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Ovadnevaite, J, Zuend A, Laaksonen A, Sanchez KJ, Roberts G, Ceburnis D, Decesari S, Rinaldi M, Hodas N, Facchini MC, Seinfeld JH, Dowd CO.  2017.  Surface tension prevails over solute effect in organic-influenced cloud droplet activation. Nature. 546:637-641.   10.1038/nature22806   AbstractWebsite

The spontaneous growth of cloud condensation nuclei (CCN) into cloud droplets under supersaturated water vapour conditions is described by classic Kohler theory(1,2). This spontaneous activation of CCN depends on the interplay between the Raoult effect, whereby activation potential increases with decreasing water activity or increasing solute concentration, and the Kelvin effect, whereby activation potential decreases with decreasing droplet size or increases with decreasing surface tension, which is sensitive to surfactants(1). Surface tension lowering caused by organic surfactants, which diminishes the Kelvin effect, is expected to be negated by a concomitant reduction in the Raoult effect, driven by the displacement of surfactant molecules from the droplet bulk to the droplet-vapour interface(3,4). Here we present observational and theoretical evidence illustrating that, in ambient air, surface tension lowering can prevail over the reduction in the Raoult effect, leading to substantial increases in cloud droplet concentrations. We suggest that consideration of liquid-liquid phase separation, leading to complete or partial engulfing of a hygroscopic particle core by a hydrophobic organic-rich phase, can explain the lack of concomitant reduction of the Raoult effect, while maintaining substantial lowering of surface tension, even for partial surface coverage. Apart from the importance of particle size and composition in droplet activation, we show by observation and modelling that incorporation of phase-separation effects into activation thermodynamics can lead to a CCN number concentration that is up to ten times what is predicted by climate models, changing the properties of clouds. An adequate representation of the CCN activation process is essential to the prediction of clouds in climate models, and given the effect of clouds on the Earth's energy balance, improved prediction of aerosol-cloud-climate interactions is likely to result in improved assessments of future climate change.

Sanchez, KJ, Russell LM, Modini RL, Frossard AA, Ahlm L, Corrigan CE, Roberts GC, Hawkins LN, Schroder JC, Bertram AK, Zhao R, Lee AKY, Lin JJ, Nenes A, Wang Z, Wonaschutz A, Sorooshian A, Noone KJ, Jonsson H, Toom D, Macdonald AM, Leaitch WR, Seinfeld JH.  2016.  Meteorological and aerosol effects on marine cloud microphysical properties. Journal of Geophysical Research-Atmospheres. 121:4142-4161.   10.1002/2015jd024595   AbstractWebsite

Meteorology and microphysics affect cloud formation, cloud droplet distributions, and shortwave reflectance. The Eastern Pacific Emitted Aerosol Cloud Experiment and the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets studies provided measurements in six case studies of cloud thermodynamic properties, initial particle number distribution and composition, and cloud drop distribution. In this study, we use simulations from a chemical and microphysical aerosol-cloud parcel (ACP) model with explicit kinetic drop activation to reproduce observed cloud droplet distributions of the case studies. Four cases had subadiabatic lapse rates, resulting in fewer activated droplets, lower liquid water content, and higher cloud base height than an adiabatic lapse rate. A weighted ensemble of simulations that reflect measured variation in updraft velocity and cloud base height was used to reproduce observed droplet distributions. Simulations show that organic hygroscopicity in internally mixed cases causes small effects on cloud reflectivity (CR) (<0.01), except for cargo ship and smoke plumes, which increased CR by 0.02 and 0.07, respectively, owing to their high organic mass fraction. Organic hygroscopicity had larger effects on droplet concentrations for cases with higher aerosol concentrations near the critical diameter (namely, polluted cases with a modal peak near 0.1 mu m). Differences in simulated droplet spectral widths (k) caused larger differences in CR than organic hygroscopicity in cases with organic mass fractions of 60% or less for the cases shown. Finally, simulations from a numerical parameterization of cloud droplet activation suitable for general circulation models compared well with the ACP model, except under high organic mass fraction.

