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 (NO₂) 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 NO₂ observations in the Beijing urban area from 1 April to 31 May 2019. The average values of the tropospheric NO₂ vertical column densities measured by MAX-DOAS and the tropospheric monitoring instrument are 15.8 × 10¹⁵ and 12.4 × 10¹⁵ 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 NO₂ vertical profile changes, although a large underestimation occurs by a factor of two. By analyzing the NO₂ vertical profile, the NO₂ concentration reveals an exponential decrease with height. The NO₂ vertical profile also coincides with the evolution of the boundary layer height. The study shows that the NO₂ 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.