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. Mt. Waliguan Observatory (WLG) is a World Meteorological Organization(WMO) Global Atmosphere Watch (GAW) global baseline station in China. WLG islocated at the northeastern part of the Tibetan Plateau (36∘17′ N, 100∘54′ E, 3816 m a.s.l.) and isrepresentative of the pristine atmosphere over the Eurasian continent. Wemade long-term ground-based multi-axis differential optical absorptionspectroscopy (MAX-DOAS) measurements at WLG during the period 2012–2015. Inthis study, we retrieve the differential slant column densities (dSCDs) andestimate the tropospheric background mixing ratios of different trace gases,including NO2, SO2, HCHO, and BrO, using the measured spectra atWLG. Averaging of 10 original spectra is found to be an “optimum option” forreducing both the statistical error of the spectral retrieval and systematicerrors in the analysis. The dSCDs of NO2, SO2, HCHO, and BrO underclear-sky and low-aerosol-load conditions are extracted from measuredspectra at different elevation angles at WLG. By performing radiativetransfer simulations with the model TRACY-2, we establish approximaterelationships between the trace gas dSCDs at 1∘ elevation angleand the corresponding average tropospheric background volume mixing ratios.Mixing ratios of these trace gases in the lower troposphere over WLG areestimated to be in a range of about 7 ppt (January) to 100 ppt (May) forNO2, below 0.5 ppb for SO2, between 0.4 and 0.9 ppb for HCHO, andlower than 0.3 ppt for BrO. The chemical box model simulations constrainedby the NO2 concentration from our MAX-DOAS measurements show that thereis a little net ozone loss (−0.8 ppb d−1) for the free-troposphericconditions and a little net ozone production (0.3 ppb d−1) for theboundary layer conditions over WLG during summertime. Our study providesvaluable information and data sets for further investigating troposphericchemistry in the background atmosphere and its links to anthropogenicactivities.

atmospheric chemistry and physics

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