A three-dimensional model calculation of the ozone depletion potential of 1-bromopropane (1-C3H7Br)

A three-dimensional chemical transport model has been used to investigate f actors affecting the potential impact of a short-lived bromine compound on lower stratospheric ozone. The model is used to calculate the ozone depleti on potential (ODP) of 1-bromopropane employing a previously used empirical approach, which depends on the lifetime of the compound and the amount reac hing the stratosphere. We show that this approach may be unsuitable for ver y short-lived compounds. Indeed for a short-lived compound the definition o f the lifetime itself is ambiguous. The lifetime varies with season, region of emission, and depends on the method of calculation. A series of tracer experiments reveals that the amount of bromine reaching the stratosphere, a nd hence the calculated ODP, can also be highly dependent on the distributi on of the surface emissions. Where emissions are located solely in the equa torial region, the calculated ODP is over 3 times greater than when the emi ssions are centered over Europe. Vigorous convection in the tropics can lif t the compound rapidly into the lower stratosphere where the bromine can be released and contribute to ozone destruction. For surface releases at high er latitudes the lifetime in the troposphere is significant compared with t he time to reach the stratosphere and a smaller ODP is calculated. This hig hlights a problem in calculating ODPs for short-lived species. Uncertaintie s in the degradation mechanisms for short-lived compounds, and the subseque nt fate of the degradation intermediates, add further uncertainty to calcul ations of their impact on the stratosphere. Additional methods need to be d eveloped to assess their potential impact on the stratosphere.