Motivation

1. T2 20-30% of days T1 T0 CCN Critical Supersaturation (%) Droplet diameter (mm) of activated CCN when they are being detected in the instrument Padró et al., ACPD; Asa-Awuku et al., ACPD CCN ACTIVITY AND THERMODYNAMIC PROPERTIES OF WATER SOLUBLE ORGANICS IN MEXICO CITY Luz T. Padró1, Chris Henningan2, Terry Lathem3, Rodney J. Weber3, and Athanasios Nenes1,3 1School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, 2School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, and 3School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA Obtaining organic properties: General Approach CCN Activity & Organic Molecular Weight Motivation Atmospheric aerosols affect climate directly by reflecting incoming solar radiation (IPCC, 2001) and indirectly through their role as cloud condensation nuclei (CCN) (Twomey, 1977; Albrecht, 1989: IPCC, 2001). Although important for aerosol cloud interactions, currently the CCN activity of ambient carbonaceous aerosol is poorly understood and largely unconstrained. In-situ measurements can fill this “knowledge gap”. Collect Particles (ambient or filter samples) Measure CCN activity and growth kinetics as a function of dry particle size (i.e., previously classified with a DMA). Supersaturation (fractional) Extraction in Water Measure inorganic composition and total carbon in aerosol Surface tension measurements Goals of thisStudy aAssuming an average organic density of 1.4 g cm-3 (Turpin and Lim, 2001) Particle Dry Diameter (nm) • We characterize CCN-relevant properties of ambient aerosols collected during MILAGRO campaign. The emphasis is placed on the water soluble organic fraction (WSOC) and specifically to: • Estimate the average molecular weight of the WSOC. • Characterize the surfactants present, in terms of their ability to affect surface tension and droplet growth kinetics. • Assess the impact of ageing processes on the above. • Aerosols collected at T1 are more CCN active than those collected at T0. • Inferred decrease in molecular weight from T0 to T1 site consistent with chemical aging (oxidation) of organics. • The inferred organic molecular weights decreases by roughly a factor of two over one day of transport. • Inferred lower molecular weight organics during the day than at night at T1 site consistent with photooxidation of the aerosol constituents. Use theory to infer the average molecular weights and the existence of films DMT Streamwise Gradient CCN Cloud Chamber for droplet formation potential and growth kinetics. CAM-100 Pendant Drop Tensiometer for surface tension measurements Köhler Theory Analysis Surfactants & Growth Kinetics General Approach Residence time (i.e., flow rate) in the measurements is the same. CCN concentrations are low enough so vapor depletion in instrument is unimportant. We select droplet size for particles with scequal to instrument s. (NH4)2SO4 particles with same sc used as a reference. • Measure size-resolved CCN activity and droplet growth kinetics of aerosol. • Dissolve aerosol in water and measure surface tension and chemical composition. • Use the above, coupled to Köhler theory, to infer molecular weight and surfactant characteristics. • Measure the saturation level (sc) required for a particle of dry size d to form a droplet. • Fit the measurements to a power law expression. • Relate fitted coefficients to aerosol properties (e.g. molecular weight, solubility) by using Köhler theory: Sample Collection & Analysis (NH4)2SO4 *With respect to pure water at the same temperature as the CCN measurement. Surfactants are present in both sites. For the same water soluble organic carbon (WSOC) concentration, organics at T0 are more surface active than at T1. Surfactants are stronger at T0. • Measurement Sites: • Instituto Mexicano de Petróleo (T0) • Universidad Tecnológica de Tecámac (T1) • Measurement Period: • March 7 – 31, 2007. CCN grow to roughly the same size, similar to (NH4)2SO4 . Inferring Molecular Weights organics: ~ 1 “Take-home” messages Acknowledgements • Compared to T1 organics at T0 are less CCN-active. • Ageing tends to decrease their molecular weight over a day of processing. • Consistent with having a larger molecular weight, surfactants are stronger at T0. • Growth kinetics of the activated CCN are similar to (NH4)2SO4. NSF, NOAA, NASA Gayle Hagler Evan Cobb PM 2.5 was collected with High Volume Samplers for 12 and 24 hour integration periods. Aerosols collected on quartz filters Constants References Molar Volume This is what you need to know about organics for CCN activity Measured (inferred) surface tension Extract Water-Soluble Component of Aerosol OC/EC Analysis Albrecht, B. A.: Aerosols, cloud mycrophysics, and fractional cloudiness, Science, 245,1227-1230,1989. Asa-Awuku, A., Nenes, A., Sullivan, A. P., Hennigan, C., and Weber, R. J.: Investigation of Molar Volume and Surfactant Characteristics of Water-Soluble Organic Compounds in Biomass Burning Aerosol, ACPD, in review. IPCC. Climate Change (2001): The Scientific Basis.: Cambridge University Press. United Kingdom,2001. Padró, L. T., Asa-Awuku, A., Morrison, R., and Nenes, A.: Inferring thermodynamic properties from CCN activation experiments: a) single-component and binary aerosols, ACPD, in review. Turpin, B. J., and Lim, H. J.: Species contributions to PM2.5 mass concentrations: Revisiting common assumptions for estimating organic mass, Aerosol Sci. Tech., 35, 602-610, 2001. Twomey, S.: Minimum size of particle for nucleation in clouds, Journal of Atmospheric Science, 34, 1832-1835, 1977. From IC/WSOC measurement From CCN activity data Characterize Measure CCN properties, WSOC & Inorganic concentration and surface tension depression Method shown to work well for laboratory-generated aerosol (Padróet al., ACPD) and SOA generated from ozonolysis of biogenic VOC (Asa-Awuku et al., in preparation).