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The chlorine atom (Cl·) has a high potential to perturb atmospheric photochemistry by oxidizing volatile organic compounds (VOCs), but the exact role it plays in the polluted troposphere remains unclear. The Master Chemical Mechanism (MCM) is a near-explicit mechanism that has been widely applied in the atmospheric chemistry research. While it addresses comprehensively the chemistry initiated by the OH, O3 and NO3 radicals, its representation of the Cl· chemistry is incomplete as it only considers the reactions for alkanes. In this paper, we develop a more comprehensive Cl· chemistry module that can be directly incorporated within the MCM framework. A suite of 205 chemical reactions describes the Cl·-initiated degradation of alkenes, aromatics, alkynes, aldehydes, ketones, alcohols, and some organic acids and nitrates, along with the inorganic chemistry involving Cl· and its precursors. To demonstrate the potential influence of the new chemistry module, it was incorporated into a MCM box model to evaluate the impacts of nitryl chloride (ClNO2), a product of nocturnal halogen activation by nitrogen oxides (NOX), on the following day's atmospheric photochemistry. With constraints of recent observations collected at a coastal site in Hong Kong, southern China, the modeling analyses suggest that the Cl· produced from ClNO2 photolysis may substantially enhance the atmospheric oxidative capacity, VOC oxidation and O3 formation, particularly in the early morning period. The results demonstrate the critical need for photochemical models to include more detailed chlorine chemistry in order to better understand the atmospheric photochemistry in polluted environments subject to intense emissions of NOX, VOCs and chlorine-containing constituents.

We analyzed the measurements of ozone (O3) and its precursors made at rural/suburban sites downwind of four large Chinese cities – Beijing, Shanghai, Guangzhou and Lanzhou, to elucidate their pollution characteristics, regional transport, in situ production, and impacts of heterogeneous processes. The same measurement techniques and observation-based model were used to minimize uncertainties in comparison of the results due to difference in methodologies. All four cities suffered from serious O3 pollution but showed different precursor distributions. The model-calculated in situ O3 production rates were compared with the observed change rates to infer the relative contributions of on-site photochemistry and transport. At the rural site downwind of Beijing, export of the well-processed urban plumes contributed to the extremely high O3 levels (up to an hourly value of 286 ppbv), while the O3 pollution observed at suburban sites of Shanghai, Guangzhou and Lanzhou was dominated by intense in situ production. The O3 production was in a volatile organic compound (VOC)-limited regime in both Shanghai and Guangzhou, and a NOx-limited regime in Lanzhou. The key VOC precursors are aromatics and alkenes in Shanghai, and aromatics in Guangzhou. The potential impacts on O3 production of several heterogeneous processes, namely, hydrolysis of dinitrogen pentoxide (N2O5), uptake of hydro peroxy radical (HO2) on particles and surface reactions of NO2 forming nitrous acid (HONO), were assessed. The analyses indicate the varying and considerable impacts of these processes in different areas of China depending on the atmospheric abundances of aerosol and NOx, and suggest the urgent need to better understand these processes and represent them in photochemical models.

The chemistry of the natural atmosphere and the influence by long-range transport of air pollution are key issues in the atmospheric sciences. Here we present two intensive field measurements of volatile organic compounds (VOCs) in late spring and summer of 2003 at Mt. Waliguan (WLG, 36.28° N, 100.90° E, 3816 m a.s.l.), a baseline station in the northeast part of the Qinghai-Tibetan Plateau. Most VOC species exhibited higher concentrations in late spring than in summer. A typical diurnal variation was observed with higher nighttime levels, in contrast to results from other mountainous sites. Five different air masses were identified from backward trajectory analysis showing distinct VOC speciation. Air masses originating from the central Eurasian continent contained the lowest VOC levels compared to the others that were impacted by anthropogenic emissions from China and the Indian subcontinent. A photochemical box model based on the Master Chemical Mechanism (version 3.2) and constrained by a full suite of measurements was developed to probe the photochemistry of atmosphere at WLG. Our results show net ozone production from in situ photochemistry during both late spring and summer. Oxidation of nitric oxide (NO) by the hydroperoxyl radical (HO2) dominates the ozone production relative to the oxidation by the organic peroxy radicals (RO2), and the ozone is primarily destroyed by photolysis and reactions with the HOx (HOx = OH + HO2) radicals. Ozone photolysis is the predominant primary source of radicals (ROx = OH + HO2 + RO2), followed by the photolysis of secondary oxygenated VOCs and hydrogen peroxides. The radical losses are governed by the self and cross reactions among the radicals. Overall, the findings of the present study provide insights into the background chemistry and the impacts of pollution transport on the pristine atmosphere over the Eurasian continent.

