Light absorption properties of brown carbon aerosol after following the switch from coal to natural gas policy in China: A case study of winter particulate matter over Xi'an, northwestern China

Abstract

To control haze pollution, the Xi'an city in northwestern China replaced coal by natural gas in industrial and household heating at the end of 2017. Following this change we studied here the light absorption properties of brown carbon (BrC)in atmospheric particulate matter (PM2.5) over Xi'an in 2018. PM2.5 samples were collected from November 15 to December 25, 2018, and the light absorption properties of brown carbon in methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC) were analyzed. Compared to a previous study from the coal combustion period, the mass absorption efficiency (MAE) increased in the sampling period while the radiation-forced absorption fraction of brown carbon relative to elemental carbon reduced, indicating that the light-absorbing capacity and efficiency of brown carbon has improved. Furthermore, highly positive linear relationships (r > 0.80) occur between the absorption coefficient and typical products of biomass burning and vehicle emission, except for coal combustion (r = 0.57). Additionally, an estimation from the positive matrix factorization and multivariable linear regression model results showed that biomass burning, motor vehicle emission, and secondary source took up approximately 89.53% of AbsWSOC,365. The contribution of coal combustion to light absorption BrC was the lowest (7.36% for AbsMSOC,365 and 10.24% for AbsWSOC,365) in our study, which differs from previous studies in Xi'an. Therefore, the emission source of bulk coal combustion has a relatively small influence on the light absorption of BrC in Xi'an.