GMD Publications for 2014
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Halogens released from very short-lived substances (VSLS) can deplete ozone in the upper-troposphere and lower stratosphere where the perturbation can exert a large climate impact. In addition to the known ozone loss from natural biogenic bromine VSLS, such as bromoform (CHBr3), using a global atmospheric model we show that anthropogenic chlorine VSLS such as dichloromethane (CH2Cl2) – not regulated by the Montreal Protocol – also contribute. Although this impact is small compared to bromine VSLS at present, CH2Cl2 has industrial sources and observations show its atmospheric loading is increasing rapidly. We estimate a signiﬁcant radiative effect of the bromine and chlorine VSLS-driven lower stratospheric ozone destruction of -0.11 Wm-2. The largest impact comes from ozone loss at high latitudes, where column ozone decreases due to VSLS are up to 6%. The trend in anthropogenic chlorine VSLS could cause a signiﬁcant radiative forcing, especially if augmented by any trend in natural bromine VSLS. We also used the model to study the impact of iodine-containing VSLS such as methyl iodide (CH3I). Of the three halogens iodine has the largest leverage to destroy lower stratospheric ozone, but current limits based on IO observations indicate only a minor impact at present.
Aerosols, transported from distant source regions, influence the Arctic surface radiation budget. When deposited on snow and ice, carbonaceous particles can reduce the surface albedo, which accelerates melting, leading to a temperature-albedo feedback that amplifies Arctic warming. Black carbon (BC), in particular, has been implicated as a major warming agent at high latitudes. BC and co-emitted aerosols in the atmosphere, however, attenuate sunlight and radiatively cool the surface. Warming by soot deposition and cooling by atmospheric aerosols are referred to as “darkening” and “dimming” effects, respectively. In this study, climatologies of spectral aerosol optical depth AOD (2001–2011) and Equivalent BC (EBC) (1989–2011) from three Arctic observatories and from a number of aircraft campaigns are used to characterize Arctic aerosols. Since the 1980s, concentrations of BC in the Arctic have decreased by more than 50% at ground stations where in situ observations are made. AOD has increased slightly during the past decade, with variations attributed to changing emission inventories and source strengths of natural aerosols, including biomass smoke and volcanic aerosol, further influenced by deposition rates and airflow patterns.