Observation based study on atmospheric oxidation capacity in Shanghai during late-autumn: Contribution from nitryl chloride

Atmospheric oxidation processes are important to convert freshly emitted trace gases to secondary pollutants. ROx radicals (including hydroxyl radical (OH), hydroperoxyl radical (HO2), and organic peroxyl radical (RO2)) dominate the atmospheric oxidation capacity. In this study, measurements of RO(x & nbsp;& nbsp;)chemistry precursors and products are presented in Shanghai, a megacity in eastern China during late autumn. During the campaign, two types of air masses are identified, one of which is warm air parcel characterized by relatively higher concentrations of particulate matters and ozone. An observational based chemical box model is utilized to simulate the radical chemistry. HONO is the most important radical source contributing more than one-third of the total radical primary source. However, alkene ozonolysis reactions not only serve as a non-photolytic source but also become relative important during polluted cases indicating the important role in polluted conditions. The maximum ClNO2 was 0.4 ppbv (5min averaged) with a 1h diurnal averaged maximum of 0.16 ppbv and 0.06 ppbv for polluted and clean cases, respectively. The contribution of ClNO2 photolysis to the total primary radical production is < 1% on 24h averaged based. Though the campaign was conducted in late autumn when ClNO2 was considered as a relatively important radical source compared to summer conditions, our results suggest the contribution of ClNO2 may vary from case to case. The role of Cl radical and OH radical in secondary pollution formation is explored. Although the model predicts that the concentrations of Cl radicals are orders of magnitude smaller than that of OH radicals, further model analysis highlight that the ozone production per VOC oxidation is more efficient due to Cl radical initiated the reaction chain compared to OH radical. The ubiquitous presence of ClNO2 with large spatial and temporal variability demonstrates that the contribution of ClNO2 to photochemical processes should be further investigated with field observations in variable environments/seasons.