Measurement report: Enhanced photochemical formation of formic and isocyanic acids in urban region aloft: insights from tower-based online gradient measurements

Abstract. Formic acid is the most abundant organic acid in the troposphere and has significant environmental and climatic impacts. Isocyanic acid poses severe threats to human health and could be formed through the degradation of formic acid. However, the lack of vertical observation information has strongly limited the understanding of their sources, particularly in urban regions with complex pollutant emissions. To address this issue, continuous (27 days) vertical gradient measurements (five heights between 5–320 m) of formic and isocyanic acids were made based on a tall tower in Beijing, China in summer of 2021. Results show that the respective mean mixing ratios of formic and isocyanic acids were 1.3±1.3 ppbv and 0.28±0.16 ppbv at 5 m and were 2.1±1.9 ppbv and 0.43±0.21 ppbv at 320 m during the campaign. The mixing ratios of formic and isocyanic acids were substantially enhanced in daytime and correlated with the diurnal change of ozone. Upon sunrise, the mixing ratios of formic and isocyanic acids at different heights simultaneously increased even in the residual layer. In addition, positive vertical gradients were observed for formic and isocyanic acids throughout the day. The afternoon peaks and positive vertical gradients of formic and isocyanic acids in nighttime indicate their dominant contributions from photochemical formations. Furthermore, the positive vertical gradients of formic and isocyanic acids in daytime imply the enhancement of their secondary formation in urban regions aloft, predominantly due to the enhancements of oxygenated volatile organic compounds. The formation pathway of isocyanic acid through HCOOH-CH₃NO-HNCO was enhanced with height but only accounted for a tiny fraction of its ambient abundance. The abundance and source contributions of formic and isocyanic acids in the atmospheric boundary layer may be highly underestimated when being derived from their ground-level measurements. With the aid of numerical modeling techniques, future studies could further identify key precursors that drive the rapid formation of formic and isocyanic acids, and quantitatively assess the impacts of the enhanced formation of the two acids aloft on their budgets at ground level.