Total OH Reactivity Measurements in a Suburban Site of Shanghai

Abstract

AbstractTotal hydroxyl radicals (OH) reactivity, as a measure of the OH total loss rate and the significance of atmospheric oxidation, is broadly used to evaluate the OH budget in the atmosphere. Here, measurements of the total OH reactivity were performed at a suburban air quality monitoring supersite (Dianshan Lake) of Shanghai, between 1 August and 15 September, 2020, using the Comparative Reactivity Method. The measured total OH reactivity ranged from 13.9 to 107.4 s−1 with an average of 38.4 s−1. Meanwhile, the calculated total OH reactivity varied between 8.9 and 84.4 s−1 from concentrations of atmospheric trace gases measured by multiple in‐situ techniques including an online gas chromatograph equipped with mass spectrometry and flame ionization detection (GC‐MS/FID) and two proton‐transfer‐reaction time‐of‐flight mass spectrometers (PTR‐ToF‐MSs). A 12.1% missing OH reactivity was evident from the comparison between the measured and calculated total OH reactivities. Inorganic trace gases, methane, and 54 observed Photochemical Assessment Monitoring Station compounds detected by GC‐MS/FID, accounted for 25.7%, 1.3%, and 10%, respectively, of the calculated total OH reactivity. In addition, 292 species detected by PTR‐ToF‐MSs contributed the rest 63%. The top 10 VOC contributors to the measured total OH reactivity were C5H8, CH2O, C2H4O, C6H6O, C4H8O, C3H6O, C10H16, C5H10O, C5H8O2, and C8H10, respectively, whose total contribution reached 42%. Correlation analysis for the missing OH reactivity suggests that primary biogenic VOCs could be a potential source for the missing OH reactivity during the daytime, and that secondary oxidation products might also be connected to the missing OH reactivity.