[Characteristics and Meteorological Factors of Complex Nonattainment Pollution of Atmospheric Photochemical Oxidant (O) and PM in the Pearl River Delta Region, China].

Based on the atmospheric pollutant data from twelve monitoring sites in the Guangdong-Hong Kong-Macao Pearl River Delta Regional Air Quality Monitoring Network, the mass concentration trends of atmospheric photochemical oxidants (O, NO+O) and PM during 2013-2017 were studied. The complex nonattainment pollution of O and PM is defined as the daily average mass concentration of NO and PM and daily maximum 8 h average (O MDA8) mass concentration of O simultaneously that exceeds the Chinese grade Ⅱ national air quality standard. The characteristics and meteorological factors that influence the complex nonattainment pollution of O and PM at different types of areas were analyzed. The results indicate that from 2013 to 2017, the annual average mass concentration of PM in the Pearl River Delta (PRD) region decreased from (44±7) μg·m to (32±4) μg·m, which met the annual standard for three consecutive years. The annual average mass concentration of O decreased from (127±14) μg·m in 2013 to (114±12) μg·m in 2016 and then showed a general rebound trend to (129±13) μg·m in 2017 when O concentrations increased significantly (10 μg·m). The proportion of pollution processes with O as the primary pollutant increased from 33% in 2013 to 78% in 2017, and the regional characteristics of simultaneous pollution in multiple cities have been highlighted. The complex nonattainment pollution of O and PM occurred 60 times during the study period, primarily in urban sites (78%) and suburban sites (22%). The largest number of days of complex nonattainment pollution occurred in autumn (52%) because of strong solar radiation that was conducive to ozone formation, and consequently, the high oxidization of the atmosphere promoted the secondary generation of PM. The weather conditions that caused the complex nonattainment pollution in the PRD mainly include outflow-high-pressures (43%), subtropical-high-pressures(30%), and tropical-depressions (27%). In terms of specific meteorological conditions, when the temperature was in the range of 20-25℃ and relative humidity was in the range of 60%-75%, the proportion of complex nonattainment pollution was the highest (22%). When O pollution was substantial, the high relative humidity and low wind speed during the nighttime caused the concentration of NO and PM to rise significantly, and then the high temperatures during the day aggravated the complex nonattainment pollution.