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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, D20208, doi:10.1029/2004JD005573, 2005

Direct radiative forcing of anthropogenic organic aerosol

Yi Ming

Visiting Scientist Program, University Corporation for Atmospheric Research, Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA

V. Ramaswamy

Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA

Paul A. Ginoux

Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA

Larry H. Horowitz

Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA

Abstract

This study simulates the direct radiative forcing of organic aerosol using the GFDL AM2 GCM. The aerosol climatology is provided by the MOZART chemical transport model (CTM). The approach to calculating aerosol optical properties explicitly considers relative humidity–dependent hygroscopic growth by employing a functional group–based thermodynamic model, and makes use of the size distribution derived from AERONET measurements. The preindustrial (PI) and present-day (PD) global burdens of organic carbon are 0.17 and 1.36 Tg OC, respectively. The annual global mean total-sky and clear-sky top-of-the atmosphere (TOA) forcings (PI to PD) are estimated as −0.34 and −0.71 W m⁻², respectively. Geographically the radiative cooling largely lies over the source regions, namely part of South America, Central Africa, Europe and South and East Asia. The annual global mean total-sky and clear-sky surface forcings are −0.63 and −0.98 W m⁻², respectively. A series of sensitivity analyses shows that the treatments of hygroscopic growth and optical properties of organic aerosol are intertwined in the determination of the global organic aerosol forcing. For example, complete deprivation of water uptake by hydrophilic organic particles reduces the standard (total-sky) and clear-sky TOA forcing estimates by 18% and 20%, respectively, while the uptake by a highly soluble organic compound (malonic acid) enhances them by 18% and 32%, respectively. Treating particles as non-absorbing enhances aerosol reflection and increases the total-sky and clear-sky TOA forcing by 47% and 18%, respectively, while neglecting the scattering brought about by the water associated with particles reduces them by 24% and 7%, respectively.

Received 4 November 2004; accepted 3 August 2005; published 28 October 2005.

Keywords: organic aerosols; direct forcing; hygroscopicity.

Index Terms: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801, 4906); 3359 Atmospheric Processes: Radiative processes; 3367 Atmospheric Processes: Theoretical modeling.

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Citation: Ming, Y., V. Ramaswamy, P. A. Ginoux, and L. H. Horowitz (2005), Direct radiative forcing of anthropogenic organic aerosol, J. Geophys. Res., 110, D20208, doi:10.1029/2004JD005573.

Copyright 2005 by the American Geophysical Union.