The authors conducted on-line measurements of water soluble inorganic ions and their gas phase precursors for more than 30 days at Wangdu, Baoding City in summer in 2014 with a Gas and Aerosol Collector (GAC). It was found that, the observation site was always subjected with an ammonia rich conditions; the averaged PM_2.5 concentration was 68.2 μg/m3, of which the detected SO₄^2-, NO₃^-, Cl^-, NH₄⁺ and K⁺ by GAC was 12.6, 8.5, 1.4, 11.7 and 0.7 μg/m³, respectively, consisted 51% of the total mass of PM_2.5. All these observed pollutants showed distinct diurnal variations: SO₂, SO₄^2-, NO₃^-, NH₄⁺ and Cl^- showed a morning maximum, HCl and HNO₃ presented an afternoon peak, and NH₃ possessed a day-night variations. As depicted by the analysis of Sulfur and Nitrogen Oxidation Rates (SOR, NOR), fast transformations of trace gases into secondary inorganic aerosols were indicated. The averaged value of SOR and NOR are 0.43 and 0.22, respectively. The variation of SOR and NOR inferred that both gas phase and aqueous oxidations contributed to the fast transformation that formed water soluble inorganic salts.

The Comprehensive Air Quality Model with extensions (CAMx), a 3-D regional chemical transport model, was used to simulate ozone concentrations in the Pearl River Delta (PRD) in November, 2009. Ozone Source Apportionment Technology (OSAT) was applied to investigate sources of two typical ozone episodes and a clean day as comparison. The results showed that emissions from urban Guangzhou and Dongguan mainly contributed to local areas and the southwestern part of PRD (15–30 μL/m3), while emissions from Shenzhen Baoan district (15–25 μL/m3) mainly affected the Pearl River Estuary. Mobile sources and solvent usage sources were two major contributors to ozone formation and had an influence in most areas of the central and western PRD. Mobile sources could contribute as much as 50 μL/m3 hourly ozone concentration to the boarding area of Foshan and Guangzhou. Trans-boundary import of ozone made episodes more likely to occur in PRD under unfavorable weather conditions, however, the ozone precursors emitted within the PRD region were the main cause of high ozone concentrations during ozone episodes. As a result, the strategies on controlling emissions within the PRD region are the key to the reduction of ozone pollutions in PRD.

The Community Multi-Scale Air Quality (CMAQ) modeling system was applied to simulate the meteorology field, O₃ and PM10 in the Pearl River Delta (PRD) region in the autumn of 2008, in order to elucidate the impact of the typical meteorological conditions on the O₃ and PM10 pollution episodes. It was found that PRD pollution characteristics presents periodic variation affected by cold air processes in the autumn of 2008. In general, good air quality was presented when cold air passing through while serious pollution was took place before and after the cold air invasion. 1) Before the cold air invasion, when the weather system “cold front front part” was presented, the height of the mixing layer was often low due to a presence of temperature inversion. Therefore, the PM10 emitted or formed at night was continuously accumulated in the morning hours causing the air pollution in northern and central PRD; the afterwards transport of the O₃ during daytime and PM10 during nighttime with northern wind would cause the air pollution in southern PRD. 2) Before the cold air invasion, when the weather system “high-pressured base” was presented, an inversion layer was built up to prevent vertical mixing; both O₃ and PM10 was then mainly horizontally transported with northeasterly wind; consequently forming air pollution in southwestern PRD. 3) After the cold air invasion, under the control of a ridge high pressure type of weather condition, temperature inversion was built up near the surface and a stagnant condition was presented; thus, O₃ and PM10 was trapped in the source regions such as western, northwestern and central PRD, resulted in serious local pollutions.

Current status of nitrogen oxides-related pollution in China was analyzed, including NO_x pollution and the related pollution of ozone, acid rain，particulate matter. Then the NO_x and related air quality standards were reviewed. Finally, strategy for integrated and multi-target control of NO_x pollution were put forward: executing ozone air quality standard, establishing the regional photochemical smog monitoring network; pursuing air quality modeling and control for Beijing, Pearl River Delta etc megacities; strengthening the control of NO_x emission from power station and vehicles and setting NO_x-related scientific research program.