME model was developed to predict the daily O 3 concentration. • LUR/BME model outperformed ordinary spatio-temporal kriging model. [ABSTRACT FROM AUTHOR]

The secondary aerosol formation mechanism in the presence of ammonia (NH 3), is poorly understood, especially under high relative humidity (RH) conditions. In this study, a total of seven experiments were conducted from toluene/NOx photo-oxidation in the presence/absence of NH 3 under dry (~7% RH) and wet (>60% RH) conditions in a ~3 m3 smog chamber. A series of instruments including gas analysers, scanning mobility particle sizer (SMPS), aerosol mass spectrometry (HR-ToF-AMS) etc. were applied to measure the NOx and O 3 concentrations, the mass concentration and chemical composition of secondary aerosol. It was found that NH 3 could enhance the mass loading of secondary aerosol, especially under wet condition. However, the presence of NH 3 or increasing RH did not have a significant influence on SOA yield. The organic aerosol mass spectrum from AMS showed that the most abundant fragment was at m / z = 44, which was mainly from the fragmentation of carboxylic acids. Compared to the absence of NH 3 , the fraction of fragment at m / z = 44 and O:C was higher in the presence of NH 3 , regardless of dry or wet conditions. The highest O:C value of 0.71–0.75 was observed in the presence of NH 3 under wet condition, suggesting there could be a synergetic effect between the high RH and the presence of NH 3 , which jointly contributed to the photochemical aging process of SOA. The N:C increased in the presence of NH 3 under both dry and wet conditions, which might be attributed to the carboxylates and organic nitrates formed from the reaction between NH 3 and carboxylic acids. The results implied that SOA modelling should consider the role of NH 3 and water vapour, which might fill the gap of O:C between laboratory studies and field measurements. Unlabelled Image • NH 3 significantly enhanced SOA formation from toluene/NOx photo-oxidation under high RH condition. • High RH and NH 3 could jointly accelerate the photochemical aging process of SOA. • Both the addition and replacement of carboxylic functional group contributed to the formation of carboxylic acids. • NH 3 could react with the carboxylic acids formed from toluene, generating carboxylates and other organic nitrates. [ABSTRACT FROM AUTHOR]