OH, HO 2 , and NO in two biomass burning plumes: Sources of HO x and implications for ozone production

The ER‐2 made two descents through upper tropospheric biomass burning plumes during ASHOE/MAESA. HO x (= OH + HO 2 ) concentrations are largely self‐limited outside the plumes, but become progressively more limited by reactions with NO x (= NO + NO 2 ) at the higher NO x concentrations inside the plumes. Sources of HO x in addition to H 2 O and CH 4 oxidation are required to balance the known HO x sinks both in the plumes and in the background upper troposphere. HO x concentrations were consistently underestimated by a model constrained by observed NO x concentrations. The size of the model underestimate is reduced when acetone photolysis is included. Models which do not include the additional HO x sources required to balance the HO x budget are likely to underestimate ozone production rates.