Tropospheric bromine chemistry and its impacts on ozone: A model study

Abstract

An off‐line three‐dimensional tropospheric chemical transport model, parallel–Tropospheric Off‐Line Model of Chemistry and Transport (p‐TOMCAT), has been extended by incorporating a detailed bromine chemistry scheme that contains gas‐phase reactions and heterogeneous reactions on both cloud particles and background aerosols. Bromine emission from bromocarbon photo‐oxidation and from sea‐salt bromine depletion and bromine removal through dry and wet deposition are included. Using this model, tropospheric bromine chemistry and ozone budgets are studied. The zonal mean of the inorganic gas‐phase bromine compounds (Brx) is calculated to be high (4–8 pptv) in the lower troposphere of the midlatitudes to high latitudes in each hemisphere, with decreasing trends with altitude (down to ∼2–3 pptv in the upper troposphere). The lowest Brx (<2 pptv) is over low latitudes, corresponding to small sea‐salt Br emission and a high rate of precipitation scavenging. A mean lifetime of ∼5 days is obtained for the tropospheric Brx. Sea‐salt emission plays the dominant role in total Brx in the lower troposphere while organic Br‐containing compounds are important in upper layers. High daytime BrO mixing ratios (>1 pptv) are found over the high‐latitude ocean surface, corresponding to high tropospheric column BrO values of up to 1.6 × 10 13 molecules/cm 2 in the monthly mean. The addition of bromine chemistry to the model leads to a reduction in tropospheric ozone amounts by 4–6% in the Northern Hemisphere and up to ∼30% in the Southern Hemisphere high latitudes. The net ozone loss depends not only on total Brx, but also on solar irradiance, especially at high latitudes. The hydrolysis reaction of bromine nitrate, which occurs on cloud and aerosol surfaces (BrONO 2 + H 2 O aq → HOBr + HNO 3), has a significant influence on ozone chemistry through its effect on NOx as well as on reactive BrO and Br.