Numerical analysis of the chemical kinetic mechanisms of ozone depletion and halogen release in the polar troposphere

Abstract

The role of halogen species (e. g., Br, Cl) in the troposphere of polar regions has been investigated since the discovery of their importance for boundary layer ozone destruction in the polar spring about 25 years ago. Halogen species take part in an auto-catalytic chemical reaction cycle, which releases Br-2 and BrCl from the sea salt aerosols, fresh sea ice or snowpack, leading to ozone depletion. In this study, three different chemical reaction schemes are investigated: a bromine-only reaction scheme, which then is subsequently extended to include nitrogen-containing compounds and chlorine species and corresponding chemical reactions. The importance of specific reactions and their rate constants is identified by a sensitivity analysis. The heterogeneous reaction rates are parameterized by considering the aerodynamic resistance, a reactive surface ratio, beta, i.e., the ratio of reactive surface area to total ground surface area, and the boundary layer height, L-mix. It is found that for beta = 1, a substantial ozone decrease occurs after five days and ozone depletion lasts for 40 h for L-mix = 200 m. For about beta >= 20, the time required for major ozone depletion ([O-3] < 4 ppb) to occur becomes independent of the height of the boundary layer, and for beta = 100 it approaches two days, 28 h of which are attributable to the induction and 20 h to the depletion time. In polar regions, a small amount of NOx may exist, which stems from nitrate contained in the snow, and may have a strong impact on the ozone depletion. Therefore, the role of nitrogen-containing species on the ozone depletion rate is studied. The results show that the NOx concentrations are influenced by different chemical reactions over different time periods. During ozone depletion, the reaction cycle involving the BrONO2 hydrolysis is dominant. A critical value of 0.0004 of the uptake coefficient of the BrONO2 hydrolysis reaction at the aerosol and saline surfaces is identified, beyond which the existence of NOx species accelerates the ozone depletion event, whereas for lower values, deceleration occurs

Tropospheric Halogen Chemistry: Sources, Cycling, and Impacts

“…58,133−136 Bloss and co-workers 134 simulated Antarctic boundary layer observations and found good replication of mean levels and diurnal cycles of observed HO x , NO x and halogen oxides. These simulations indicated that photolysis and reaction of Br with HCHO, along with ozone photolysis followed by reaction with water were the principal sources of HO x radicals.…”

“…In the presence of significant reactive surface area, bromine explosions were found to occur after a few days induction period and with ozone depletion time scales of 1–2 days. 136 The inclusion of chlorine in this modeling study had a minor influence in ozone depletion as compared to bromine and NO x . 136 However, no snowpack source of chlorine was included in the model, as has been indicated by the study of Liao and co-workers.…”

“…136 The inclusion of chlorine in this modeling study had a minor influence in ozone depletion as compared to bromine and NO x . 136 However, no snowpack source of chlorine was included in the model, as has been indicated by the study of Liao and co-workers. 53 …”

“…64 A similar zero-dimensional box model was applied at Summit Greenland, again constrained by radical measurements, found generally good agreement with observations that were improved by including bromine chemistry, although the agreement was not quantitative. 135 Cao and co-workers 136 recently carried out an extensive sensitivity modeling study on halogen release and ozone depletion chemistry. Three reaction schemes were used.…”

Bromine atom production and chain propagation during springtime Arctic ozone depletion events in Barrow, Alaska

“…Additionally, the current knowledge of the physical properties of the QLL and the location of liquid-like surfaces on snow grains would seem to invalidate the aforementioned assumptions on which many of the current heterogeneous models are based (Domine et al, 2013), specifically that the chemistry occurs in a liquid-like environment on snow grains. Indeed, Cao et al (2014) adopted a simplified heterogeneous chemistry mechanism in their modeling of Arctic ozone depletion, wherein they use the mass transfer of HOBr to the surface as the rate-limiting step in Br 2 production, citing the lack of suitable reaction mechanisms with which to properly simulate condensed-phase chemistry on snow/ice. Ad- …”