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.

We performed a comprehensive and intensive field experiment including ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurement at Raoyang (115°44′ E, 38°14′ N; 20 m altitude) in summer (13 June–20 August) 2014. The NO₂ and HCHO profiles retrieved by MAX-DOAS take on different vertical distribution shapes, with the former declining with the increasing altitude and the latter having an elevated layer. The average levels of vertical column densities (VCDs) and near-surface volume mixing ratios (VMRs) were 1.02 ± 0.51 × 10¹⁶ molec·cm⁻² and 3.23 ± 2.70 ppb for NO₂ and 2.32 ± 0.56 × 10¹⁶ molec·cm⁻² and 5.62 ± 2.11 ppb for HCHO, respectively. The NO₂ and HCHO levels are closely connected with meteorological conditions, with the larger NO₂ VCDs being associated with lower temperature, higher relative humidity (RH) and lower planetary boundary layer height (PBLH). With respect to the diurnal variations of vertical distribution, the NO₂ in the residual layer gradually disappeared from 1.2 km height to the surface during the period of 7:00–11:00 Beijing time (BJ), and the near-surface NO₂ had larger VMRs in the early morning and evening than in the later morning and afternoon. An elevated HCHO layer was observed to occur persistently with the lifted layer height rising from ~0.5 km to ~1.0 km before 10:00 BJ; the near-surface HCHO VMRs gradually increased and peaked around 10:00 BJ. The ratios of HCHO to NO₂ (R_HCHO-NO2) were generally larger than two in the boundary layer from 11:00 BJ until 19:00 BJ, the time period when ozone photochemistry was most active. Thus, ozone (O₃) production was mainly in the NO_x-limited regime during the observation campaign, which was closely related to relatively high temperatures and low RH. The O₃ production regimes also changed with the wind’s direction. These results are significant to reveal the formation mechanism of O₃ pollution and develop strategies for controlling the O₃ photochemical pollution over the North China Plain.