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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.

Ramanathan, V, Ramana MV, Roberts G, Kim D, Corrigan C, Chung C, Winker D.  2007.  Warming trends in Asia amplified by brown cloud solar absorption. Nature. 448:575-578.   10.1038/nature06019   AbstractWebsite

Atmospheric brown clouds are mostly the result of biomass burning and fossil fuel consumption(1). They consist of a mixture of light-absorbing and light-scattering aerosols(1) and therefore contribute to atmospheric solar heating and surface cooling. The sum of the two climate forcing terms-the net aerosol forcing effect is thought to be negative and may have masked as much as half of the global warming attributed to the recent rapid rise in greenhouse gases(2). There is, however, at least a fourfold uncertainty(2) in the aerosol forcing effect. Atmospheric solar heating is a significant source of the uncertainty, because current estimates are largely derived from model studies. Here we use three lightweight unmanned aerial vehicles that were vertically stacked between 0.5 and 3 km over the polluted Indian Ocean. These unmanned aerial vehicles deployed miniaturized instruments measuring aerosol concentrations, soot amount and solar fluxes. During 18 flight missions the three unmanned aerial vehicles were flown with a horizontal separation of tens of metres or less and a temporal separation of less than ten seconds, which made it possible to measure the atmospheric solar heating rates directly. We found that atmospheric brown clouds enhanced lower atmospheric solar heating by about 50 per cent. Our general circulation model simulations, which take into account the recently observed widespread occurrence of vertically extended atmospheric brown clouds over the Indian Ocean and Asia(3), suggest that atmospheric brown clouds contribute as much as the recent increase in anthropogenic greenhouse gases to regional lower atmospheric warming trends. We propose that the combined warming trend of 0.25 K per decade may be sufficient to account for the observed retreat of the Himalayan glaciers(4-6).

Rissler, J, Swietlicki E, Zhou J, Roberts G, Andreae MO, Gatti LV, Artaxo P.  2004.  Physical properties of the sub-micrometer aerosol over the Amazon rain forest during the wet-to-dry season transition - comparison of modeled and measured CCN concentrations. Atmospheric Chemistry and Physics. 4:2119-2143.   10.5194/acp-4-2119-2004   AbstractWebsite

Sub-micrometer atmospheric aerosol particles were studied in the Amazon region, 125 km northeast of Manaus, Brazil (-1degrees55.2'S, 59degrees28.1'W). The measurements were performed during the wet-to-dry transition period, 4-28 July 2001 as part of the LBA (Large-Scale Biosphere Atmosphere Experiment in Amazonia) CLAIRE-2001 (Cooperative LBA Airborne Regional Experiment) experiment. The number size distribution was measured with two parallel differential mobility analyzers, the hygroscopic growth at 90% RH with a Hygroscopic Tandem Mobility Analyzer (H-TDMA) and the concentrations of cloud condensation nuclei (CCN) with a cloud condensation nuclei counter. A model was developed that uses the H-TDMA data to predict the number of soluble molecules or ions in the individual particles and the corresponding minimum particle diameter for activation into a cloud droplet at a certain supersaturation. Integrating the number size distribution above this diameter, CCN concentrations were predicted with a time resolution of 10 min and compared to the measured concentrations. During the study period, three different air masses were identified and compared: clean background, air influenced by aged biomass burning, and moderately polluted air from recent local biomass burning. For the clean period 2001, similar number size distributions and hygroscopic behavior were observed as during the wet season at the same site in 1998, with mostly internally mixed particles of low diameter growth factor (similar to1.3 taken from dry to 90% RH). During the periods influenced by biomass burning the hygroscopic growth changed slightly, but the largest difference was seen in the number size distribution. The CCN model was found to be successful in predicting the measured CCN concentrations, typically within 25%. A sensitivity study showed relatively small dependence on the assumption of which model salt that was used to predict CCN concentrations from H-TDMA data. One strength of using H-TDMA data to predict CCN concentrations is that the model can also take into account soluble organic compounds, insofar as they go into solution at 90% RH. Another advantage is the higher time resolution compared to using size-resolved chemical composition data.

Roberts, GC, Ramana MV, Corrigan C, Kim D, Ramanathan V.  2008.  Simultaneous observations of aerosol-cloud-albedo interactions with three stacked unmanned aerial vehicles. Proceedings of the National Academy of Sciences of the United States of America. 105:7370-7375.   10.1073/pnas.0710308105   AbstractWebsite

Aerosol impacts on climate change are still poorly understood, in part, because the few observations and methods for detecting their effects are not well established. For the first time, the enhancement in cloud albedo is directly measured on a cloud-by-cloud basis and linked to increasing aerosol concentrations by using multiple autonomous unmanned aerial vehicles to simultaneously observe the cloud microphysics, vertical aerosol distribution, and associated solar radiative fluxes. In the presence of long-range transport of dust and anthropogenic pollution, the trade cumuli have higher droplet concentrations and are on average brighter. Our observations suggest a higher sensitivity of radiative forcing by trade cumuli to increases in cloud droplet concentrations. than previously reported owing to a constrained droplet radius such that increases in droplet concentrations also increase cloud liquid water content. This aerosol-cloud forcing efficiency is as much as -60 W m(-2) per 100% percent cloud fraction for a doubling of droplet concentrations and associated increase of liquid water content. Finally, we develop a strategy for detecting aerosol-cloud interactions based on a nondimensional scaling analysis that relates the contribution of single clouds to albedo measurements and illustrates the significance of characterizing cloud morphology in resolving radiometric measurements. This study demonstrates that aerosol-cloud-albedo interactions can be directly observed by simultaneous observations below, in, and above the clouds.

