Particle Hygroscopic Growth Research Articles

Comparison of methods for deriving aerosol asymmetry parameter

Values for Mie‐equivalent aerosol asymmetry parameter (g) were derived using a variety of methods from the large suite of measurements (in situ and remote from surface and aircraft) made in Oklahoma during the 2003 aerosol Intensive Operations Period (IOP). Median values derived for dry asymmetry parameter at 550 nm ranged between 0.55 and 0.63 over all instruments and for all derivation methods, with the exception of one instrument which did not measure over the full size range of optically important aerosol. Median values for the “wet” asymmetry parameter (i.e., asymmetry parameter at humidity conditions closer to ambient) were between 0.59 and 0.72. Values for g derived for surface and airborne in situ measurements were highly correlated, but in situ and remote sensing measurements both at the surface and aloft did not agree as well because of vertical inhomogeneity of the aerosol. Radiative forcing calculations suggest that a 10% decrease in g would result in a 19% reduction in top of atmosphere radiative forcing for the conditions observed during the IOP. Comparison of the different methods for deriving g suggests that in computing the asymmetry parameter, aerosol size is the most important parameter to measure; composition is less important except for how it influences the hygroscopic growth (i.e., size) of particles.

Cloud droplet activation of polymerized organic aerosol

High-molecular-weight organic compounds represent an important fraction in atmospheric aerosols, but their interactions with atmospheric water vapour are not well understood. Specifically, the hygroscopicity of polymerized atmospheric aerosols is larger than expected for their high molecular weights and is not a strong function of the degree of polymerization, but their cloud-nucleating activity declines as polymerization progresses.We apply a Flory–Huggins based theory for polymer–water interactions to describe water activity in aqueous solutions of atmospherically relevant oligomers and polymers, and show via experiments on particles composed of commercial polymers that it is consistent with observed particle hygroscopic growth and cloud formation behaviour.

Size-resolved soluble volume fractions of submicrometer particles in air masses of different character

As a contribution to the joint research project FEBUKO, hygroscopic properties of atmospheric Aitken and accumulation mode particles were measured in the Thueringer Wald, Germany, using a hygroscopicity-tandem differential mobility analyser (H-TDMA). The hygroscopic growth of particles with initial dry diameters of D p = 50 , 150, and 250 nm at 90% relative humidity was used to calculate average distributions of the soluble volume fraction with respect to the hygroscopic growth of ammonium sulphate. The application of this parameterisation procedure was tested by analysing the dataset with respect to the dependence of the soluble volume fraction on particle size and air masses character. With increasing dry particle size, the fraction of particles containing high soluble volume fractions was found to increase. The number of accumulation mode particles in marine air masses passing the sampling site having a large soluble volume fraction was significantly higher than in air masses of more continental character. For particles with D p = 50 nm , no air mass dependence of the soluble volume fraction was found. In marine air masses, particles with D p = 150 and 250 nm are assumed to undergo similar evolution processes, whereas in continental air masses this seemed to be the case for particles with D p = 50 nm and 150 nm.

Link between aerosol hygroscopic growth and droplet activation observed for hill-capped clouds at connected flow conditions during FEBUKO

Abstract Within the ground-based cloud passage experiment FEBUKO, which was carried out at the mountain ridge Thuringer Wald (Germany) during October 2001 and 2002, the dry number size distribution and hygroscopic growth of aerosol particles upwind cloud and the dry number size distributions of interstitial particles and cloud droplet residuals inside cloud were measured at connected flow conditions. The connected flow between the upwind and in-cloud summit site was meteorologically predicted and experimentally confirmed for three selected cloud events. For these events, it could be verified that entrainment and droplet deposition had only a minor influence on the evolution of the particle size distribution between the two sites. Hence, the size resolved soluble volume fraction of the cloud input aerosol particles determined from the hygroscopic growth measurements could be related to the particle activation inferred from the particle size distributions observed inside cloud. The shape and steepness of the scavenging fraction as a function of particle diameter was found to correlate with the increase of soluble volume fraction with size, which had implications for the droplet activation diameter of the cloud condensation nuclei (CCN) that ranged between 110 and 180 nm. The minimum soluble volume fraction e min that was required to serve as CCN was determined for three different dry diameters from the relation of the particle volume fraction and scavenging fraction. From the comparison with e min predictions from classical Kohler theory it is inferred that aerosol particles remained in the interstitial phase although they should have been activated. A discussion of different processes which have the general ability to explain this finding favoured the hypothesis of organic surface films retarding the uptake of water molecules.

