Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a powerful remote sensing measurement technique which provides valuable vertical distribution information on atmospheric aerosols and trace gases ( Platt and Stutz, 2008). Consequently, it is important to understand the spatial and temporal variations in atmospheric NO 2 and HCHO for better air pollution management and control. The spatial distribution of NO 2 and HCHO is strongly related to their emissions due to their short atmospheric lifetimes. NO 2 and HCHO are also toxic to humans in high concentrations. Both NO 2 and HCHO contribute to the formation of secondary aerosols ( Jang and Kamens, 2001), while the contribution from HCHO is relatively small. Oxidation of methane ( CH 4) emitted from the ocean is the main source of HCHO over water. Major HCHO sources over the continents include the oxidation of VOCs emitted from plants, biomass burning, traffic and industrial emissions. VOCs also have significant impacts on the abundance of hydroxyl (OH) radicals in the atmosphere, which is the major oxidant in the troposphere. Therefore, it is widely used as an indicator of nonmethane volatile organic compounds (NMVOCs) ( Fried et al., 2011). HCHO is an intermediate product of the oxidation of almost all volatile organic compounds (VOCs). The emission of NO x shows a significant increasing trend in China due to the rapid industrialization and economic development in the last 2 decades ( Zhang et al., 2007 van der A et al., 2008 Zhao et al., 2013), making it one of the most severe air pollution problems. Major NO 2 sources include fossil fuel combustion, biomass burning, lightning and oxidation of ammonia ( Bond et al., 2001 Zhang et al., 2003). NO 2 is a catalyst for ozone ( O 3) formation in the troposphere, while also participating in the catalytic destruction of stratospheric O 3 ( Crutzen, 1970). Nitrogen dioxide ( NO 2) and formaldehyde (HCHO) are major atmospheric pollutants playing crucial roles in atmospheric chemistry. Our results provide a better understanding of the transportation and sources of pollutants over the Yangtze River Delta as well as the effect of emission control measures during large international events, which are important for the future design of air pollution control policies. The MAX-DOAS data show a significant reduction of ambient aerosol, NO 2 and HCHO (30 %–50 %) during the Youth Olympic Games. The MAX-DOAS data are also used to evaluate the effectiveness of air pollution control measures implemented during the Youth Olympic Games 2014. The result shows that the MAX-DOAS measurements are sensitive to the air pollution transportation in the Yangtze River Delta, indicating the air quality in Nanjing is significantly influenced by regional transportation of air pollutants. However, due to the short atmospheric lifetime of HCHO, the backward-propagated HCHO data do not show a strong spatial correlation with the OMI HCHO observations. The reconstructed NO 2 fields show a distinct agreement with OMI satellite observations. An age-weighted backward-propagation approach is applied to the MAX-DOAS measurements of NO 2 and HCHO to reconstruct the spatial distribution of NO 2 and HCHO over the Yangtze River Delta during summer and winter time. The comparison result of MAX-DOAS and OMI observations of HCHO VCD shows a good agreement with R of 0.75 and the slope of the regression line is 0.99. Replacing the a priori NO 2 profiles by the MAX-DOAS profiles in the OMI NO 2 VCD retrieval would increase the OMI NO 2 VCDs by ∼30 % with correlation nearly unchanged. However, OMI observations are on average a factor of 3 lower than the MAX-DOAS measurements. The comparison shows that the OMI observations of NO 2 correlate well with the MAX-DOAS data with Pearson correlation coefficient ( R) of 0.91. The MAX-DOAS measurements of NO 2 and HCHO vertical column densities (VCDs) are used to validate ozone monitoring instrument (OMI) satellite observations over Nanjing. Ground-based MAX-DOAS measurements were performed from April 2013 to February 2017. Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument. In this paper, we present long-term observations of atmospheric nitrogen dioxide ( NO 2) and formaldehyde (HCHO) in Nanjing using a
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