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Sanchez, KJ, Roberts GC, Calmer R, Nicoll K, Hashimshoni E, Rosenfeld D, Ovadnevaite J, Preissler J, Ceburnis D, O'Dowd C, Russell LM.  2017.  Top-down and bottom-up aerosol-cloud closure: towards understanding sources of uncertainty in deriving cloud shortwave radiative flux. Atmospheric Chemistry and Physics. 17:9797-9814.   10.5194/acp-17-9797-2017   AbstractWebsite

Top-down and bottom-up aerosol-cloud shortwave radiative flux closures were conducted at the Mace Head Atmospheric Research Station in Galway, Ireland, in August 2015. This study is part of the BACCHUS (Impact of Biogenic versus Anthropogenic emissions on Clouds and Climate: towards a Holistic UnderStanding) European collaborative project, with the goal of understanding key processes affecting aerosol-cloud shortwave radiative flux closures to improve future climate predictions and develop sustainable policies for Europe. Instrument platforms include ground-based unmanned aerial vehicles (UAVs)(1) and satellite measurements of aerosols, clouds and meteorological variables. The ground-based and airborne measurements of aerosol size distributions and cloud condensation nuclei (CCN) concentration were used to initiate a 1-D microphysical aerosol-cloud parcel model (ACPM). UAVs were equipped for a specific science mission, with an optical particle counter for aerosol distribution profiles, a cloud sensor to measure cloud extinction or a five-hole probe for 3D wind vectors. UAV cloud measurements are rare and have only become possible in recent years through the miniaturization of instrumentation. These are the first UAV measurements at Mace Head. ACPM simulations are compared to in situ cloud extinction measurements from UAVs to quantify closure in terms of cloud shortwave radiative flux. Two out of seven cases exhibit sub-adiabatic vertical temperature profiles within the cloud, which suggests that entrainment processes affect cloud microphysical properties and lead to an overestimate of simulated cloud shortwave radiative flux. Including an entrainment parameterization and explicitly calculating the entrainment fraction in the ACPM simulations both improved cloud-top radiative closure. Entrainment reduced the difference between simulated and observation-derived cloud-top shortwave radiative flux (delta RF) by between 25 and 60Wm(-2). After accounting for entrainment, satellite-derived cloud droplet number concentrations (CDNCs) were within 30% of simulated CDNC. In cases with a well-mixed boundary layer, delta RF is no greater than 20Wm(-2) after accounting for cloud-top entrainment and up to 50Wm(-2) when entrainment is not taken into account. In cases with a decoupled boundary layer, cloud microphysical properties are inconsistent with ground-based aerosol measurements, as expected, and delta RF is as high as 88Wm(-2), even high (> 30Wm(-2)) after accounting for cloud-top entrainment. This work demonstrates the need to take in situ measurements of aerosol properties for cases where the boundary layer is decoupled as well as consider cloud-top entrainment to accurately model stratocumulus cloud radiative flux.

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.

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.

Siebert, H, Beals M, Bethke J, Bierwirth E, Conrath T, Dieckmann K, Ditas F, Ehrlich A, Farrell D, Hartmann S, Izaguirre MA, Katzwinkel J, Nuijens L, Roberts G, Schafer M, Shaw RA, Schmeissner T, Serikov I, Stevens B, Stratmann F, Wehner B, Wendisch M, Werner F, Wex H.  2013.  The fine-scale structure of the trade wind cumuli over Barbados - an introduction to the CARRIBA project. Atmospheric Chemistry and Physics. 13:10061-10077.   10.5194/acp-13-10061-2013   AbstractWebsite

The CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) project, focused on high resolution and collocated measurements of thermodynamic, turbulent, microphysical, and radiative properties of trade wind cumuli over Barbados, is introduced. The project is based on two one-month field campaigns in November 2010 (climatic wet season) and April 2011 (climatic dry season). Observations are based on helicopterborne and ground-based measurements in an area of 100 km(2) off the coast of Barbados. CARRIBA is accompanied by long-term observations at the Barbados Cloud Observatory located at the East coast of Barbados since early in 2010 and which provides a longer-term context for the CARRIBA measurements. The deployed instrumentation and sampling strategy are presented together with a classification of the meteorological conditions. The two campaigns were influenced by different air masses advected from the Caribbean area, the Atlantic Ocean, and the African continent which led to distinct aerosol conditions. Pristine conditions with low aerosol particle number concentrations of similar to 100 cm(3) were alternating with periods influenced by Saharan dust or aerosol from biomass burning resulting in comparably high number concentrations of similar to 500 cm(3). The biomass burning aerosol was originating from both the Caribbean area and Africa. The shallow cumulus clouds responded to the different aerosol conditions with a wide range of mean droplet sizes and number concentrations. Two days with different aerosol and cloud microphysical properties but almost identical meteorological conditions have been analyzed in detail. The differences in the droplet number concentration and droplet sizes appear not to show any significant change for turbulent cloud mixing, but the relative roles of droplet inertia and sedimentation in initiating coalescence, as well as the cloud reflectivity, do change substantially.

