PM1 chemical composition and light absorption properties in urban and rural areas within Sichuan Basin, southwest China.

Sichuan Basin is encircled by high mountains and plateaus with the heights ranging from 1 km to 3 km, and is one of the most polluted regions in China. However, the dominant chemical species and light absorption properties of aerosol particles is still not clear in rural areas. Chemical composition in PM1 (airborne particulate matter with an aerodynamic diameter less than 1 µm) and light-absorbing properties were determined in Chengdu (urban) and Sanbacun (rural) in western Sichuan Basin (WSB), Southwest China. Carbonaceous aerosols and secondary inorganic ions (NH4+, NO3- and SO42-) dominate PM1 pollution, contributing more than 85% to PM1 mass at WSB. The mean concentrations of organic and elemental carbon (OC, EC), K+ and Cl- are 19.69 µg m-3, 8.00 µg m-3, 1.32 µg m-3, 1.16 µg m-3 at the rural site, which are 26.2%, 65.3%, 34.7% and 48.7% higher than those at the urban site, respectively. BrC (brown carbon) light absorption coefficient at 405 nm is 63.90 ± 27.81 M m-1 at the rural site, contributing more than half of total absorption, which is about five times higher than that at urban site (10.43 ± 4.74 M m-1). Compared with secondary OC, rural BrC light absorption more depends on primary OC from biomass and coal burning. The rural MAEBrC (BrC mass absorption efficiency) at 405 nm ranges from 0.6 to 5.1 m2 g-1 with mean value of 3.5 ± 0.8 m2 g-1, which is about three times higher than the urban site.

PM2.5 and O3 pollution during 2015-2019 over 367 Chinese cities: Spatiotemporal variations, meteorological and topographical impacts.

The strict Clean Air Action Plan has been in place by central and local government in China since 2013 to alleviate haze pollution. In response to implementation of the Plan, daytime PM2.5 (particulate matter with aerodynamic diameter less than 2.5 µm) showed significant downward trends from 2015 to 2019, with the largest reduction during spring and winter in the North China Plain. Unlike PM2.5, O3 (ozone) showed a general increasing trend, reaching 29.7 µg m-3 on summer afternoons. Increased O3 and reduced PM2.5 simultaneously occurred in more than half of Chinese cities, increasing to approximately three-fourths in summer. Declining trends in both PM2.5 and O3 occurred in only a few cities, varying from 19.1% of cities in summer to 33.7% in fall. Meteorological variables helped to decrease PM2.5 and O3 in some cities and increase PM2.5 and O3 in others, which is closely related to terrain. High wind speed and 24 h changing pressure favored PM2.5 dispersion and dilution, especially in winter in southern China. However, O3 was mainly affected by 24 h maximum temperature over most cities. Soil temperature was found to be a key factor modulating air pollution. Its impact on PM2.5 concentrations depended largely on soil depth and seasons; spring and fall soil temperature at 80 cm below the surface had largely negative impacts. Compared with PM2.5, O3 was more significantly affected by soil temperature, with the largest impact at 20 cm below the surface and with less seasonal variation.

Spatial patterns and temporal variations of six criteria air pollutants during 2015 to 2017 in the city clusters of Sichuan Basin, China.

Spatiotemporal variations of six criteria air pollutants and influencing factors in the city clusters of Sichuan Basin were studied based on real-time hourly concentrations of PM2.5 (the particles with diameters smaller than 2.5µm), PM10 (the particles with diameters smaller than 10µm), SO2, NO2, CO and O3 and routine meteorological data during the years from 2015 to 2017. The Sichuan Basin was further categorized into four regions: West, south, northeast Sichuan Basin (WSB, SSB and NESB) and plateau of west Sichuan Basin (PWSB) to better understand regional air pollution characteristics. Heavy air pollution was mainly induced by high PM2.5 or ozone concentrations in the cities clusters of Sichuan Basin. The compound air pollution characteristics existed in WSB with simultaneously high concentrations of PM2.5 and ozone, while PM2.5 concentrations in SSB were the highest among the four regions and especially in the city of Zigong with maximum PM2.5 concentration of 109.3µgm-3 in winter. The MDA8 (daily maximum 8-hour average surface O3 concentrations) more frequently exceeded CAAQS (Chinese Ambient Air Quality Standards) Grade I and II standards in Ziyang, Guang'an and Liangshan than the other cities maybe due to joint effects of industry emissions and regional transportation from surrounding cities. Annual (diurnal) variations of the pollutants with the exception of ozone showed "U" (flat "W") shape, while the ozone exhibited the opposite trends inside Sichuan Basin (WSB, SSB and NESB). Ozone pollution was more dependent on vehicle emissions inside Sichuan Basin, and industry had more important effects on ozone in the cities of PWSB with less vehicles. Severe ozone pollution can be formed easily under the weather conditions of high temperature, long sunshine duration and low RH (relative humidity) inside Sichuan Basin. High ozone concentrations in winter in PWSB may be partly transported from the other surrounding cities.