Hammer, E, Gysel M, Roberts GC, Elias T, Hofer J, Hoyle CR, Bukowiecki N, Dupont JC, Burnet F, Baltensperger U, Weingartner E.  2014.  Size-dependent particle activation properties in fog during the ParisFog 2012/13 field campaign. Atmospheric Chemistry and Physics. 14:10517-10533.   10.5194/acp-14-10517-2014   AbstractWebsite

Fog-induced visibility reduction is responsible for a variety of hazards in the transport sector. Therefore there is a large demand for an improved understanding of fog formation and thus improved forecasts. Improved fog forecasts require a better understanding of the numerous complex mechanisms during the fog life cycle. During winter 2012/13 a field campaign called ParisFog aiming at fog research took place at SIRTA (Instrumented Site for Atmospheric Remote Sensing Research). SIRTA is located about 20 km southwest of the Paris city center, France, in a semi-urban environment. In situ activation properties of the prevailing fog were investigated by measuring (1) total and interstitial (non-activated) dry particle number size distributions behind two different inlet systems; (2) interstitial hydrated aerosol and fog droplet size distributions at ambient conditions; and (3) cloud condensation nuclei (CCN) number concentration at different supersaturations (SS) with a CCN counter. The aerosol particles were characterized regarding their hygroscopic properties, fog droplet activation behavior and contribution to light scattering for 17 developed fog events. Low particle hygroscopicity with an overall median of the hygroscopicity parameter, kappa, of 0.14 was found, likely caused by substantial influence from local traffic and wood burning emissions. Measurements of the aerosol size distribution at ambient RH re-vealed that the critical wet diameter, above which the hydrated aerosols activate to fog droplets, is rather large (with a median value of 2.6 mu m) and is highly variable (ranging from 1 to 5 mu m) between the different fog events. Thus, the number of activated fog droplets was very small and the nonactivated hydrated particles were found to contribute significantly to the observed light scattering and thus to the reduction in visibility. Combining all experimental data, the effective peak supersaturation, SSpeak, a measure of the peak supersaturation during the fog formation, was determined. The median SSpeak value was estimated to be in the range from 0.031 to 0.046% (upper and lower limit estimations), which is in good agreement with previous experimental and modeling studies of fog.

Collins, DB, Ault AP, Moffet RC, Ruppel MJ, Cuadra-Rodriguez LA, Guasco TL, Corrigan CE, Pedler BE, Azam F, Aluwihare LI, Bertram TH, Roberts GC, Grassian VH, Prather KA.  2013.  Impact of marine biogeochemistry on the chemical mixing state and cloud forming ability of nascent sea spray aerosol. Journal of Geophysical Research-Atmospheres. 118:8553-8565.   10.1002/jgrd.50598   AbstractWebsite

The composition and properties of sea spray aerosol, a major component of the atmosphere, are often controlled by marine biological activity; however, the scope of impacts that ocean chemistry has on the ability for sea spray aerosol to act as cloud condensation nuclei (CCN) is not well understood. In this study, we utilize a mesocosm experiment to investigate the impact of marine biogeochemical processes on the composition and mixing state of sea spray aerosol particles with diameters<0.2 mu m produced by controlled breaking waves in a unique ocean-atmosphere facility. An increase in relative abundance of a distinct, insoluble organic particle type was observed after concentrations of heterotrophic bacteria increased in the seawater, leading to an 86 +/- 5% reduction in the hygroscopicity parameter () at 0.2% supersaturation. Aerosol size distributions showed very little change and the submicron organic mass fraction increased by less than 15% throughout the experiment; as such, neither of these typical metrics can explain the observed reduction in hygroscopicity. Predictions of the hygroscopicity parameter that make the common assumption that all particles have the same bulk organic volume fractions lead to overpredictions of CCN concentrations by 25% in these experiments. Importantly, key changes in sea spray aerosol mixing state that ultimately influenced CCN activity were driven by bacteria-mediated alterations to the organic composition of seawater.

