Emission and transport of atmospheric very short-lived halogens in the tropics

Abstract

This thesis investigated the emission and transport of very short–lived halogens (VSLS) over the tropics. VSLS are described as organic halogen gases with lifetimes of less than 6 months. In areas of rapid convective transport they reach the upper troposphere and lower stratosphere where they contribute to total bromine loading (~20 pptv) in the stratosphere that is a cause of ozone (O3) depletion. This thesis investigated speed of transport in the tropics using model age of air (Chapter 3), the strength of VSLS source regions in tropical troposphere (Chapter 4), and quantification of their monthly emission fluxes (Chapter 5). The two most abundant VSLS bromoform (CHBr3) and CH2Br2 were focussed on. A new model age of air calculation was used to describe transport of ocean emissions in the tropical latitudes. Age of air describes how long an air mass has been out of contact with the emission source region. The two most rapid convection regions of the Indian Ocean (InO) and Western Pacific (WPa) showed age of air in the tropical tropopause layer (TTL) to be 25 days. This is similar to the lifetime of CHBr3 (24 days). Using age of air estimated from simulations covering 1989–2013, it was shown how strong El Niño events can increase the age of air over the WPa by 5–7 days in the mid–troposphere, and up to 12 days in the TTL. This increase in age was due to a change in the Walker Circulation, weakening convection in the WPa and increasing convection over the CPa. Over this period, it was shown that age decreases in the tropical circulation system (the Hadley Cell). Decreasing age results from increasing convection, and more rapid transport of VSLS to the upper troposphere, lower stratosphere (UTLS). To study regional emission sources over the WPa, a tagged CHBr3 and CH2Br2 model was developed. It is the first study to quantify how open and coastal emissions contribute separately to the vertical profiles of CHBr3 and CH2Br2 in the WPa. Variability over the WPa is dominated by an open oceanic emission source, with enhanced coastal emissions influencing concentrations in the upper troposphere. Estimations of 3.14 pptv of CHBr3 and CH2Br2 contribution to TTL Bry were in agreement with recent observational studies (3.27 pptv, Navarro et al. (2015)) over the same region. Comparison with aircraft observations showed that the model has a positive bias and this is attributed to over estimation of model emissions. Ground–based observations were used in an inverse model to estimate surface emissions of CHBr3. This method has not been previously used to estimate CHBr3 emissions. The monthly a posteriori emissions had seasonal cycles in the northern and southern hemisphere coastal emissions, and reductions over tropical open oceans. A posteriori emissions were put in to the model and the predicted volume mixing ratios were able to reproduce ground stations observations over the mid–latitude and tropical stations, important for convective transport of VSLS. The model still showed a bias when compared to CAST and CONTRAST aircraft observations over the Western Pacific, but the mean model minus observed residual was reduced by around 0.3 pptv and 0.1 pptv for respective CAST and CONTRAST campaigns from the a priori emissions.