Assessment of regional air quality resulting from emission control in the Pearl River Delta region, southern China

Modeling assessment of air pollution control measures and COVID-19 pandemic on air quality improvements over Greater Bay Area of China

A remarkable progress has been made toward the air quality improvements over the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) of China from 2017 to 2020. In this study, for the first time, the emission reductions of regional control measures together with the COVID-19 pandemic were considered simultaneously into the development of the GBA's emission inventories for the years of 2017 and 2020. Based on these collective emission inventories, the impacts of control measures, meteorological variations together with temporary COVID-19 lockdowns on the five major air quality index pollutants (SO2, NO2, PM2.5, PM10, and O3, excluding CO) were evaluated using the WRF-CMAQ and SMAT-CE model attainment assessment tool over the GBA region. Our results revealed that control measures in the Pearl River Delta (PRD) region affected significantly the GBA, resulting in pollutant reductions ranging from 48 % to 64 %. In contrast, control measures in Hong Kong and Macao contributed to pollutant reductions up to 10 %. In PRD emission sectors, stationary combustion, on-road, industrial processes and dust sectors stand out as the primary contributors to overall air quality improvements. Moreover, the COVID-19 pandemic during period I (Jan 23-Feb 23) led to a reduction of NO2 concentration by 7.4 %, resulting in a negative contribution (disbenefit) for O3 with an increase by 2.4 %. Our findings highlight the significance of PRD control measures for the air quality improvements over the GBA, emphasizing the necessity of implementing more refined and feasible manageable joint prevention and control policies.

Chronic and acute health effects of PM2.5 exposure and the basis of pollution control targets

Ho Chi Minh City (HCMC) is changing and expanding quickly, leading to environmental consequences that seriously threaten human health. PM2.5 pollution is one of the main causes of premature death. In this context, studies have evaluated strategies to control and reduce air pollution; such pollution-control measures need to be economically justified. The objective of this study was to assess the socio-economic damage caused by exposure to the current pollution scenario, taking 2019 as the base year. A methodology for calculating and evaluating the economic and environmental benefits of air pollution reduction was implemented. This study aimed to simultaneously evaluate the impacts of both short-term (acute) and long-term (chronic) PM2.5 pollution exposure on human health, providing a comprehensive overview of economic losses attributable to such pollution. Spatial partitioning (inner-city and suburban) on health risks of PM2.5 and detailed construction of health impact maps by age group and sex on a spatial resolution grid (3.0 km × 3.0 km) was performed. The calculation results show that the economic loss from premature deaths due to short-term exposure (approximately 38.86 trillion VND) is higher than that from long-term exposure (approximately 14.89 trillion VND). As the government of HCMC has been developing control and mitigation solutions for the Air Quality Action Plan towards short- and medium-term goals in 2030, focusing mainly on PM2.5, the results of this study will help policymakers develop a roadmap to reduce the impact of PM2.5 during 2025-2030.

Spatial association network of PM2.5 and its influencing factors in the Beijing-Tianjin-Hebei urban agglomeration

In this paper, we empirically study the spatial association network of PM_2.5 and the factors influencing those correlations using the gravity model, social network analysis (SNA), and the quadratic assignment procedure (QAP) based on data from the Beijing-Tianjin-Hebei urban agglomeration (BTHUA) in China from 2005 to 2018. We draw the following conclusions. First, the spatial association network of PM_2.5 exhibits relatively typical network structure characteristics: the network density and network correlations are highly sensitive to efforts to control air pollution, and there are obvious spatial correlations within the network. Second, cities in the center of the BTHUA have large network centrality values, while cities in the peripheral region have small centrality values. Tianjin is a core city in the network, and the spillover effect of PM_2.5 pollution in Shijiazhuang and Hengshui is the most noticeable. Third, the 14 cities can be divided into four plates, with each plate having obvious geographical location characteristics and linkage effects. The cities in the association network are divided into three tiers. Beijing, Tianjin, and Shijiazhuang are located in the first tier, and a considerable number of PM_2.5 connections are completed through these cities. Fourth, differences in geographical distance and urbanization are the main drivers of the spatial correlations of PM_2.5. The greater the urbanization differences, the more likely the generation of PM_2.5 links is, while the opposite is true for differences in geographical distance.

Research on the ozone formation sensitivity indicator of four urban agglomerations of China using Ozone Monitoring Instrument (OMI) satellite data and ground-based measurements

Near surface ozone is a typical secondary pollutant, and is mostly generated by a series of complex photochemical reactions of volatile organic compounds (VOCs) and nitrogen oxides (NO_x) in the air under sunlight. At present, a large number of studies have applied FNR (a ratio of formaldehyde (HCHO) to nitrogen dioxide (NO₂) retrieved by satellite) indicator to study the ozone formation sensitivity (OFS). OFS analysis is critical for taking targeted ozone pollution prevention and control measures. Regional OFS can be more accurately diagnosed by utilizing localized FNR threshold. In this study, localized FNR thresholds were established for four severe ozone polluted urban agglomerations in China (Beijing-Tianjin-Hebei (BTH) region, Yangtze River Delta (YRD) region, Pearl River Delta (PRD) region, and Chengdu-Chongqing (CY) region), based on the statistical analysis between FNR (obtained from OMI observation, with daily transit time of approximately 13:45 local standard time) and ΔO₃/ΔNO₂ (the ratio of ozone change to nitrogen dioxide change between two consecutive months, obtained from ground measurements) from 2014 to 2016. And these thresholds were verified by the statistical analysis between FNR and ΔO₃/O₃ (ozone change rate between two consecutive months), and between FNR and O₃ concentration during the OFS significant shift months. Furthermore, the results were also compared and verified with the method proposed by previous studies. The results indicate that there are significant regional dependences in the FNR threshold, and the lower-upper limits for the four urban agglomerations are as follows: 0.65-1.21 for BTH, 0.64-1.48 for the YRD, 1.25-2.39 for the PRD, and 1.44-3.69 for CY (FNR < lower limit indicates VOCs-limited regime; lower limit < FNR < upper limit indicates transitional regime; FNR > upper limit indicates NO_x-limited regime). This method eliminates the problems associated with the undifferentiated use of FNR thresholds in different regions and significantly reduces the deviations for OFS.

Why did air quality experience little improvement during the COVID-19 lockdown in megacities, northeast China?

