Atmospheric Environment 41 (2007) 7806–7820 An assessment of the polar HO x photochemical budget based on 2003 Summit Greenland field observations G. Chen a,Ã , L.G. Huey b , J.H. Crawford a , J.R. Olson a , M.A. Hutterli c,d , S. Sjostedt b , D. Tanner b , J. Dibb e , B. Lefer f , N. Blake g , Douglas Davis b , A. Stohl h a Chemistry and Dynamics Branch, Science Directorate, NASA Langley Research Center, Hampton, VA 23681, USA b School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA c Physical Sciences Division, British Antarctic Survey, Cambridge, UK d Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ, USA e Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, USA f Department of Geosciences, University of Houston, TX, USA g Department of Chemistry, University of California, Irvine, CA, USA h Norwegian Institute for Air Research, Kjeller, Norway Received 6 November 2006; received in revised form 9 June 2007; accepted 9 June 2007 Abstract An interpretative modeling analysis is conducted to simulate the diurnal variations in OH and HO 2 +RO 2 observed at Summit, Greenland in 2003. The main goal is to assess the HO x budget and to quantify the impact of snow emissions on ambient HO x as well as on CH 2 O and H 2 O 2 . This analysis is based on composite diurnal profiles of HO x precursors recorded during a 3-day period (July 7–9), which were generally compatible with values reported in earlier studies. The model simulations can reproduce the observed diurnal variation in HO 2 +RO 2 when they are constrained by observations of H 2 O 2 and CH 2 O. By contrast, model predictions of OH were about factor of 2 higher than the observed values. Modeling analysis of H 2 O 2 suggests that its distinct diurnal variation is likely controlled by snow emissions and loss by deposition and/or scavenging. Similarly, deposition and/or scavenging sinks are needed to reproduce the observed diel profile in CH 2 O. This study suggests that for the Summit 2003 period snow emissions contribute 25% of the total CH 2 O production, while photochemical oxidation of hydrocarbon appears to be the dominant source. A budget assessment of HO x radicals shows that primary production from O( 1 D)+H 2 O and photolysis of snow emitted precursors (i.e., H 2 O 2 and CH 2 O) are the largest primary HO x sources at Summit, contributing 41% and 40%, respectively. The snow contribution to the HO x budget is mostly in the form of emissions of H 2 O 2 . The dominant HO x sink involves the HO 2 +HO 2 reaction forming H 2 O 2 , followed by its deposition to snow. These results differ from those previously reported for the South Pole (SP), in that primary production of HO x was shown to be largely driven by both the photolysis of CH 2 O and H 2 O 2 emissions (46%) with smaller contributions coming from the oxidation of CH 4 and the O( 1 D)+H 2 O reaction (i.e., 27% each). In sharp contrast to the findings at Summit in 2003, due to the much higher levels of NO x , the SP HO x sinks are dominated by HO x –NO x reactions, leading to the formation and deposition of HNO 3 and HO 2 NO 2 . Thus, a comparison between SP and Summit studies suggests that snow emissions appear to play a prominent role in controlling primary HO x production in both environments. However, as regards to maintaining highly elevated levels of OH, the two ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.06.014 Ã Corresponding author. Tel.: +1 757 864 2290. E-mail address: gao.chen@nasa.gov (G. Chen).