The factors controlling the concentrations of HO radicals (= OH + peroxy) in the upper troposphere (8–12 km) are examined using concurrent aircraft observations of OH, HO, HO, CHOOH, and CHO made during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) at northern midlatitudes in the fall. These observations, complemented by concurrent measurements of O, HO, NO, peroxyacetyl nitrate (PAN), HNO, CH, CO, acetone, hydrocarbons, actinic fluxes, and aerosols, allow a highly constrained mass balance analysis of HO and of the larger chemical family HO (= HO + 2 HO + 2 CHOOH + HNO + HNO). Observations of OH and HO are successfully simulated to within 40% by a diel steady state model constrained with observed HO and CHOOH. The model captures 85% of the observed HO variance, which is driven mainly by the concentrations of NO (= NO + NO) and by the strength of the HO primary sources. Exceptions to the good agreement between modeled and observed HO are at sunrise and sunset, where the model is too low by factors of 2–5, and inside cirrus clouds, where the model is too high by factors of 1.2–2. Heterogeneous conversion of NO to HONO on aerosols (γ = 10) during the night followed by photolysis of HONO could explain part of the discrepancy at sunrise. Heterogeneous loss of HO on ice crystals (γ = 0.025) could explain the discrepancy in cirrus. Primary sources of HO from O()+HO and acetone photolysis were of comparable magnitude during SONEX. The dominant sinks of HO were OH+HO (NO50 pptv). Observed HO concentrations are reproduced by model calculations to within 50% if one allows in the model for heterogeneous conversion of HO to HO on aerosols (γ = 0.2). Observed CHOOH concentrations are underestimated by a factor of 2 on average. Observed CHO concentrations were usually below the 50 pptv detection limit, consistent with model results; however, frequent occurrences of high values in the observations (up to 350 pptv) are not captured by the model. These high values are correlated with high CHOH and with cirrus clouds. Heterogeneous oxidation of CHOH to CHO on aerosols or ice crystals might provide an explanation (γ ∼ 0.01 would be needed).Engineering and Applied Science