To inhibit the COVID-19 (Coronavirus disease 2019) outbreak, unprecedented nationwide lockdowns were implemented in China in early 2020, resulting in a marked reduction of anthropogenic emissions. However, reasons for the insignificant improvement in air quality in megacities of northeast China, including Shenyang, Changchun, Jilin, Harbin, and Daqing, were scarcely reported. We assessed the influences of meteorological conditions and changes in emissions on air quality in the five megacities during the COVID-19 lockdown (February 2020) using the WRF-CMAQ model. Modeling results indicated that meteorology contributed a 14.7% increment in Air Quality Index (AQI) averaged over the five megacities, thus, the local unfavorable meteorology was one of the causes to yield little improved air quality. In terms of emission changes, the increase in residential emissions (+15%) accompanied by declining industry emissions (-15%) and transportation (-90%) emissions resulted in a slight AQI decrease of 3.1%, demonstrating the decrease in emissions associated with the lockdown were largely offset by the increment in residential emissions. Also, residential emissions contributed 42.3% to PM2.5 concentration on average based on the Integrated Source Apportionment tool. These results demonstrated the key role residential emissions played in determining air quality. The findings of this study provide a scenario that helps make appropriate emission mitigation measures for improving air quality in this part of China.

Ground-level ozone in the Mekong Delta region: precursors, meteorological factors, and regional transport

The Mekong Delta region (MDR), also known as Vietnam's rice bowl, produced a bountiful harvest of about 23.8 million tons in 2020, accounting for 55.7% of the country's total production, providing food security for 20% of the world population. With the rapid pace of industrialisation and urbanisation, the concentration of ozone in the lower atmosphere has risen to a level that reduces crop yields, especially rice, and is therefore the subject of research. This study aims to simulate the spatiotemporal distribution of ground-level ozone in the area and evaluate the impact of precursor emissions and meteorological factors on the spatiotemporal distributions of ozone concentrations. The study area was divided into seven zones, including six agro-ecological zones (AEZs) and one low-mountainous area, mainly to clarify the role of emissions in each AEZ. The simulation results showed that ground-level O₃ in the MDR ranged from 40.39 to 52.13 µg/m³. In six agro-ecological zones, the average annual ground-level O₃ concentration was relatively high and was the highest in zone 6 (CPZ) and zone 3 (LXZ) with values of 96.18 µg/m³ (exceeding 1.60 times the WHO Guidelines 2021) and 94.86 µg/m³ (exceeding 1.58 times the WHO Guidelines 2021), respectively. In each zone, the annual average O₃ concentration tended to gradually increase from the inner delta to coastal areas. Two types of precursors, NO_x and NMVOCs, are the main contributors to O₃ pollution, with the largest contribution coming from zone 1 (FAZ) with 91.5 thousand tons of NO_x/year and 455.2 thousand tons of NMVOCs/year. Among the meteorological factors considered, temperature (T), relative humidity (RH), and surface pressure (P) were the three main factors that contributed to the increase in ground-level ozone. The spatio-temporal distribution of ground-level O₃ in the MDR was influenced by emission precursors from different zones as well as meteorological factors. The present results can help policy-makers formulate plans for agro-industrial development in the entire region.

Synergistic effect of reductions in multiple gaseous precursors on secondary inorganic aerosols in winter under a meteorology-based redistributed daily NH3 emission inventory within the Beijing-Tianjin-Hebei region, China

Secondary inorganic aerosols (SIA) account for 20-60% of the total fine particulates in the Beijing-Tianjin-Hebei (BTH) region of China, indicating an urgent need to clarify the relationship among such compounds. The purpose of this study was to quantify the relationship between emissions of NH₃, NOx, SO₂, VOCs and SIA concentrations during a severe winter haze episode using an air quality model and a meteorology-based redistributed NH₃ emission inventory within the BTH region. The results showed that the model performance regarding the NH₃ simulations in January by the four emission inventories improved after the redistribution of daily NH₃ emissions, with an increase of 0.02-0.13 in R, a 9-56% decrease in NMB, and a 7-51% decrease in NME. The updated simulations reproduced the daily observations of SIA, SO₂, and NO₂ well. A total of 125 sets of sensitivity simulations showed that a synergistic reduction in NH₃ and VOCs was more efficient in terms of SIA control than simply reducing SO₂ or NOx in the BTH region. If only NOx emissions were reduced, the SIA concentration would first increase and then decrease, and it could decline by another 0.86-8.03% in parallel with an equal NH₃ emission cut. SIA could be reduced by approximately 22.68% with the most stringent inorganic precursors' control. Moreover, VOCs emission reductions could lead to a decrease in SIA, and the impact of VOCs on SIA was similar to that of NH₃. The collaborative control of both inorganic precursors and VOCs was more effective than single-factor control measures for decreasing SIA, and the decline rate was approximately 29.26% under minimum emission conditions. This improved effectiveness was obtained because VOCs mitigation effectively decreases the ozone concentration, which in turn influences SIA formation. Finally, on the premise of a 60% SO₂ cut, the reduction scheme NH₃:VOCs:NOx = 4:4:1 was suggested for SIA control.

Diagnostic analysis of regional ozone pollution in Yangtze River Delta, China: A case study in summer 2020

A regional ozone (O₃) pollution event occurred in the Yangtze River Delta region during August 17-23, 2020 (except on August 21). This study aims to understand the causes of O₃ pollution during the event using an emission-based model (i.e., the Community Multiscale Air Quality (CMAQ) model) and an observation-based model (OBM). The OBM was used to investigate O₃ sensitivity to its precursors during the O₃ pollution, concluding that O₃ formation was limited by volatile organic compounds (VOCs) on August 19, but was co-limited by VOCs and nitrogen oxides (NOx) on other polluted days. Aromatics and alkenes were the two main VOC groups contributing to the O₃ formation, with trans-2-butene and m/p-xylene as the key species among the VOCs measured at the Nanjing urban site. The source apportionment results estimated using the source-oriented CMAQ model suggest that the transportation and industry sources dominated the non-background O₃ production in Nanjing, which were responsible for 52% and 24.7%, respectively. The O₃ concentration attributed to NOx (~70%) was significantly higher than that attributed to VOCs (approximately 30%). The process analysis revealed that vertical mixing increased the O₃ concentrations in the early morning, and photochemical reactions promoted O₃ formation and accumulation during the daytime within the planetary boundary layer. At night, outflow from horizontal transport and nocturnal chemistry jointly resulted the O₃ depletion. The contributions of inter-city transport during the O₃ pollution period in Nanjing were also estimated. The predicted O₃ concentration was largely recorded from long-distance regions, reaching 46%, followed by local sources (38%) and surrounding cities (16%). The results indicate that both NOx and VOCs contributed significantly to O₃ pollution during this event, and the emissions controls of NOx and the key VOC species of aromatics and alkenes from a cooperative regional perspective should be considered to mitigate O₃ pollution.

