An assessment of the polar HO_x photochemical budget based on 2003 Summit Greenland field observations

摘要

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_2O and H_2O_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_2O_2 and CH_2O. By contrast, model predictions of OH were about factor of 2 higher than the observed values. Modeling analysis of H_2O_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_2O. This study suggests that for the Summit 2003 period snow emissions contribute ～25% of the total CH_2O 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(~1D) + H_2O and photolysis of snow emitted precursors (i.e., H_2O_2 and CH_2O) 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_2O_2. The dominant HO_x sink involves the HO2 + HO2 reaction forming H_2O_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_2O and H_2O_2 emissions (46%) with smaller contributions coming from the oxidation of CH_4 and the O(~1D) + H_2O 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_2NO_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 environments differ substantially. At Summit the elevated rate for primary production of HO_x is most important; whereas, at SP it is the rapid recycling of the more prevalent HO_2 radical, through reaction with NO, back to OH that is primarily responsible.