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Wex, H, Dieckmann K, Roberts GC, Conrath T, Izaguirre MA, Hartmann S, Herenz P, Schafer M, Ditas F, Schmeissner T, Henning S, Wehner B, Siebert H, Stratmann F.  2016.  Aerosol arriving on the Caribbean island of Barbados: physical properties and origin. Atmospheric Chemistry and Physics. 16:14107-14130.   10.5194/acp-16-14107-2016   AbstractWebsite

The marine aerosol arriving at Barbados (Ragged Point) was characterized during two 3-week long measurement periods in November 2010 and April 2011, in the context of the measurement campaign CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados). Through a comparison between ground-based and airborne measurements it was shown that the former are representative of the marine boundary layer at least up to cloud base. In general, total particle number concentrations (N-total) ranged from as low as 100 up to 800 cm(-3), while number concentrations for cloud condensation nuclei (N-CCN) at a supersaturation of 0.26% ranged from some 10 to 600 cm(-3). N-total and N-CCN depended on the air mass origin. Three distinct types of air masses were found. One type showed elevated values for both N-total and N-CCN and could be attributed to long-range transport from Africa, by which biomass burning particles from the Sahel region and/or mineral dust particles from the Sahara were advected. The second and third type both had values for N-CCN below 200 cm(-3) and a clear minimum in the particle number size distribution (NSD) around 70 to 80 nm (Hoppel minimum). While for one of these two types the accumulation mode was dominating (albeit less so than for air masses advected from Africa), the Aitken mode dominated the other and contributed more than 50% of all particles. These Aitken mode particles likely were formed by new particle formation no more than 3 days prior to the measurements. Hygroscopicity of particles in the CCN size range was determined from CCN measurements to be kappa = 0.66 on average, which suggests that these particles contain mainly sulfate and do not show a strong influence from organic material, which might generally be the case for the months during which measurements were made. The average kappa could be used to derive N-CCN from measured number size distributions, showing that this is a valid approach to obtain N-CCN. Although the total particulate mass sampled on filters was found to be dominated by Na+ and Cl-, this was found to be contributed by a small number of large particles (> 500 nm, mostly even in the super-micron size range). Based on a three-modal fit, a sea spray mode observed in the NSDs was found to contribute 90% to the total particulate mass but only 4 to 10% to N-total and up to 15% to N-CCN. This is in accordance with finding no correlation between N-total and wind speed.

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.

Conant, WC, VanReken TM, Rissman TA, Varutbangkul V, Jonsson HH, Nenes A, Jimenez JL, Delia AE, Bahreini R, Roberts GC, Flagan RC, Seinfeld JH.  2004.  Aerosol-cloud drop concentration closure in warm cumulus. Journal of Geophysical Research-Atmospheres. 109   10.1029/2003jd004324   AbstractWebsite

[1] Our understanding of the activation of aerosol particles into cloud drops during the formation of warm cumulus clouds presently has a limited observational foundation. Detailed observations of aerosol size and composition, cloud microphysics and dynamics, and atmospheric thermodynamic state were collected in a systematic study of 21 cumulus clouds by the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft during NASA's Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE). An "aerosol-cloud'' closure study was carried out in which a detailed cloud activation parcel model, which predicts cloud drop concentration using observed aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against observations. On average, measured droplet concentration in adiabatic cloud regions is within 15% of the predictions. This agreement is corroborated by independent measurements of aerosol activation carried out by two cloud condensation nucleus (CCN) counters on the aircraft. Variations in aerosol concentration, which ranged from 300 to 3300 cm(-3), drives large microphysical differences ( 250 2300 cm(-3)) observed among continental and maritime clouds in the South Florida region. This is the first known study in which a cloud parcel model is evaluated in a closure study using a constraining set of data collected from a single platform. Likewise, this is the first known study in which relationships among aerosol size distribution, CCN spectrum, and cloud droplet concentration are all found to be consistent with theory within experimental uncertainties much less than 50%. Vertical profiles of cloud microphysical properties ( effective radius, droplet concentration, dispersion) clearly demonstrate the boundary layer aerosol's effect on cloud microphysics throughout the lowest 1 km of cloud depth. Onboard measurements of aerosol hygroscopic growth and the organic to sulfate mass ratio are related to CCN properties. These chemical data are used to quantify the range of uncertainty associated with the simplified treatment of aerosol composition assumed in the closure study.

