The First Observation of HOx radicals in the Chengyu Urban Agglomeration

A comprehensive field campaign was carried out in summer 2019 in Chengdu, which obtained the first complete radical dataset of Chengyu Urban Agglomeration. Observed daily concentration maxima of radicals by the laser-induced-fluorescence (LIF) technique were in the range of (2-10)×10⁶ cm^-3 for OH and (4-15)×10⁸ cm^-3 for HO₂. During daytime, OH reactivities were generally high (5-32 s^-1). The missing reactivity was not be observed within uncertainty, and inorganics, observed VOCs and the calculated oxidation products contributed about one-third in total reactivity, respectively.

The chemical box model RACM 2 was used to interpret the observed radical concentrations. The model over-predicted OH and HO₂ at noon during the O₃ polluted episode. Constraining the model by the observed HO₂ concentration, the overestimation of OH can be explained almost by the overestimation of HO₂. Besides, as in the previous field campaigns (e.g. Pennsylvania, Mexico City, New York and so on), the underestimation of the net conversion of OH into HO₂ enlarged with the increasing NO concentration, indicating the conversion of HO₂ into OH still need to be studied based on the discussion above. Different schemes to improve the agreement between observed and modelled HO₂ were explored in this work. The sensitivity tests indicated observed and modelled HO₂ can be agreed well by reducing the HO₂ yield in the reaction of OH and HCHO a half.

The oxidation rate of primary pollutants dominated by OH radicals was significantly higher than that in winter Beijing, which contributes significantly to secondary pollution, especially O₃. Besides, the atmospheric self-cleaning ability and recycling efficiency both peaked for about 600 pptv of NO, indicating small amounts of NO can help to maintain the atmospheric oxidation. The campaign emphasizes the important role of HO₂ yield in the reaction channels of OH and VOCs especially, and the need for further laboratory experiments of the HO₂ yield measurement in order to understand radical chemistry in VOC-rich air.