Agricultural ammonia emissions and its impact on PM2.5 concentrations in the Beijing-Tianjin-Hebei region from 2000 to 2018

Ammonia (NH₃) discharged from agricultural activities to the atmosphere plays a crucial role in the formation of secondary inorganic aerosols. This study analyzed the temporal-spatial development of agricultural NH₃ emissions from 2000 to 2018 in the Beijing-Tianjin-Hebei (BTH) region and assessed the effects of reducing PM_2.5 by removing agricultural NH₃ using an air quality model. The results showed that the interannual agricultural NH₃ emissions in the BTH region exhibited a stairs trend from 2000 to 2018, with an average of 971.63 Gg. In particular, agricultural NH₃ emissions in the BTH region reached a maximum in summer when the temperature was high and were more concentrated in the southern plains compared to the northern areas. Under the reduction scenario (RS), the agricultural NH₃ emissions in the BTH region in 2015, 2016, 2017, and 2018 were reduced by 2.95%, 4.10%, 18.75%, and 10.21%, resulting in a reduction of 0.5%, 0.5%, 2.5%, and 1.2% of annual mean PM_2.5 concentration, respectively, compared with the baseline scenario (BS). Furthermore, agricultural NH₃ emissions contributed 12.6, 12.1, 11.9, and 11.3 μg m^-3 to PM_2.5 concentrations in 2015, 2016, 2017, and 2018 under the zero-emission scenario (ZS), respectively. However, the contribution rates exhibited a slightly increasing trend from 20.5% in 2015 to 24.6% in 2018. These findings could provide a new understanding of agricultural NH₃ emission trends and their impacts on PM_2.5 concentration based on actual NH₃ mitigation ratios in recent years, thereby guiding the formulation of future control strategies.

Anthropogenic emission inventory of multiple air pollutants and their spatiotemporal variations in 2017 for the Shandong Province, China

Shandong is the most populous and highly industrialized province in eastern China, and the resultant poor air quality is a cause for widespread concern. This study combines bottom-up and top-down approaches to develop a high-resolution anthropogenic emission inventory of air pollutants for 2017. The inventory was developed based on updated emission factors and detailed activity data. The emissions of sulfur dioxide (SO₂), nitrogen oxides (NO_x), particulate matter with aerodynamic diameters smaller than 2.5 and 10 μm (PM_2.5 and PM₁₀, respectively), carbon monoxide (CO), volatile organic compounds (VOCs), and ammonia (NH₃) were estimated to be 1387.8, 2488.6, 5281.7, 3193.0, 9250.7, 2254.7, and 1210.6 kt, respectively. Power plants were the largest contributors of SO₂ and NO_x emissions accounting for 43.7% and 41.9% of the total emissions, respectively. CO emissions mainly originated from industrial processes (40.1%), mobile sources (24.8%), and fossil fuel burning (21.2%). The major sources of PM₁₀ and PM_2.5 emissions were industrial processes and fugitive dust, contributing 83.0% and 86.9% of their total emissions, respectively. Industrial processes (60.0%) contributed the largest VOC emissions, followed by mobile sources (16.8%) and solvent use (14.5%). Livestock and N-fertilizers were major emitters of NH₃, accounting for 69.9% and 21.2% of the total emissions, respectively. Emissions were spatially allocated to grid cells with a resolution of 0.05 ° × 0.05 ° based on spatial surrogates, using Geographic Information System (GIS). Heavy pollutant emissions were mainly concentrated in the central and eastern areas of Shandong, while high NH₃-emissions occurred in the western region. Most pollutant emissions from industrial sectors occurred in June and July, while low emissions were recorded between January and February. Range uncertainties in emission inventory were quantified using Monte Carlo simulations. Our inventory provides effective information to understand local pollutant emission characteristics, perform air quality simulations, and formulate pollution control measures.

Air Quality During COVID-19 Lockdown in the Yangtze River Delta and the Pearl River Delta: Two Different Responsive Mechanisms to Emission Reductions in China

Despite the large reduction in anthropogenic activities due to the outbreak of COVID-19, air quality in China has witnessed little improvement and featured great regional disparities. Here, by combining observational data and simulations, this work aims to understand the diverse air quality response in two city clusters, Yangtze River Delta region (YRD) and Pearl River Delta region (PRD), China. Though there was a noticeable drop in primary pollutants in both the regions, differently, the maximum daily 8 h average ozone (O3) soared by 20.6-76.8% in YRD but decreased by 15.5-28.1% in PRD. In YRD, nitrogen oxide (NOx) reductions enhanced O3 accumulation and hence increased secondary aerosol formation. Such an increment in secondary organic and inorganic aerosols under stationary weather reached up to 36.4 and 10.2%, respectively, which was further intensified by regional transport. PRD was quite the opposite. The emission reductions benefited PRD air quality, while regional transport corresponded to an increase of 17.3 and 9.3% in secondary organic and inorganic aerosols, respectively. Apart from meteorology, the discrepancy in O3-VOCs-NOx relationships determined the different O3 responses, indicating that future emission control shall be regionally specific, instead of one-size-fits-all cut. Overall, the importance of regionally coordinated and balanced control strategy for multiple pollutants is highly emphasized.

Individual particle investigation on the chloride depletion of inland transported sea spray aerosols during East Asian summer monsoon

Inland transported sea spray aerosol (SSA) particles along with multiphase reactions are essential to drive the regional circulation of nitrogen, sulfur and halogen species in the atmosphere. Specially, the physicochemical properties of SSA will be significantly affected by the displacement reaction of chloride. However, the role of organic species and the mixing state on the chloride depletion of SSA during long-range inland transport remains unclear. Hence, a single particle aerosol mass spectrometer (SPAMS) was employed to investigate the particle size and chemical composition of individual SSA particles over inland southern China during the East Asian summer monsoon. Based on the variation of chemical composition, SSA particles were clustered into SSA-Aged, SSA-Bio and SSA-Ca. SSA-Aged was regarded as the aged Na-rich SSA particles. In comparison to the SSA-Aged, SSA-Bio involved some extra organic species associated with biological origin (i.e., organic nitrogen and phosphate). Each type occupies for approximately 50% of total detected SSA particles. Besides, SSA-Ca may relate to organic shell of Na-rich SSA particles, which is negligible (~3%). Tight correlation between Na and diverse organic acids was exhibited for the SSA-Aged (r² = 0.52, p < 0.01) and SSA-Bio (r² = 0.61, p < 0.01), reflecting the impact of organic acids to the chloride displacement during inland transport SSA particles. The chloride depletion occupied by organic acids is estimated to be up to 34%. It is noted that distinctly different degree of chloride depletion was observed between SSA-Aged and SSA-Bio. It is more likely to be attributed to the associated organic coatings for the SSA-Bio particles, which inhibits the displacement reactions between acids and chloride. As revealed from the mixing state of SSA-Bio, defined hourly mean peak area ratio of Cl / Na increases with the increasing phosphate and organic nitrogen. This finding provides additional basis for the improvement of modeling simulations in chlorine circulation and a comprehensive understanding of the effects of organics on chloride depletion of SSA particles.

