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Abstract: The updated SAPRC-07 mechanism was evaluated against data from experiments performed in the CSIRO smog chamber. The mechanism predictions have been compared to experimental results as well as predictions by SAPRC-99. Experiments were performed using either toluene or m-xylene in the presence of NOx at sub-0.1 ppmv concentrations. For the majority of m-xylene experiments, the modelled Δ(O3–NO) concentration was within 20% of observed values for both SAPRC mechanisms. However during the oxidation of toluene the production of radicals was poorly predicted, with final Δ(O3–NO) concentration under-predicted by up to 60%. The predictions of major oxidants from isoprene oxidation were in good agreement with observed values. For the NOx-limited conditions however, the ozone concentration predicted by both mechanisms were under-predicted by approximately 20% in the five experiments tested. The performance of the SAPRC-07 mechanism was also evaluated against twelve evaporated fuel experiments. Two types of evaporative mode experiments were performed: headspace evaporated fuel and wholly evaporated fuel. The major difference was a significantly higher concentration of aromatic hydrocarbons and larger alkane products in wholly evaporated fuels. For headspace evaporated fuel experiments both SAPRC mechanisms were in good agreement with experimental results. For wholly evaporated experiments the average Δ(O3–NO) model error was −25% with SAPRC-07 compared to less than −5% for SAPRC-99. Updates to the photolysis data for dicarbonyls, the light source used and the experimental conditions under which these experiments were performed are possible causes for the discrepancy between SAPRC-99 and -07 predictions for wholly evaporated experiments. [Copyright &y& Elsevier]

A new state-of-the-art indoor smog chamber facility (CAPS-ZJU) has been constructed and characterized at Zhejiang University, which is designed for chemical mechanism evaluation under well-controlled conditions. A series of characterization experiments were performed to validate the well-established experimental protocols, including temperature variation pattern, light spectrum and equivalent intensity (J NO2), injection and mixing performance, as well as gases and particle wall loss. In addition, based on some characterization experiments, the auxiliary wall mechanism has been setup and examined. Fifty chamber experiments were performed across a broad range of experimental scenarios, and we demonstrated the ability to utilize these chamber data for evaluating SAPRC chemical mechanism. It was found that the SAPRC-11 can well predict the O 3 formation and NO oxidation for almost all propene runs, with 6 hr Δ(O 3 – NO) model error of –3% ± 7%, while the final O 3 was underestimated by ~20% for isoprene experiments. As for toluene and p -xylene experiments, it was confirmed that SAPRC-11 has significant improvement on aromatic chemistry than earlier version of SAPRC-07, although the aromatic decay rate was still underestimated to some extent. The model sensitivity test has been carried out, and the most sensitive parameters identified are the initial concentrations of reactants and the light intensity as well as HONO offgasing rate and O 3 wall loss rate. All of which demonstrated that CAPS-ZJU smog chamber could derive high quality experimental data, and could provide insights on chamber studies and chemical mechanism development. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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]

The formation and growth of new particles has recently been shown to have a significant influence on Chinese haze pollution, and sulfuric acid has long been recognized as a major contributor to new particle formation. In this study, four comparison groups of experiments related to SO 2 photooxidation, as well as aerosol nucleation and growth, have been conducted in the CAPS-ZJU (Complex Air Pollution Study-Zhejiang University) smog chamber. These were conducted either under SO 2 /NOx or SO 2 /toluene gas-phase environments in the absence of seed particles. During aerosol nucleation and growth process, several physical properties such as mass, size and effective density were measured simultaneously by Scanning Mobility Particle Sizer (SMPS) and Differential Mobility Analyzer-Aerosol Particle Mass Analyzer-Condensation Particle Counter (DMA-APM-CPC). The effective density of new particles decreased from 1.8 to 1.35 g/cm 3 as the particle size increased from 20 to 65 nm. The single particle mass showed good power-law relationship with mobility diameter, with an average mass-mobility exponent of 2.885. A new algorithm and a reference density of 1.38 g/cm 3 based on size-resolved single particle mass (SPM) were proposed to calculate the mass concentration of new particles. Two methods based on Log Normal and Max Concentration were applied to derive particle growth rate (GR), and data merging from both methods was implemented to decrease calculation uncertainty. Meanwhile, both continuous nucleation and inhibition of further growth in sub-20 nm size range were observed in different experiments depending on composition, and possible reasons were analyzed. The presence of NO was found to suppress nucleation and subsequent aerosol growth; while the presence of NO 2 or toluene promoted it. It was concluded that decreasing NOx (NO or NO 2 ) or increasing toluene may promote SO 2 photooxidation, nucleation and subsequent aerosol growth, all of which is significant for deeper understanding of complex air pollution in China. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) is an important base gas in the atmosphere and has received extensive research interest due to its role on secondary aerosol formation. However, the effects of NH 3 on the photochemical smog oxidants formation including ozone and secondary aerosol did not attract much attention. In this study, a set of smog chamber experiments was conducted under toluene/NOx photo-oxidation base condition and adding NH 3 in the following photochemical regimes referred as hydrocarbon, NOx-limited and transitional regimes. Dedicated instruments including gas analysers, scanning mobility particle sizer (SMPS), aerosol mass spectrometry (HR-ToF-AMS) were used to measure the gaseous compounds, aerosol mass concentration and chemical composition. It was found that the presence of NH 3 did not affect the O 3 formation rate. However, the particle number concentration increased dramatically after NH 3 was injected in any photochemical regimes. This effect has been mainly attributed to the rapid formation of organic ammonium and ammonium nitrate. The mass spectra of organic aerosols from AMS showed that the most abundant fragments were at m/z 28, 29, 43 and 44, which became higher after the addition of NH 3 in the transition and NOx-limited regimes, indicating that NH 3 could enhance the formation of the compounds containing carbonyl and carboxylic acid functional groups. Compared to hydrocarbon-limited regime, higher atomic nitrogen-to-carbon (N:C) ratios were observed when NH 3 were injected into transition and NOx-limited regimes, and this is mainly due to more organic nitrate formation. Four factors, which were assigned to inorganic nitrate aerosols, carboxylic acid compounds, carbonyl compounds and N-containing organic compounds (NOC), were identified and interpreted by applying the positive matrix factorization (PMF) to the AMS dataset. These results provide new insights into the complex chemistry of toluene/NOx/NH 3 during different O 3 formation regimes, and imply the benefit of controlling NH 3 to mitigate PM 2.5 when O 3 formation is under VOC-limited regime. [Display omitted] • How NH 3 affects particle formation in different photochemical regimes was investigated. • NH 3 does not affect O 3 formation but affect particle formation and composition. • Higher O:C and N:C were observed after NH 3 injection in NOx-limited regime. • Four factors contributing to the secondary aerosols were identified by PMF analysis. [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]