Measurements of OH and RO₂ radicals at Dome C, East Antarctica

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.