Ramana, MV, Ramanathan V, Kim D, Roberts GC, Corrigan CE.  2007.  Albedo, atmospheric solar absorption and heating rate measurements with stacked UAVs. Quarterly Journal of the Royal Meteorological Society. 133:1913-1931.   10.1002/qj.172   AbstractWebsite

This paper reports unique measurements of albedo, atmospheric solar absorption, and heating rates in the visible (0.4 to 0.7 mu m) and broadband (0.3 to 2.8 mu m) spectral regions using vertically stacked multiple lightweight autonomous unmanned aerial vehicles (UAVs). The most significant finding of this study is that when absorbing aerosols and water vapour concentrations are measured accurately and accounted for in models, and when heating rates are measured directly with stacked aircraft, the simulated clear sky heating rates are consistent with the observed broadband heating rates within experimental errors (about 15%). We conclude that there is no need to invoke anomalous or excess absorption or unknown physics in clear skies. Aerosol-radiation-cloud measurements were made over the tropical Indian Ocean within the lowest 3 km of the atmosphere during the Maldives Autonomous UAV Campaign (MAC). The UAVs and ground-based remote sensing instruments determined most of the parameters required for calculating the albedo and vertical distribution of solar fluxes. The paper provides a refined analytical procedure to reduce errors and biases due to the offset errors arising from mounting of the radiometers on the aircraft and due to the aircraft attitude. Measured fluxes have been compared with those derived from a Monte-Carlo radiative transfer algorithm which can incorporate both gaseous and aerosol components. Under cloud-free conditions the calculated and measured incoming fluxes agree within 2-10 W m(-2) (<1%) depending upon the altitudes. Similarly, the measured and calculated reflected fluxes agreed within 2-5 W m(-2) (<5%). The analysis focuses on a cloud-free day when the air was polluted due to long-range transport from India, and the mean aerosol optical depth (AOD) was 0.31 and mean single scattering albedo was 0.92. The UAV-measured absorption AOD, was 0.019 which agreed within 20% of the value of 0.024 reported by a ground-based instrument. The observed and simulated solar absorption agreed within 5% above 1.0 km and aerosol absorption accounted for 30% to 50% of the absorption depending upon the altitude and solar zenith angle. Thus there was no need to invoke spurious or anomalous absorption, provided we accounted for aerosol black carbon. The diurnal mean absorption values for altitudes between 0.5 and 3.0 km above mean sea level were observed to be 41 +/- 3 W m(-2) (1.5 K/day) in the broadband region and 8 +/- 2 W m(-2) (0.3 K/day) in the visible region. The contribution of absorbing aerosols to the heating rate was an order of magnitude larger than the contribution of CO2 and one-third that of the water vapour. In the lowest 3 km of the tropical atmosphere, aerosols accounted for more than 80% of the atmospheric absorption in the visible region. Copyright (c) 2007 Royal Meteorological Society.

Crispel, P, Roberts G.  2018.  All-sky photogrammetry techniques to georeference a cloud field. Atmospheric Measurement Techniques. 11:593-609.   10.5194/amt-11-593-2018   AbstractWebsite

In this study, we present a novel method of identifying and geolocalizing cloud field elements from a portable all-sky camera stereo network based on the ground and oriented towards zenith. The methodology is mainly based on stereophotogrammetry which is a 3-D reconstruction technique based on triangulation from corresponding stereo pixels in rectified images. In cases where clouds are horizontally separated, identifying individual positions is performed with segmentation techniques based on hue filtering and contour detection algorithms. Macroscopic cloud field characteristics such as cloud layer base heights and velocity fields are also deduced. In addition, the methodology is fitted to the context of measurement campaigns which impose simplicity of implementation, auto-calibration, and portability. Camera internal geometry models are achieved a priori in the laboratory and validated to ensure a certain accuracy in the peripheral parts of the all-sky image. Then, stereophotogrammetry with dense 3-D reconstruction is applied with cameras spaced 150m apart for two validation cases. The first validation case is carried out with cumulus clouds having a cloud base height at 1500ma. g.l. The second validation case is carried out with two cloud layers: a cumulus fractus layer with a base height at 1000ma. g.l. and an altocumulus stratiformis layer with a base height of 2300ma. g.l. Velocity fields at cloud base are computed by tracking image rectangular patterns through successive shots. The height uncertainty is estimated by comparison with a Vaisala CL31 ceilometer located on the site. The uncertainty on the horizontal coordinates and on the velocity field are theoretically quantified by using the experimental uncertainties of the cloud base height and camera orientation. In the first cumulus case, segmentation of the image is performed to identify individuals clouds in the cloud field and determine the horizontal positions of the cloud centers.