Emission Inventories and Particulate Matter Air Quality Modeling over the Pearl River Delta Region

The Pearl River Delta (PRD) region is located on the southeast coast of mainland China and it is an important economic hub. The high levels of particulate matter (PM) in the atmosphere, however, and poor visibility have become a complex environmental problem for the region. Air quality modeling systems are useful to understand the temporal and spatial distribution of air pollution, making use of atmospheric emission data as inputs. Over the years, several atmospheric emission inventories have been developed for the Asia region. The main purpose of this work is to evaluate the performance of the air quality modeling system for simulating PM concentrations over the PRD using three atmospheric emission inventories (i.e., EDGAR, REAS and MIX) during a winter and a summer period. In general, there is a tendency to underestimate PM levels, but results based on the EDGAR emission inventory show slightly better accuracy. However, improvements in the spatial and temporal disaggregation of emissions are still needed to properly represent PRD air quality. This study’s comparison of the three emission inventories’ data, as well as their PM simulating outcomes, generates recommendations for future improvements to atmospheric emission inventories and our understanding of air pollution problems in the PRD region.

Photochemistry of ozone pollution in autumn in Pearl River Estuary, South China

To explore the photochemical O₃ pollution over the Pearl River Estuary (PRE), intensive measurements of O₃ and its precursors, including trace gases and volatile organic compounds (VOCs), were simultaneously conducted at a suburban site on the east bank of PRE (Tung Chung, TC) in Hong Kong and a rural site on the west bank (Qi'ao, QA) in Zhuhai, Guangdong in autumn 2016. Throughout the sampling period, 3 days with high O₃ levels (maximum hourly O₃ > 100 ppbv) were captured at both sites (pattern 1) and 13 days with O₃ episodes occurred only at QA (pattern 2). It was found that O₃ formation at TC was VOC-limited in both patterns because of the large local NO_x emissions. However, the O₃ formation at QA was co-limited by VOCs and NO_x in pattern 1, but VOC-limited in pattern 2. In both patterns, isoprene, formaldehyde, xylenes and trimethylbenzenes were the top 4 VOCs that modulated local O₃ formation at QA, while they were isoprene, formaldehyde, xylenes and toluene at TC. In pattern 1, the net O₃ production rate at QA (13.1 ± 1.6 ppbv h^-1) was high, and comparable (p = 0.40) to that at TC (12.1 ± 1.5 ppbv h^-1), so was the hydroxyl radical (i.e., OH), implying high atmospheric oxidative capacity over PRE. In contrast, the net O₃ production rate was significantly higher (p < 0.05) at QA (16.3 ± 0.4 ppbv h^-1) than that at TC (4.7 ± 0.2 ppbv h^-1) in pattern 2, and the OH concentration and cycling rate were also higher, indicating much stronger photochemical reactions at QA. These findings enhanced our understanding of O₃ photochemistry in the Pearl River estuary, which could be extended to other estuaries.

Single particle diversity and mixing state of carbonaceous aerosols in Guangzhou, China

Many field studies have investigated the formation mechanisms of organic aerosol (OA) based on bulk analysis, yet the source and formation process of individual organic particles may be quite different due to the diversity of chemical composition and mixing state in single particles. Here we present the observation results of chemical composition and mixing state of carbonaceous single particles at an urban site in Guangzhou. The carbonaceous particles accounted for 74.6% of the total detected single particles, and were grouped into four types including elemental carbon-aged (EC-aged), elemental and organic carbon (ECOC), organic carbon-rich (OC-rich) and secondary ions-rich (SEC) particles. The formation of EC-aged particles was closely associated with the absorption of organics onto fresh EC particles from primary sources, and the further enrichment of organics in EC-aged particles resulted in the production of ECOC particles. In the daytime OC-rich and SEC particles were mainly produced from the photochemical reactions, while in the nighttime their sharp increases were found along with the enrichment of nitrate and organic nitrogen fragments, suggesting the heterogeneous formation of nitrate and organic nitrogen in OC-rich and SEC particles. The production rates of carbonaceous particles were also investigated in an episodic event, and the EC-aged particles showed the highest production rate compared to the other carbonaceous particles both in the daytime and nighttime, suggesting a significant role of EC in the formation and aging process of carbonaceous particles. The results from this work have revealed different formation processes and production rates of carbonaceous particles due to their diversity in mixing state, providing further insights into the formation mechanisms of OA in field studies.

Insights into source origins and formation mechanisms of nitrate during winter haze episodes in the Yangtze River Delta

Nitrate became the most significant component of secondary inorganic aerosols (SNA, the sum of sulfate, nitrate and ammonium ions) as the emissions of sulfur dioxide (SO₂) have been greatly reduced in China in recent years. In the Yangtze River Delta (YRD), nitrate could contribute 56% of SNA and 34% of total PM_2.5 during haze episodes. In this study, a modified Community Multiscale Air Quality (CMAQ) model was used to provide a comprehensive understanding of nitrate source and formation under severe pollution during winter 2015 and 2016. Three haze episodes (HEP1, HEP2 and HEP3) and one clean episode (CEP) were selected to investigate the emission sector and regional contributions to nitrate at six environmental monitoring sites in the YRD. Source apportionment results showed that industry (35%), transportation (32%) and power (28%) sectors were the important sources of nitrate during haze episodes. Regional transport (60-98%) was responsible for the high nitrate concentrations in the YRD. During haze episodes, the high ozone production (PO₃) rate (up to 700 ppb/h) and hydroxyl radicals (OH) removal rate (up to 9 ppb/h) were observed in the daytime indicating the important atmospheric oxidation capacity in the YRD. Also, the nitrogen oxidation ratio (NOR) analysis elucidated that daytime photochemistry played an important role in nitrate formation and the heterogeneous chemistry enhanced the high nitrate at night. Results from emission scenario analysis demonstrated that averaged nitrate concentration in Shanghai decreased by 18% during haze episodes under emission reductions of 20% NO_x, NH₃ and VOC in the YRD, and Shandong, Shanxi, Henan and Hebei provinces. Emission reduction on the regional scale (one city and its surrounding areas) is an efficient strategy to reduce nitrate concentration in the YRD.

