Sources and variability of surface ozone over the Tibetan Plateau revealed by in situ observations and EMAC model simulations

Abstract. Ozone variability over the Tibetan Plateau (TP) is complex, and our understanding of the factors driving surface ozone variations on the plateau remains incomplete. We combine in-situ observations from the Nam Co (NMC) and Nepal Climate Observatory-Pyramid (PYR) stations with the global atmospheric chemistry-climate model EMAC to analyze the sources and spatiotemporal variations of surface ozone over TP. The three ozone source regions, i.e., the Northern Hemisphere and Tropical Stratospheric Source (NTST), Northern Hemisphere Mid-High Latitude Tropospheric Source (NHTS), and Tropical Tropospheric Source (TRTS), are considered. The results reveal strong seasonal and daily variability in the contributions of these sources to TP surface ozone. Regulated by the changes in the position and intensity of the subtropical westerly jet, surface NTST-O3 is highest in spring, reaching a daily maximum contribution of 30 % at both stations. The efficient transport of NHTS-O3 by zonal circulation from source areas, including Central Asia, West Asia, and Europe, results in a summertime maximum of surface ozone in the northern TP, and even influences surface ozone on the central and southern TP, with a daily maximum contribution of 62 % at NMC and 58 % at PYR in summer. During the pre-monsoon period, enhanced TRTS-O3 in South and Southeast Asia leads to annual surface ozone peaks in the southern and central TP, with a daily maximum contribution of 93 % at NMC and 98 % at PYR. With ongoing changes in the emissions of ozone precursors from the aforementioned areas, impacts on surface ozone over TP require continued investigation.