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.

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Amouroux, D, Roberts G, Rapsomanikis S, Andreae MO.  2002.  Biogenic gas (CH4, N2O, DMS) emission to the atmosphere from near-shore and shelf waters of the north-western Black Sea. Estuarine Coastal and Shelf Science. 54:575-587.   10.1006/ecss.2000.0666   AbstractWebsite

The marine emissions of biogenic gases (CH4, N2O, DMS) were determined on the north-western shelf of the Black Sea in July 1995. Methane (CH,) saturation ratios (SR) averaged 930% (173-10 500%) and were inversely proportional to salinity. The mean nitrous oxide (N2O) SR was 111% (96-149%), which is similar to the values found in other coastal water masses, and did not present any significant variability as a function of salinity. Dimethyl sulphide (DMS) concentrations ranged between 2.96 and 11.9 nM and chlorophyll a-normalized DIMS concentrations were found to increase exponentially with salinity, illustrating the influence of plankton speciation. Coastal and shelf waters affected by riverine inputs, which represent 30% of the Black Sea surface area, contributed up to 53% of the CH4 flux during the summer period. Biogenic sulphur emissions, primarily as DMS, showed a strong gradient, increasing from open Black Sea waters to estuarine Danube waters. There was less variation in the flux densities of N2O for the different water masses. By extrapolating values obtained during summer 1995, upper limits for the emissions from the Black Sea to the atmosphere were estimated to be 6.7-11.7 Gmol yr(-1) of CH4, 0.45-0.75 Gmol yr(-1) of N2O and 0.30-0.80 Gmol yr(-1) of DMS, using two different air-sea gas exchange models. (C) 2002 Elsevier Science Ltd. All rights reserved.

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Corrigan, CE, Roberts GC, Ramana MV, Kim D, Ramanathan V.  2008.  Capturing vertical profiles of aerosols and black carbon over the Indian Ocean using autonomous unmanned aerial vehicles. Atmospheric Chemistry and Physics. 8:737-747.   10.5194/acp-8-737-2008   AbstractWebsite

Measurements of the vertical distribution of aerosol properties provide essential information for generating more accurate model estimates of radiative forcing and atmospheric heating rates compared with employing remotely sensed column averaged properties. A month long campaign over the Indian Ocean during March 2006 investigated the interaction of aerosol, clouds, and radiative effects. Routine vertical profiles of aerosol and water vapor were determined using autonomous unmanned aerial vehicles equipped with miniaturized instruments. Comparisons of these airborne instruments with established ground-based instruments and in aircraft-to-aircraft comparisons demonstrated an agreement within 10%. Aerosol absorption optical depths measured directly using the unmanned aircraft differed from columnar AERONET sun-photometer results by only 20%. Measurements of total particle concentration, particle size distributions, aerosol absorption and black carbon concentrations are presented along with the trade wind thermodynamic structure from the surface to 3000 m above sea level. Early March revealed a well-mixed layer up to the cloud base at 500 m above mean sea level (m a.s.l.), followed by a decrease of aerosol concentrations with altitude. The second half of March saw the arrival of a high altitude plume existing above the mixed layer that originated from a continental source and increased aerosol concentrations by more than tenfold, yet the surface air mass showed little change in aerosol concentrations and was still predominantly influenced by marine sources. Black carbon concentrations at 1500 m above sea level increased from 70 ng/m(3) to more than 800 ng/m(3) with the arrival of this polluted plume. The absorption aerosol optical depth increased from as low as 0.005 to as much as 0.035 over the same period. The spectral dependence of the aerosol absorption revealed an absorption Angstrom exponent of 1.0, which is typical of an aerosol with most of its absorption attributed to black carbon and generally indicates the absorbing component originated from fossil fuel sources and other high-temperature combustion sources. The results indicate that surface measurements do not represent the aerosol properties within the elevated layers, especially if these layers are influenced by long range transport.

