Abstract. Guangdong Province (GD), one of the most prosperous and populous regions in China, still experiences haze events and growing ozone pollution in spite of the substantial air-quality improvement in recent years. Integrated control of fine particulate matter (PM2.5) and ozone in GD calls for a systematic review of historical emissions. In this study, emission trends, spatial variations, source-contribution variations, and reduction potentials of sulfur dioxide (SO2), nitrogen oxides (NOx), PM2.5, inhalable particles (PM10), carbon monoxide (CO), ammonia (NH3), and volatile organic compounds (VOCs) in GD from 2006 to 2015 were first examined using a dynamic methodology, taking into account economic development, technology penetration, and emission controls. The relative change rates of anthropogenic emissions in GD during 2006–2015 are −48 % for SO2, −0.5 % for NOx, −16 % for PM2.5, −22 % for PM10, 13 % for CO, 3 % for NH3, and 13 % for VOCs. The declines of SO2, NOx, PM2.5, and PM10 emissions in the whole province mainly resulted from the stringent emission control in the Pearl River delta (PRD) region, where most previous control measures were focused, especially on power plants (SO2 and NOx), industrial combustion (SO2, PM2.5, PM10), on-road mobile sources (NOx), and dust sources (PM2.5 and PM10). Emissions from other areas (non-PRD, NPRD), nevertheless, remain relatively stable due to the lax control measures and rapidly growing energy consumption. In addition, emission leaks of SO2 and NOx from industries are observed from PRD to NPRD in 2010 and 2011. As a result, emissions in NPRD are increasingly important in GD, particularly those from industrial combustion. The contribution of NPRD to the total SO2 emissions in GD, for example, increased from 27 % in 2006 to 48 % in 2015. On-road mobile sources and solvent use are the two key sources that should receive more effective control measures in GD. Current control-driven emission reductions from on-road mobile sources are neutralized by the substantial growth of the vehicle population, while VOC emissions in GD steadily increase due to the growth of solvent use and the absence of effective control measures. Besides, future work could focus on power plants and industrial combustion in GD and industrial process sources in NPRD, which still have large emission reduction potentials. The historical emission inventory developed in this study not only helps to understand the emission evolution in GD, but also provides robust data to quantify the impact of emission and meteorology variations on air quality and unveil the primary cause of significant air-quality change in GD in the recent decade.

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Abstract. China is experiencing increasingly serious ambient ozone pollution, including the economically developed Pearl River Delta (PRD) region. However, the underlying reasons for the ozone increase remain largely unclear, leading to perplexity regarding formulating effective ozone control strategies. In this study, we quantitatively examine the impacts of meteorology and precursor emissions from within and outside of the PRD on the evolution of ozone during the past decade by developing a statistical analysis framework combining meteorological adjustment and source apportionment. We found that meteorological conditions mitigated ozone increase, and that their variation can account for a maximum of 15 % of the annual ozone concentration in the PRD. Precursor emissions from outside the PRD (“nonlocal”) have the largest contribution to ambient ozone in the region and show a consistently increasing trend, whereas emissions from within the PRD (“local”) show a significant spatial heterogeneity and play a more important role during ozone episodes over the southwest of the region. Under general conditions, the impact on the northeastern PRD is positive but decreasing, and in the southwest it is negative but increasing. During ozone episodes, the impact on the northeastern PRD is negative and decreasing, whereas in the southwestern PRD it is positive but decreasing. The central and western PRD are the only areas with an increasing local ozone contribution. The spatial heterogeneity in both the local ozone contribution and its trend under general conditions and during ozone episodes is well interpreted by a conceptual diagram that collectively takes ozone precursor emissions and their changing trends, ozone formation regimes, and the monsoonal and microscale synoptic conditions over different subregions of the PRD into consideration. In particular, we conclude that an inappropriate NOx∕VOC control ratio within the PRD over the past few years is most likely responsible for the ozone increase over southwest of this region, both under general conditions and during ozone episodes. By investigating the ozone evolution influenced by emission changes within and outside of the PRD during the past decade, this study highlights the importance of establishing a dichotomous ozone control strategy to tackle general conditions and pollution events separately. NOx emission control should be further strengthened to alleviate the peak ozone level during episodes. Detailed investigation is needed to retrieve appropriate NOx∕VOC ratios for different emission and meteorological conditions, so as to maximize the ozone reduction efficiency in the PRD.

Abstract. A typical multi-day ozone (O3) pollution event was chosen to explore the atmospheric oxidation capacity (AOC), OH reactivity, radical chemistry, and O3 pollution mechanism in a coastal city of southeastern China, with an observation-based model coupled to the Master Chemical Mechanism (OBM-MCM). The hydroxyl radical (OH) was the predominant oxidant (90 ± 25 %) for daytime AOC, while the NO3 radical played an important role in AOC during the nighttime (72 ± 9 %). Oxygenated volatile organic compounds (OVOCs; 30 ± 8 %), NO2 (29 ± 8 %), and CO (25 ± 5 %) were the dominant contributors to OH reactivity, accelerating the production of O3 and recycling of ROx radicals (ROx = OH + HO2 + RO2). Photolysis of nitrous acid (HONO, 33 ± 14 %), O3 (25 ± 13 %), formaldehyde (HCHO, 20 ± 5 %), and other OVOCs (17 ± 2 %) was a major ROx source, which played an initiation role in atmospheric oxidation processes. Combined with regional transport analysis, the reasons for this O3 episode were the accumulation of local photochemical production and regional transport. The results of sensitivity analysis showed that volatile organic compounds (VOCs) were the limiting factor of radical recycling and O3 formation, and the 5 % reduction of O3 would be achieved by decreasing 20 % anthropogenic VOCs. Controlling emissions of aromatics, alkenes, and alkanes with ≥4 carbons was beneficial for ozone pollution mitigation. The findings of this study provide significant guidance for emission reduction and regional collaboration for future photochemical pollution control in the relatively clean coastal cities of China and similar countries.