Observations of the vertical distributions of summertime atmospheric pollutants in Nam Co: OH production and source analysis

Abstract. The Tibetan Plateau (TP) plays a key role in the regional environment and global climate change; however, the lack of vertical observations of atmospheric species, such as HONO and O3, hinders a deeper understanding of the atmospheric chemistry and atmospheric oxidation capacity (AOC) on the TP. In this study, we conducted multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements at Nam Co, the central TP, to observe the vertical profiles of aerosol, water vapor (H2O), NO2, HONO and O3 from May to July 2019. In addition to NO2 mainly exhibiting a Gaussian shape with the maximum value appearing at 300=-400 m, the other four species all showed an exponential shape and decreased with the increase in height. The maximum values of monthly averaged aerosol (0.17 km−1) and O3 (66.71 ppb) occurred in May, H2O (3.68 × 1017 molec. cm−3) and HONO (0.13 ppb) appeared in July, and NO2 (0.39 ppb) occurred in June at the 200–400 m layer. H2O, HONO and O3 all exhibited a multi-peak pattern, and aerosol appeared to have a bi-peak pattern for its averaged diurnal variations. The averaged vertical profiles of OH production rates from O3 and HONO all exhibited an exponential shape decreasing with the increase in height, with maximum values of 2.61 and 0.49 ppb h−1 at the bottom layer, respectively. The total OH production rate contributed by HONO and O3 on the TP was obviously larger than that in low-altitude areas. In addition, source analysis was conducted for HONO and O3 at different height layers. The heterogeneous reaction of NO2 on wet surfaces was a significant source of HONO. The maximum values of HONO/NO2 appeared when H2O concentrations were approximately 1.0 × 1017 molec. cm−3 and aerosol concentrations were larger than 0.15 km−1 below 1.0 km. The maximum values were usually accompanied by H2O concentrations of 1.0–2.0 × 1017 molec. cm−3 and aerosol concentrations greater than 0.02 km−1 at 1.0–2.0 km. O3 was potentially sourced from the South Asian subcontinent and Himalayas through long-range transport. Our results contribute to the new understanding of vertical distribution of atmospheric components and explain the strong AOC on the TP.