Aerosol direct radiative effects over the northwest Atlantic, northwest Pacific, and North Indian Oceans: estimates based on in-situ chemical and optical measurements and chemical transport modeling

Abstract

Abstract. The largest uncertainty in the radiative forcing of climate change over the industrial era is that due to aerosols, a substantial fraction of which is the uncertainty associated with scattering and absorption of shortwave (solar) radiation by anthropogenic aerosols in cloud-free conditions (IPCC, 2001). Quantifying and reducing the uncertainty in aerosol influences on climate is critical to understanding climate change over the industrial period and to improving predictions of future climate change for assumed emission scenarios. Measurements of aerosol properties during major field campaigns in several regions of the globe during the past decade are contributing to an enhanced understanding of atmospheric aerosols and their effects on light scattering and climate. The present study, which focuses on three regions downwind of major urban/population centers (North Indian Ocean (NIO) during INDOEX, the Northwest Pacific Ocean (NWP) during ACE-Asia, and the Northwest Atlantic Ocean (NWA) during ICARTT), incorporates understanding gained from field observations of aerosol distributions and properties into calculations of perturbations in radiative fluxes due to these aerosols. This study evaluates the current state of observations and of two chemical transport models (STEM and MOZART). Measurements of burdens, extinction optical depth (AOD), and direct radiative effect of aerosols (DRE – change in radiative flux due to total aerosols) are used as measurement-model check points to assess uncertainties. In-situ measured and remotely sensed aerosol properties for each region (mixing state, mass scattering efficiency, single scattering albedo, and angular scattering properties and their dependences on relative humidity) are used as input parameters to two radiative transfer models (GFDL and University of Michigan) to constrain estimates of aerosol radiative effects, with uncertainties in each step propagated through the analysis. Constraining the radiative transfer calculations by observational inputs increases the clear-sky, 24-h averaged AOD (34±8%), top of atmosphere (TOA) DRE (32±12%), and TOA direct climate forcing of aerosols (DCF – change in radiative flux due to anthropogenic aerosols) (37±7%) relative to values obtained with "a priori" parameterizations of aerosol loadings and properties (GFDL RTM). The resulting constrained TOA DCF is −3.3±0.47, −14±2.6, −6.4±2.1 Wm−2 for the NIO, NWP, and NWA, respectively. Constraining the radiative transfer calculations by observational inputs reduces the uncertainty range in the DCF in these regions relative to global IPCC (2001) estimates by a factor of approximately 2. Such comparisons with observations and resultant reductions in uncertainties are essential for improving and developing confidence in climate model calculations incorporating aerosol forcing.

How Different Calculations of the Refractive Index Affect Estimates of the Radiative Forcing Efficiency of Ammonium Sulfate Aerosols

“…where, in terms of g (Wiscombe and Grams 1976; Bates et al 2006), b 5 1 2 g 2 2g 1 ffiffiffiffiffiffiffiffiffiffiffiffi ffi 1 1 g 2 p 2 1 1 1 g ! .…”

“…The latter internal mixture is intended to be suggestive of sulfate-carbonaceous pollution aerosols (Bates et al 2006). We perform two sets of calculations, one for aerosols consisting of an internal mixture of AS and water only, and one for aerosols consisting of an internal mixture of AS, water, and 2% black carbon by volume.…”

Biomass burning and pollution aerosol over North America: Organic components and their influence on spectral optical properties and humidification response

“…The importance of aerosol physical, chemical and optical properties and their role in regional and global climate forcing is well recognized. Yet there remains considerable uncertainty about the contribution of both natural and anthropogenic aerosols to their overall radiative effects [ Bates et al , 2006]. Some uncertainty is due to the complex and varied properties of organic aerosols and more information is needed to better model their optical properties [ Kanakidou et al , 2004].…”

Parameterization for the relative humidity dependence of light extinction: Organic‐ammonium sulfate aerosol

“…Aerosols, which are ubiquitous in the Earth's atmosphere, are composed of a variety of inorganic and organic compounds. The composition of the organic fraction is complex and can vary by region [ Saxena and Hildemann , 1996; Jacobson et al , 2000; Kanakidou et al , 2005; Bates et al , 2006]. In some cases, organic material can dominate the atmospheric aerosol composition [ Saxena and Hildemann , 1996; Middlebrook et al , 1998; Molnar et al , 1999; Turpin et al , 2000] with a substantial amount of the organic fraction being water‐soluble [ Saxena and Hildemann , 1996; Zappoli et al , 1999; Mayol‐Bracero et al , 2002].…”