MAX-DOAS Measurements of Tropospheric NO(2) and HCHO Vertical Profiles at the Longfengshan Regional Background Station in Northeastern China

The vertical profiles of nitrogen dioxide (NO(2)) and formaldehyde (HCHO) in the troposphere at the Longfengshan (LFS) regional atmospheric background station (127°36′ E, 44°44′ N, 330.5 m above sea level) from 24 October 2020 to 13 October 2021 were retrieved from solar scattering spectra by multi-axis differential optical absorption spectroscopy (MAX-DOAS). We analyzed the temporal variations of NO(2) and HCHO as well as the sensitivity of ozone (O(3)) production to the concentration ratio of HCHO to NO(2). The largest NO(2) volume mixing ratios (VMRs) occur in the near-surface layer for each month, with high values concentrated in the morning and evening. HCHO has an elevated layer around the altitude of 1.4 km consistently. The means ± standard deviations of vertical column densities (VCDs) and near-surface VMRs were 4.69 ± 3.72 ×10(15) molecule·cm(−2) and 1.22 ± 1.09 ppb for NO(2), and they were 1.19 ± 8.35 × 10(16) molecule·cm(−2) and 2.41 ± 3.26 ppb for HCHO. The VCDs and near-surface VMRs for NO(2) were high in the cold months and low in the warm months, while HCHO presented the opposite. The larger near-surface NO(2) VMRs appeared in the condition associated with lower temperature and higher humidity, but this relationship was not found between HCHO and temperature. We also found the O(3) production at the Longfengshan station was mainly in the NO(x)-limited regime. This is the first study presenting the vertical distributions of NO(2) and HCHO in the regional background atmosphere of northeastern China, which are significant to enhancing the understanding of background atmospheric chemistry and regional ozone pollution processes.