Rapid Increases of Ozone Concentrations over Tibetan Plateau Caused by Local and Non-Local Factors

Abstract. Changes in tropospheric ozone over the Tibetan Plateau (TP) profoundly affect the local ecosystems and human health. Yet previous studies on the TP ozone have focused on the background regions, with much less attention on the urban ozone. Here we quantify the ozone trends over the whole TP from 2015 to 2019 in the context of its long-term trends, with a focus on urban ozone. We use ozone measurements from 30 urban stations in 17 cities, the Waliguan baseline station, and four satellite products of tropospheric ozone. We further analyze the drivers of ozone trends through a combination of chemical transport model simulations, back-trajectory calculations, a bottom-up emission inventory, and a satellite-derived emission dataset of nitrogen oxides (NO_x). We find a strong increase in deseasonalized urban ozone at the MEE stations from 2015 to 2019 (by 1.71 ppb yr^-1). The urban ozone trend far exceeds the trend at Waliguan (by 0.26 ppb yr^-1) and the TP average trend (by up to 0.08 ppb yr^-1) derived from the four satellite products. Interannual variations in meteorology do not produce significant ozone trends over the TP. Non-local factors contribute positively to the urban ozone trends, due mainly to more frequent transport passing through the footprint layers (0–300 m above the ground) of non-local high-emission regions. Another important contributor to the urban ozone growth is the 26.5 % increase in local anthropogenic NO_x emissions. Emission reductions in both the local and non-local source regions can help mitigate the rapid urban ozone growth over the plateau.