Particle-Phase Photoreactions of HULIS and TMIs Establish a Strong Source of HO and Particulate Sulfate in the Winter North China Plain.

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase HO formation through HO recombination. Surprisingly, HO mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of HO, driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state HO mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, HO formation rates of about 0.2 ppbv h during the initial irradiation periods are consistent with the HO rates observed in the field. A novel chemical mechanism was developed explaining the in-particle HO formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM ≥ 75 μg m agree with the observed HO concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO reaction with OH and HSO oxidation by HO. Overall, under high pollution, the HO-caused sulfate formation rate is above 250 ng m h, contributing to the sulfate formation by more than 70%.