Characteristics and Sources of Organic Aerosol in PM2.5 at Yangbajing in Tibetan Plateau

Abstract

The climate and environment of the Tibetan Plateau (TP) are sensitive to human activities. As a key component highly influencing global climate and environment, organic aerosol over the TP is investigated in this study, especially in organic composition and its sources. This study aims to explore the composition characteristics, seasonal variation, and potential sources of organic aerosol including two episodes through one-year sampling at the Yangbajing (YBJ) site in the TP. A total of 120 primary and secondary organic species as well as organic carbon (OC) and elemental carbon (EC) were measured in PM2.5. The annual average concentrations of OC, EC, and organic species were 3.12 ± 3.73 μgC/m3, 0.28 ± 0.18 μgC/m3 and 104 ± 36.6 ng/m3, respectively. High ratio of OC to EC (OC/EC) and low ratio of fatty acids to n-alkanes were found for aerosol in the TP. The OC/EC ratio was 11.8 ± 10.2 on average at the YBJ site and strongly correlated with levoglucosan, a tracer for biomass burning, but not so with typical secondary species, indicating that the high ratio of OC/EC over the TP was more influenced by biomass burning than secondary formation. The ratio of fatty acids to n-alkanes at YBJ was 1.39 ± 0.29 on average, lower than those reported over urban areas (3.13 ± 1.49), probably owing to less residential cooking activities. Seasonally, the non-monsoon season had significant increases in primary organic aerosol (POA) tracers, among which biomass burning tracers had a higher increase percentage than other specific tracers. On the contrary, the levels of secondary organic aerosol (SOA) tracers, especially isoprene- and monoterpene-derived SOA tracers were enhanced in the monsoon season (summer time), largely resulting from high emissions of biogenic volatile organic compounds. Two episodes characterized with much higher concentrations of organic species (1215 and 688 ng/m3) were observed in this study. One was characterized by significant enhancement of POA tracers with the air masses originated from South Asia, and the other showed large increases in SOA and POA tracers with the air masses transported across multiple regions. This study could help to better understand the influence of human activities on atmospheric organic aerosol in remote regions and how POA and SOA vary with seasons and in different episodes.