A Tandem DMA for highly temperature-stabilized hygroscopic particle growth measurements between 90% and 98% relative humidity

This development describes a design of a high humidity tandem differential mobility analyzer (HHTDMA) to determine aerosol hygroscopic growth factors at relative humidities (RH) between 90% and 98% under highly temperature-stabilized conditions. Two temperature baths are used to thermally stabilize the humidity conditioning section and the second DMA and to maintain a slightly lowered temperature in the second DMA relative to the particle humidity conditioning section. The reduced temperature of the second DMA results in constant and higher RHs between 90% and 98%. We describe the basic system design and results from temperature and RH stability tests. In the HHTDMA-system, the absolute temperatures can be maintained within ± 0.1 K and with a long-term stability of 0.02 K. The RH in the second DMA reaches an absolute accuracy of ± 1.2 % for 98% and a long-term stability of ± 0.1 –0.4% of set point values. To demonstrate the capability of the HHTDMA, hygroscopic growth factors of ammonium sulfate aerosol between 84% and 98% RH were determined. The experimental results agree with theoretical values within the uncertainties of the system, with values based on measurements using a single-particle levitation technique, and growth factors of other TDMA experiments in the range up to 94% RH.

A modified hygroscopic tandem DMA and a data retrieval method based on optimal estimation

We present a new design of hygroscopic tandem differential mobility analyser (HTDMA) capable of rapidly switching between measurement at constant humidity and the determination of humidograms. Both technical aspects of the instrumental setup and the software control are discussed. The optimal estimation method (OEM) developed by Rodgers (Rev. Geophys. 14 (1976), J. Geophys. Res. 95 (1990), Inverse Methods for Atmospheric Sounding, World Scientific, Singapore 2000) for solving atmospheric data inversion problems has been successfully developed to retrieve measurements of hygroscopic particle growth using the tandem DMA. The technique makes no assumptions about the shape of the hygroscopic growth distribution and is able to determine the resolution and shape of the retrieved distribution. The technique has been shown to be robust throughout extensive tests and a thorough error analysis has been performed. The largest source of error arises from the under-sampling of the measurements, the so-called ‘smoothing error’. However, the uncertainties in the measurements and error in the forward model are also significant. The technique allows hygroscopic growth measurements to be retrieved under a variety of atmospheric conditions and over an extended time period in a reliable and consistent manner. A comparison is made between the OEM routine and the well-documented TDMAfit method of data analysis, showing the limitations and subjectiveness of TDMAfit in certain situations. The OEM data retrieval process is a transferable routine that could also be used in other applications, such as the volatility tandem DMA instrument.

Seasonal variability of the aerosol parameters over Kanpur, an urban site in Indo-Gangetic basin

With the growing antropogenic activities and urbanization, pollution level is found to be increasing in the Indo-Gangetic (IG) basin. This trend of increasing pollution has direct impact on the climatic conditions, especially the increase of haze, fog and cloudy conditions in the region. A ground-based CIMEL radiometer has been deployed on the campus of the Indian Institute of Technology (IIT) Kanpur, India, as a part of the Aerosol Robotic Network (AERONET) program with an objective to characterize the aerosols in the IG basin. The optical properties of the aerosols over Kanpur, which is one of the largest industrial cities in the IG basin, show strong seasonal and inter-annual variations. The fine mode urban/industrial aerosols contribute more than 75% to the observed aerosol optical depth (AOD) during the post-monsoon and winter seasons, whereas the natural dusts contribute ∼60% to the AOD during the pre-monsoon and monsoon seasons. Effective radius ( R eff) and volume weighted mean radius ( R V), the two best representative parameter of the coarse and fine modes of aerosol size distribution show maximum values during winter season. The increase in the R V is attributed to the hygroscopic growth of fine mode particles in presence of high relative humidity and favorable temperature condition; but the increase in r eff is due to the association of the fine mode absorbing particles to the coarser dusts. This is also reflected in low values (<0.8) of single scattering albedo (SSA) at coarse mode during the season. Higher values of SSA at fine mode indicate dominance of water-soluble aerosols scattering in nature. The linear regression equations between AOD (at 0.67 μm wavelength) and extinction coefficient due to fine and coarse modes also reveal the relative contribution of the fine and coarse mode particles to the observed optical properties during different seasons in Kanpur.

