Light absorption properties of brown carbon aerosol after coal to natural gas switch policy in China: a case study in winter PM2.5 over Xi'an, northwestern China

In order to control the haze, Xi'an took the tentative measure of replacing coal with gas at end of 2017. As the previous studies reported, coal combustion was an important contributor to aerosol in Xi'an (Lei et al., 2018; Shen et al., 2017). And the implementation of abrogating coal combustion may change the composition and physicochemical properties, as well as the emission sources apportionment of aerosols in the atmosphere (Liu et al., 2019). This study concentrated on the light absorption properties of light-absorbing organic compounds (Brown Carbon, BrC) in PM_2.5 over Xi'an after the policy. The ambient PM_2.5 samples were collected in the period from November 15 to December 25. Using solvent extraction (methanol and water) for off-line membrane to characterize the particulate BrC in PM_2.5 samples. By measuring the optical parameters of the absorption coefficient, mass absorption efficiency and the absorption Angstrom exponent, the results were compared with the previous studies (Huang et al., 2018; Shen et al., 2017). And it showed that the ability and efficiency of light-absorbing of BrC had increased in the sampling period after the policy. Additionally, there were highly positive liner relationships between the typical products of biomass burning and vehicle emission except the coal combustion. And the sources identification by Positive matrix factorization (PMF) and multivariable linear regression model verified that biomass burning, motor vehicle emission and secondary source are the dominating contributors to the light absorption of BrC, taking up approximately 80%. Different from previous studies in Xi'an (Lei et al., 2018; Shen et al., 2017), the contribution of coal combustion to light absorption BrC was the smallest portion in our case. Moreover, the solar radiation forcing of BrC in methanol soluble organic carbon was 53.56%at the ultraviolet-visible range, which was less than the reported results in Xi'an. We inferred that the policy change the emission sources apportionment of BrC, as coal combustion was just a micro contributor to BrC over Xi'an. In order to more effectively reduce the source of light-absorbing BrC in PM_2.5, it is possible to start from biomass burning, motor vehicle emission, secondary source and other main sources.