An interpretative modeling analysis is conducted to simulate the diurnal variations in OH and HO2 + RO2 observed at Summit, Greenland in 2003. The main goal is to assess the HOx budget and to quantify the impact of snow emissions on ambient HOx as well as on CH2O and H2O2. This analysis is based on composite diurnal profiles of HOx 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 HO2 + RO2 when they are constrained by observations of H2O2 and CH2O. By contrast, model predictions of OH were about factor of 2 higher than the observed values. Modeling analysis of H2O2 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 CH2O. This study suggests that for the Summit 2003 period snow emissions contribute similar to 25% of the total CH2O production, while photochemical oxidation of hydrocarbon appears to be the dominant source. A budget assessment of HOx radicals shows that primary production from O(D-1) + H2O and photolysis of snow emitted precursors (i.e., H2O2 and CH2O)are the largest primary HOx sources at Summit, contributing 41% and 40%, respectively. The snow contribution to the HOx budget is mostly in the form of emissions of H2O2. The dominant HO, sink involves the HO2 + HO2 reaction forming H2O2, followed by its deposition to snow. These results differ from those previously reported for the South Pole (SP), in that primary production of HOx was shown to be largely driven by both the photolysis of CH2O and H2O2 emissions (46%) with smaller contributions coming from the oxidation of CH4 and the 0(D-1) + H2O reaction (i.e., 27% each). In sharp contrast to the findings at Summit in 2003, due to the much higher levels of NOx the SP HO, sinks are dominated by HOx-NOx reactions, leading to the formation and deposition of HNO3 and HO2NO2. Thus, a comparison between SP and Summit studies suggests that snow emissions appear to play a prominent role in controlling primary HOx production in both environments. However, as regards to maintaining highly elevated levels of OH, the two environments differ substantially. At Summit the elevated rate for primary production of HOx is most important; whereas, at SP it is the rapid recycling of the more prevalent HO2 radical, through reaction with NO, back to OH that is primarily responsible
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