PNNL

Abstract

Anthropogenic emissions of aerosols and precursor compounds are known to significantly affect the energy balance of the Earth-atmosphere system, alter the formation of clouds and precipitation, and have substantial impact on human health and the environment. Global models are an essential tool used to examine the impacts of these emissions. In this study, we examine the sensitivity of model results to the assumed height of SO2 injection, seasonality of SO2 and BC, and the assumed fraction of SO2 that is injected into the grid cell as SO4 in 11 climate and chemistry models, including both chemical transport models and the atmospheric components of Earth system models. The objective is to quantify the influence of these emission characteristics on model simulations and to better understand the extent to which these characteristics affect results across different models. Each model simulation reported gas and aerosol concentration and deposition rate variables as well as radiative fluxes. We find a large variation in atmospheric lifetime across models for SO2, SO4, and BC, with a particularly large relative variation for SO2. Of the perturbations examined in this study, the assumed height of SO2 injection had the largest overall impacts, particularly on net radiative flux (maximum absolute difference of -0.35 W/m2), SO2 lifetime over northern hemisphere land (maximum absolute difference of 0.8 days), surface SO2 concentration (up to 59% drop), and surface sulfate concentration (up to 23% increase). Emitting SO2 at height consistently increased SO2 and SO4 column burdens and shortwave cooling, although with varying magnitudes, but had inconsistent effects across models on the sign of the change in implied cloud forcing. The assumed SO4 fraction also had a significant impact in some models. Because these properties are not standardized across models this is a source of inter-model variability. These results imply a need to assure that anthropogenic emission injection height and SO4 fraction are accurately and consistently represented in global models.