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 NO_x-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 NO_x, and suggest the urgent need to better understand these processes and represent them in photochemical models.

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 (NO_x) 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 PM_2.5 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+PM_2.5 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 PM_2.5, fine sulfate, total odd reactive nitrogen (NO_y), 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 growing interest in the visible light‐absorbing organic component of atmospheric aerosols, referred to as “brown” carbon, our knowledge of its chemical composition remains limited. It is well accepted that biomass burning is one important source of “brown” carbon in the atmosphere. In this study, cloud water samples heavily affected by biomass burning were collected at Mount Tai (1534 m, ASL), located in Shandong province in the North China Plain in summer 2008. The samples were analyzed with high performance liquid chromatography equipped with a UV/Vis absorbance detector immediately followed by electrospray ionization and analysis using a time‐of‐flight (ToF) mass spectrometer. The high mass resolution and accuracy provided by the ToF mass spectrometer allow determination of the elemental composition of detected ions. Using this approach, the elemental compositions of 16 major light‐absorbing compounds, which together accounted for approximately half of measured sample absorption between 300 and 400 nm, were determined. The most important classes of light‐absorbing compounds were found to be nitrophenols and aromatic carbonyls. Light absorption over this wavelength range by reduced nitrogen compounds was insignificant in these samples.

We analyze a photochemical smog episode to understand the oxidative capacity and radical chemistry of the polluted atmosphere in Hong Kong and the Pearl River Delta (PRD) region. A photochemical box model based on the Master Chemical Mechanism (MCM v3.2) is constrained by an intensive set of field observations to elucidate the budgets of RO_x (RO_x = OH+HO₂+RO₂) and NO₃ radicals. Highly abundant radical precursors (i.e. O₃, HONO and carbonyls), nitrogen oxides (NO_x) and volatile organic compounds (VOCs) facilitate strong production and efficient recycling of RO_x radicals. The OH reactivity is dominated by oxygenated VOCs (OVOCs), followed by aromatics, alkenes and alkanes. Photolysis of OVOCs (except for formaldehyde) is the dominant primary source of RO_x with average daytime contributions of 34–47 %. HONO photolysis is the largest contributor to OH and the second-most significant source (19–22 %) of RO_x. Other considerable RO_x sources include O₃ photolysis (11–20 %), formaldehyde photolysis (10–16 %), and ozonolysis reactions of unsaturated VOCs (3.9–6.2 %). In one case when solar irradiation was attenuated, possibly by the high aerosol loadings, NO₃ became an important oxidant and the NO₃-initiated VOC oxidation presented another significant RO_x source (6.2 %) even during daytime. This study suggests the possible impacts of daytime NO₃ chemistry in the polluted atmospheres under conditions with the co-existence of abundant O₃, NO₂, VOCs and aerosols, and also provides new insights into the radical chemistry that essentially drives the formation of photochemical smog in the high-NO_x environment of Hong Kong and the PRD region.

Tropospheric ozone (O₃) is a trace gas playing important roles in atmospheric chemistry, air quality and climate change. In contrast to North America and Europe, long-term measurements of surface O₃ are very limited in China. We compile available O₃ observations at Mt. Tai – the highest mountain over the North China Plain – during 2003–2015 and analyze the decadal change of O₃ and its sources. A linear regression analysis shows that summertime O₃ measured at Mt. Tai has increased significantly by 1.7 ppbv yr⁻¹ for June and 2.1 ppbv yr⁻¹ for the July–August average. The observed increase is supported by a global chemistry-climate model hindcast (GFDL-AM3) with O₃ precursor emissions varying from year to year over 1980–2014. Analysis of satellite data indicates that the O₃ increase was mainly due to the increased emissions of O₃ precursors, in particular volatile organic compounds (VOCs). An important finding is that the emissions of nitrogen oxides (NO_x) have diminished since 2011, but the increase of VOCs appears to have enhanced the ozone production efficiency and contributed to the observed O₃ increase in central eastern China. We present evidence that controlling NO_x alone, in the absence of VOC controls, is not sufficient to reduce regional O₃ levels in North China in a short period.

Nitryl chloride (ClNO₂) is a dominant source of chlorine radical in polluted environment, and can significantly affect the atmospheric oxidative chemistry. However, the abundance of ClNO₂ and its exact role are not fully understood under different environmental conditions. During the summer of 2014, we deployed a chemical ionization mass spectrometer to measure ClNO₂ and dinitrogen pentoxide (N₂O₅) at a rural site in the polluted North China Plain. Elevated mixing ratios of ClNO₂ (> 350 pptv) were observed at most of the nights with low levels of N₂O₅ (< 200 pptv). The highest ClNO₂ mixing ratio of 2070 pptv (1 min average) was observed in a plume from a megacity (Tianjin), and was characterized with a faster N₂O₅ heterogeneous loss rate and ClNO₂ production rate compared to average conditions. The abundant ClNO₂ concentration kept increasing even after sunrise, and reached a peak 4 h later. Such highly sustained ClNO₂ peaks after sunrise are discrepant from the previously observed typical diurnal pattern. Meteorological and chemical analysis shows that the sustained ClNO₂ morning peaks are caused by significant ClNO₂ production in the residual layer at night followed by downward mixing after breakup of the nocturnal inversion layer in the morning. We estimated that ∼ 1.7–4.0 ppbv of ClNO₂ would exist in the residual layer in order to maintain the observed morning ClNO₂ peaks at the surface site. Observation-based box model analysis show that photolysis of ClNO₂ produced chlorine radical with a rate up to 1.12 ppbv h⁻¹, accounting for 10–30 % of primary RO_x production in the morning hours. The perturbation in total radical production leads to an increase of integrated daytime net ozone production by 3 % (4.3 ppbv) on average, and with a larger increase of 13 % (11 ppbv) in megacity outflow that was characterized with higher ClNO₂ and a relatively lower oxygenated hydrocarbon (OVOC) to non-methane hydrocarbon (NMHC) ratio.