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

The Tibetan Plateau (TP) plays a key role in regional environment and global climate change, however, the lack of vertical observation hinders a deeper understanding of the atmospheric chemistry and atmospheric oxidation capacity (AOC) on the TP. In this study, we conducted MAX-DOAS measurements at Nam Co, central TP, to observe the vertical profiles of aerosol, water vapor, NO ₂, HONO and O ₃ from May to July 2019. In addition to NO ₂ mainly exhibiting a Gaussian shape with the maximum value appearing at 300–400 m, other four species all showed an exponential shape and decreased with the increase of height. The maximum values of monthly averaged aerosol (0.17 km ^-1) and O ₃ (66.71 ppb) occurred on May, water vapor (3.68×10 ¹⁷ molec cm ^-3) and HONO (0.13 ppb) appeared on July, while NO ₂ (0.39 ppb) occurred on June at 200–400 m layer. Water vapor, HONO and O ₃ all exhibited a multi-peak pattern, and aerosol appeared a bi-peak pattern for their averaged diurnal variation. Moreover, we found O ₃ and HONO were the main contributors to OH on the TP. The averaged vertical profiles of OH production rates from O ₃ and HONO all exhibited an exponential shape, and decreased with the increase of height with the maximum values of 2.61 ppb/h and 0.49 ppb/h at the bottom layer, respectively. In addition, source analysis for HONO and O ₃ were conducted based on vertical observations. The heterogeneous reaction of NO ₂ on wet surfaces was a significant source of HONO, which obviously associated with water vapor concentration and aerosol extinction. The maximum values of HONO/NO ₂ appeared around water vapor being 1.0×10 ¹⁷ molec cm ^-3 and aerosol being lager 0.15 km ^-1 under 1.0 km, and the maximum values usually accompanied with water vapor being 1.0–2.0×10 ¹⁷ molec cm ^-3 and aerosol being lager 0.02 km ^-1 at 1.0–2.0 km. O ₃ was potentially sourced from south Asian subcontinent and Himalayas through long-range transport. Our results enrich the new understanding of vertical distribution of atmospheric components and explained the strong AOC on the TP.