Measurements of OH and RO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; radicals at Dome C, East Antarctica

J. Gil Roca,A. Kukui,J. L. France,S. Preunkert,M. D. King,M. Legrand,B. Jourdain,M. M. Frey,G. Ancellet,R. Loisil

doi:10.5194/acp-14-12373-2014

Abstract

Abstract. Concentrations of OH radicals and the sum of peroxy radicals, RO2, were measured in the boundary layer for the first time on the East Antarctic Plateau at the Concordia Station (Dome C, 75.10° S, 123.31° E) during the austral summer 2011/2012. The median concentrations of OH and RO2 radicals were 3.1 × 106 molecule cm−3 and 9.9 × 106 molecule cm−3, respectively. These values are comparable to those observed at the South Pole, confirming that the elevated oxidative capacity of the Antarctic atmospheric boundary layer found at the South Pole is not restricted to the South Pole but common over the high Antarctic plateau. At Concordia, the concentration of radicals showed distinct diurnal profiles with the median maximum of 5.2 × 106 molecule cm−3 at 11:00 and the median minimum of 1.1 × 106 molecule cm−3 at 01:00 for OH radicals and 1.7 × 108 molecule cm−3 and 2.5 × 107 molecule cm−3 for RO2 radicals at 13:00 and 23:00, respectively (all times are local times). Concurrent measurements of O3, HONO, NO, NO2, HCHO and H2O2 demonstrated that the major primary source of OH and RO2 radicals at Dome C was the photolysis of HONO, HCHO and H2O2, with the photolysis of HONO contributing ~75% of total primary radical production. However, photochemical modelling with accounting for all these radical sources overestimates the concentrations of OH and RO2 radicals by a factor of 2 compared to field observations. Neglecting the net OH production from HONO in the photochemical modelling results in an underestimation of the concentrations of OH and RO2 radicals by a factor of 2. To explain the observations of radicals in this case an additional source of OH equivalent to about (25–35)% of measured photolysis of HONO is required. Even with a factor of 5 reduction in the concentrations of HONO, the photolysis of HONO represents the major primary radical source at Dome C. To account for a possibility of an overestimation of NO2 observed at Dome C the calculations were also performed with NO2 concentrations estimated by assuming steady-state NO2 / NO ratios. In this case the net radical production from the photolysis of HONO should be reduced by a factor of 5 or completely removed based on the photochemical budget of OH or 0-D modelling, respectively. Another major factor leading to the large concentration of OH radicals measured at Dome C was large concentrations of NO molecules and fast recycling of peroxy radicals to OH radicals.

Highlights

Atmospheric chemistry in polar regions has gained growing interest over the two last decades due to the discovery of surprisingly high photochemical activity in both Antarctic and Arctic, i.e. at the South Pole
The concentrations of OH and RO2 radicals were found to be comparable to those observed previously at the South Pole (Mauldin et al, 2001, 2004, 2010) confirming that the elevated oxidative capacity found at the SP is not unique but a common characteristic of near-surface atmospheric layer for a large part of the high Antarctic plateau
Similar to the findings at the SP the major explanation for the large concentrations of OH radical at Dome C was found to be the large concentrations of NO (Frey et al, 2013, 2014) leading to fast recycling of peroxy radicals to OH radicals

Summary

Introduction

The photochemistry of the boundary layer atmosphere (BL) at these snow covered regions is significantly influenced by the emissions of reactive gases from the snowpack. At the South Pole (SP) unexpectedly large concentrations of OH radicals in the boundary layer, about 2 × 106 molecule cm−3, were observed for the first time during ISCAT 1998 (Mauldin et al, 2001) and were confirmed in later campaigns (ISCAT 2000, Mauldin et al, 2004; ANTCI 2003, Mauldin et al, 2010). The large concentration of NO molecules exceeded free tropospheric concentrations (Davis et al, 2001) and was attributed to the release of NOx from snowpack (following UV photolysis of the nitrate anion (NO−3 ) on/in snow grains; Jones et al, 2001) and its accumulation in a stable and shallow BL at the SP (Davis et al, 2001, 2004, 2008). The build up of large concentrations of NOx at the SP was suggested to be enhanced by the continuous sunlight during summer and the location at the bottom of a large air drainage basin (Davis et al, 2004, 2008)

Methods

Results

Conclusion