MAX-DOAS Measurements of Tropospheric NO₂ and HCHO Vertical Profiles at the Longfengshan Regional Background Station in Northeastern China

The vertical profiles of nitrogen dioxide (NO₂) 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 NO₂ and HCHO as well as the sensitivity of ozone (O₃) production to the concentration ratio of HCHO to NO₂. The largest NO₂ 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 ×10¹⁵ molecule·cm^-2 and 1.22 ± 1.09 ppb for NO₂, and they were 1.19 ± 8.35 × 10¹⁶ molecule·cm^-2 and 2.41 ± 3.26 ppb for HCHO. The VCDs and near-surface VMRs for NO₂ were high in the cold months and low in the warm months, while HCHO presented the opposite. The larger near-surface NO₂ 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 O₃ production at the Longfengshan station was mainly in the NO_x-limited regime. This is the first study presenting the vertical distributions of NO₂ 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.