Kubatova, A, Vermeylen R, Claeys M, Cafmeyer J, Maenhaut W, Roberts G, Artaxo P.  2000.  Carbonaceous aerosol characterization in the Amazon basin, Brazil: novel dicarboxylic acids and related compounds. Atmospheric Environment. 34:5037-5051.   10.1016/s1352-2310(00)00320-4   AbstractWebsite

High-resolution capillary gas chromatography (GC) and GC/mass spectrometry (MS) were employed for the quantitative determination of dichloromethane-extractable organic compounds in total and size-fractionated aerosol samples which were collected in the Amazon basin, Brazil, during the wet season, as part of the LBA-CLAIRE-98 experiment. Special emphasis was placed on the characterization and identification of several novel unknown dicarboxylic acids and related oxidative degradation products. This class of acidic products was enriched in the fine size fraction, suggesting that they were secondary organic aerosol products formed by gas-to-particle conversion. Some of the unknowns contributed more to the class of dicarboxylic acids than the major known compound, nonadioic acid (azelaic acid). The same unknowns were also observed in urban aerosol samples collected on hot summer days in Gent, Belgium. For the characterization and structure elucidation of the unknowns, various types of derivatizations and Fractionation by solid-phase extraction were employed in combination with GC/MS. Four unknowns were identified. The most abundant were two derivatives of glutaric acid, 3-isopropyl pentanedioic acid and 3-acetyl pentanedioic acid. The other two identified unknowns were another oxo homologue, 3-acetyl hexanedioic acid, and, interestingly, 3-carboxy heptanedioic acid. To our knowledge, the occurrence of these four compounds in atmospheric aerosols has not yet been reported. The biogenic precursors of the novel identified compounds could not be pinpointed. but most likely include monoterpenes and fatty acids. (C) 2000 Elsevier Science Ltd. All rights reserved.

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.

DeMott, PJ, Hudson JG, Roberts GC, Bundke U.  2013.  Cloud condensation and ice nuclei. Airborne Measurements for Environmental Research. ( Wendisch M, Brenguier JL, Kohkhanovsky A, Eds.).:157-223.: John Wiley & Sons Abstract
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Roberts, GC, Andreae MO, Zhou J, Artaxo P.  2001.  Cloud condensation nuclei in the Amazon Basin: “marine” conditions over a continent? Geophysical Research Letters. 28:2807-2810.   10.1029/2000GL012585   AbstractWebsite

Cloud condensation nuclei (CCN) are linked to radiative forcing, precipitation, and cloud structure; yet, their role in tropical climates remains largely unknown. CCN concentrations (NCCN) measured during the wet season in the Amazon Basin were surprisingly low (mean NCCN at 1% supersaturation: 267±132 cm−3) and resembled concentrations more typical of marine locations than most continental sites. At low background CCN concentrations, cloud properties are more sensitive to an increase in NCCN. Therefore, enhanced aerosol emissions due to human activity in the Amazon Basin may have a stronger impact on climate than emissions in other continental regions. In spite of the large organic fraction in the Amazonian aerosol, a detailed analysis of number distributions and size-dependent chemical composition indicates that sulfate plays an important role in CCN activity.

Roberts, GC, Andreae MO, Maenhaut W, Fernandez-Jimenez MT.  2001.  Composition and sources of aerosol in a central African rain forest during the dry season. Journal of Geophysical Research-Atmospheres. 106:14423-14434.   10.1029/2000jd900774   AbstractWebsite

During the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO-96), size-fractionated aerosol samples were collected in November and December 1996 at a ground site in the tropical rain forest at the N'doki National Park (NNP) in the Republic of Congo. The samples were analyzed for up to 26 elements using particle-induced X-ray emission. Elements related to mineral dust and pyrogenic aerosol exhibited greater concentrations during the daytime, while aerosol produced by the rain forest exhibited higher concentrations at night. Samples were also collected at two levels on the tower, above and below the canopy, to characterize vegetation sources. Absolute principal component analysis (APCA) identified three major aerosol source types in each size fraction, which explained more than 90% of the data variance. The fine-size fraction contained mineral dust (Al, Si, Ca, Ti, and Fe), pyrogenic (black carbon, K, and Zn), and marine/anthropogenic sulfur components. The coarse-size fraction included a mineral dust (Al, Si, Ca, Ti, Mn, and Fe) and two primary biogenic components consisting of K, P, Zn, and S. Absolute principal component scores were calculated for the components of APCA, and temporal trends were compared to 7 day isopycnic backward trajectories. Consistent relationships between the temporal trends of the fine fraction aerosol components and meteorological patterns were observed. Trade wind air masses transported biomass burning and mineral dust aerosol to NNP during the first half of the experiment. The fine fraction sulfur component correlated well with the pyrogenic activity before the change in meteorological patterns halfway through the experiment. The fine and coarse sulfur concentrations nearly doubled in the latter part of the experiment as a monsoon circulation brought sulfur-enriched aerosol from the Atlantic Ocean. Various industrial activities on the coast of Cameroon and Gabon probably contributed to the high sulfur concentrations as well.

