HETEROGENOUS BRONO2 HYDROLYSIS - EFFECT ON NO2 COLUMNS AND OZONE AT HIGH-LATITUDES IN SUMMER

The heterogeneous reaction, N2O5 + H2Oaerosol --> 2HNO(3), is responsi ble for increasing [HOx] and repartitioning of active NOx into HNO3. T hroughout much of the atmosphere, N2O5 is formed predominantly at nigh t owing to the rapid photolysis of its precursor, NO3, in sunlit hours . Laboratory measurements have shown that BrONO2 + H2Oaerosol --> HOBr + HNO3 (reaction (3)) also has the potential to cause repartitioning of ozone-depleting species, although better determination of gamma, th e reaction probability, is still required for some stratospheric condi tions. The diurnal behavior of N2O5 and BrONO2 are entirely different. In contrast to N2O5, BrONO2 is formed predominantly during the daytim e. The result of (3) is to increase [HOBr] at the expense of [BrONO]. Photolysis of HOBr then leads to increased [OH] and increased O-3 loss . In this work two-dimensional calculations show clearly that the impa ct of (3) is greatest for high aerosol levels and for high latitudes i n summer. The calculations have been used to determine the effects of increased aerosol loading on calculated NO2 columns in the Antarctic d uring summer and autumn of 1990, 1991, 1992 and 1993. It is shown that (3) could be responsible for reductions in NO2 columns during polar d ay comparable to those measured in 1992 and 1993 following the eruptio n of Mount Pinatubo. Reaction (3) results in only marginal changes to ozone catalytic loss cycles in 1990. However, for the high aerosol lev els of 1992, the inclusion of this reaction results in up to 50% highe r ozone loss rates in the 12 to 20 km range. This is caused predominan tly by a large increase in [HOx] tempered by a reduction in loss due t o NOx. Calculations in which transport terms were switched off showed that, between 12 and 20 km at 77.5 degrees N, local chemistry removes about 30% of the ozone between April and September compared with 20% w hen the effects of (3) are not included.