Airborne cloud condensation nuclei measurementsduring the 2006 Texas Air Quality StudyAkua Asa‐Awuku,1,2Richard H. Moore,1Athanasios Nenes,1,3Roya Bahreini,4,5John S. Holloway,4,5Charles A. Brock,4Ann M. Middlebrook,4Thomas B. Ryerson,4Jose L. Jimenez,5,6Peter F. DeCarlo,5,6Arsineh Hecobian,3Rodney J. Weber,3Robert Stickel,3Dave J. Tanner,3and Lewis G. Huey3Received 8 August 2010; revised 1 March 2011; accepted 9 March 2011; published 1 June 2011.[1] Airborne measurements of aerosol and cloud condensation nuclei (CCN) wereconducted aboard the National Oceanic and Atmospheric Administration WP‐3D platformduring the 2006 Texas Air Quality Study/Gulf of Mexico Atmospheric Composition andClimate Study (TexAQS/GoMACCS). The measurements were conducted in regionsinfluenced by industrial and urban sources. Observations show significant local variabilityof CCN activity (CCN/CN from 0.1 to 0.5 at s = 0.43%), while variability is lesssignificant across regional scales (∼100 km × 100 km; CCN/CN is ∼0.1 at s = 0.43%).CCN activity can increase with increasing plume age and oxygenated organic fraction.CCN measurements are compared to predictions for a number of mixing state andcomposition assumptions. Mixing state assumptions that assumed internally mixed aerosolpredict CCN concentrations well. Assuming organics are as hygroscopic as ammoniumsulfate consistently overpredicted CCN concentrations. On average, the water‐solubleorganic carbon (WSOC) fraction is 60 ± 14% of the organic aerosol. We show that CCNclosure can be significantly improved by incorporating knowledge of the WSOC fractionwith a prescribed organic hygroscopicity parameter ( = 0.16 or effective  ∼ 0.3). Thisimplies that the hygroscopicity of organic mass is primarily a function of the WSOCfraction. The overall aerosol hygroscopicity parameter varies between 0.08 and 0.88.Furthermore, droplet activation kinetics are variable and 60% of particles are smaller thanthe size characteristic of rapid droplet growth.Citation: Asa‐Awuku, A., et al. (2011), Airborne cloud condensation nuclei measurements during the 2006 Texas Air QualityStudy, J. Geophys. Res., 116, D11201, doi:10.1029/2010JD014874.1. Introduction[2] Atmospheric particles, by acting as cloud condensa-tion nuclei (CCN), can indirectly influence climate throughtheir impact on cloud radiative properties and the hydro-logical cycle [e.g., Lohmann and Feichter, 2005, and refer-ences therein]. The complexity and incomplete descriptionof aerosol‐cloud interactions in models result in largeuncertainties in assessments of the anthropogenic indi-rect aerosol effect [e.g., Haywood and Boucher, 2000;Ramanathan et al., 2001; Lohmann and Feichter, 2005;Intergovernmental Panel on Climate Change (IPCC), 2007].Much of this uncertainty in global climate models arises fromthe subgrid nature of cloud processes and the effect of poorlyconstrained parameters [e.g., IPCC, 2007], one of which isthe CCN concentration. The prime factor controlling CCNconcentration is the aerosol size distribution [Twomey, 1977;Dusek et al., 2006]; however the variability of aerosol com-position has also been shown to play a significant role in CCNactivity [Jimenez et al., 2009]. Predictions of CCN con-centrations in climate models require simplifying assump-tions, particularly in the description of chemical composition,and the resulting uncertainty in indirect forcing from theirapplication need to be quantified [e.g., Sotiropoulou et al.,2006].[3] Each aerosol particle requires exposure to a “critical”level of water vapor supersaturation, sc, before it can act as aCCN and activate into a cloud droplet. The scdepends onthe aerosol dry size and chemical composition, and is1School of Chemical and Biomolecular Engineering, Georgia Instituteof Technology, Atlanta, Georgia, USA.2Department of Chemical and Environmental Engineering, BournsCollege of Engineering‐Center for Environmental Research andTechnology, University of California, Riverside, California, USA.3School of Earth and Atmospheric Sciences, Georgia Institute ofTechnology, Atlanta, Georgia, USA.4Chemical Sciences Division, Earth System Research Laboratory,National Oceanic and Atmospheric Administration, Boulder, Colorado,USA.5Cooperative Institute for Research in Environmental Sciences,University of Colorado at Boulder, Boulder, Colorado, USA.6Department of Chemistry and Biochemistry and Department ofAtmospheric and Oceanic Sciences, University of Colorado at Boulder,Boulder, Colorado, USA.Copyright 2011 by the American Geophysical Union.0148‐0227/11/2010JD014874JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D11201, doi:10.1029/2010JD014874, 2011D11201 1of18computed from considering solute and curvature effects onthe equilibrium water vapor pressure [Köhler, 1936].“Köhler theory” remains to date the basis for linking CCNactivity with aerosol thermophysical properties, and is usedin all physically based models of the indirect effect topredict CCN number concentrations from knowledge ofthe aerosol size distribution, chemical composition anddynamical forcing. The simpler forms of Köhler theoryinvolve aerosol composed of an “insoluble” and completelysoluble fraction, and have been successfully applied towater‐soluble inorganic and low molecular‐ weight organicaerosol. Simple forms of the theory may be subject touncertainty when partially soluble compounds are present orwhen the aerosol is a complex mixture of inorganic andorganic compounds. A comprehensive theory can becomequite complex, as the presence of multiple phases and all theinteractions of organics and inorganics with water must beaccounted for, and requires the knowledge of poorly con-strained parameters. This is especially true if the aerosolcontains substantial amounts of ambient water‐solubleorganic carbon (WSOC), which can contribute solute[Shulman et al., 1996], act as a surfactant that depressesdroplet surface tension [Facchini et al., 1999; Decesari et al.,2003; Asa‐Awuku et al., 2008] and, potentially affect thecondensation rate of water onto growing droplets [e.g.,Feingold and Chuang, 2002; Chuang, 2003 ; Nenes et al.,2002b; Asa‐Awuku et al., 2009].[4] “CCN closure,” or comparison of predictions withobservations of ambient CCN concentrations, has been thefocus of numerous studies and is the ultimate test of Köhlertheory [e.g., Li u et al.,1996;Covert et al., 1998; Ji et al.,1998; Snider and Brenguier, 2000; Cantrell et al., 2001;Dusek et