Roberts, GC, Nenes A.  2005.  A continuous-flow streamwise thermal-gradient CCN chamber for atmospheric measurements. Aerosol Science and Technology. 39:206-221.   10.1080/027868290913988   AbstractWebsite

We have addressed the need for improved measurements of cloud condensation nuclei (CCN) by developing a continuous-flow instrument that provides in situ measurements of CCN. The design presented in this article can operate between 0.1 and 3% supersaturation, at sampling rates sufficient for airborne operation. The design constitutes a cylindrical continuous-flow thermal-gradient diffusion chamber employing a novel technique of generating a supersaturation: by establishing a constant streamwise temperature gradient so that the difference in water vapor and thermal diffusivity yield a quasi-uniform centerline supersaturation. Our design maximizes the growth rate of activated droplets, thereby enhancing the performance of the instrument. The temperature gradient and the flow through the column control the supersaturation and may be modified to retrieve CCN spectra. The principle of the CCN instrument was validated in controlled laboratory experiments at different operating conditions using a monodisperse aerosols with known composition and size. These experiments yield sharp activation curves, even for those kinetically limited particles that have not exceeded their critical diameter. The performance of the CCN instrument was also assessed using polydisperse laboratory-generated aerosol of known composition and size distributions similar to ambient particulate matter. In all tests, the measured CCN concentrations compared well with predicted values and highlight the instrument's ability to measure CCN at various size distributions. The full potential of the new design has yet to be explored; however, model simulations suggest that direct measurements in the climatically important range of supersaturations of less than 0.1% (certainly down to 0.07%) are possible. The new instrument clearly offers a unique level of design simplicity, robustness, and flexilibity (temperature control, large range of supersaturations without flow reversal, and multiple configurations for same supersaturation) necessary for atmospheric studies.

Williams, E, Rosenfeld D, Madden N, Gerlach J, Gears N, Atkinson L, Dunnemann N, Frostrom G, Antonio M, Biazon B, Camargo R, Franca H, Gomes A, Lima M, Machado R, Manhaes S, Nachtigall L, Piva H, Quintiliano W, Machado L, Artaxo P, Roberts G, Renno N, Blakeslee R, Bailey J, Boccippio D, Betts A, Wolff D, Roy B, Halverson J, Rickenbach T, Fuentes J, Avelino E.  2002.  Contrasting convective regimes over the Amazon: Implications for cloud electrification. Journal of Geophysical Research-Atmospheres. 107   10.1029/2001jd000380   AbstractWebsite

Four distinct meteorological regimes in the Amazon basin have been examined to distinguish the contributions from boundary layer aerosol and convective available potential energy (CAPE) to continental cloud structure and electrification. The lack of distinction in the electrical parameters (peak flash rate, lightning yield per unit rainfall) between aerosol-rich October and aerosol-poor November in the premonsoon regime casts doubt on a primary role for the aerosol in enhancing cloud electrification. Evidence for a substantial role for the aerosol in suppressing warm rain coalescence is identified in the most highly polluted period in early October. The electrical activity in this stage is qualitatively peculiar. During the easterly and westerly wind regimes of the wet season, the lightning yield per unit of rainfall is positively correlated with the aerosol concentration, but the electrical parameters are also correlated with CAPE, with a similar degree of scatter. Here cause and effect are difficult to establish with available observations. This ambiguity extends to the "green ocean" westerly regime, a distinctly maritime regime over a major continent with minimum aerosol concentration, minimum CAPE, and little if any lightning.

