Elevated levels of OH observed in haze events during wintertime in central Beijing

Wintertime in situ measurements of OH, HO2 and RO2 radicals and OH reactivity were made in central Beijing during November and December 2016. Exceptionally elevated NO was observed on occasions, up to ∼250 ppbv. The daily maximum mixing ratios for radical species varied significantly day-to-day over the ranges 1–8×106 cm−3 (OH), 0.2–1.5×108 cm−3 (HO2) and 0.3–2.5×108 cm−3 (RO2). Averaged over the full observation period, the mean daytime peak in radicals was 2.7×106, 0.39×108 and 0.88×108 cm−3 for OH, HO2 and total RO2, respectively. The main daytime source of new radicals via initiation processes (primary production) was the photolysis of HONO (∼83 %), and the dominant termination pathways were the reactions of OH with NO and NO2, particularly under polluted haze conditions. The Master Chemical Mechanism (MCM) v3.3.1 operating within a box model was used to simulate the concentrations of OH, HO2 and RO2. The model underpredicted OH, HO2 and RO2, especially when NO mixing ratios were high (above 6 ppbv). The observation-to-model ratio of OH, HO2 and RO2 increased from ∼1 (for all radicals) at 3 ppbv of NO to a factor of ∼3, ∼20 and ∼91 for OH, HO2 and RO2, respectively, at ∼200 ppbv of NO. The significant underprediction of radical concentrations by the MCM suggests a deficiency in the representation of gas-phase chemistry at high NOx. The OH concentrations were surprisingly similar (within 20 % during the day) in and outside of haze events, despite j(O1D) decreasing by 50 % during haze periods. These observations provide strong evidence that gas-phase oxidation by OH can continue to generate secondary pollutants even under high-pollution episodes, despite the reduction in photolysis rates within haze