On the effect of dust particles on global cloud condensation nuclei and cloud droplet number

Aerosol-cloud interaction studies to date consider aerosol with a substantial fraction of soluble material as the sole source of cloud condensation nuclei (CCN). Emerging evidence suggests that mineral dust can act as good CCN through water adsorption onto the surface of particles. This study provides a first assessment of the contribution of insoluble dust to global CCN and cloud droplet number concentration (CDNC). Simulations are carried out with the NASA Global Modeling Initiative chemical transport model with an online aerosol simulation, considering emissions from fossil fuel, biomass burning, marine, and dust sources. CDNC is calculated online and explicitly considers the competition of soluble and insoluble CCN for water vapor. The predicted annual average contribution of insoluble mineral dust to CCN and CDNC in cloud-forming areas is up to 40 and 23.8%, respectively. Sensitivity tests suggest that uncertainties in dust size distribution and water adsorption parameters modulate the contribution of mineral dust to CDNC by 23 and 56%, respectively. Coating of dust by hygroscopic salts during the atmospheric aging causes a twofold enhancement of the dust contribution to CCN; the aged dust, however, can substantially deplete in-cloud supersaturation during the initial stages of cloud formation and can eventually reduce CDNC. Considering the hydrophilicity from adsorption and hygroscopicity from solute is required to comprehensively capture the dust-warm cloud interactions. The framework presented here addresses this need and can be easily integrated in atmospheric models. Copyright 2011 by the American Geophysical Union.

Biomass-burning impact on CCN number, hygroscopicity and cloud formation during summertime in the eastern Mediterranean

Athanasios Nenes

This study investigates the concentration, cloud condensation nuclei (CCN) activity and hygroscopic properties of particles influenced by biomass burning in the eastern Mediterranean and their impacts on cloud droplet formation. Air masses sampled were subject to a range of atmospheric processing (several hours up to 3 days). Values of the hygroscopicity parameter, κ, were derived from CCN measurements and a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA). An Aerosol Chemical Speciation Monitor (ACSM) was also used to determine the chemical composition and mass concentration of non-refractory components of the submicron aerosol fraction. During fire events, the increased organic content (and lower inorganic fraction) of the aerosol decreases the values of κ, for all particle sizes. Particle sizes smaller than 80 nm exhibited considerable chemical dispersion (where hygroscopicity varied up to 100% for particles of same size); larger particles, however, exhibited considerably less dispersion owing to the effects of condensational growth and cloud processing. ACSM measurements indicate that the bulk composition reflects the hygroscopicity and chemical nature of the largest particles (having a diameter of ∼ 100 nm at dry conditions) sampled. Based on positive matrix factorization (PMF) analysis of the organic ACSM spectra, CCN concentrations follow a similar trend as the biomass-burning organic aerosol (BBOA) component, with the former being enhanced between 65 and 150% (for supersaturations ranging between 0.2 and 0.7%) with the arrival of the smoke plumes. Using multilinear regression of the PMF factors (BBOA, OOA-BB and OOA) and the observed hygroscopicity parameter, the inferred hygroscopicity of the oxygenated organic aerosol components is determined. We find that the transformation of freshly emitted biomass burning (BBOA) to more oxidized organic aerosol (OOA-BB) can result in a 2-fold increase of the inferred organic hygroscopicity; about 10% of the total aerosol hygroscopicity is related to the two biomass-burning components (BBOA and OOA-BB), which in turn contribute almost 35% to the fine-particle organic water of the aerosol. Observation-derived calculations of the cloud droplet concentrations that develop for typical boundary layer cloud conditions suggest that biomass burning increases droplet number, on average by 8.5%. The strongly sublinear response of clouds to biomass-burning (BB) influences is a result of strong competition of CCN for water vapor, which results in very low maximum supersaturation (0.08% on average). Attributing droplet number variations to the total aerosol number and the chemical composition variations shows that the importance of chemical composition increases with distance, contributing up to 25% of the total droplet variability. Therefore, although BB may strongly elevate CCN numbers, the impact on droplet number is limited by water vapor availability and depends on the aerosol particle concentration levels associated with the background. © 2016 Author(s).

