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2016
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.

2015
Modini, RL, Frossard AA, Ahlm L, Russell LM, Corrigan CE, Roberts GC, Hawkins LN, Schroder JC, Bertram AK, Zhao R, Lee AKY, Abbatt JPD, Lin J, Nenes A, Wang Z, Wonaschutz A, Sorooshian A, Noone KJ, Jonsson H, Seinfeld JH, Toom-Sauntry D, Macdonald AM, Leaitch WR.  2015.  Primary marine aerosol-cloud interactions off the coast of California. Journal of Geophysical Research-Atmospheres. 120:4282-4303.   10.1002/2014jd022963   AbstractWebsite

Primary marine aerosol (PMA)-cloud interactions off the coast of California were investigated using observations of marine aerosol, cloud condensation nuclei (CCN), and stratocumulus clouds during the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) and the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) studies. Based on recently reported measurements of PMA size distributions, a constrained lognormal-mode-fitting procedure was devised to isolate PMA number size distributions from total aerosol size distributions and applied to E-PEACE measurements. During the 12 day E-PEACE cruise on the R/V Point Sur, PMA typically contributed less than 15% of total particle concentrations. PMA number concentrations averaged 12 cm(-3) during a relatively calmer period (average wind speed 12m/s(1)) lasting 8 days, and 71cm(-3) during a period of higher wind speeds (average 16m/s(1)) lasting 5 days. On average, PMA contributed less than 10% of total CCN at supersaturations up to 0.9% during the calmer period; however, during the higher wind speed period, PMA comprised 5-63% of CCN (average 16-28%) at supersaturations less than 0.3%. Sea salt was measured directly in the dried residuals of cloud droplets during the SOLEDAD study. The mass fractions of sea salt in the residuals averaged 12 to 24% during three cloud events. Comparing the marine stratocumulus clouds sampled in the two campaigns, measured peak supersaturations were 0.20.04% during E-PEACE and 0.05-0.1% during SOLEDAD. The available measurements show that cloud droplet number concentrations increased with >100 nm particles in E-PEACE but decreased in the three SOLEDAD cloud events.

2013
Juranyi, Z, Tritscher T, Gysel M, Laborde M, Gomes L, Roberts G, Baltensperger U, Weingartner E.  2013.  Hygroscopic mixing state of urban aerosol derived from size-resolved cloud condensation nuclei measurements during the MEGAPOLI campaign in Paris. Atmospheric Chemistry and Physics. 13:6431-6446.   10.5194/acp-13-6431-2013   AbstractWebsite

Ambient aerosols are a complex mixture of particles with different physical and chemical properties and consequently distinct hygroscopic behaviour. The hygroscopicity of a particle determines its water uptake at subsaturated relative humidity (RH) and its ability to form a cloud droplet at supersaturated RH. These processes influence Earth's climate and the atmospheric lifetime of the particles. Cloud condensation nuclei (CCN) number size distributions (i.e. CCN number concentrations as a function of dry particle diameter) were measured close to Paris during the MEGAPOLI campaign in January-February 2010, covering 10 different supersaturations (SS = 0.1-1.0%). The time-resolved hygroscopic mixing state with respect to CCN activation was also derived from these measurements. Simultaneously, a hygroscopicity tandem differential mobility analyser (HTDMA) was used to measure the hygroscopic growth factor (ratio of wet to dry mobility diameter) distributions at RH = 90%. The aerosol was highly externally mixed and its mixing state showed significant temporal variability. The average particle hygroscopicity was relatively low at subsaturation (RH = 90%; mean hygroscopicity parameter kappa = 0.12-0.27) and increased with increasing dry diameter in the range 35-265 nm. The mean kappa value, derived from the CCN measurements at supersaturation, ranged from 0.08 to 0.24 at SS = 1.0-0.1%. Two types of mixing-state resolved hygroscopicity closure studies were performed, comparing the water uptake ability measured below and above saturation. In the first type the CCN counter was connected in series with the HTDMA and and closure was achieved over the whole range of probed dry diameters, growth factors and supersaturations using the kappa-parametrization for the water activity and assuming surface tension of pure water in the Kohler theory. In the second closure type we compared hygroscopicity distributions derived from parallel monodisperse CCN measurements and HTDMA measurements. Very good agreement was found at all supersaturations, which shows that monodisperse CCN measurements are a reliable alternative to determine the hygroscopic mixing state of ambient aerosols.

2008
Furutani, H, Dall'osto M, Roberts GC, Prather KA.  2008.  Assessment of the relative importance of atmospheric aging on CCN activity derived from field observations. Atmospheric Environment. 42:3130-3142.   10.1016/j.atmosenv.2007.09.024   AbstractWebsite

The effect of atmospheric aging on the cloud condensation nuclei (CCN) activity of atmospheric aerosols was studied by comparing different air masses with different degrees of aging along the southern coast of California over the Pacific Ocean during a research cruise on the R/V Roger Revelle from 2-19 November 2004. Activation diameters (D(act)) were calculated using the measured CCN concentrations, condensation nuclei (CN) concentrations, and particle size distributions. Measurements of single particle size and chemistry, as well as black carbon (BC) concentrations with an aethalometer, were made to provide further insight into aerosol chemistry. A gradient of aerosol concentrations was encountered: along the coast of California, the highest BC and CN concentrations (1000-6000 ng m(-3) and 2000-15,000 cm(-3)) were measured which decreased as the ship moved away from shore to much lower values (<100 ng m(-3), similar to 300 cm(-3)). In all regions, external mixtures of organic carbon, elemental carbon, sea salt, and dust aerosols frequently associated with nitrate and sulfate were observed. A correlation plot between the CCN/CN ratio and D(act) exhibits a clear linear correlation, showing a distinct relationship between the extent of anthropogenic aging and CCN activity with the most highly aged air masses showing the highest CCN activity and smallest D(act). These results show changes in aerosol chemistry due to atmospheric aging that play an important role in determining the CCN activity of atmospheric aerosols. The present study demonstrates that variations in aerosol chemistry must be taken into account in models to adequately account for the physicochemical properties of atmospheric aerosols and their CCN activity. (C) 2007 Published by Elsevier Ltd.