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Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (Max-DOAS) measurements of nitrogen dioxide (NO 2) were continuously obtained from January to November 2019 in northeastern China (NEC). Seasonal variations in the mean NO 2 vertical column densities (VCDs) were apparent, with a maximum of 2.9 × 1016 molecules cm−2 in the winter due to enhanced NO 2 emissions from coal-fired winter heating, a longer photochemical lifetime and atmospheric transport. Daily maximum and minimum NO 2 VCDs were observed, independent of the season, at around 11:00 and 13:00 local time, respectively, and the most obvious increases and decreases occurred in the winter and autumn, respectively. The mean diurnal NO 2 VCDs at 11:00 increased to at 08:00 by 1.6, 5.8, and 6.7 × 1015 molecules cm−2 in the summer, autumn and winter, respectively, due to increased NO 2 emissions, and then decreased by 2.8, 4.2, and 5.1 × 1015 molecules cm−2 at 13:00 in the spring, summer, and autumn, respectively. This was due to strong solar radiation and increased planetary boundary layer height. There was no obvious weekend effect, and the NO 2 VCDs only decreased by about 10% on the weekends. We evaluated the contributions of emissions and transport in the different seasons to the NO 2 VCDs using a generalized additive model, where the contributions of local emissions to the total in the spring, summer, autumn, and winter were 89 ± 12%, 92 ± 11%, 86 ± 12%, and 72 ± 16%, respectively. The contribution of regional transport reached 26% in the winter, and this high contribution value was mainly correlated with the northeast wind, which was due to the transport channel of air pollutants along the Changbai Mountains in NEC. The NO 2 /SO 2 ratio was used to identify NO 2 from industrial sources and vehicle exhaust. The contribution of industrial NO 2 VCD sources was >66.3 ± 16% in Shenyang due to the large amount of coal combustion from heavy industrial activity, which emitted large amounts of NO 2. Our results suggest that air quality management in Shenyang should consider reductions in local NO 2 emissions from industrial sources along with regional cooperative control. [Display omitted] • NO 2 vertical column densities (VCDs) peaked in winter due to enhanced heating emissions. • NO 2 VCDs showed an uptrend from 08:00 to 11:00 and had a slight decrease at noon. • NO 2 was mainly sourced from local industrial NO 2 emissions in Shenyang. • Regional transport from the northeastern direction contributed 26% to NO 2 in winter. [ABSTRACT FROM AUTHOR]

[Display omitted] • HCHO VCDs peaked in summer affected by oxidation of BVOCs in Shenyang. • The contribution of primary and secondary sources of HCHO were 8.36% and 73.74%, respectively. • The accuracy of O 3 sensitivity was improved 12% by using HCHO sec /NO 2 in summer. • The hourly O 3 sensitivities were mainly VOC-limited during the whole year. In this study, tropospheric formaldehyde (HCHO) vertical column densities (VCDs) were measured using multi-axis differential optical absorption spectroscopy (MAX-DOAS) from January to November 2019 in Shenyang, Northeast China. The maximum HCHO VCD value appeared in the summer (1.74 × 1016 molec/cm2), due to increased photo-oxidation of volatile organic compounds (VOCs). HCHO concentrations increased from 08:00 and peaked near 13:00, which was mainly attributed to the increased release of isoprene from plants and enhanced photolysis at noon. The HCHO VCDs observed by MAX-DOAS and OMI have a good correlation coefficient (R) of 0.78, and the contributions from primary and secondary HCHO sources were distinguished by the multi-linear regression model. The anthropogenic emissions showed unobvious seasonal variations, and the primary HCHO was relatively stable in Shenyang. Secondary HCHO contributed 82.62%, 83.90%, 78.90%, and 41.53% to the total measured ambient HCHO during the winter, spring, summer, and autumn, respectively. We also found a good correlation (R = 0.78) between enhanced vegetation index (EVI) and HCHO VCDs, indicating that the oxidation of biogenic volatile organic compounds (BVOCs) was the main source of HCHO. The ratio of secondary HCHO to nitrogen dioxide (NO 2) was used as the tracer to analyze O 3 -NO x -VOC sensitivities. We found that the VOC-limited, VOC-NO x -limited, and NO x -limited regimes made up 93.67%, 6.23%, 0.11% of the overall measurements, respectively. In addition, summertime ozone (O 3) sensitivity changed from VOC-limited in the morning to VOC-NO x -limited in the afternoon. Therefore, this study offers information on HCHO sources and corresponding O 3 production sensitivities to support strategic management decisions. [ABSTRACT FROM AUTHOR]