Identification Of Ozone Sensitivity For NO₂ And Secondary HCHO Based On MAX-DOAS Measurements In Northeast China

Formaldehyde（HCHO）can potentially influence tumor promotion within a specific concentration range and is highly associated with lung,leukemia and nasopharyngeal cancer.As one of the most abundant volatile organic compounds（VOCs）in the Earth’s atmosphere,HCHO plays a significant role in atmospheric photochemistry.For example,HCHO photolysis generates HO_x radical（OH+HO₂）,which strongly drive ozone（O₃）formation.Recently,the HCHO and O₃ concentrations have been increasing in China,affecting the atmospheric oxidative capacity and resulting in photochemical smog.Therefore,an investigation into variation characteristics is greatly needed,to distinguish the sources of HCHO and better understand O3-NOx-VOC sensitivity to improve air quality.Multi-axis Differential Optical Absorption Spectroscopy（MAX-DOAS）has been successfully used to detect the three-dimensional distribution of atmospheric HCHO as a convenient and fast remote sensing method.In this paper,the tropospheric HCHO concentration was observed in northeast China through MAX-DOAS observation,which is of great significance to understand the spatio-temporal variation characteristics,formation mechanism and source of air pollution in Northeast China.Based on MAX-DOAS observation,the vertical column concentration of tropospheric HCHO over Shenyang was inverted,the seasonal and diurnal variation characteristics of HCHO vertical column concentration in Shenyang were analyzed and the source contribution of HCHO was discussed.The results showed that the seasonal variation of HCHO VCD in troposphere was the maximum in August and the minimum in September,and the concentration of HCHO VCD was the maximum in summer due to plant emission and solar radiation intensity.From the diurnal variation of HCHO,HCHO VCD showed an overall upward trend from 8:00 to 12:00.HCHO VCD decreased slightly from 14:00 to 16:00 noon,which was influenced by the change of PBL except in summer.Correlation analysis was used to compare the concentration of HCHO VCD observed by MAX-DOAS and OMI.The correlation coefficient R reached0.76,and the OMI observation was higher than the concentration of MAX-DOAS HCHO VCD.This may be due to the different space coverage and the influence of aerosols and clouds on the sensitivity of OMI satellite observation.Multiple regression was used to analyze the relative contributions of directMultiple regression was used to analyze the relative contributions of direct emissions and secondary generation to HCHO level in different seasons in Shenyang.The results showed that the atmospheric HCHO in each season mainly came from secondary generation.The contribution of secondary generation to HCHO level in winter,spring,summer and autumn were 82.62%,83.90%,78.90%and 41.53%,respectively.The contribution of secondary emission in summer was the highest,reaching 6.30 ppbv,which may be caused by strong photochemical reaction in summer.The level of HCHO was almost unchanged in winter,indicating that primary discharge was not an important source of HCHO.The sensitivity of O₃generation in different seasons was estimated by using the ratio of secondary HCHO to NO₂.The generation control type of O₃accounted for93.67%,6.23%and 0.11%of the total measured value of VOCs control area,VOC-NO_x-limited area and NO_x-limited area system,respectively.In addition,the sensitivity of O₃generation in summer changes from the VOC-limited area in the morning to the VOC-NO_xco-control area in the afternoon.Therefore,this study provides information on HCHO sources and O₃production sensitivity to support strategic management decisions.