Source and sectoral contribution analysis of PM2.5 based on efficient response surface modeling technique over Pearl River Delta Region of China

Identifying and quantifying source contributions of pollutant emissions are crucial for an effective control strategy to break through the bottleneck in reducing ambient PM_2.5 levels over the Pearl River Delta (PRD) region of China. In this study, an innovative response surface modeling technique with differential method (RSM-DM) has been developed and applied to investigate the PM_2.5 contributions from multiple regions, sectors, and pollutants over the PRD region in 2015. The new differential method, with the ability to reproduce the nonlinear response surface of PM_2.5 to precursor emissions by dissecting the emission changes into a series of small intervals, has shown to overcome the issue of the traditional brute force method in overestimating the accumulative contribution of precursor emissions to PM_2.5. The results of this case study showed that PM_2.5 in the PRD region was generally dominated by local emission sources (39-64%). Among the contributions of PM_2.5 from various sectors and pollutants, the primary PM_2.5 emissions from fugitive dust source contributed most (25-42%) to PM_2.5 levels. The contributions of agriculture NH₃ emissions (6-13%) could also play a significant role compared to other sectoral precursor emissions. Among the NO_X sectors, the emissions control of stationary combustion source could be most effective in reducing PM_2.5 levels over the PRD region.

Quantifying the impact of particle matter on mortality and hospitalizations in four Brazilian metropolitan areas

Air quality management involves investigating areas where pollutant concentrations are above guideline or standard values to minimize its effect on human health. Particulate matter (PM) is one of the most studied pollutants, and its relationship with health has been widely outlined. To guide the construction and improvement of air quality policies, the impact of PM on the four Brazilian southeast metropolitan areas was investigated. One-year long modeling of PM₁₀ and PM_2.5 was performed with the WRF-Chem model for 2015 to quantify daily and annual PM concentrations in 102 cities. Avoidable mortality due to diverse causes and morbidity due to respiratory and circular system diseases were estimated concerning WHO guidelines, which was adopted in Brazil as a final standard to be reached in the future; although there is no deadline set for its implementation yet. Results showed satisfactory representation of meteorology and ambient PM concentrations. An overestimation in PM concentrations for some monitoring stations was observed, mainly in São Paulo metropolitan area. Cities around capitals with high modelled annual PM_2.5 concentrations do not monitor this pollutant. The total avoidable deaths estimated for the region, related to PM_2.5, were 32,000 ± 5,300 due to all-cause mortality, between 16,000 ± 2,100 and 51,000 ± 3,000 due non-accidental causes, between 7,300 ± 1,300 and 16,700 ± 1,500 due to cardiovascular disease, between 4,750 ± 900 and 10,950 ± 870 due ischemic heart diseases and 1,220 ± 330 avoidable deaths due to lung cancer. Avoidable respiratory hospitalizations were greater for PM_2.5 among 'children' age group than for PM₁₀ (all age group) except in São Paulo metropolitan area. For circulatory system diseases, 9,840 ± 3,950 avoidable hospitalizations in the elderly related to a decrease in PM_2.5 concentrations were estimated. This study endorses that more restrictive air quality standards, human exposure, and health effects are essential factors to consider in urban air quality management.

Real-time source contribution analysis of ambient ozone using an enhanced meta-modeling approach over the Pearl River Delta Region of China

The nonlinear response of O₃ to nitrogen oxides (NO_x) and volatile organic compounds (VOC) is not conducive to accurately identify the various source contributions and O₃-NO_x-VOC relationships. An enhanced meta-modeling approach, polynomial functions based response surface modeling coupled with the sectoral linear fitting technique (pf-ERSM-SL), integrating a new differential method (DM), was proposed to break through the limitation. The pf-ERSM-SL with DM was applied for analysis of O₃ formation regime and real-time source contributions in July and October 2015 over the Pearl River Delta Region (PRD) of Mainland China. According to evaluations, the pf-ERSM-SL with DM was proven to be effective in source apportionment when the traditional sensitivity analysis was unsuitable for deriving the source contributions in the nonlinear system. After diagnosing the O₃-NO_x-VOC relationships, O₃ formation in most regions of the PRD was identified as a distinctive NO_x-limited regime in July; in October, the initial VOC-limited regime was found at small emission reductions (less than 22-44%), but it will transit to NO_x-limited when further reductions were implemented. Investigation of the source contributions suggested that NO_x emissions were the dominated contributor when turning-off the anthropogenic emissions, occupying 85.41-94.90% and 52.60-75.37% of the peak O₃ responses in July and October respectively in the receptor regions of the PRD; NO_x emissions from the on-road mobile source (NO_x_ORM) in Guangzhou (GZ), Dongguan&Shenzhen (DG&SZ) and Zhongshan (ZS) were identified as the main contributors. Consequently, the reinforced control of NO_x_ORM is highly recommended to lower the ambient O₃ in the PRD effectively.

Impact of the ‘13th Five-Year Plan’ Policy on Air Quality in Pearl River Delta, China: A Case Study of Haizhu District in Guangzhou City Using WRF-Chem

Due to increasingly stringent control policy, air quality has generally improved in major cities in China during the past decade. However, the standards of national regulation and the World Health Organization are yet to be fulfilled in certain areas (in some urban districts among the cities) and/or certain periods (during pollution episode event). A further control policy, hence, has been issued in the 13th Five-Year Plan (2016–2020, hereafter 13th FYP). It will be of interest to evaluate the air quality before the 13th FYP (2015) and to estimate the potential air quality by the end of the 13th FYP (2020) with a focus on the area of an urban district and the periods of severe pollution episodes. Based on observation data of major air pollutants, including SO2 (sulphur dioxide), NO2 (nitrogen dioxide), CO (carbon monoxide), PM10 (particulate matter with aerodynamic diameter equal to or less than 10 μm), PM2.5 (particulate matter with aerodynamic diameter equal to or less than 2.5 µm) and O3 (Ozone), the air quality of Haizhu district [an urban district in the Pearl River Delta (PRD), China] in 2015 suggested that typical heavy pollution occurred in winter and the hot season, with NO2 or PM2.5 as the key pollutants in winter and O3 as the key pollutant in the hot season. We also adopted a state-of-the-art chemical transport model, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), to predict the air quality in Haizhu District 2020 under different scenarios. The simulation results suggested that among the emission control scenarios, comprehensive measures taken in the whole of Guangzhou city would improve air quality more significantly than measures taken just in Haizhu, under all conditions. In the urban district, vehicle emission control would account more than half of the influence of all source emission control on air quality. Based on our simulation, by the end of the 13th FYP, it is noticeable that O3 pollution would increase, which indicates that the control ratio of volatile organic compounds (VOCs) and nitrogen oxides (NOx) may be unsuitable and therefore should be adjusted. Our study highlights the significance of evaluating the efficacy of current policy in reducing the air pollutants and recommends possible directions for further air pollution control for urban areas during the 13th FYP.

