| Summary: | The vertical profiles of nitrogen dioxide (NO<sub>2</sub>) 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<sub>2</sub> and HCHO as well as the sensitivity of ozone (O<sub>3</sub>) production to the concentration ratio of HCHO to NO<sub>2</sub>. The largest NO<sub>2</sub> 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<sup>15</sup> molecule·cm<sup>−2</sup> and 1.22 ± 1.09 ppb for NO<sub>2</sub>, and they were 1.19 ± 8.35 × 10<sup>16</sup> molecule·cm<sup>−2</sup> and 2.41 ± 3.26 ppb for HCHO. The VCDs and near-surface VMRs for NO<sub>2</sub> were high in the cold months and low in the warm months, while HCHO presented the opposite. The larger near-surface NO<sub>2</sub> 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<sub>3</sub> production at the Longfengshan station was mainly in the NO<sub>x</sub>-limited regime. This is the first study presenting the vertical distributions of NO<sub>2</sub> 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.
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