Sorensen, CM, Roberts GC.  1997.  The prefactor of fractal aggregates. Journal of Colloid and Interface Science. 186:447-452.   10.1006/jcis.1996.4664   AbstractWebsite

The prefactor k(0) of the fractal aggregate scaling relationship N = k(0)(R(g)/a)(D)f is determined for both Diffusion Limited and Diffusion Limited Cluster Aggregation processes in spatial dimensions of 2, 3, 4, and 5. For the physically relevant case of DLCA aggregates in three dimensions we find k(0) = 1.19 +/- 0.1 when D-f = 1.82 +/- 0.04. Comparison of all aggregation types shows that the prefactor k(0) displays uniform trends with the fractal dimension D-f. Attempts to explain these trends are made based on either a common small N limit for all clusters or the packing of spheres in space. (C) 1997 Academic Press.

Stith, JL, Ramanathan V, Cooper WA, Roberts GC, DeMott PJ, Carmichael G, Hatch CD, Adhikary B, Twohy CH, Rogers DC, Baumgardner D, Prenni AJ, Campos T, Gao R, Anderson J, Feng Y.  2009.  An overview of aircraft observations from the Pacific Dust Experiment campaign. Journal of Geophysical Research-Atmospheres. 114   10.1029/2008jd010924   AbstractWebsite

Fourteen research flights were conducted in the Pacific Dust Experiment (PACDEX) during April and May 2007 to sample pollution and dust outbreaks from east Asia as they traveled across the northern Pacific Ocean into North America and interacted with maritime storms. Significant concentrations of black carbon (BC, consisting of soot and other light-absorbing particles measured with a soot photometer 2 instrument) and dust were observed both in the west and east Pacific Ocean from Asian plumes of dust and pollution. BC particles were observed through much of the troposphere, but the major finding is that the percentage of these particles compared with the total number of accumulation mode particles increased significantly (by a factor of 2-4) with increasing altitude, with peak values occurring between 5 and 10 km. Dust plumes had only a small impact on total cloud condensation nuclei at the sampling supersaturations but did exhibit high concentrations of ice nuclei (IN). IN concentrations in dust plumes exceeded typical tropospheric values by 4-20 times and were similar to previous studies in the Saharan aerosol layer when differences in the number concentrations of dust are accounted for. Enhanced IN concentrations were found in the upper troposphere off the coast of North America, providing a first direct validation of the transport of high-IN-containing dust layers near the tropopause entering the North American continent from distant sources. A source-specific chemical transport model was used to predict dust and other aerosols during PACDEX. The model was able to predict several features of the in situ observations, including the general altitudes where BC was found and a peak in the ratio of BC to sulfate between 5 and 10 km.

Sullivan, RC, Moore MJK, Petters MD, Kreidenweis SM, Qafoku O, Laskin A, Roberts GC, Prather KA.  2010.  Impact of particle generation method on the apparent hygroscopicity of insoluble mineral particles. Aerosol Science and Technology. 44:830-846.   10.1080/02786826.2010.497514   AbstractWebsite

Calcite (CaCO(3)) mineral particles are commonly generated by atomization techniques to study their heterogeneous chemistry, hygroscopicity, and cloud nucleation properties. Here we investigate the significant artifact introduced in generating calcium mineral particles through the atomization of a saturated suspension of the powder in water, by measuring particle hygroscopicity via CCN activation curves. Particles produced from atomization displayed hygroscopicities as large as kappa(app) > 0.1, 100 times more hygroscopic than that obtained for dry-generated calcite, kappa(app) = 0.0011. The hygroscopicity of the wet-generated particles increased as a function of time the calcite powder spent in water, and with decreasing particle size. Wet-generated calcium oxalate was more hygroscopic through wet- (kappa(app) = 0.34) versus dry-generation (kappa(app) = 0.048). Atomized calcium sulfate particles, however, were only slightly more hygroscopic (kappa(app) = 0.0045) than those generated dry (kappa(app) = 0.0016). Single-particle analysis by ATOFMS and SEM/EDX, and bulk analysis of the calcite powders by ICP-MS and IC revealed no significant soluble contaminants. The atomized particles were likely composed of components that dissolved from the powder and then re-precipitated, and appeared to contain little of the original mineral powder. The increased hygroscopicity of atomized calcite may have been caused by aqueous carbonate chemistry producing Ca(OH)(2), Ca(HCO(3))(2), and metastable hydrates with increased solubility. Surface water adsorption may have also played a role, in addition to uncharacterized soluble components produced by wet-generation, and the precipitation of amorphous phases including glassy states. This study suggests that using wet-generation methods to suspend mineral dust samples will not produce particles with the correct physicochemical properties in laboratory studies, a finding which has important implications for past and future laboratory studies focusing on understanding relationships between the hygroscopicity and chemistry of mineral dust particles.

