The influence of chemical composition and mixing state of Los Angeles urban aerosol on CCN number and cloud properties

Abstract

Abstract. The relationship between cloud condensation nuclei (CCN) number and the physical and chemical properties of the atmospheric aerosol distribution is explored for a polluted urban data set from the Study of Organic Aerosols at Riverside I (SOAR-1) campaign conducted at Riverside, California, USA during summer 2005. The mixing state and, to a lesser degree, the average chemical composition are shown to be important parameters in determining the activation properties of those particles around the critical activation diameters for atmospherically-realistic supersaturation values. Closure between predictions and measurements of CCN number at several supersaturations is attempted by modeling a number of aerosol chemical composition and mixing state schemes of increasing complexity. It is shown that a realistic treatment of the state of mixing of the urban aerosol distribution is critical in order to eliminate model bias. Fresh emissions such as elemental carbon and small organic particles must be treated as non-activating and explicitly accounted for in the model scheme. The relative number concentration of these particles compared to inorganics and oxygenated organic compounds of limited hygroscopicity plays an important role in determining the CCN number. Furthermore, expanding the different composition/mixing state schemes to predictions of cloud droplet number concentration in a cloud parcel model highlights the dependence of cloud optical properties on the state of mixing and hygroscopic properties of the different aerosol modes, but shows that the relative differences between the different schemes are reduced compared to those from the CCN model.

Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN)

“…In most earlier studies of cloud droplet formation, the number concentration of aerosol particles did not exceed 10 4 cm −3 (e.g. Hjelmfelt et al, 1978;Hegg, 1999;Nenes et al, 2001;Feingold, 2003;Lance et al, 2004;Lohmann et al, 2004;Ervens et al, 2005;Segal and Khain, 2006;Kivekas et al, 2008;Cubison et al, 2008;Altaratz et al, 2008). This is realistic for regions with low or moderate air pollution, but in biomass burning plumes the aerosol particle number concentrations can reach up to ∼10 5 cm −3 Reid et al, 2005;Janhäll et al, 2009).…”

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

“…Single-particle measurements reveal mixtures of soot, dust, and organic carbon, with secondary species such as sulfate, ammonium, nitrate, and oxidized organics (12)(13)(14). Although, in some heavily aged environments, the diversity in particle hygroscopicity is relatively small (15), other studies demonstrate clear impacts of particle mixing state on CCN behavior (16)(17)(18).…”

Surface Characterization of Aerosol Particles in Guangzhou, China: A Study by XPS

“…Atmospheric particles play a direct key role in the radiative budget of the earth's atmosphere (Maria et al 2004;Satheesh and Moorthy 2005), by absorbing, scattering, and reflecting infrared radiation (Andreae et al 2005;Andreae and Gelencer 2006), and indirectly through the formation of cloud condensation nuclei (CCN) (Penner et al 2004;Lohmann and Feichter 2005;Sun and Ariya 2006;Fuzzi et al 2006;Bréon 2006), thereby influencing global as well as regional climate. In addition, atmospheric particles are also important in global carbon cycles, air quality, and the transformation and fate of organic and inorganic pollutants in natural systems (Andreae and Gelencer 2006;Fuzzi et al 2006;Cubison et al 2008).…”