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Russell, LM, Sorooshian A, Seinfeld JH, Albrecht BA, Nenes A, Ahlm L, Chen YC, Coggon M, Craven JS, Flagan RC, Frossard AA, Jonsson H, Jung E, Lin JJ, Metcalf AR, Modini R, Mulmenstadt J, Roberts GC, Shingler T, Song S, Wang Z, Wonaschutz A.  2013.  Eastern Pacific emitted aerosol cloud experiment. Bulletin of the American Meteorological Society. 94:709-+.   10.1175/bams-d-12-00015.1   AbstractWebsite

Aerosol-cloud-radiation interactions are widely held to be the largest single source of uncertainty in climate model projections of future radiative forcing due to increasing anthropogenic emissions. The underlying causes of this uncertainty among modeled predictions of climate are the gaps in our fundamental understanding of cloud processes. There has been significant progress with both observations and models in addressing these important questions but quantifying them correctly is nontrivial, thus limiting our ability to represent them in global climate models. The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) 2011 was a targeted aircraft campaign with embedded modeling studies, using the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft and the research vessel Point Sur in July and August 2011 off the central coast of California, with a full payload of instruments to measure particle and cloud number, mass, composition, and water uptake distributions. E-PEACE used three emitted particle sources to separate particle-induced feedbacks from dynamical variability, namely 1) shipboard smoke-generated particles with 0.05-1-mu m diameters (which produced tracks measured by satellite and had drop composition characteristic of organic smoke), 2) combustion particles from container ships with 0.05-0.2-mu m diameters (which were measured in a variety of conditions with droplets containing both organic and sulfate components), and 3) aircraft-based milled salt particles with 3-5-mu m diameters (which showed enhanced drizzle rates in some clouds). The aircraft observations were consistent with past large-eddy simulations of deeper clouds in ship tracks and aerosol cloud parcel modeling of cloud drop number and composition, providing quantitative constraints on aerosol effects on warm-cloud microphysics.

Russell, LM, Sorooshian A, Seinfeld JH, Albrecht BA, Nenes A, Ahlm L, Chen Y-C, Coggon M, Craven JS, Flagan RC, Frossard AA, Jonsson H, Jung E, Lin JJ, Metcalf AR, Modini R, Mülmenstädt J, Roberts GC, Shingler T, Song S, Wang Z, Wonaschütz A.  2012.  Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). Bulletin of the American Meteorological Society. : American Meteorological Society   10.1175/BAMS-D-12-00015   AbstractWebsite

Aerosol-cloud-radiation interactions are widely held to be the largest single source of uncertainty in climate model projections of future radiative forcing due to increasing anthropogenic emissions. The underlying causes of this uncertainty among modeled predictions of climate are the gaps in our fundamental understanding of cloud processes. There has been significant progress with both observations and models on addressing these important questions, but quantifying them correctly is nontrivial thus limiting our ability to represent them in global climate models. The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) 2011 was a targeted aircraft campaign with embedded modeling studies, using the CIRPAS Twin Otter aircraft and the Research Vessel Point Sur in July and August 2011 off the central coast of California, with a full payload of instruments to measure particle and cloud number, mass, composition, and water uptake distributions. E-PEACE used three emitted particle sources to separate particle-induced feedbacks from dynamical variability, namely (i) shipboard smoke-generated particles with 0.05–1 μm diameters (which produced tracks measured by satellite and had drop composition characteristic of organic smoke), (ii) combustion particles from container ships with 0.05–0.2 μm diameters (which were measured in a variety of conditions with droplets containing both organic and sulfate components), and (iii) aircraft-based milled salt particles with 3–5 μm diameters (which showed enhanced drizzle rates in some clouds). The aircraft observations were consistent with past large eddy simulations of deeper clouds in ship tracks and aerosol-cloud parcel modeling of cloud drop number and composition, providing quantitative constraints on aerosol effects on warm cloud microphysics.

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.

