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 is
representative of the pristine atmosphere over the Eurasian continent. We
made long-term ground-based multi-axis differential optical absorption
spectroscopy (MAX-DOAS) measurements at WLG during the period 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. Averaging of 10 original spectra is found to be an “optimum option” for
reducing both the statistical error of the spectral retrieval and systematic
errors in the analysis. The dSCDs of NO2, SO2, HCHO, and BrO under
clear-sky and low-aerosol-load conditions are extracted from measured
spectra at different elevation angles 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 in a range of about 7 ppt (January) to 100 ppt (May) for
NO2, below 0.5 ppb for SO2, between 0.4 and 0.9 ppb for HCHO, and
lower than 0.3 ppt for BrO. The chemical box model simulations constrained
by the NO2 concentration from our MAX-DOAS measurements show that there
is a little net ozone loss (−0.8 ppb d−1) for the free-tropospheric
conditions and a little net ozone production (0.3 ppb d−1) for the
boundary layer conditions over WLG during summertime. Our study provides
valuable information and data sets for further investigating tropospheric
chemistry in the background atmosphere and its links to anthropogenic
activities.