A Method for the In Situ Measurement of Fine Aerosol Water Content of Ambient Aerosols: The Dry-Ambient Aerosol Size Spectrometer (DAASS) Special Issue ofAerosol Science and Technologyon Findings from the Fine Particulate Matter Supersites Program

Hygroscopic growth of atmospheric particles affects a number of environmentally important aerosol properties. Due to the hysteresis exhibited by the aerosol hygroscopic growth, the physical state of particles and the amount of aerosol water are uncertain within a wide range of relative humidities (RHs) found in the troposphere, leading to uncertainties in optical and chemical properties of the aerosol. Here we report the design and tests of an automated system that was built to assess the amount of aerosol water at atmospheric conditions. The system consists of two scanning mobility particle sizers (SMPS) and an aerodynamic particle sizer (APS) that measure the aerosol size distribution between 3 nm and 10 μm in diameter. The inlets of the instruments and their sheath air lines are equipped with computer-controlled valves that direct air through Nafion dryers or bypass them. The Nafion dryers dehydrate the air streams to below 30% RH at which point ambient particles are expected to lose most or all water....

Organic aerosol effects on fog droplet spectra

Organic aerosol alters cloud and fog properties through surface tension and solubility effects. This study characterizes the role of organic compounds in affecting fog droplet number concentration by initializing and comparing detailed particle microphysical simulations with two field campaigns in the Po Valley. The size distribution and chemical composition of aerosol were based on the measurements made in the Po Valley Fog Experiments in 1989 and 1998–1999. Two types of aerosol with different hygroscopicity were considered: the less hygroscopic particles, composed mainly of organic compounds, and the more hygroscopic particles, composed mainly of inorganic salts. The organic fraction of aerosol mass was explicitly modeled as a mixture of seven soluble compounds [Fuzzi et al., 2001] by employing a functional group‐based thermodynamic model [Ming and Russell, 2002]. Condensable gases in the vapor phase included nitric acid, sulfuric acid, and ammonia. The maximum supersaturation in the simulation is 0.030% and is comparable to the calculation by Noone et al. [1992] inferred from measured residual particle fractions. The minimum activation diameters of the less and more hygroscopic particles are 0.49 μm and 0.40 μm, respectively. The predicted residual particle fractions are in agreement with measurements. The organic components of aerosol account for 34% of the droplet residual particle mass and change the average droplet number concentration by −10–6%, depending on the lowering of droplet surface tension and the interactions among dissolving ions. The hygroscopic growth of particles due to the presence of water‐soluble organic compounds enhances the condensation of nitric acid and ammonia due to the increased surface area, resulting in a 9% increase in the average droplet number concentration. Assuming ideal behavior of aqueous solutions of water‐soluble organic compounds overestimates the hygroscopic growth of particles and increases droplet numbers by 6%. The results are sensitive to microphysical processes such as condensation of soluble gases, which increases the average droplet number concentration by 26%. Wet deposition plays an important role in controlling liquid water content in this shallow fog.