Heterogeneous processes on dust particles are important for understanding the chemistry and radiative balance of the atmosphere. This paper investigates an intense Asian dust storm episode observed at Mount Heng (1269 m a.s.l.) in southern China on 24–26 April 2009. A set of aerosol and trace gas data collected during the study was analyzed to investigate their chemical evolution and heterogeneous photochemistry as the dust traveled to southern China. Results show that the mineral dust arriving at Mt. Heng experienced significant modifications during transport, with large enrichments in secondary species (sulfate, nitrate, and ammonium) compared with the dust composition collected at an upwind mountain top site (Mount Hua). A photochemical age "clock" (−Log10(NOx/NOy)) was employed to quantify the atmospheric processing time. The result indicates an obvious increase in the abundance of secondary water-soluble ions in dust particles with the air mass atmospheric processing time. Based on the observations, a 4-stage evolution process is proposed for carbonate-containing Asian dust, starting from fresh dust to particles coated with hydrophilic and acidic materials. Daytime-enhanced nitrite formation on the dust particles was also observed, which indicates the recent laboratory result of the TiO2 photocatalysis of NO2 as a potential source of nitrite and nitrous acid.

This paper presents the first results of the measurements of trace gases and aerosols at three surface sites in and outside Beijing before and during the 2008 Olympics. The official air pollution index near the Olympic Stadium and the data from our nearby site revealed an obvious association between air quality and meteorology and different responses of secondary and primary pollutants to the control measures. Ambient concentrations of vehicle-related nitrogen oxides (NOx) and volatile organic compounds (VOCs) at an urban site dropped by 25% and 20–45% in the first two weeks after full control was put in place, but the levels of ozone, sulfate and nitrate in PM2.5 increased by 16%, 64%, 37%, respectively, compared to the period prior to the full control; wind data and back trajectories indicated the contribution of regional pollution from the North China Plain. Air quality (for both primary and secondary pollutants) improved significantly during the Games, which were also associated with the changes in weather conditions (prolonged rainfall, decreased temperature, and more frequent air masses from clean regions). A comparison of the ozone data at three sites on eight ozone-pollution days, when the air masses were from the southeast-south-southwest sector, showed that regional pollution sources contributed >34–88% to the peak ozone concentrations at the urban site in Beijing. Regional sources also contributed significantly to the CO concentrations in urban Beijing. Ozone production efficiencies at two sites were low (~3 ppbv/ppbv), indicating that ozone formation was being controlled by VOCs. Compared with data collected in 2005 at a downwind site, the concentrations of ozone, sulfur dioxide (SO2), total sulfur (SO2+PM2.5 sulfate), carbon monoxide (CO), reactive aromatics (toluene and xylenes) sharply decreased (by 8–64%) in 2008, but no significant changes were observed for the concentrations of PM2.5, fine sulfate, total odd reactive nitrogen (NOy), and longer lived alkanes and benzene. We suggest that these results indicate the success of the government's efforts in reducing emissions of SO2, CO, and VOCs in Beijing, but increased regional emissions during 2005–2008. More stringent control of regional emissions will be needed for significant reductions of ozone and fine particulate pollution in Beijing.

Despite a large volume of research over a number of years, our understandings of the key precursors that control tropospheric ozone production and the impacts of heterogeneous processes remain incomplete. In this study, we analyze measurements of ozone and its precursors made at rural/suburban sites downwind of four large Chinese cities -Beijing, Shanghai, Guangzhou and Lanzhou. At each site the same measurement techniques were utilized and a photochemical box model based on the Master Chemical Mechanism (v3.2) was applied, to minimize uncertainties in comparison of the results due to differences in methodology. All four cities suffered from severe ozone pollution. At the rural site of Beijing, export of the well-processed urban plumes contributed to the extremely high ozone levels (up to an hourly value of 286 ppbv), while the pollution observed at the suburban sites of Shanghai, Guangzhou and Lanzhou was characterized by intense in-situ ozone production. The major anthropogenic hydrocarbons were alkenes and aromatics in Beijing and Shanghai, aromatics in Guangzhou, and alkenes in Lanzhou. The ozone production was found to be in a VOCs-limited regime in both Shanghai and Guangzhou, and a mixed regime in Lanzhou. In Shanghai, the ozone formation was most sensitive to aromatics and alkenes, while in Guangzhou aromatics were the predominant ozone precursors. In Lanzhou, either controlling NOx or reducing emissions of olefins from the petrochemical industry would mitigate the local ozone production. The potential impacts of several heterogeneous processes on the ozone formation were assessed. The hydrolysis of dinitrogen pentoxide (N2O5), uptake of the hydroperoxyl radical (HO2) on particles, and surface reactions of NO2 forming nitrous acid (HONO) present considerable sources of uncertainty in the current studies of ozone chemistry. Further efforts are urgently required to better understand these processes and refine atmospheric models. [ABSTRACT FROM AUTHOR]