Dominant Influence of Biomass Combustion and Cross-Border Transport on Nitrogen-Containing Organic Compound Levels in the Southeastern Tibetan Plateau

Abstract. The Tibetan Plateau (TP) is highly susceptible to climate change and the nitrogen-containing organic compounds (NOCs) in fine particulate matter (PM_2.5) represent one of the large uncertainties in affecting the climate in high-altitude areas. Previous studies have shown that NOCs play a vital role in the nitrogen budget of PM_2.5. However, our understanding of the composition and sources of NOCs in PM_2.5, particularly in TP, is limited. Here, we aim to enhance our understanding of NOCs in the TP region by examining their identification, concentration levels, sources, and origins. We conducted field sampling at a regional background sampling site in Gaomeigu, in the southeastern margin of TP from March 11^th to May 13^th in 2017. The daily mass concentrations of NOCs ranged from 714.4 to 3887.1 ng m^-3, with an average of (2119.4 ± 875.0 ng m^-3) during the campaign. This average concentration was approximately 40 % higher than that reported at a typical regional site in the North China Plain (NCP), highlighting a more significant presence of NOCs in the Tibetan area. Biomass burning and secondary sources were identified as the major contributors to total NOCs. This was further substantiated by a regional air quality model, which indicated that over 80 % of the aerosol in the southeast of TP originated from neighboring countries. This study enhances our understanding of NOCs’ contribution to PM_2.5 in TP and their potential impacts on the climate stability in high-altitude areas.