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. Wintertime in situ measurements of OH, HO2 andRO2 radicals and OH reactivity were made in central Beijingduring November and December 2016. Exceptionally elevated NO wasobserved on occasions, up to ∼250 ppbv. The dailymaximum mixing ratios for radical species varied significantlyday-to-day over the ranges 1–8×106 cm−3 (OH),0.2–1.5×108 cm−3 (HO2) and0.3–2.5×108 cm−3 (RO2). Averaged overthe full observation period, the mean daytime peak in radicals was2.7×106, 0.39×108 and 0.88×108 cm−3 for OH, HO2 and total RO2,respectively. The main daytime source of new radicals viainitiation processes (primary production) was the photolysis of HONO(∼83 %), and the dominant termination pathways were thereactions of OH with NO and NO2, particularly underpolluted haze conditions. The Master Chemical Mechanism (MCM) v3.3.1operating within a box model was used to simulate the concentrationsof OH, HO2 and RO2. The model underpredicted OH,HO2 and RO2, especially when NO mixing ratios werehigh (above 6 ppbv). The observation-to-model ratio of OH,HO2 and RO2 increased from ∼1 (for allradicals) at 3 ppbv of NO to a factor of ∼3, ∼20and ∼91 for OH, HO2 and RO2, respectively,at ∼200 ppbv of NO. The significant underprediction ofradical concentrations by the MCM suggests a deficiency in therepresentation of gas-phase chemistry at high NOx. The OHconcentrations were surprisingly similar (within 20 % during theday) in and outside of haze events, despite j(O1D)decreasing by 50 % during haze periods. These observations providestrong evidence that gas-phase oxidation by OH can continue togenerate secondary pollutants even under high-pollution episodes,despite the reduction in photolysis rates within haze.

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