Two winter PM2.5 pollution types and the causes in the city clusters of Sichuan Basin, Western China.

PM2.5 pollution types were analyzed and the causes were uncovered in Sichuan Basin using PM2.5 data from Chinese Ministry of Environmental Protection (MEP) and multiple meteorology data during January 2015 to February 2017. The events that PM2.5 increased gradually in the periods longer than 10 days and then decreased sharply were defined as "Type I", while the symmetrical variations of PM2.5 during increasing and decreasing periods were defined as "Type II" of PM2.5 pollution. Five cases of Type I and two cases of Type II were identified during the study period inside the basin. The increasing rates were almost comparable between the two PM2.5 pollution types with the range from 4 µg m-3 d-1 to 8 µg m-3 d-1, while the decreasing rates of Type I were between 25 µg m-3 d-1 and 40 µg m-3 d-1, which were 3-5 times higher than those of Type II (~8 µg m-3 d-1). The rapid reduction of PM2.5 for Type I was mainly related to improvement of vertical and horizontal diffusion conditions induced by invasion of cold air masses, while slowly decreased PM2.5 for Type II was due largely to elevated horizontal wind speeds and shifted wind directions in the city clusters of the basin.

Annual and diurnal variations of gaseous and particulate pollutants in 31 provincial capital cities based on in situ air quality monitoring data from China National Environmental Monitoring Center.

Long-term air quality data with high temporal and spatial resolutions are needed to understand some important processes affecting the air quality and corresponding environmental and health effects. The annual and diurnal variations of each criteria pollutant including PM2.5 and PM10 (particulate matter with aerodynamic diameter less than 2.5 µm and 10 µm, respectively), CO (carbon monoxide), NO2 (nitrogen dioxide), SO2 (sulfur dioxide) and O3 (ozone) in 31 provincial capital cities between April 2014 and March 2015 were investigated by cluster analysis to evaluate current air pollution situations in China, and the cities were classified as severely, moderately, and slightly polluted cities according to the variations. The concentrations of air pollutants in winter months were significantly higher than those in other months with the exception of O3, and the cities with the highest CO and SO2 concentrations were located in northern China. The annual variation of PM2.5 concentrations in northern cities was bimodal with comparable peaks in October 2014 and January 2015, while that in southern China was unobvious with slightly high PM2.5 concentrations in winter months. The concentrations of particulate matter and trace gases from primary emissions (SO2 and CO) and NO2 were low in the afternoon (~16:00), while diurnal variation of O3 concentrations was opposite to that of other pollutants with the highest values in the afternoon. The most polluted cities were mainly located in North China Plain, while slightly polluted cities mostly focus on southern China and the cities with high altitude such as Lasa. This study provides a basis for the formulation of future urban air pollution control measures in China.

Urban particle size distributions during two contrasting dust events originating from Taklimakan and Gobi Deserts.

The dust origins of the two events were identified using HYSPLIT trajectory model and MODIS and CALIPSO satellite data to understand the particle size distribution during two contrasting dust events originated from Taklimakan and Gobi deserts. The supermicron particles significantly increased during the dust events. The dust event from Gobi desert affected significantly on the particles larger than 2.5 µm, while that from Taklimakan desert impacted obviously on the particles in 1.0-2.5 µm. It is found that the particle size distributions and their modal parameters such as VMD (volume median diameter) have significant difference for varying dust origins. The dust from Taklimakan desert was finer than that from Gobi desert also probably due to other influencing factors such as mixing between dust and urban emissions. Our findings illustrated the capacity of combining in situ, satellite data and trajectory model to characterize large-scale dust plumes with a variety of aerosol parameters.