Characteristics of Black Carbon and Fine Particle Concentrations and Influencing Factors over the Suburban Area of Southwest Chengdu City, China

Background, aim, and scope Sichuan Basin is one of the heavy air pollution regions in China due to the special topographic features and anthropogenic pollution emissions. Limited studies on black carbon aerosol （BC） and its ratio in PM_2.5 were available in this region.

Materials and methods Hourly concentrations of BC, PM_2.5, gaseous pollutants data and meteorological observation data were obtained from December 1, 2017 to November 30, 2018 in suburban area of southwest Chengdu city, Sichuan province. In this study, seasonal，monthly，and diurnal variations of BC, PM_2.5 concentrations, the ratio of BC in PM_2.5 were analyzed, and the relationship between BC and gaseous pollutants were studied. The influences of meteorological conditions on BC concentrations were also analyzed. The sources of BC were explored based on the Aethalometer model and the Hybrid Single Particle Lagrangian Integrated Trajectory model (Hysplit-4). Then the source apportionment of BC was investigated using potential source contribution function (PSCF) and concentration weighted trajectory (CWT).

Results In this study area, the BC hourly concentration ranged from 0.18 to 40.51 μg·m^-3, the annual average concentration of BC and PM_2.5 were 5.26±4.68 μg·m^-3 and 60.02 ± 46.91 μg·m^-3 respectively.The background concentrations of BC and PM_2.5 were 3.34 μg·m^-3 and 33.38 μg·m^-3 respectively.The BC concentration was the highest in the Winter (8.18 μg/m³) with the monthly mean of 11.11 μg/m³ peaking in December, followed by the Spring and Autumn, and the lowest in Summer (3.28 μg/m³) .The diurnal variations of BC and PM_2.5 showed typical bimodal patterns in four seasons mainly due to the influence of the boundary layer and human activities. The average ratio of BC in PM_2.5 was 9.16% ± 5.13%, which presented the diurnal change pattern were low in daytime and high at night. When the PM_2.5 concentrations increased, the ratio of BC in PM_2.5 decreased. The BC and PM_2.5 concentrations were highest in winter and lowest in summer, while the ratio of BC in PM₂.₅ varied in the opposite trend (the ratio is much higher in summer than that in winter). Gaseous pollutants CO and NO₂ had the strongest correlation with BC, while the correlation coefficient between BC and SO₂ was lower. The source apportionment of BC showed that the liquid fuel (e.g., vehicle emission) had higher contribution to total BC concentration during all seasons (ranging from 69% in winter to 82% in summer) than that of the solid fuel (e.g., coal and biomass combustion). The results PSCF and CWT showed that the BC in Chengdu was mainly affected by local and surrounding emissions. Pollutants from downtown area in the NW direction and outer suburbs in in the SW direction contribute more to the BC concentration in southwest suburban of Chengdu.

Conclusions and perspectives This study explored the influencing factors of BC and PM_2.5 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.The further study of BC source might have great significance for the accurate assessment of BC radiative and climate effects in Chengdu.

Key words: black carbon; the ratio of BC in PM_2.5; fine particles; characteristics; meteorological factors;source apportionment.

Acknowledgments:The study was supported by the National Key Research and Development Program of China [grant No.2016YFA0602004,No.2018YFC0214002] and Projects of Science and Technology Plan of Sichuan Province（grant No.2018JY0011,No.2018SZDZX0023)