Abstract: Background, aim, and scope Black carbon (BC) is an aerosol component produced by the incomplete combustion of fossil fuels and biomass. As a component of PM2.5, it has been associated with air quality, climate change and human health. Due to its twofold effects on air quality and climate change, BC is of wide concern in the atmospheric sciences. Sichuan Basin is experiencing severe air pollution during wintertime in recent years, while limited studies focusing on the BC aerosols in this region. The objective of this study is to reveal the basic feature, main sources of BC and influencing factors, to provide a reliable knowledge for air pollution and BC contral. Materials and methods Black carbon was measured at Chengdu Suburban site from November 2017 to December 2018 using a seven-channel Aethalometer (AE-33). Only BC concentration with 880 nm wavelength was employed in this study. PM2.5, SO2, NO, CO2 and ground-based meteorological data were derived from Shuangliu weather station in Chengdu.In this study, seasonal, monthly, and diurnal changes of BC and PM2.5 mass concentrations, the ratio of BC in PM2.5 were analyzed over suburban of southwest Chengdu. The relationship between BC and gaseous pollutants and the influences of meteorological conditions on BC concentrations were analyzed. Results BC concentration ranged from 0.18 to 40.51 μg·m-3, with an average of 5.26±4.68 μg·m-3. PM2.5 concentration ranged from 1 to 44.5 μg·m-3, with an average of 60.02 ± 46.91 μg·m-3.The background concentrations of BC and PM2.5 were 3.34 μg·m-3 and 33.38 μg·m-3 respectively. The average ratio of BC in PM2.5 was 9.16% ± 5.13%. Both the concentrations and the ratio of BC in PM2.5 presented clear diurnal variations, characterized by bimodal types.Two peaks appeared at 7:00~9:00 in the morning and 22:00~23:00 in the evening and the lowest value was 15:00~17:00 in the afternoon. When PM2.5 concentrations increased, the ratio of BC in PM2.5 decreased. The BC and PM2.5 concentrations were highest in winter and lowest in summer, while the ratio of BC in PM2.5 varied in the opposite trend. Gaseous pollutants CO and NO2 had the strongest correlation with BC, while the correlation coefficient between BC and SO2 was lower. The BC concentration generally higher when the wind speed lower than 2 m/s. High BC concentration (>20 μg·m-3 ) mainly occurred in NE, SW and NE winds. Discussion In this study, we only used one site data observed one year in suburban of southwest Chengdu to investigate and discuss the characteristics of BC, PM2.5 and their relationship as well as influencing factors. Based on statistical analysis on BC and gaseous pollutants concentrations, we identified the important contributions of the biomass burning and motor vehicle exhaust to BC emission of this region. In this study, when the PM2.5 concentrations increased, the ratio of BC in PM2.5 decreased. Compared with other research results, the characteristics of the ratio of BC in PM2.5 is different owing to the differences in the meteorological factors and sources of BC in various regions. Conclusions The temporal and spatial characteristics of BC over suburban of southwest Chengdu were influenced by the local source emissions and meteorological conditions. Pollutants from downtown area in the NW direction and outer suburbs in in the SW direction contribute more to the BC concentration. Recommendations and perspectives This study explored the influencing factors of BC concentration level, revealed the air quality situation in Chengdu suburban, which provides some reference for the government to make scientific policy to prevent and control the air pollution in Chengdu.
Keywords: black carbon; the ratio of BC in PM2.5; fine particles; characteristics; meteorological factors