Moore, MJK, Furutani H, Roberts GC, Moffet RC, Gilles MK, Palenik B, Prather KA.  2011.  Effect of organic compounds on cloud condensation nuclei (CCN) activity of sea spray aerosol produced by bubble bursting. Atmospheric Environment. 45:7462-7469.   10.1016/j.atmosenv.2011.04.034   AbstractWebsite

The ocean comprises over 70% of the surface of the earth and thus sea spray aerosols generated by wave processes represent a critical component of our climate system. The manner in which different complex oceanic mixtures of organic species and inorganic salts are distributed between individual particles in sea spray directly determines which particles will effectively form cloud nuclei. Controlled laboratory experiments were undertaken to better understand the full range of particle properties produced by bubbling solutions composed of simplistic model organic species, oleic acid and sodium dodecyl sulfate (SDS), mixed with NaCl to more complex artificial seawater mixed with complex organic mixtures produced by common oceanic microorganisms. Simple mixtures of NaCl and oleic acid or SDS had a significant effect on CCN activity, even in relatively small amounts. However, an artificial seawater (ASW) solution containing microorganisms, the common cyanobacteria (Synechococcus) and DMS-producing green algae (Ostreococcus), produced particles containing similar to 34 times more carbon than the particles produced from pure ASW, yet no significant change was observed in the overall CCN activity. We hypothesize that these microorganisms produce diverse mixtures of organic species with a wide range of properties that produced offsetting effects, leading to no net change in the overall average measured hygroscopicity of the collection of sea spray particles. Based on these observations, changes in CCN activity due to "bloom" conditions would be predicted to lead to small changes in the average CCN activity, and thus have a negligible impact on cloud formation. However, each sea spray particle will contain a broad spectrum of different species, and thus further studies are needed of the CCN activity of individual sea spray particles and biological processes under a wide range of controllable conditions. (C) 2011 Published by Elsevier Ltd.

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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.

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Kulmala, M, Asmi A, Lappalainen HK, Baltensperger U, Brenguier JL, Facchini MC, Hansson HC, Hov O, O'Dowd CD, Poschl U et al..  2011.  General overview: European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) - integrating aerosol research from nano to global scales. Atmos Chem Phys. 11:13061-13143.   doi:10.5194/acp-11-13061-2011   Abstract

In this paper we describe and summarize the main achievements of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). EUCAARI started on 1 January 2007 and ended on 31 December 2010 leaving a rich legacy including: (a) a comprehensive database with a year of observations of the physical, chemical and optical properties of aerosol particles over Europe, (b) comprehensive aerosol measurements in four developing countries, (c) a database of airborne measurements of aerosols and clouds over Europe during May 2008, (d) comprehensive modeling tools to study aerosol processes fron nano to global scale and their effects on climate and air quality. In addition a new Pan-European aerosol emissions inventory was developed and evaluated, a new cluster spectrometer was built and tested in the field and several new aerosol parameterizations and computations modules for chemical transport and global climate models were developed and evaluated. These achievements and related studies have substantially improved our understanding and reduced the uncertainties of aerosol radiative forcing and air quality-climate interactions. The EUCAARI results can be utilized in European and global environmental policy to assess the aerosol impacts and the corresponding abatement strategies.

Kettle, AJ, Andreae MO, Amouroux D, Andreae TW, Bates TS, Berresheim H, Bingemer H, Boniforti R, Curran MAJ, DiTullio GR, Helas G, Jones GB, Keller MD, Kiene RP, Leck C, Levasseur M, Malin G, Maspero M, Matrai P, McTaggart AR, Mihalopoulos N, Nguyen BC, Novo A, Putaud JP, Rapsomanikis S, Roberts G, Schebeske G, Sharma S, Simo R, Staubes R, Turner S, Uher G.  1999.  A global database of sea surface dimethylsulfide (DMS) measurements and a procedure to predict sea surface DMS as a function of latitude, longitude, and month. Global Biogeochemical Cycles. 13:399-444.   10.1029/1999gb900004   AbstractWebsite

A database of 15,617 point measurements of dimethylsulfide (DMS) in surface waters along with lesser amounts of data for aqueous and particulate dimethylsulfoniopropionate concentration, chlorophyll concentration, sea surface salinity and temperature, and wind speed has been assembled. The database was processed to create a series of climatological annual and monthly 1 degrees x1 degrees latitude-longitude squares of data. The results were compared to published fields of geophysical and biological parameters. No significant correlation was found between DMS and these parameters, and no simple algorithm could be found to create monthly fields of sea surface DMS concentration based on these parameters. Instead, an annual map of sea surface DMS was produced using an algorithm similar to that employed by Conkright et al. [1994]. In this approach, a first-guess field of DMS sea surface concentration measurements is created and then a correction to this field is generated based on actual measurements. Monthly sea surface grids of DMS were obtained using a similar scheme, but the sparsity of DMS measurements made the method difficult to implement. A scheme was used which projected actual data into months of the year where no data were otherwise present.