Large-scale optimization of multi-pollutant control strategies in the Pearl River Delta region of China using a genetic algorithm in machine learning

A scientifically sound integrated assessment modeling (IAM) system capable of providing optimized cost-benefit analysis is essential in effective air quality management and control strategy development. Yet scenario optimization for large-scale applications is limited by the computational expense of optimization over many control factors. In this study, a multi-pollutant cost-benefit optimization system based on a genetic algorithm (GA) in machine learning has been developed to provide cost-effective air quality control strategies for large-scale applications (e.g., solution spaces of ~1035). The method was demonstrated by providing optimal cost-benefit control pathways to attain air quality goals for fine particulate matter (PM2.5) and ozone (O3) over the Pearl River Delta (PRD) region of China. The GA was found to be >99% more efficient than the commonly used grid searching method while providing the same combination of optimized multi-pollutant control strategies. The GA method can therefore address air quality management problems that are intractable using the grid searching method. The annual attainment goals for PM2.5 (< 35 μg m-3) and O3 (< 80 ppb) can be achieved simultaneously over the PRD region and surrounding areas by reducing NOx (22%), volatile organic compounds (VOCs, 12%), and primary PM (30%) emissions. However, to attain stricter PM2.5 goals, SO2 reductions (> 9%) are needed as well. The estimated benefit-to-cost ratio of the optimal control strategy reached 17.7 in our application, demonstrating the value of multi-pollutant control for cost-effective air quality management in the PRD region.

Monitoring history and change trends of ambient air quality in China during the past four decades

This study summarized the history of ambient air quality monitoring and air pollution prevention and control, and it analyzed the spatiotemporal patterns of ambient air pollutants during 1981-2017 in China. The results showed that monitoring of ambient air quality has changed dramatically in terms of determinants, sampling methods, monitoring extent, and evaluation basis during the previous four decades. Annual average concentrations of total suspended particulates, PM₁₀ and SO₂ have shown obvious decreasing trends during the studied period. These improvements have been closely related to the considerable efforts and various approaches undertaken to prevent and control air pollution. However, although policy implementation has been decisive and, at least in part, it has been enforced effectively, significant challenges remain. Air pollution control cannot be accomplished without a long-term strategy designed to achieve clean air in all parts of China.

Source and exposure apportionments of ambient PM2.5 under different synoptic patterns in the Pearl River Delta region

PM_2.5 is one of the most notorious ambient pollutants in the Pearl River Delta (PRD) region during episodic conditions. In this work, the Comprehensive Air Quality Model with extension (CAMx) was used together with the Particulate Source Apportionment Technology (PSAT) module to analyze the influences of different sources on PM_2.5 concentration in the PRD region under different synoptic patterns (sea high pressure, sub-tropical high pressure and equalizing pressure field). The result shows that the PM_2.5 concentration increases to different degrees under the three synoptic patterns. The emissions outside the PRD region contribute more than 54% under episodic conditions. The source category contribution varies little under different synoptic patterns. Area (46%), mobile (21%) and industry point source (16%) are the major contributors over the three episodic cases. The regional source contributions (from other cities within the PRD) to Foshan, Zhongshan and Zhaoqing are larger and can reach up to 33%. People living in the PRD region are more exposed to pollutants produced from the area and mobile sources. About 80% of the population is exposed to PM_2.5 levels exceeding the IT-3 standard during the pollution episodes.

Effects of air pollution control measures on air quality improvement in Guangzhou, China

The ambient air quality of Guangzhou in 2016 has significantly improved since Guangzhou and its surrounding cities implemented a series of air pollution control measures from 2014 to 2016. This study not only estimated the effects of meteorology and emission control measures on air quality improvement in Guangzhou but also assessed the contributions of emissions reduction from various sources through the combination of observation data and simulation results from Weather Research and Forecasting - Community Multiscale Air Quality (WRF-CMAQ) modeling system. Results showed that the favorable meteorological conditions in 2016 alleviated the air pollution. Compared to change in meteorology, implementing emission control measures in Guangzhou and surrounding cities was more beneficial for air quality improvement, and it could reduce the concentrations of SO₂, NO₂, PM_2.5, PM₁₀, and O₃ by 9.7 μg m^-3 (48.4%), 9.2 μg m^-3 (17.7%), 7.7 μg m^-3 (14.6%), 9.7 μg m^-3 (13.4%), and 12.0 μg m^-3 (7.7%), respectively. Furthermore, emission control measures that implemented in Guangzhou contributed most to the concentration reduction of SO₂, NO₂, PM_2.5, and PM₁₀ (46.0% for SO₂, 15.2% for NO₂, 9.4% for PM_2.5, and 9.1% for PM₁₀), and it increased O₃ concentration by 2.4%. With respect to the individual contributions of source emissions reduction, power sector emissions reduction showed the greatest contribution in reducing the concentrations of SO₂, NO₂, PM_2.5, and PM₁₀ due to the implementation of Ultra-Clean control technology. As for O₃ mitigation, VOCs product-related source emissions reduction was most effective, and followed by transportation source emissions reduction, while the reductions of power sector, industrial boiler, and industrial process source might not be as effective. Our findings provide scientific advice for the Guangzhou government to formulate air pollution prevention and control policies in the future.

Aggravating O3 pollution due to NOx emission control in eastern China

During the past five years, China has witnessed a rapid drop of nitrogen oxides (NO_x) owing to the wildly-applied rigorous emission control strategies across the country. However, ozone (O₃) pollution was found to steadily deteriorate in most part of eastern China, especially in developed regions such as Jing-Jin-Ji (JJJ), Yangtze River Delta region (YRD) and Pearl River Delta region (PRD). To shed more light on current O₃ pollution and its responses to precursor emissions, we integrate satellite retrievals, ground-based measurements together with regional numerical simulation in this study. It is indicated by multiple sets of observational data that NO_x in eastern China has declined more than 25% from 2012 to 2016. Based on chemical transport modeling, we find that O₃ formation in eastern China has changed from volatile organic compounds (VOCs) sensitive regime to the mixed sensitive regime due to NO_x reductions, substantially contributing to the recent increasing trend in urban O₃. In addition, such transitions tend to bring about an ~1-1.5 h earlier peak of net O₃ formation rate. We further studied the O₃ precursors relationships by conducting tens of sensitivity simulations to explore potential ways for effective O₃ mitigation. It is suggested that the past control measures that only focused on NO_x may not work or even aggravate O₃ pollution in the city clusters. In practice, O₃ pollution in the three regions is expected to be effectively mitigated only when the reduction ratio of VOCs/NO_x is greater than 2:1, indicating VOCs-targeted control is a more practical and feasible way.