Roberts, GC, Day DA, Russell LM, Dunlea EJ, Jimenez JL, Tomlinson JM, Collins DR, Shinozuka Y, Clarke AD.  2010.  Characterization of particle cloud droplet activity and composition in the free troposphere and the boundary layer during INTEX-B. Atmospheric Chemistry and Physics. 10:6627-6644.   10.5194/acp-10-6627-2010   AbstractWebsite

Measurements of cloud condensation nuclei (CCN), aerosol size distributions, and submicron aerosol composition were made as part of the Intercontinental Chemical Transport Experiment Phase B (INTEX-B) campaign during spring 2006. Measurements were conducted from an aircraft platform over the northeastern Pacific and western North America with a focus on how the transport and evolution of Asian pollution across the Pacific Ocean affected CCN properties. A broad range of air masses were sampled and here we focus on three distinct air mass types defined geographically: the Pacific free troposphere (FT), the marine boundary layer (MBL), and the polluted continental boundary layer in the California Central Valley (CCV). These observations add to the few observations of CCN in the FT. CCN concentrations showed a large range of concentrations between air masses, however CCN activity was similar for the MBL and CCV (kappa similar to 0.2-0.25). FT air masses showed evidence of long-range transport from Asia and CCN activity was consistently higher than for the boundary layer air masses. Bulk chemical measurements predicted CCN activity reasonably well for the CCV and FT air masses. Decreasing trends in kappa with organic mass fraction were observed for the combination of the FT and CCV air masses and can be explained by the measured soluble inorganic chemical components. Changes in hygroscopicity associated with differences in the non-refractory organic composition were too small to be distinguished from the simultaneous changes in inorganic ion composition in the FT and MBL, although measurements for the large organic fractions (0.6-0.8) found in the CCV showed values of the organic fraction hygroscopicity consistent with other polluted regions (kappa(org)similar to 0.1-0.2). A comparison of CCN-derived kappa (for particles at the critical diameter) to H-TDMA-derived kappa (for particles at 100 nm diameter) showed similar trends, however the CCN-derived kappa values were significantly higher.

Shinozuka, Y, Clarke AD, DeCarlo PF, Jimenez JL, Dunlea EJ, Roberts GC, Tomlinson JM, Collins DR, Howell SG, Kapustin VN, McNaughton CS, Zhou J.  2009.  Aerosol optical properties relevant to regional remote sensing of CCN activity and links to their organic mass fraction: airborne observations over Central Mexico and the US West Coast during MILAGRO/INTEX-B. Atmospheric Chemistry and Physics. 9:6727-6742.   10.5194/acp-9-6727-2009   AbstractWebsite

Remote sensing of cloud condensation nuclei (CCN) would help evaluate the indirect effects of tropospheric aerosols on clouds and climate. To assess its feasibility, we examined relationships of submicron aerosol composition to CCN activity and optical properties observed during the MILAGRO/INTEX-B aircraft campaigns. An indicator of CCN activity, kappa, was calculated from hygroscopicity measured under saturation. kappa for dry 100 nm particles decreased with increasing organic fraction of non-refractory mass of submicron particles (OMF) as 0.34-0.20xOMF over Central Mexico and 0.47-0.43xOMF over the US West Coast. These fits represent the critical dry diameter, centered near 100 nm for 0.2% supersaturation but varied as kappa((-1/3)), within measurement uncertainty (similar to 20%). The decreasing trends of CCN activity with the organic content, evident also in our direct CCN counts, were consistent with previous ground and laboratory observations of highly organic particles. The wider range of OMF, 0-0.8, for our research areas means that aerosol composition will be more critical for estimation of CCN concentration than at the fixed sites previously studied. Furthermore, the wavelength dependence of extinction was anti-correlated with OMF as -0.70xOMF+2.0 for Central Mexico's urban and industrial pollution air masses, for unclear reasons. The Angstrom exponent of absorption increased with OMF, more rapidly under higher single scattering albedo, as expected for the interplay between soot and colored weak absorbers ( some organic species and dust). Because remote sensing products currently use the wavelength dependence of extinction albeit in the column integral form and may potentially include that of absorption, these regional spectral dependencies are expected to facilitate retrievals of aerosol bulk chemical composition and CCN activity over Central Mexico.