Photochemistry of HO_<i>x</i> in the upper troposphere at northern midlatitudes

The factors controlling the concentrations of HO x radicals (= OH + peroxy) in the upper troposphere (8–12 km) are examined using concurrent aircraft observations of OH, HO 2 , H 2 O 2 , CH 3 OOH, and CH 2 O 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 3 , H 2 O, NO, peroxyacetyl nitrate (PAN), HNO 3 , CH 4 , CO, acetone, hydrocarbons, actinic fluxes, and aerosols, allow a highly constrained mass balance analysis of HO x and of the larger chemical family HO y (= HO x + 2 H 2 O 2 + 2 CH 3 OOH + HNO 2 + HNO 4 ). Observations of OH and HO 2 are successfully simulated to within 40% by a diel steady state model constrained with observed H 2 O 2 and CH 3 OOH. The model captures 85% of the observed HO x variance, which is driven mainly by the concentrations of NO x (= NO + NO 2 ) and by the strength of the HO x primary sources. Exceptions to the good agreement between modeled and observed HO x 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 2 to HONO on aerosols (γ NO2 = 10 −3 ) during the night followed by photolysis of HONO could explain part of the discrepancy at sunrise. Heterogeneous loss of HO 2 on ice crystals (γ ice_HO2 = 0.025) could explain the discrepancy in cirrus. Primary sources of HO x from O( 1 D )+H 2 O and acetone photolysis were of comparable magnitude during SONEX. The dominant sinks of HO y were OH+HO 2 (NO x &lt;50 parts per trillion by volume (pptv)) and OH+HNO 4 (NO x &gt;50 pptv). Observed H 2 O 2 concentrations are reproduced by model calculations to within 50% if one allows in the model for heterogeneous conversion of HO 2 to H 2 O 2 on aerosols (γ HO2 = 0.2). Observed CH 3 OOH concentrations are underestimated by a factor of 2 on average. Observed CH 2 O 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 CH 3 OH and with cirrus clouds. Heterogeneous oxidation of CH 3 OH to CH 2 O on aerosols or ice crystals might provide an explanation (γ ice_CH3OH ∼ 0.01 would be needed).