Interaction of aerosol particles composed of protein and saltswith water vapor: hygroscopic growth and microstructural rearrangement

Abstract. The interaction of aerosol particles composed of the protein bovine serum albumin (BSA) and the inorganic salts sodium chloride and ammonium nitrate with water vapor has been investigated by hygroscopicity tandem differential mobility analyzer (H-TDMA) experiments complemented by transmission electron microscopy (TEM) and Köhler theory calculations (100-300nm particle size range, 298K, 960hPa). BSA was chosen as a well-defined model substance for proteins and other macromolecular compounds, which constitute a large fraction of the water-soluble organic component of air particulate matter. Pure BSA particles exhibited deliquescence and efflorescence transitions at 35% relative humidity () and a hygroscopic diameter increase by up to 10% at 95% in good agreement with model calculations based on a simple parameterisation of the osmotic coefficient. Pure NaCl particles were converted from near-cubic to near-spherical shape upon interaction with water vapor at relative humidities below the deliquescence threshold (partial surface dissolution and recrystallisation), and the diameters of pure NH4NO3 particles decreased by up to 10% due to chemical decomposition and evaporation. Mixed NaCl-BSA and NH4NO3-BSA particles interacting with water vapor exhibited mobility equivalent diameter reductions of up to 20%, depending on particle generation, conditioning, size, and chemical composition (BSA dry mass fraction 10-90%). These observations can be explained by formation of porous agglomerates (envelope void fractions up to 50%) due to ion-protein interactions and electric charge effects on the one hand, and by compaction of the agglomerate structure due to capillary condensation effects on the other. The size of NH4NO3-BSA particles was apparently also influenced by volatilisation of NH4NO3, but not as much as for pure salt particles, i.e. the protein inhibited the decomposition of NH4NO3 or the evaporation of the decomposition products NH3 and HNO3. The efflorescence threshold of NaCl-BSA particles decreased with increasing BSA dry mass fraction, i.e. the protein inhibited the formation of salt crystals and enhanced the stability of supersaturated solution droplets. The H-TDMA and TEM results indicate that the protein was enriched at the surface of the mixed particles and formed an envelope, which inhibits the access of water vapor to the particle core and leads to kinetic limitations of hygroscopic growth, phase transitions, and microstructural rearrangement processes. The Köhler theory calculations performed with different types of models demonstrate that the hygroscopic growth of particles composed of inorganic salts and proteins can be efficiently described with a simple volume additivity approach, provided that the correct dry solute mass equivalent diameter and composition are known. A parameterisation for the osmotic coefficient of macromolecular substances has been derived from an osmotic pressure virial equation. For its application only the density and molar mass of the substance have to be known or estimated, and it is fully compatible with traditional volume additivity models for salt mixtures.

Eddy correlation measurements of aerosol deposition to grass

An experiment was conducted to measure aerosol turbulent fluxes to a grass field. A new high-flow-rate aerosol sensor was deployed from a tower to make eddy correlation (EC) measurements of aerosol turbulent flux and deposition velocity. The EC data were screened and analysed for uncertainties associated with advection, boundary layer growth, instrument separation and counting particles. An apparent bias in the aerosol flux due to particle hygroscopic growth was evaluated from chemical and microphysical measurements and removed from the results based on derived corrections. The resulting aerosol deposition velocity for 0.52 μm diameter particles depended on atmospheric stability with values of 0.3 cm s−1 during near-neutral stability, 0.44 cm s−1 during unstable periods and 0.16 cm s−1 during stable periods with an estimated uncertainty of ±0.07 cm s−1 due to chemical composition and particle counting.

Hygroscopic properties of different aerosol types over the Atlantic and Indian Oceans

