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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
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.