One-Year Characterization and Reactivity of Isoprene and Its Impact on Surface Ozone Formation at A Suburban Site in Guangzhou, China

Isoprene has a potentially large effect on ozone (O3) formation in the subtropical, highly polluted city of Guangzhou. Online measurements of isoprene in Guangzhou city are scarce; thus, isoprene levels were monitored for one year at the Guangzhou Panyu Atmospheric Composition Station (GPACS), a suburban site in Guangzhou, using an online gas chromatography-flame ionization detector (GC–FID) system to investigate the characterization and reactivity of isoprene and its effect on the O3 peak profile in different seasons. The results showed that the daily average mixing ratios of isoprene at GPACS were 0.40, 2.20, 1.40, and 0.13 mixing ratio by volume (ppbv) in spring, summer, autumn, and winter, respectively. These values were considerably higher than the mixing ratios of isoprene in the numerous other subtropical and temperate cities around the world. Furthermore, isoprene ranked first with regard to O3 formation potential (OFP) and propylene-equivalent mixing ratio among 56 measured non–methane hydrocarbons (NMHCs). The ratios of isoprene to cis-2-butene, an exhaust tracer, were determined to estimate the fractions of biogenic and anthropogenic emissions. The results revealed a much greater contribution from biogenic than anthropogenic factors during the daytime in all four seasons. In addition, night-time isoprene emissions were mostly associated with vehicles in winter, and the residual isoprene that remained after photochemical loss during the daytime also persisted into the night. The high levels of isoprene in summer and autumn may cause the strong and broad peaks of the O3 profile because of its association with the most favorable meteorological conditions (e.g., high temperature and intense solar radiation) and the highest OH mixing ratio, which could affect human health by exposing people to a high O3 mixing ratio for prolonged periods. The lower mixing ratios of isoprene resulted in a weak and sharp peak in the O3 profile in both spring and winter. The high level of isoprene in the subtropical zone could accentuate its large impact on atmospheric oxidant capacity and air quality in Guangzhou city.

Characteristics of atmospheric PM2.5 composition during the implementation of stringent pollution control measures in shanghai for the 2016 G20 summit

To reduce air pollution within a 300 km radius from Hangzhou (the capital city of Zhejiang Province in East China) for the 2016 G20 summit (9/4-9/5), the 14-day (8/24-9/6) stringent pollution control measures were implemented in Shanghai. Changes in atmospheric concentrations during the same 14-day period from 2014 to 2016 were examined at two Supersites, i.e., urban Pudong site (PD) and Dianshan Lake regional site (DSL). Up to 50% reductions were found for PM_2.5, with 13.1% and 9.7% reductions for SO₂ and NO₂, respectively. No apparent improvements were found for 8-h average O₃ concentrations. Large reductions were also found for SO₄^2- (51.4%), NO₃^- (68.8%), and NH₄⁺ (84.4%), on average. Elevated coefficient of divergence values (0.52-0.56) suggested that pollutant sources differed at the two sites. Biomass burning, resuspended dust, combustion, iron and steel industry, sea salt, secondary aerosol, and vehicle exhaust were identified at the DSL site by Positive Matrix Factorization (PMF). Secondary aerosol and vehicle exhaust accounted for 45.7% of PM_2.5 mass, followed 11.2%-13.7% each by industry, resuspended dust, and coal and oil combustion.

Characteristics and Trends of Ambient Ozone and Nitrogen Oxides at Urban, Suburban, and Rural Sites from 2011 to 2017 in Shenzhen, China

The emissions of nitrogen oxides (NOx) decreased under China’s air quality control policies. However, concern remains regarding the response of ozone (O3) in the metropolitan areas. The characteristics and trends of ambient O3 and NOx in Shenzhen were investigated during the 2011–2017 period. Both the human population and vegetation are exposed to higher O3 at suburban and rural sites than at the urban site. The O3 weekend effect is significant (p = 0.062) at the urban site, with O3 levels 1.19 ppb higher on Sunday than on weekdays. Solar radiation, precipitation, and relative humidity are the most relevant meteorological factors that affect O3 daily variations. Wind speed is the least relevant factor, but wind direction is related to the presence of high O3 air concentrations. Both 1-h and 8-h O3 exhibit an increase, opposite to the trend of NOx. A slight decline in O3 occurs in autumn at less urbanized sites. The increase in O3 is more prevalent and rapid in the winter at more urbanized sites. This can be due to the transport of increased O3 from northern China, as well as a lowered O3 titration effect with NOx reduction. O3 increases fastest at the urban site, with an estimated rate of 4.3% (95% confidence intervals (CIs): 0.96, 8.25) per year (p < 0.05) for 8-h O3 and 2.5% (95% CIs: −0.46, 6.12) per year (p > 0.1) for 1-h O3, posing greater human health risks to areas with high population density.

PAN–Precursor Relationship and Process Analysis of PAN Variations in the Pearl River Delta Region

Peroxy acetyl nitrate (PAN) is an important photochemical product formed from the reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx) under sunlight. In this study, a field measurement was conducted at a rural site (the backgarden site, or BGS) of the Pearl River Delta (PRD) region in 2006, with the 10 min maximum PAN mixing ratios of 3.9 ppbv observed. The factors influencing the abundance of PAN at the BGS site was evaluated by the process analysis through the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model. The results suggested that the increase of PAN abundance at the BGS site was mainly controlled by the gas-phase chemistry, followed by vertical transport, while its loss was modulated mainly by dry deposition and horizontal transport. As the dominant important role of gas-phase chemistry, to provide detailed information on the photochemical formation of PAN, a photochemical box model with near-explicit chemical mechanism (i.e., the master chemical mechanism, MCM) was used to explore the relationship of photochemical PAN formation with its precursors based on the measured data at the BGS site. It was found that PAN formation was VOC-limited at the BGS site, with the oxidation of acetaldehyde the most important pathway for photochemical PAN production, followed by the oxidation and photolysis of methylglyoxal (MGLY). Among all the primary VOC precursors, isoprene and xylenes were the main contributors to PAN formation. Overall, our study provides new insights into the PAN photochemical formation and its controlling factors, and highlighted the importance of gas chemistry on the PAN abundance in the PRD region.

