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

Wintertime in situ measurements of OH, inline-formulaHO₂ and inline-formulaRO₂ radicals and OH reactivity were made in central Beijing during November and December 2016. Exceptionally elevated NO was observed on occasions, up to inline-formula∼250 inline-formulappbv. The daily maximum mixing ratios for radical species varied significantly day-to-day over the ranges 1–inline-formula8×10⁶ inline-formulacm⁻³ (OH), 0.2–inline-formula1.5×10⁸ inline-formulacm⁻³ (inline-formulaHO₂) and 0.3–inline-formula2.5×10⁸ inline-formulacm⁻³ (inline-formulaRO₂). Averaged over the full observation period, the mean daytime peak in radicals was inline-formula2.7×10⁶, inline-formula0.39×10⁸ and inline-formula0.88×10⁸ inline-formulacm⁻³ for OH, inline-formulaHO₂ and total inline-formulaRO₂, respectively. The main daytime source of new radicals via initiation processes (primary production) was the photolysis of HONO (inline-formula∼83 %), and the dominant termination pathways were the reactions of OH with NO and inline-formulaNO₂, 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, inline-formulaHO₂ and inline-formulaRO₂. The model underpredicted OH, inline-formulaHO₂ and inline-formulaRO₂, especially when NO mixing ratios were high (above 6 inline-formulappbv). The observation-to-model ratio of OH, inline-formulaHO₂ and inline-formulaRO₂ increased from inline-formula∼1 (for all radicals) at 3 inline-formulappbv of NO to a factor of inline-formula∼3, inline-formula∼20 and inline-formula∼91 for OH, inline-formulaHO₂ and inline-formulaRO₂, respectively, at inline-formula∼200 inline-formulappbv of NO. The significant underprediction of radical concentrations by the MCM suggests a deficiency in the representation of gas-phase chemistry at high NOinline-formula_x. The OH concentrations were surprisingly similar (within 20 % during the day) in and outside of haze events, despite inline-formulaj(inline-formulaO¹D) 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.