Quantifying variability, source, and transport of CO over the Himalayas and Tibetan Plateau

Abstract. Atmospheric pollutants over the Himalayas and Tibetan Plateau (HTP) have potential implications for accelerating the melting of glaciers, damaging air quality, water sources and grasslands, and threatening climate on regional and global scales. Improved knowledge of the variabilities, sources, drivers, and transport pathways of atmospheric pollutants over the HTP is significant for regulatory and control purpose. In this study, we first quantify the variability, source, and transport of CO over the HTP by using in situ measurement, GEOS-Chem model tagged CO simulation, and atmospheric circulation pattern techniques. Diurnal, seasonal, and interannual variability of CO over the HTP are investigated with ~ 6 years (January 2015 to July 2020) of surface CO measurements in eight cities over the HTP. Annual mean of surface CO volume mixing ratio (VMR) over the HTP varied over 318.3 ± 71.6 to 901.6 ± 472.2 ppbv, and a large seasonal cycle was observed with high levels of CO in the late autumn to spring and low levels of CO in summer to early autumn. The diurnal cycle is characterized by a bimodal pattern with two maximums in later morning and midnight, respectively. Surface CO VMR from 2015–2020 in most cities over the HTP showed negative trends. The in situ CO measurements are for the first time used to assess the performance of GEOS-Chem full-chemistry model for the specifics of the HTP. Generally, GEOS-Chem can capture the measured variability of low CO levels but shows large discrepancies in high CO levels. Distinct dependencies on a short lifetime species of NO2 almost in all cities over the HTP were observed, implying local emissions to be predominant. By turning off the emission inventories within the HTP in GEOS-Chem tagged CO simulation, the relative contribution of long range transport was evaluated. The results disclosed that transport ratios of primary anthropogenic source, primary biomass burning (BB) source, and secondary oxidation source to the surface CO VMR over the HTP varied over 35 to 61 %, 5 to 21 %, and 30 to 56 %, respectively. The anthropogenic contribution is dominated by the South Asia and East Asia (SEAS) region throughout the year (58 % to 91 %). The BB contribution is dominated by the SEAS region in spring (25 to 80 %) and the Africa (AF) region in July–February (30–70 %). This study concluded that the anthropogenic and oxidation sources originating either local or in SEAS region dominated the surface CO over the HTP, which is different from the black carbon that is mainly attributed to BB source from SEAS region. The decreasing trends in surface CO VMR since 2015 in most cities over the HTP are attributed to the reduction in local and transported CO emissions in recent years.