Description: Black carbon emitted by incomplete combustion of fossil fuels and biomass has a net warming effect in the atmosphere and reduces the albedo when deposited on ice and snow; accurate knowledge of past emissions is essential to quantify and model associated global climate forcing. Although bottom-up inventories provide historical Black Carbon emission estimates that are widely used in Earth System Models, they are poorly constrained by observations prior to the late 20th century. Here we use an objective inversion technique based on detailed atmospheric transport and deposition modeling to reconstruct 1850 to 2000 emissions from thirteen Northern Hemisphere ice-core records. We find substantial discrepancies between reconstructed Black Carbon emissions and existing bottom-up inventories which do not fully capture the complex spatial-temporal emission patterns. Our findings imply changes to existing historical Black Carbon radiative forcing estimates are necessary, with potential implications for observation-constrained climate sensitivity.

Description: Halogenated very short-lived substances (VSLS) are expected to contribute significantly to the stratospheric halogen loading and therefore to the stratospheric ozone chemistry. Our understanding of the highly variable emission of VSLS and their transport from the surface into the stratosphere is crucial to estimate their contribution to stratospheric halogen loading. In this study we investigate the relative impacts of emission rates, large scale and convective transport as well as wet and dry deposition on the contribution of VSLS to stratospheric ozone depletion. Therefore we have simulated the transport, washout and photochemical decay of VSLS with the Lagrangian particle dispersion model FLEXPART. The transport simulations are based on VSLS sea-to-air flux measurements obtained from the tropical Atlantic in October/November 2002 and from the tropical Western Pacific in October 2009. We show, that the spatial and temporal variability of emission rates, convective transport and dehydration processes in the TTL leads to strong variability in the overall transport of VSLS into the stratosphere. We will give estimates of the amount of VSLS and their organic product gases transported into the stratosphere and compare those to measurements form aircraft and balloon campaigns. In particular we will discuss the importance of methyl iodide emitted in the Western Pacific as a carrier of iodine into the upper troposphere and lower stratosphere. We will estimate the ozone depleting potential of the brominated and iodinated VSLS and discuss the relevance of VSLS emissions in the tropical Atlantic and tropical Western Pacific for stratospheric ozone chemistry.

Description: Bromoform (CHBr3), a recognized contributor to stratospheric ozone depletion, has been largely exempt from the Montreal Protocol's regulation due to its short atmospheric lifetime and large natural emissions. Using our recent CHBr3 emission inventory containing both natural and anthropogenic sources, we reevaluated the role played by the latter in the total CHBr3 flux into the Northern Hemisphere extratropical stratosphere. Derived mainly from ship ballast, power plant cooling and desalination plant brine water, these anthropogenic sources suggest a substantial underestimation in previous global CHBr3 emission estimates. Anthropogenic sources have been underestimated by 31.5% globally, and more alarmingly, this underestimation escalates to 70.5% when focusing on the Northern Hemisphere. Consequently, atmospheric CHBr3 concentrations are also significantly higher than previous estimates, especially over the NH extratropics during boreal winter. The ODP-weighted emissions in the NH based on historical ECMWF meteorology are ~28.2 Gg Br/year, increased by ~78% above previous estimates, suggesting a more significant contribution of anthropogenic CHBr3 to stratospheric ozone depletion, especially in the NH lowermost stratosphere. To study the potential impact of these revised emission inventories, we employ the Whole Atmosphere Community Climate Model (WACCM), which enables us to project the future ozone depletion from CHBr3 under climate change scenarios and evaluate the necessity for regulatory measures to manage anthropogenic sources.

Description: To mitigate the rumen enteric methane (CH4) produced by ruminant livestock, Asparagopsis taxiformis is proposed as an additive to ruminant feed. During the cultivation of Asparagopsis taxiformis in the sea or in terrestrially based systems, this macroalgae, like most seaweeds and phytoplankton, produces a large amount of bromoform (CHBr3), which contributes to ozone depletion once released into the atmosphere. In this study, we focus on the impact of CHBr3 on the stratospheric ozone layer resulting from potential emissions from proposed Asparagopsis cultivation in Australia. The impact is assessed by weighting the emissions of CHBr3 with its ozone depletion potential (ODP), which is traditionally defined for long-lived halocarbons but has also been applied to very short-lived substances (VSLSs). An annual yield of ∼3.5 × 104 Mg dry weight is required to meet the needs of 50 % of the beef feedlot and dairy cattle in Australia. Our study shows that the intensity and impact of CHBr3 emissions vary, depending on location and cultivation scenarios. Of the proposed locations, tropical farms near the Darwin region are associated with the largest CHBr3 ODP values. However, farming of Asparagopsis using either ocean or terrestrial cultivation systems at any of the proposed locations does not have the potential to significantly impact the ozone layer. Even if all Asparagopsis farming were performed in Darwin, the CHBr3 emitted into the atmosphere would amount to less than 0.02 % of the global ODP-weighted emissions. The impact of remaining farming scenarios is also relatively small even if the intended annual yield in Darwin is scaled by a factor of 30 to meet the global requirements, which will increase the global ODP-weighted emissions up to ∼0.5 %.