Impacts of aerosol-cloud interactions on past and future changes in tropospheric composition

Abstract

Abstract. The development of effective emissions control policies that are beneficial to both climate and air quality requires a detailed understanding of all the feedbacks in the atmospheric composition and climate system. We perform sensitivity studies with a global atmospheric composition-climate model to assess the impact of aerosols on tropospheric chemistry through their modification on clouds, aerosol-cloud interactions (ACI). The model includes coupling between both tropospheric gas-phase and aerosol chemistry and aerosols and liquid-phase clouds. We investigate past impacts from preindustrial (PI) to present day (PD) and future impacts from PD to 2050 (for the moderate IPCC A1B scenario) that embrace a wide spectrum of precursor emission changes and consequential ACI. The aerosol indirect effect (AIE) is estimated to be −2.0 Wm −2 for PD-PI and −0.6 Wm −2 for 2050-PD, at the high end of current estimates. Inclusion of ACI substantially impacts changes in global mean methane lifetime across both time periods, enhancing the past and future increases by 10% and 30%, respectively. In regions where pollution emissions increase, inclusion of ACI leads to 20% enhancements in in-cloud sulfate production and ∼10% enhancements in sulfate wet deposition that is displaced away from the immediate source regions. The enhanced in-cloud sulfate formation leads to larger increases in surface sulfate across polluted regions (∼10-30%). Nitric acid wet deposition is dampened by 15-20% across the industrialized regions due to ACI allowing additional re-release of reactive nitrogen that contributes to 1-2 ppbv increases in surface ozone in outflow regions. Our model findings indicate that ACI must be considered in studCorrespondence to: N. Unger (nunger@giss.nasa.gov) ies of methane trends and projections of future changes to particulate matter air quality.

Climate versus emission drivers of methane lifetime against loss by tropospheric OH from 1860–2100

“…We then report the mean and standard deviation over the full length of the simulations. For our 1860 CONTROL simulation (Table 3), the mean and standard deviation of τ CH4 OH averaged over 200 yr is 8.06 ± 0.08 yr, which is within the range of preindustrial estimates (6.71-12.8 yr) simulated by other models (Martinerie et al, 1995;Unger et al, 2009;Sövde et al, 2011), and 25 % lower than obtained in a recent multi-model study (10.1 ± 1.7 yr, Naik et al, 2012b). Figure 1 shows the temporal evolution of the global annual mean τ CH4 OH in the CM3/CMIP5 historical simulations and future projections, and in Table 4 we report the mean and standard deviation of τ CH4 OH , as well as the correlation coefficient of τ CH4 OH with the global mean lower tropospheric (below 500 hPa) airmass-weighted OH concentration and temperature, over the full length of the model simulations.…”

“…The consistency between the AEROSOL INDIRECT and AEROSOL in the evolution of τ CH4 OH (identical correlations of τ CH4 OH against temperature and OH below 500 hPa in Table 4; similar percentage changes in Table 5 and correlations of OH versus the climate factors in Table 6), imply that aerosol-cloud interactions (indirect effect) dominate the CM3 climate response to aerosols. τ CH4 OH in the model is therefore sensitive to the aerosol indirect effect (Unger et al, 2009).…”

Aerosol Optical Depth measurements at 340 nm with a Brewer spectrophotometer and comparison with Cimel sunphotometer observations at Uccle, Belgium

“…They affect the tropospheric chemical composition, they can reduce visibility and they have important impacts on human health (Unger et al, 2009;Lyamani et al, 2010;Raghavendra Kumar et al, 2010). Aerosols also influence the Earth's radiation budget in a direct, semi-direct and indirect manner.…”

“…This will allow more solar radiation to reach the surface (Ramanathan et al, 2001;Cazorla et al, 2009). The indirect effect concerns the ability of aerosols to act as cloud condensation nuclei which influences the microphysical and optical properties of clouds, thus changing the radiative and precipitation properties and the lifetime of clouds (Ramanathan et al, 2001;Kaufman et al, 2002;Lohmann, 2002;Lohmann and Feichter, 2005;Unger et al, 2009). Because of a lack of information concerning the temporal and spatial distribution of aerosols, they are key contributors to the uncertainties in current climate studies (Andreae et al, 2005;IPCC, 2007).…”

Comparison of the optical depth of total ozone and atmospheric aerosols in Poprad-Gánovce, Slovakia

“…This is a change of their radiation properties, a change of the properties of atmospheric precipitation and also changes the life of clouds. It is true that an increase in the number of cloudy condensation nuclei leads to an increase in the number of cloud drops and to 30 a reduction in their size under given conditions of the water content in the atmosphere, which causes the growth of the albedo and the lifetime of the cloud (Lohmann and Feichter, 2005;Unger et al, 2009). For these reasons, anthropogenically emitted aerosol particles have a significant role to play in the ongoing global climate change and their impact on the radiation balance still has high uncertainties (IPCC, 2014, and references therein).…”