Abstract. Hygroscopic properties of atmospheric particles were studied in the marine tropospheric boundary layer over the Atlantic and Indian Oceans during two consecutive field studies: the Aerosols99 cruise (Atlantic Ocean) from 15 January to 20 February 1999, and the INDOEX cruise (Indian Ocean Experiment) from 23 February to 30 March 1999. The hygroscopic properties were compared to optical and chemical properties, such as absorption, chemical inorganic composition, and mass concentration of organic and elemental carbon, to identify the influence of these parameters on hygroscopicity. During the two field studies, four types of aerosol-sampling instruments were used on board the NOAA (National Oceanic and Atmospheric Administration) Research Vessel Ronald H. Brown: Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA), seven-stage cascade impactor, two-stage cascade impactor, and Particle Soot Absorption Photometer (PSAP). The HTDMA was used to determine the hygroscopic properties of atmospheric particles at initial dry sizes (Dp) of 50, 150, and 250 nm and at relative humidities (RH) of 30, 55, 75, and 90%. Simultaneously, a seven-stage cascade impactor of which 3 stages were in the sub-mm size range was used to determine the molar composition of the major inorganic ions such as ammonium and sulfate ions. A two-stage cascade impactor (1 in the sub-mm size range, 1 in the sup-mm size range) was used to determine the mass concentration of organic and elemental carbon. The PSAP was used (at a wavelength of 565 nm) to measure the light absorption coefficient of the aerosol. During the two field studies, air masses of several different origins passed the ship's cruise path. The occurrence of different air masses was classified into special time periods signifying the origin of the observed aerosol. All time periods showed a group of particles with high hygroscopic growth. The measured average hygroscopic growth factors defined by the ratio of dry and wet particle diameter at 90% RH ranged from 1.6 to 2.0, depending on the dry particle size and on the type of air mass. Particles with low hygroscopic growth occurred only when continentally influenced air masses arrived at the ship's position. Distinctions in hygroscopic growth of particles of different air masses were more significant for small relative humidities (30% or 55% RH). High concentrations of elemental carbon corresponded with high light absorption coefficients and with the occurrence of less-hygroscopic and nearly hydrophobic particle fractions in the hygroscopic growth distributions. A key finding is that clean marine air masses that had no land contact for five to six days could clearly be distinguished from polluted air masses that had passed over a continent several days before reaching the ship.

Hygroscopic behavior of atmospheric aerosol in Taipei

Hygroscopic growth of particles of different sizes and the resultant size distribution changes were observed as a function of the relative humidity (RH). A particle generation device, RH module, and a Tandem Differential Mobility Analyzer system were set up to measure the particle size distributions under different RH conditions. Adopting Nafion as an RH adjusting module, the aerosol hygroscopic observations were successfully performed without the interference caused by blending sample stream with humidified air. The measured deliquescence humidity of model compounds, NaCl and (NH 4) 2SO 4, agree with the theoretical values reported by other investigators. The particle growth factor is enhanced around the RH of 70%. In addition, particle size distribution behaves as two split groups of particles with the RH >76%. The average growth factors of hygroscopic ambient particles in Taiwan are similar to those reported elsewhere. There are several hygroscopic salt compositions in ambient aerosols, (NH 4) 2SO 4 is the most abundant one. Observed particle deliquescence behaviors showed limited alternation of organics on particle growth at higher RH.

Effect of Electrolyte Diffusion on the Growth of NaCl Particles by Water Vapour Condensation

The hygroscopic growth of NaCl particles was investigated both experimentally and theoretically. Straightforward experiments done with a humidity controlled microbalance show that a macroscopic time (hours to days) is required before the final (equilibrium) size is reached. The developed model shows that the droplets are not homogeneous salt solutions and that a finite liquid phase diffusion resistance exists. This is clearly evident from the experimental data also: the initial stage of growth shows a linear mass growth in time. This would not be possible without the liquid-phase resistance. It was also possible to determine the liquid-phase diffusion coefficient. Although the value depends on composition and temperature, the best fit with experimental data was obtained by using a value of 1.5 × 10-9 m2/s, which agrees well with available experimental data for strong NaCl−water electrolytes.

Characterization and parameterization of atmospheric particle number‐, mass‐, and chemical‐size distributions in central Europe during LACE 98 and MINT