Numerical simulations for the sources apportionment and control strategies of PM2.5 over Pearl River Delta, China, part I: Inventory and PM2.5 sources apportionment

This article uses the WRF-CMAQ model to systematically study the source apportionment of PM_2.5 under typical meteorological conditions in the dry season (November 2010) in the Pearl River Delta (PRD). According to the geographical location and the relative magnitude of pollutant emission, Guangdong Province is divided into eight subdomains for source apportionment study. The Brute-Force Method (BFM) method was implemented to simulate the contribution from different regions to the PM_2.5 pollution in the PRD. Results show that the industrial sources accounted for the largest proportion. For emission species, the total amount of NO_x and VOC in Guangdong Province, and NH₃ and VOC in Hunan Province are relatively larger. In Guangdong Province, the emission of SO₂, NO_x and VOC in the PRD are relatively larger, and the NH₃ emissions are higher outside the PRD. In northerly-controlled episodes, model simulations demonstrate that local emissions are important for PM_2.5 pollution in Guangzhou and Foshan. Meanwhile, emissions from Dongguan and Huizhou (DH), and out of Guangdong Province (SW) are important contributors for PM_2.5 pollution in Guangzhou. For PM_2.5 pollution in Foshan, emissions in Guangzhou and DH are the major contributors. In addition, high contribution ratio from DH only occurs in severe pollution periods. In southerly-controlled episode, contribution from the southern PRD increases. Local emissions and emissions from Shenzhen, DH, Zhuhai-Jiangmen-Zhongshan (ZJZ) are the major contributors. Regional contribution to the chemical compositions of PM_2.5 indicates that the sources of chemical components are similar to those of PM_2.5. In particular, SO₄^2- is mainly sourced from emissions out of Guangdong Province, while the NO^3- and NH⁴⁺ are more linked to agricultural emissions.

Numerical simulations for the sources apportionment and control strategies of PM2.5 over Pearl River Delta, China, part II: Vertical distribution and emission reduction strategies

The contribution of various emission sources to the vertical structure of the PM_2.5 concentration and the modeling of emission reduction strategies are emphasized in this study. Analysis of vertical distribution of PM_2.5 concentration in the planetary boundary layer (PBL) reveals that strong diurnal cycle exists during the pollution episodes, with heavier surface pollution in nocturnal periods. Contributions from transportation and agriculture are mainly restricted to the surface, while contributions from industry and power are distributed in a relatively higher layer. In the northerly-controlled episodes, the contribution of local emissions mainly accumulates below 300 m and the impact of the emissions from surrounding cities can reach 500-600 m during nocturnal periods. The contributions outside of Guangdong are uniformly distributed within 1000 m altitude. In the daytime, the contribution of emissions is basically uniform throughout the PBL. In the southerly-controlled episodes, the contribution of local emission mainly concentrates below 400 m during the nocturnal periods. Emissions from surrounding cities can exert the influence below 1000 m height, and the contribution outside of Guangdong reaches even 1500 m. In the daytime, the contribution of emissions in the PBL is distributed evenly. The highest altitude of the contribution from different subdomains that can reach is closely related to the physical property of the PBL. The industrial and agricultural emissions are the most important contributors for the surface PM_2.5 concentration. Results from emission reduction experiments show that PM_2.5 reduces significantly near the pollution center. Although control efficiency decreases with the increasing reduction ratio, the efficiency differences between 30% and 50% reduction is limited. In particular, 10% reduction in industrial emission causes PM_2.5 concentration to be slightly higher in the afternoon. Furthermore, below 200-m height, emission reduction experiments perform the effective reduction in PM_2.5 concentration, and higher reduction ratio results in larger reduced PM_2.5 concentration on almost all layers in the PBL.

Development of emissions inventory and identification of sources for priority control in the middle reaches of Yangtze River Urban Agglomerations

This paper presents the first attempt to investigate the emission source control of the Middle Reaches of Yangtze River Urban Agglomerations (MRYRUA), one of the national urban agglomerations in China. An emission inventory of the MRYRUA was developed as inputs to the CAMx model based on county-level activity data obtained by full-coverage investigation and source-based spatial surrogates. A classification technology method for priority control of atmospheric emission sources was introduced and applied in the MRYRUA for the evaluation of the emission sources control on the region-scale and city-scale, respectively. The results demonstrated that the emission sources in the Hefei-centered urban agglomerations contributed the biggest on the mean PM_2.5 concentrations of the MRYRUA and should be taken the priority to control. The emission sources in the Ma'anshan city, Xiangtan city, Hefei city and Wuhan city were the bigger contributors on the mean PM_2.5 concentrations of the MRYRUA among the cities and should be taken the priority to control. In generally, emission sources in cities along the Yangtze River and the tributary should be given the special attention for the regional air quality target attainments. This study can give an understanding of Chinese emissions and provide a valuable preference to policy makers for finding effective mitigation measures and control strategies for reducing national and regional air pollution in China.

Column-integrated aerosol optical properties of coarse- and fine-mode particles over the Pearl River Delta region in China

The sun-photometer data from 2011 to 2013 at Panyu site (Panyu) and from 2007 to 2013 at Dongguan site (Dg) in the Pearl River Delta region, were used for the retrieving of the aerosol optical depth (AOD), single scattering albedo (SSA), Ångström exponent (AE) and volume size distribution of coarse- and fine-mode particles. The coarse-mode particles presented low AOD (ranging from 0.05±0.03 to 0.08±0.05) but a strong absorption property (SSA ranged from 0.70±0.03 to 0.90±0.02) for the wavelengths between 440 and 1020nm. However, these coarse particles accounted for <10% of the total particles. The AOD of fine particles (AODf) was over 3 times as large as that of coarse particles (AODc). The fine particles SSA (SSAf) generally decreased as a function of wavelength, and the relatively lower SSAf value in summer was likely to be due to the stronger solar radiation and higher temperature. More than 70% of the aerosols at Panyu site were dominated by fine-mode absorbing particles, whereas about 70% of the particles at Dg site were attributed to fine-mode scattering particles. The differences of the aerosol optical properties between the two sites are likely associated with local emissions of the light-absorbing carbonaceous aerosols and the scattering aerosols (e.g., sulfate and nitrate particles) caused by the gas-phase oxidation of gaseous precursors (e.g., SO₂ and NO₂). The size distribution exhibited bimodal structures in which the accumulation mode was predominant. The fine-mode volume showed positive dependence on AOD (500nm), and the growth of peak value of the fine-mode volume was higher than that of the coarse volume. Both the AOD and SSA increased with increasing relative humidity (RH), while the AE decreased with increasing RH. These correlations imply that the aerosol properties are greatly modified by condensation growth.