MAX-DOAS measurements of NO 2 , SO 2 , HCHO and BrO at the Mt. Waliguan WMO/GAW global baseline station in the Tibetan Plateau

Abstract. Mt. Waliguan Observatory (WLG) is a World Meteorological Organization (WMO)/Global Atmosphere Watch (GAW) global baseline station in China. WLG is located at the northeastern part of the Tibetan plateau (36°17' N, 100°54' E, 3816 m a.s.l.) and has a representativeness of the pristine atmosphere over the Eurasian continent. We made long-term ground-based MAX-DOAS measurements at WLG during the years 2012–2015. In this study, we retrieve the differential slant column densities (dSCDs) and estimate the tropospheric background mixing ratios of different trace gases, including NO2, SO2, HCHO and BrO, using the measured spectra at WLG. We find averaging of 10 original spectra to be an optimum option for reducing both the statistical error of the spectral retrieval and systematic errors in the analysis. We retrieve the dSCDs of NO2, SO2, HCHO and BrO from measured spectra at different elevation angles under clear sky and low aerosol load conditions at WLG. By performing radiative transfer simulations with the model TRACY-2, we establish approximate relationships between the trace gas dSCDs at 1° elevation angle and the corresponding average tropospheric background volume mixing ratios. Mixing ratios of these trace gases in the lower troposphere over WLG are estimated to be between about 5 ppt (winter) and 70 ppt (summer) for NO2, fall below 0.5 ppb for SO2, range between about 0.3 and 0.7 ppb for HCHO, and be close to ~ 0 ppt for BrO. Our study provides valuable information and data set for further investigating tropospheric background levels of these trace gases and their relationship to anthropogenic activities.