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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.

Wonaschutz, A, Coggon M, Sorooshian A, Modini R, Frossard AA, Ahlm L, Mulmenstadt J, Roberts GC, Russell LM, Dey S, Brechtel FJ, Seinfeld JH.  2013.  Hygroscopic properties of smoke-generated organic aerosol particles emitted in the marine atmosphere. Atmospheric Chemistry and Physics. 13:9819-9835.   10.5194/acp-13-9819-2013   AbstractWebsite

During the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE), a plume of organic aerosol was produced by a smoke generator and emitted into the marine atmosphere from aboard the R/V Point Sur. In this study, the hygroscopic properties and the chemical composition of the plume were studied at plume ages between 0 and 4 h in different meteorological conditions. In sunny conditions, the plume particles had very low hygroscopic growth factors (GFs): between 1.05 and 1.09 for 30 nm and between 1.02 and 1.1 for 150 nm dry size at a relative humidity (RH) of 92 %, contrasted by an average marine background GF of 1.6. New particles were produced in large quantities (several 10 000 cm(-3)), which lead to substantially increased cloud condensation nuclei (CCN) concentrations at super-saturations between 0.07 and 0.88 %. Ratios of oxygen to carbon (O : C) and water-soluble organic mass (WSOM) increased with plume age: from <0.001 to 0.2, and from 2.42 to 4.96 mu g m(-3), respectively, while organic mass fractions decreased slightly (similar to 0.97 to similar to 0.94). High-resolution aerosol mass spectrometer (AMS) spectra show that the organic fragment m/z 43 was dominated by C2H3O+ in the small, new particle mode and by C3H7+ in the large particle mode. In the marine background aerosol, GFs for 150 nm particles at 40% RH were found to be enhanced at higher organic mass fractions: an average GF of 1.06 was observed for aerosols with an organic mass fraction of 0.53, and a GF of 1.04 for an organic mass fraction of 0.35.

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Roberts, GC, Nenes A, Seinfeld JH, Andreae MO.  2003.  Impact of biomass burning on cloud properties in the Amazon Basin. Journal of Geophysical Research-Atmospheres. 108   10.1029/2001jd000985   AbstractWebsite

[1] We used a one-dimensional (1-D) cloud parcel model to assess the impact of biomass-burning aerosol on cloud properties in the Amazon Basin and to identify the physical and chemical properties of the aerosol that influence droplet growth. Cloud condensation nuclei (CCN) measurements were performed between 0.15% and 1.5% supersaturation at ground-based sites in the states of Amazonas and Rondonia, Brazil during several field campaigns in 1998 and 1999 as part of the Large-Scale Biosphere - Atmosphere (LBA) Experiment in Amazonia. CCN concentrations measured during the wet season were low and resembled concentrations more typical of marine conditions than most continental sites. During the dry season, smoke aerosol from biomass burning dramatically increased CCN concentrations. The modification of cloud properties, such as cloud droplet effective radius and maximum supersaturation, is most sensitive at low CCN concentrations. Hence, we could expect larger interannual variation of cloud properties during the wet season that the dry season. We found that differences between CCN spectra from forested and deforested regions during the wet season are modest and result in modifications of cloud properties that are small compared to those between wet and dry seasons. Our study suggests that the differences in surface albedo, rather than cloud albedo, between forested and deforested regions may dominate the impact of deforestation on the hydrological cycle and convective activity during the wet season. During the dry season, on the other hand, cloud droplet concentrations may increase by up to 7 times, which leads to a model-predicted decrease in cloud effective radius by a factor of 2. This could imply a maximum indirect radiative forcing due to aerosol as high as ca. -27 W m(-2) for a nonabsorbing cloud. Light-absorbing substances in smoke darken the Amazonian clouds and reduce the net radiative forcing, and a comparison of the Advanced Very High Resolution Radiometer (AVHRR) analysis and our modeling studies suggests that absorption of sunlight due to smoke aerosol may compensate for about half of the maximum aerosol effect. Sensitivity tests show that complete characterization of the aerosol is necessary when kinetic growth limitations become important. Subtle differences in the chemical and physical makeup are shown to be particularly influential in the activation and growth behavior of the aerosol. Knowledge of the CCN spectrum alone is not sufficient to fully capture the climatic influence of biomass burning.