Roberts, G, Mauger G, Hadley O, Ramanathan V.  2006.  North American and Asian aerosols over the eastern Pacific Ocean and their role in regulating cloud condensation nuclei. Journal of Geophysical Research-Atmospheres. 111   10.1029/2005jd006661   AbstractWebsite

[ 1] Measurements of aerosol and cloud properties in the Eastern Pacific Ocean were taken during an airborne experiment on the University of Wyoming's King Air during April 2004 as part of the Cloud Indirect Forcing Experiment (CIFEX). We observed a wide variety of aerosols, including those of long-range transport from Asia, clean marine boundary layer, and North American emissions. These aerosols, classified by their size distribution and history, were found in stratified layers between 500 to 7500 m above sea level and thicknesses from 100 to 3000 m. A comparison of the aerosol size distributions to measurements of cloud condensation nuclei (CCN) provides insight to the CCN activity of the different aerosol types. The overall ratio of measured to predicted CCN concentration (NCCN) is 0.56 +/- 0.41 with a relationship of N-CCN,N- measured = N-CCN, predicted(0.846 +/- 0.002) for 23 research flights and 1884 comparisons. Such a relationship does not accurately describe a CCN closure; however, it is consistent with our measurements that high CCN concentrations are more influenced by anthropogenic sources, which are less CCN active. While other CCN closures have obtained results closer to the expected 1: 1 relationship, the different aerosol types ( and presumably differences in aerosol chemistry) are responsible for the discrepancy. The measured N-CCN at 0.3% supersaturation (S-c) ranged from 20 cm(-3) (pristine) to 350 cm(-3) ( anthropogenic) with an average of 106 +/- 54 cm(-3) over the experiment. The inferred supersaturation in the clouds sampled during this experiment is similar to 0.3%. CCN concentrations of cloud-processed aerosol were well predicted using an ammonium sulfate approximation for S-c <= 0.4%. Predicted N-CCN for other aerosol types (i.e., Asian and North American aerosols) were high compared to measured values indicating a less CCN active aerosol. This study highlights the importance of chemical effects on CCN measurements and introduces a CCN activation index as a method of classifying the efficiency of an aerosol to serve as CCN relative to an ammonium sulfate particle. This index ranged from close to unity for cloud processed aerosols to as low as 0.31 for aged aerosols transported from Asia. We also compare the performance of two CCN instruments ( static thermal diffusion chamber and streamwise continuous flow chamber) on a 45 minute level leg where we observe an aged layer and a nucleation event. More than 50% of the aged aerosol served as CCN at 0.2% S-c, primarily owing to their large size, while CCN concentrations during the nucleation event were close to 0 cm(-3). CCN concentrations from both instruments agreed within instrument errors; however, the continuous flow chamber effectively captured the rapid transition in aerosol properties.

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.

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.

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, 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, Artaxo P, Zhou JC, Swietlicki E, Andreae MO.  2002.  Sensitivity of CCN spectra on chemical and physical properties of aerosol: A case study from the Amazon Basin. Journal of Geophysical Research-Atmospheres. 107   10.1029/2001jd000583   AbstractWebsite

Organic material, about half of which is water soluble, constitutes nearly 80% of the wet-season aerosol mass in the Amazon Basin, while soluble inorganic salts (predominantly ammonium bisulfate) represent about 15%. A detailed analysis of number distributions and the size-dependent chemical composition of the aerosol indicates that, in principle, the sulfate fraction could account for most of the cloud condensation nuclei (CCN) activity. Uncertainty about the chemical speciation of the water-soluble organic component precludes a rigorous analysis of its contribution to nucleation activity. Within reasonable assumptions, we can, however, predict a similar contribution of the organic component to CCN activity as that from sulfate. Because of the nonlinear dependence of droplet nucleation behavior on solute amount, the nucleation activity cannot be attributed uniquely to the inorganic or organic fractions. The role of water-soluble organic compounds as surfactants, however, may be significant (especially in the case of biomass-burning aerosol) and more field measurements are needed to quantify their effects on the surface tension of ambient aerosols. The parametric dependence of the CCN spectra on the physical and chemical properties of the aerosol show that the number distribution, soluble content of the aerosol, and surface tension effects all play an important role in determining CCN spectra.

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.

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.