Sullivan, RC, Moore MJK, Petters MD, Kreidenweis SM, Roberts GC, Prather KA.  2009.  Effect of chemical mixing state on the hygroscopicity and cloud nucleation properties of calcium mineral dust particles. Atmospheric Chemistry and Physics. 9:3303-3316.   10.5194/acp-9-3303-2009   AbstractWebsite

Atmospheric mineral dust particles can alter cloud properties and thus climate by acting as cloud condensation nuclei (CCN) that form cloud droplets. The CCN activation properties of various calcium mineral dust particles were studied experimentally to investigate the consequences of field observations showing the segregation of sulphate from nitrate and chloride between individual aged Asian dust particles, and the enrichment of oxalic acid in Asian dust. Each mineral's observed apparent hygroscopicity was primarily controlled by its solubility, which determines the degree to which the mineral's intrinsic hygroscopicity can be expressed. The significant increase in hygroscopicity caused by mixing soluble hygroscopic material with insoluble mineral particles is also presented. Insoluble minerals including calcium carbonate, representing fresh unprocessed dust, and calcium sulphate, representing atmospherically processed dust, had similarly small apparent hygroscopicities. Their activation is accurately described by a deliquescence limit following the Kelvin effect and corresponded to an apparent single-hygroscopicity parameter, kappa, of similar to 0.001. Soluble calcium chloride and calcium nitrate, representing atmospherically processed mineral dust particles, were much more hygroscopic, activating similar to ammonium sulphate with kappa similar to 0.5. Calcium oxalate monohydrate (kappa=0.05) was significantly less CCN-active than oxalic acid (kappa=0.3), but not as inactive as its low solubility would predict. These results indicate that the common assumption that all mineral dust particles become more hygroscopic and CCN-active after atmospheric processing should be revisited. Calcium sulphate and calcium oxalate are two realistic proxies for aged mineral dust that remain non-hygroscopic. The dust's apparent hygroscopicity will be controlled by its chemical mixing state, which is determined by its mineralogy and the chemical reaction pathways it experiences during transport.

Sullivan, RC, Moore MJK, Petters MD, Kreidenweis SM, Roberts GC, Prather KA.  2009.  Timescale for hygroscopic conversion of calcite mineral particles through heterogeneous reaction with nitric acid. Physical Chemistry Chemical Physics. 11:7826-7837.   10.1039/b904217b   AbstractWebsite

Atmospheric heterogeneous reactions can potentially change the hygroscopicity of atmospheric aerosols as they undergo chemical aging processes in the atmosphere. A particle's hygroscopicity influences its cloud condensation nuclei (CCN) properties with potential impacts on cloud formation and climate. In this study, size-selected calcite mineral particles were reacted with controlled amounts of nitric acid vapour over a wide range of relative humidities in an aerosol flow tube to study the conversion of insoluble and thus apparently non-hygroscopic calcium carbonate into soluble and hygroscopic calcium nitrate. The rate of hygroscopic change particles undergo during a heterogeneous reaction is derived from experimental measurements for the first time. The chemistry of the reacted particles was determined using an ultra. ne aerosol time-of-flight mass spectrometer (UF-ATOFMS) while the particles' hygroscopicity was determined through measuring CCN activation curves fit to a single parameter of hygroscopicity, kappa. The reaction is rapid, corresponding to atmospheric timescales of hours. At low to moderate HNO(3) exposures, the increase in the hygroscopicity of the particles is a linear function of the HNO(3)(g) exposure. The experimentally observed conversion rate was used to constrain a simple but accurate kinetic model. This model predicts that calcite particles will be rapidly converted into hygroscopic particles (kappa > 0.1) within 4 h for low HNO(3) mixing ratios (10 pptv) and in less than 3 min for 1000 pptv HNO(3). This suggests that the hygroscopic conversion of the calcite component of atmospheric mineral dust aerosol will be controlled by the availability of nitric acid and similar reactants, and not by the atmospheric residence time.