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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.

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.

VanReken, TM, Rissman TA, Roberts GC, Varutbangkul V, Jonsson HH, Flagan RC, Seinfeld JH.  2003.  Toward aerosol/cloud condensation nuclei (CCN) closure during CRYSTAL-FACE. Journal of Geophysical Research-Atmospheres. 108   10.1029/2003jd003582   AbstractWebsite

[1] During July 2002, measurements of cloud condensation nuclei were made in the vicinity of southwest Florida as part of the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) field campaign. These observations, at supersaturations of 0.2 and 0.85%, are presented here. The performance of each of the two CCN counters was validated through laboratory calibration and an in situ intercomparison. The measurements indicate that the aerosol sampled during the campaign was predominantly marine in character: the median concentrations were 233 cm(-3) (at S = 0.2%) and 371 cm(-3) (at S = 0.85%). Three flights during the experiment differed from this general trend; the aerosol sampled during the two flights on 18 July was more continental in character, and the observations on 28 July indicate high spatial variability and periods of very high aerosol concentrations. This study also includes a simplified aerosol/CCN closure analysis. Aerosol size distributions were measured simultaneously with the CCN observations, and these data are used to predict a CCN concentration using Kohler theory. For the purpose of this analysis, an idealized composition of pure ammonium sulfate was assumed. The analysis indicates that in this case, there was good general agreement between the predicted and observed CCN concentrations: at S = 0.2%, N-predicted/N-observed = 1.047 (R-2 = 0.911); at S = 0.85%, N-predicted/N-observed = 1.201 (R-2 = 0.835). The impacts of the compositional assumption and of including in-cloud data in the analysis are addressed. The effect of removing the data from the 28 July flight is also examined; doing so improves the result of the closure analysis at S = 0.85%. When omitting that atypical flight, N-predicted/N-observed = 1.085 (R-2 = 0.770) at S = 0.85%.

Hadley, OL, Ramanathan V, Carmichael GR, Tang Y, Corrigan CE, Roberts GC, Mauger GS.  2007.  Trans-Pacific transport of black carbon and fine aerosols (D < 2.5 ┬Ám) into North America. Journal of Geophysical Research-Atmospheres. 112   10.1029/2006jd007632   AbstractWebsite

[1] This study presents estimates of long-range transport of black carbon (BC) and aerosol fine mass (diameter less than 2.5 mm) across the Pacific Ocean into North America during April 2004. These transport estimates are based on simulations by the Chemical Weather Forecast System (CFORS) model and evaluated across 130 degrees W, (30 degrees N-60 degrees N) from 26 March through 25 April 2004. CFORS calculates BC transport into North America at 25-32 Gg of which over 75% originates from Asia. Modeled fine aerosol mass transport is between 900 and 1100 Gg. The BC transport amounts to about 77% of the published estimates of North American BC emissions. Approximately 78% of the BC and 82% of the fine aerosol mass transport occur in the midtroposphere above 2 km. Given the relatively large magnitude of the estimated BC transport, we undertake a detailed validation of the model simulations of fine aerosol mass and BC over the west coast of North America. In situ aircraft data were available for the month of April 2004 to assess the accuracy of model simulations of aerosols in the lower troposphere. Aircraft data for aerosol mass collected in the eastern Pacific Ocean during April 2004 as part of the Cloud Indirect Forcing Experiment, as well as surface measurements of fine mass and BC at 30 west coast locations, are compared to CFORS predictions. These surface sites are part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. Both the aircraft and the IMPROVE data sets reveal similar patterns of good agreement near and above the boundary layer accompanied by large overprediction within the boundary layer. The observational data validate the CFORS simulations of BC and fine aerosol mass above the boundary layer. The near-surface overprediction does not impair the major conclusions of this study regarding long-range aerosol and BC transport, as most of the long-range transport occurs above 2 km. From this we conclude that the transport of BC from Asia and other regions west is a major source of BC at high elevations over North America. The simulated concentrations of BC between 1 and 3 km, as well as the measured BC concentrations over the elevated IMPROVE sites, range from 0.1 to 0.3 mu g/m(3). Direct radiative forcing over North America due to the modeled BC concentration between 1 and 15 km is estimated at an additional 2.04-2.55 W/m(2) absorbed in the atmosphere and a dimming of-1.45 to-1.47 W/m(2) at the surface. The impact of transported BC on the regional radiation budget through direct and indirect effects of the transported BC and other aerosols warrants further study.

Werner, F, Ditas F, Siebert H, Simmel M, Wehner B, Pilewskie P, Schmeissner T, Shaw RA, Hartmann S, Wex H, Roberts GC, Wendisch M.  2014.  Twomey effect observed from collocated microphysical and remote sensing measurements over shallow cumulus. Journal of Geophysical Research-Atmospheres. 119:1534-1545.   10.1002/2013jd020131   AbstractWebsite

Clear experimental evidence of the Twomey effect for shallow trade wind cumuli near Barbados is presented. Effective droplet radius (r(eff)) and cloud optical thickness (), retrieved from helicopter-borne spectral cloud-reflected radiance measurements, and spectral cloud reflectivity () are correlated with collocated in situ observations of the number concentration of aerosol particles from the subcloud layer (N). N denotes the concentration of particles larger than 80 nm in diameter and represents particles in the activation mode. In situ cloud microphysical and aerosol parameters were sampled by the Airborne Cloud Turbulence Observation System (ACTOS). Spectral cloud-reflected radiance data were collected by the Spectral Modular Airborne Radiation measurement sysTem (SMART-HELIOS). With increasing N a shift in the probability density functions of and toward larger values is observed, while the mean values and observed ranges of retrieved r(eff) decrease. The relative susceptibilities (RS) of r(eff), , and to N are derived for bins of constant liquid water path. The resulting values of RS are in the range of 0.35 for r(eff) and , and 0.27 for . These results are close to the maximum susceptibility possible from theory. Overall, the shallow cumuli sampled near Barbados show characteristics of homogeneous, plane-parallel clouds. Comparisons of RS derived from in situ measured r(eff) and from a microphysical parcel model are in close agreement.