Very short-lived halogenated species : modelling their tropospheric distribution, contribution to stratospheric bromine and impact on ozone

A three-dimensional (3-D) chemical transport model (CTM) has been used to study the tropospheric chemistry and troposphere-stratosphere transport of biogenic very short-lived species (VSLS). Calculations have been performed in order to quantify their contribution to stratospheric bromine loading and also the relative importance of the source gas injection (SGI) and product gas injection (PGI) pathways. Simulations with a chemistry-climate model (CCM) have also been performed to assess the impact of VSLS on stratospheric ozone (03) and how their transport to the stratosphere may respond to climate change. Five brominated VSLS were considered in this work; bromoform (CHBr3), dibromomethane (CH2Br2), dibromochloromethane (CHBr2Cl), bromodichloromethane (CHBrC12) and bromochloromethane (CH2BrCl). The CTM shows these gases contribute ~5 parts per trillion (pptv) of inorganic bromine (BryvSLS) to the stratosphere, representing ~25% of total stratospheric bromine in 2009. SGI accounts for between ~69-75% of this supply, with the remainder from PG!. The longest lived organic product gas (PG) from CHBr3 and CH2Br2 degradation is carbonyl dibromide (CBr20). Its tropospheric lifetime with respect to photolysis is ~7 days. It is unlikely that organic PGs make a significant contribution to the total PGI from VSLS. The CTM performs reasonably well in reproducing tropospheric VSLS observations from a number of aircraft campaigns. In the tropical tropopause layer (TTL), CH2Br2 is overestimated when vertical transport is diagnosed from the meteoro- logical analyses. An improved agreement is obtained when vertical transport is calculated using diabatic heating rates, resulting in slower transport through the TTL. CCM runs show during periods of background aerosol loading, the impact of BryvSLS on mid-latitude 03 is small. The impact is larger in polar regions, where BryvSLS enhances the BrO-ClO loss cycle, resulting in a reduction of up to ~15 Dobson units (DU) of the total 03 column over Antarctica. The CCM has also been used to diagnose the potential response of SGI to climate change. For 2000, the modelled SGI is ~1.7 pptv, lower than the CTM estimate but still in good agreement with observations. For 2100, the CCM estimate increases to ~1.9 and ~2.7 pptv when the model is forced with Intergovernmental Panel on Climate Change (IPCC) representative concentration pathways (RC Ps) 4.5 and 8.5, respectively. The increase is due to enhanced tropical deep convection, reducing transport time-scales in the upper troposphere, particularly over the tropical Western Pacific region.EThOS - Electronic Theses Online ServiceGBUnited Kingdo