Evaluating the sensitivity of radical chemistry and ozone formation to 1ambient VOCs and NOx in Beijing.

Measurements of OH, HO₂, RO₂-complex (alkene and aromatic-related RO₂) and total RO₂ radicals taken during the AIRPRO campaign in central Beijing in the summer of 2017, alongside observations of OH reactivity are presented. The concentrations of radicals were elevated with OH reaching up to 2.8 x 10⁷ molecule cm^-3, HO₂ peaked at 1 x 10⁹ molecule cm^-3 and the total RO₂ concentration reached 5.5 x 10⁹ molecule cm^-3. OH reactivity (k(OH)) peaked at 89 s^-1 during the night, with a minimum during the afternoons of ~22 s^-1 on average. An experimental budget analysis, in which the rates of production and destruction of the radicals are compared, highlighted that although the sources and sinks of OH were balanced under high NO concentrations, the OH sinks exceeded the known sources (by 15 ppbv hr^-1) under the very low NO conditions (<0.5 ppbv) experienced in the afternoons, demonstrating a missing OH source consistent with previous studies under high volatile organic compound (VOC), low NO loadings. Under the highest NO mixing ratios (104 ppbv), the HO₂ production rate exceeded the rate of destruction by ~ 50 ppbv hr^-1, whilst the rate of destruction of total-RO₂ exceeded the production by the same rate indicating that the net propagation rate of RO₂ to HO₂ may be substantially slower than assumed. If just 10% of the RO₂ radicals propagate to HO₂ upon reaction with NO, the HO₂ and RO₂ budgets could be closed at high NO, but at low NO this lower RO₂ to HO₂ propagation rate revealed a missing RO₂ sink that was similar in magnitude to the missing OH source. A detailed box model that incorporated the latest MCM chemical mechanism (MCM3.3.1) reproduced the observed OH concentrations well, but over-predicted the observed HO₂ under low concentrations of NO (<1 ppbv) and under-predicted RO₂ (both the complex-RO₂ fraction and other RO₂ types which we classify as simple-RO₂) most significantly at the highest NO concentrations. The model also under-predicted the observed k(OH) consistently by ~10 s^-1 across all NO[sub x] levels highlighting that the good agreement for OH was fortuitous due to a cancellation of missing OH source and sink terms in its budget. Including heterogeneous loss of HO₂ to aerosol surfaces did reduce the modelled HO₂ concentrations in-line with the observations, but only at NO mixing ratios <0.3 ppbv. The inclusion of Cl atoms, formed from the photolysis of nitryl chloride, enhanced the modelled RO₂ concentration on several mornings when the Cl atom concentration was calculated to exceed 1 x 10⁴ atoms cm^-3 and could reconcile the modelled and measured RO₂ concentrations at these times. However, on other mornings, when the Cl atom concentration was lower, large under-predictions in total RO₂ remained. Furthermore, the inclusion of Cl atom chemistry did not enhance the modelled RO₂ beyond the first few hours after sunrise and so was unable to resolve the modelled under-prediction in RO₂ observed at other times of the day. Model scenarios, in which missing VOC reactivity was included as an additional reaction that converted OH to RO₂, highlighted that the modelled OH, HO₂ and RO₂ concentrations were sensitive to the choice of RO₂ product. The level of modelled to measured agreement for HO₂ and RO₂ (both complex and simple) could be improved if the missing OH reactivity formed a larger RO₂ species that was able to undergo reaction with NO, followed by isomerisation reactions reforming other RO₂ species, before eventually generating HO₂. In this work an α-pinene-derived RO₂ species was used as an example. In this simulation, consistent with the experimental budget analysis, the model underestimated the observed OH indicating a missing OH source. The model uncertainty, with regards to the types of RO₂ species present and the radicals they form upon reaction with NO (HO₂ directly or another RO₂ species), leads to over an order of magnitude less O₃ production calculated from the predicted peroxy radicals than calculated from the observed peroxy radicals at the highest NO concentrations. This demonstrates the rate at which the larger RO₂ species propagate to HO₂ or to another RO₂ or indeed to OH needs to be understood to accurately simulate the rate of ozone production in environments such as Beijing where large multifunctional VOCs are likely present. [ABSTRACT FROM AUTHOR]