Copernicus GmbH2016

Mineral dust and its microphysical interactions with clouds

Athanasios Nenes

Our understanding of the interactions of aerosols and clouds has a strong heritage in laboratory experiments, field measurements, and process modeling. We present a review on the state of knowledge for mineral dust emitted from major global dust source regions. Laboratory studies and field measurements have given insights on processes and mechanisms taking place when mineral dust is released into the atmosphere and reacts with the atmospheric constituents. Furthermore, theoretical approaches and parameterizations have been established to interpret the observations and quantitatively express the mechanisms by which dust can act as cloud condensation nuclei (CCN) and ice nuclei (IN). Finally, model simulations have been used in order to study the effects of dust particles to different aerosol-cloud-climate interactions. Dust can act as efficient CCN in clouds solely based on their relatively large size combined with the hydrophilicity from the adsorption of water vapor on their insoluble core. When mixed with even small fractions of hygroscopic material from emission or atmospheric processing, their hygroscopicity and CCN activity are significantly enhanced. The theoretical frameworks of adsorption activation and Köhler theory are presented to explain dust CCN activity, together with a summary on the potential contributions of dust to cloud droplet number concentration (CDNC), and its role in regulating supersaturation. Mineral dust aerosol is an effective IN and, combined with their concentration, can dominate ice production in cirrus and mixed-phase clouds even at great distances from source regions. The pathways to nucleation of ice are different for different cloud types and have distinct effects in those clouds. Our fundamental understanding of ice nucleation lags behind that for CCN activation, and a key challenge is that we cannot predict a priori which aerosol materials will make effective IN. Nevertheless, numerous field and laboratory studies have shown that mineral dust from deserts is one of the most important ice-nucleating aerosol types around the globe. © 2014 Springer Science+Business Media Dordrecht. All rights are reserved.

Springer Netherlands2014

Hygroscopicity and composition of California CCN during summer 2010

Athanasios Nenes

We present an overview and analysis of cloud condensation nuclei (CCN) sampled in California by a NOAA WP-3D aircraft during the 2010 CalNex project. Four distinct geographical regions are characterized, including the Los Angeles basin, the San Joaquin and Sacramento Valleys, and the eastern Pacific Ocean west of southern California. Median size distributions in the Central Valley were unimodal (Dg ∼ 25nm) with a larger fraction of organic species and smaller fraction of nitrate species in the Sacramento Valley aerosol than in the San Joaquin Valley aerosol. Size distributions in the Los Angeles basin and marine outflow were bimodal (geometric mean diameter, Dg ∼ 30, 90-100nm) with similar organic fractions and some replacement of nitrate with sulfate in the marine outflow. Both fine particle and CCN concentrations were found to decrease rapidly above the planetary boundary layer (∼2 km altitude), with CCN concentrations in the boundary layer ranging from ∼102-104cm-3 STP, while fine particle concentrations (0.004-1 m diameters) ranged from ∼103-10 5 cm-3 STP. The CCN-active number fraction varied between 0-100% in the Los Angeles Basin and Marine Outflow, but was substantially lower (0-40%) in the San Joaquin and Sacramento Valleys. Values of the hygroscopicity parameter, , inferred from the CCN measurements varied from 0.1-0.25, with the highest values in the marine outflow and the lowest values in the Sacramento Valley. The values agreed well with the predictions based on size-resolved aerosol composition, but were overpredicted by almost twofold when size-averaged composition was used. CCN closure was assessed for simplified compositional and mixing state assumptions, and it was found that assuming the aerosol to be internally mixed overpredicted CCN concentrations by 30-75% for all air mass types except within the Sacramento Valley, where good closure (overprediction < 10%) was achieved by assuming insoluble organics. Assuming an externally-mixed aerosol fraction or incorporating size-resolved composition data improved closure in the other three regions, consistent with the bimodal nature of the aerosol size distribution. © 2012 by the American Geophysical Union.

Blackwell Publishing Ltd2012

Biomass-burning impact on CCN number, hygroscopicity and cloud formation during summertime in the eastern Mediterranean

Athanasios Nenes

Copernicus GmbH2016

Mineral dust and its microphysical interactions with clouds

Athanasios Nenes

Springer Netherlands2014

Hygroscopicity and composition of California CCN during summer 2010

Athanasios Nenes

Blackwell Publishing Ltd2012