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.

Volatile organic compounds (VOCs) are key precursors of photochemical smog. Quantitatively evaluating the contributions of VOC sources to ozone (O3) formation could provide valuable information for emissions control and photochemical pollution abatement. This study analyzed continuous measurements of VOCs during the photochemical season in 2014 at a receptor site (Heshan site, HS) in the Pearl River Delta (PRD) region, where photochemical pollution has been a long-standing issue. The averaged mixing ratio of measured VOCs was 34±3 ppbv, with the largest contribution from alkanes (17±2 ppbv, 49 %), followed by aromatics, alkenes and acetylene. The positive matrix factorization (PMF) model was applied to resolve the anthropogenic sources of VOCs, coupled with a photochemical-age-based parameterization that better considers the photochemical processing effects. Four anthropogenic emission sources were identified and quantified, with gasoline vehicular emission as the most significant contributor to the observed VOCs, followed by diesel vehicular emissions, biomass burning and solvent usage. The O3 photochemical formation regime at the HS was identified as VOC-limited by a photochemical box model with the master chemical mechanism (PBM-MCM). The PBM-MCM model results also suggested that vehicular emission was the most important source to the O3 formation, followed by biomass burning and solvent usage. Sensitivity analysis indicated that combined VOC and NOx emission controls would effectively reduce incremental O3 formation when the ratios of VOC-to-NOx emission reductions were > 3.8 for diesel vehicular emission, > 4.6 for solvent usage, > 4.6 for biomass burning and 3.3 for gasoline vehicular emission. Based on the above results, a brief review of the policies regarding the control of vehicular emissions and biomass burning in the PRD region from a regional perspective were also provided in this study. It reveals that different policies have been, and continue to be, implemented and formulated and could help to alleviate the photochemical pollution in the PRD region. Nevertheless, evaluation of the cost-benefit of each policy is still needed to improve air quality.

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.

Nitrous acid (HONO) can strongly affect atmospheric photochemistry in polluted regions through the production of hydroxyl radicals (OHs). In January 2017, a severe pollution episode occurred in the Pearl River Delta (PRD) of China, with maximum hourly PM_2.5, ozone, and HONO levels reaching 400 µg m⁻³, 150 ppb, and 8 ppb, respectively, at a suburban site. The present study investigated the sources and processes generating such high HONO concentrations and the role of HONO chemistry in this severe winter episode. Four recently reported HONO sources were added to the Community Multiscale Air Quality (CMAQ) model, including RH-dependent (relative humidity) and light-enhancing effects on heterogeneous reactions, photolysis of particulate nitrate in the atmosphere, and photolysis of HNO₃ and nitrate on surfaces. The revised model reproduced the observed HONO and significantly improved its performance for O₃ and PM_2.5. The model simulations showed that the heterogeneous generation on surfaces (with RH and light effects) was the largest contributor (72 %) to the predicted HONO concentrations, with the RH-enhancing effects more significant at nighttime and the light-enhancing effects more important in the daytime. The photolysis of total nitrate in the atmosphere and deposited on surfaces was the dominant HONO source during noon and afternoon, contributing above 50 % of the simulated HONO. The HONO photolysis was the dominant contributor to HO_x production in this episode. With all HONO sources, the daytime average O₃ at the Heshan site was increased by 24 ppb (or 70 %), compared to the simulation results without any HONO sources. Moreover, the simulated mean concentrations of TNO₃ (HNO₃+ fine particle NO₃⁻) at the Heshan site, which was the key species for this haze formation, increased by about 17 µg m⁻³ (67 %) due to the HONO chemistry, and the peak enhancement reached 55 µg m⁻³. This study highlights the key role of HONO chemistry in the formation of winter haze in a subtropical environment.