Intensive measurements of chemical and physical properties of the atmospheric aerosol have been performed at two sites in central Europe during the Melpitz‐Intensive (MINT) in November 1997 and the Lindenberg Aerosol Characterization Experiment 1998 (LACE 98) in July and August 1998. Number‐size distributions, hygroscopic particle growth, size‐segregated gravimetric mass, and size‐segregated chemical masses of water‐soluble ions and organic and elemental carbon of aerosol particles have been measured. To obtain information on the quality of the different methods, the number‐derived, gravimetric, and chemically derived mass distributions are compared. Gravimetric mass of fine particles is attributed completely to chemical composition by carbonaceous material and ions, including an estimate of the water content due to hygroscopic compounds. For the characterization of coarse particles, which contribute less to the total mass concentration, insoluble material has to be included in the mass balance. Mass concentrations calculated from the number‐size distributions are well correlated with the gravimetric mass concentration; however, the calculated mass is larger, especially for the Aitken and accumulation modes. The number‐derived mass concentration is most sensitive to the sizing uncertainty of the measured number‐size distribution. Moreover, the impactor cutoffs and the limited knowledge about the density of the particles (especially with high carbon content) account for a major part of the uncertainties. The overall uncertainty of the calculated mass, determined as the standard deviation of the average value in a Monte Carlo approach, is found to be about 10%. Lognormal parameters for the number‐size and volume‐size distributions as well as gravimetric mass‐size distribution and corresponding chemical composition are presented for different air mass types. Most of the modal parameters do not differ significantly between the air mass types. Higher mass concentrations are mostly due to an increase in size (of Aitken and accumulation mode) rather than an increase in the number of particles in a given mode. Generally, the mass percent carbon content increases with decreasing particle size. The most pronounced difference with season is an increase of carbon content from summer to winter as well as an increase in nitrate content, resulting in a decrease of sulfate. For nitrate a strong dependence on air mass direction is observed. Sulfate and nitrate are predominantly neutralized by ammonium. With the results of the two experiments, quality‐controlled mode parameters and corresponding chemical composition of atmospheric aerosol particles in central Europe are now available for application in models.

Submicrometer aerosol particle size distribution and hygroscopic growth measured in the Amazon rain forest during the wet season

The number‐size distribution and hygroscopic growth of submicrometer aerosol particles were measured in central Amazonia during the first Cooperative LBA Airborne Regional Experiment (CLAIRE) wet season experiment in March–April 1998. This was the first time ever that these types of measurements were performed in the Amazon rain forest. A Differential Mobility Particle Sizer (DMPS) was used to measure aerosol number‐size distribution with diameters in the range 3–850 nm. The observed total number concentrations were frequently between 300 and 600 cm−3 with a mean value around 450 cm−3. Two aerosol particle modes (Aitken and accumulation mode) were always present. The average particle concentrations for those two modes were 239 and 177 cm−3, with geometric diameters of 68 and 151 nm, respectively. An ultrafine mode had a number concentration and a mean diameter of 92 cm−3 and 24 nm, respectively, and only occurred at 18% of the time, causing the size distribution to be trimodal instead of bimodal. The hygroscopic growth of aerosol particles was measured in situ with a Hygroscopic Tandem Differential Mobility Analyzer (H‐TDMA) at six dry particle diameters between 35 and 265 nm. In contrast to the bimodal hygroscopic behavior found in polluted continental environments, the hygroscopic properties of aerosol particles in the Amazon rain forest is essentially unimodal with average diameter growth factors of 1.16–1.32 from dry to 90% relative humidity (RH). Aerosol soluble volume fractions were, in general, between 0.14 and 0.27, estimated by assuming that only ammonium hydrogen sulphate interacted with water vapour.

Thermodynamic equilibrium of organic‐electrolyte mixtures in aerosol particles

AbstractThis thermodynamic model describes the phase equilibria of mixtures of electrolytes and organic species in aqueous solutions existing as aerosol particles. The activity coefficient of each species in solution is explicitly related to the chemical composition by treating the (inorganic) ion–water, organic–water and ion‐organic interactions with a combined Pitzer–UNIFAC thermodynamic approach. It was parameterized with a new type of multifunctional “meta‐group” to better represent measured properties of long‐chain monofunctional compounds and short‐chain multifunctional compounds. Interactions between dissolved electrolytes and organic species are modeled using the “salting‐out” effect of NaCl with organic compounds to predict the hygroscopic growth of particles composed of a salt and diacid. The predicted growth agrees well with laboratory measurements. The presence of 50% malonic acid decreases the growth of pure (NH4)2SO4 by 20% at high relative humidities, while mixtures with 50% succinic acid and 50% glutaric acid cause decreases of 35% and 38%, respectively. The mixing of organic compounds with solubility higher than 4 mol·L−1 with salt can decrease the observed DRH. The mixture of malonic acid and (NH4)2SO4 is predicted to start taking up water at 58%, much lower than the DRH of pure (NH4)2SO4 (80%). Insoluble compounds do not change the observed DRH, while reducing the amount of water taken up. The predicted water contents for internal and external mixtures are largely similar, with small differences predicted for mixtures of soluble organic species. The largest deviation of 10% between the water contents of internal and external mixtures occurs for 50% malonic acid with 50% (NH4)2SO4. For less soluble compounds such as succinic acid and glutaric acid, the two types of growth generally agree within 3%.

