The Breathing of Halogenated Volatile Organic Compounds (HVOCs) from Human-impacted Ecosystems

Atmospheric methyl chloride (CH3Cl) and methyl bromide (CH3Br) are the largest natural gaseous carriers of reactive chlorine and bromine, which contribute to stratospheric ozone depletion. Chloroform (CHCl3) also contributes to total stratospheric chlorine load and ozone destruction, with a much smaller impact. This dissertation investigates and quantifies emissions of these halogenated volatile organic compounds (HVOCs) from some natural ecosystems impacted by human activities.Chapter two describes a study on HVOCs flux measurements at an intermediate zone of coastal ecosystems in South Carolina, which are subject to seawater intrusion induced by storm surges combined with sea level rise. The degraded forested wetland showed significant CHCl3 emission rates and remained a net CH3Cl sink and a negligible CH3Br source/sink, unlike the saltmarsh which was a significant source for both. This study suggested that sea level rise and frequent storm surges derived from global climate change, in the long term, may increase CHCl3 emissions from coastal degraded forested wetlands and of methyl halides if salt marshes expand, with potential impacts for stratospheric ozone depletion. Chapter three describes a project that measured life-cycle fluxes of CH3Cl, CH3Br and CH3I from rapeseed (Brassica napus “Empire”). The results showed that rapeseed emitted 5.3 ± 1.3 Gg CH3Cl year−1, 2.8 ± 0.7 Gg CH3Br year−1, and 4.0 ± 0.8 Gg CH3I year−1 as of 2018. Assuming the future global rapeseed production follows the same rate of increase as the past 60 years, the atmospheric sources of CH3Cl, CH3Br and CH3I may increase to 13.1 Gg year-1, 7.0 Gg year-1, and 9.8 Gg year-1, respectively by 2050, becoming an ever-larger global source. Chapter four describes a study which showed that CuSO4, in conjunction with either hydrogen peroxide (H2O2) or solar radiation, significantly enhanced CH3Cl and CH3Br production from organic matter in soil or seawater. This study identified a new abiotic pathway of methyl halide production occurring in agricultural fields and water bodies facilitated by the widespread application of Cu (II) based pesticides, herbicides and fungicides. A rough quantification suggested this mechanism had the potential to account for a significant part of the missing sources of CH3Cl and CH3Br. These aforementioned studies highlight the increasing contribution from natural sources of ozone depleting substances (ODSs) influenced by human activities, which may offset some of the ODSs reductions achieved by the Montreal Protocol