The chemistry influencing ODEs in the Polar Boundary Layer in spring: a model study

Introduction Conclusions References Tables Figures ◭ ◮ ◭ ◮ Back Close Full Screen / Esc Abstract Introduction Conclusions References Tables Figures ◭ ◮ ◭ ◮ Back Close Full Screen / Esc Abstract Near-total depletions of ozone have been observed in the Arctic spring since the mid 1980s. The autocatalytic cycles involving reactive halogens are now recognized to be of main importance for Ozone Depletion Events (ODEs) in the Polar Boundary Layer (PBL). We present sensitivity studies using the model MISTRA in the box-model mode 5 on the influence of chemical species on these ozone depletion processes. In order to test the sensitivity of the chemistry under polar conditions, we compared base runs undergoing fluxes of either Br 2 , BrCl, or Cl 2 to induce ozone depletions, with similar runs including a modification of the chemical conditions. 10 mixing ratios of HCHO, H 2 O 2 , DMS, Cl 2 , and HONO induced a shift in bromine speci-ation from Br/BrO to HOBr/HBr, while high mixing ratios of C 2 H 6 induced a shift from HOBr/HBr to Br/BrO. Cases with elevated mixing ratios of HONO, NO 2 , and RONO 2 induced a shift to BrNO 2 /BrONO 2. The shifts from Br/BrO to HOBr/HBr accelerated the aerosol debromination, but also increased the total amount of deposited bromine at the 15 surface (mainly via increased deposition of HOBr). These shifts to HOBr/HBr also hindered the BrO self-reaction. In these cases, the ozone depletion was slowed down, where increases in H 2 O 2 and HONO had the greatest effect. The tests with increased mixing ratios of C 2 H 4 highlighted the decrease in HO x which reduced the production of HOBr from bromine radicals. In addition, the direct reaction of C 2 H 4 with bromine 20 atoms led to less available reactive bromine. The aerosol debromination was therefore strongly reduced. Ozone levels were highly affected by the chemistry of C 2 H 4. Cl 2-induced ozone depletions were found unrealistic compared to field measurements due to the rapid production of CH 3 O 2 , HO x , and ROOH which rapidly convert reactive chlorine to HCl in a " chlorine counter-cycle ". This counter-cycle efficiently reduces the 25 concentration of reactive halogens in the boundary layer. Depending on the relative bromine and chlorine mixing ratios, the production of CH 3 O 2 , HO x , …