Atmospheric Aerosol Optical Properties in the Persian Gulf

Aerosol optical depth measurements over Bahrain acquired through the ground-based Aerosol Robotic Network (AERONET) are analyzed. Optical depths obtained from ground-based sun/sky radiometers showed a pronounced temporal trend, with a maximum dust aerosol loading observed during the March-July period. The aerosol optical depth probability distribution is rather narrow with a modal value of about 0.25. The Angstrom parameter frequency distribution has two peaks. One peak around 0.7 characterizes a situation when dust aerosol is more dominant, the second peak around 1.2 corresponds to relatively dust-free cases. The correlation between aerosol optical depth and water vapor content in the total atmospheric column is strong (correlation coefficient of 0.82) when dust aerosol is almost absent (Angstrom parameter is greater than 0.7), suggesting possible hygroscopic growth of fine mode particles or source region correlation, and much weaker (correlation coefficient of 0.45) in the presence of dust (Angstrom parameter is less than 0.7). Diurnal variations of the aerosol optical depth and precipitable water were insignificant. Angstrom parameter diurnal variability (;20%-25%) is evident during the April-May period, when dust dominated the atmospheric optical conditions. Variations in the aerosol volume size distributions retrieved from spectral sun and sky radiance data are mainly associated with the changes in the concentration of the coarse aerosol fraction (variation coefficient of 61%). Geometric mean radii for the fine and coarse aerosol fractions are 0.14 mm (std dev 5 0.02) and 2.57 mm (std dev 5 0.27), respectively. The geometric standard deviation of each fraction is 0.41 and 0.73, respectively. In dust-free conditions the single scattering albedo (SSA) decreases with wavelength, while in the presence of dust the SSA either stays neutral or increases slightly with wavelength. The changes in the Angstrom parameter derived from a ground-based nephelometer and a collocated sun photometer during the initial checkout period were quite similar.

Hygroscopic properties of aerosol particles over the central Arctic Ocean during summer

The hygroscopic properties of submicrometer aerosol particles in the Arctic summer marine boundary layer (MBL) were observed on board the icebreaker Oden during the Arctic Ocean Expedition 1996 (AOE‐96). The measurements were performed between July 15 and August 25 1996 and covered the region between longitudes 16°‐147°E and latitudes 70°‐87.5°N, mostly over melting pack ice. The hygroscopic tandem differential mobility analyzer (H‐TDMA) was used to determine the hygroscopic diameter growth of aerosol particles at four dry diameters (15, 35, 50, and 165 nm) and three relative humidities (50%, 70%, and 90% RH). The hygroscopic behavior of the aerosol particles over the pack ice showed large temporal variations, in contrast to previous observations in marine boundary layers over warmer oceans. These variations were mostly due to the high degree of vertical atmospheric stratification often observed over the pack ice. However, when comparing the average diameter growth factors of the more hygroscopic particle group, representing an aged aerosol with growth factors between 1.4–1.9 at 90% RH and present in 81–86% of all cases, the agreement between the measurements over the Arctic and the warmer oceans was very good and depended on the average wind speed. The average diameter growth factors of the more hygroscopic particles as a function of relative humidity were modeled empirically by power law expressions. The concentration of cloud condensation nuclei (CCN) estimated from aerosol number size distribution and hygroscopic growth data correlated well with direct measurements but overpredicted the CCN concentrations by about 30%. In 43 cases when the sampled air mass had undergone processing in Arctic Ocean MBL clouds, the minimum CCN diameter was estimated to be 76±15 nm, corresponding to effective water vapor supersaturations of 0.28±0.08%.

Hygroscopic growth of aerosol particles in the southern atlantic ocean and indian ocean

Hygroscopic growth of aerosol particles in the southern atlantic ocean and indian ocean