Evaluation and Evolution of MAX-DOAS-observed Vertical NO2 Profiles in Urban Beijing

Multiaxis differential absorption spectroscopy (MAX-DOAS) is a newly developed advanced vertical profile detection method, but the vertical nitrogen dioxide (NO 2 ) profiles measured by MAX-DOAS have not yet been fully verified. In this study, we perform MAX-DOAS and tower gradient observations to simultaneously acquire tropospheric NO 2 observations in the Beijing urban area from 1 April to 31 May 2019. The average values of the tropospheric NO 2 vertical column densities measured by MAX-DOAS and the tropospheric monitoring instrument are 15.8 × 10 15 and 12.4 × 10 15 molecules cm −2 , respectively, and the correlation coefficient R reaches 0.87. The MAX-DOAS measurements are highly consistent with the tower-based in situ measurements, and the correlation coefficients R from the ground to the upper air are 0.89 (60 m), 0.87 (160 m), and 0.76 (280 m). MAX-DOAS accurately measures the trend of NO 2 vertical profile changes, although a large underestimation occurs by a factor of two. By analyzing the NO 2 vertical profile, the NO 2 concentration reveals an exponential decrease with height. The NO 2 vertical profile also coincides with the evolution of the boundary layer height. The study shows that the NO 2 over Beijing mainly originates from local sources and occurs in the boundary layer, and its vertical evolution pattern has an important guiding significance to better understand nitrate production and ozone pollution.