A23B-1251
Global distribution of solid and aqueous sulfate aerosols: effect of the hysteresis of particle phase transitions
The partitioning between solid and aqueous phases of tropospheric sulfate-ammonium particles is simulated with a global 3D chemical transport model (CTM). The simulation explicitly accounts for the hysteresis of particle phase transitions by transporting aqueous sulfate and three solid sulfate forms (ammonium sulfate, letovicite, and ammonium bisulfate). Composition-dependent deliquescence relative humidities (DRH) and crystallization relative humidities (CRH) are based on recent laboratory data. We find that the solids mass fraction on a sulfate basis is 0.34, partitioned as 93% ammonium sulfate, 6% letovicite, and 1% ammonium bisulfate. The solids mass fraction increases with altitude from 0.10-0.20 in the boundary layer to 0.60-0.80 in the upper troposphere. The dominance of solids in the upper troposphere arises in part from high sulfate neutralization, reflecting in our simulation a low retention efficiency of NH3 upon cloud freezing. High sulfate neutralization is consistent with the few available observations. High acidity with a dominant aqueous phase, however, can occur in the upper troposphere during volcanic eruptions. Seasonal variation of the solids mass fraction in global average is modulated by emissions of NH3 from the terrestrial biosphere and biomass burning and of dimethylsulfide from the ocean biosphere. The timescale of phase transitions as driven by changes in relative humidity varies from 10- 50 hr in the boundary layer to 150-400 hr in the upper troposphere. Omission of the hysteresis effect in global models by assuming that particle phase follows the lower side of the hysteresis loop increases the solids mass fraction from 0.34 to 0.56. An upper-side assumption decreases the fraction to 0.17. Lower- and upper-side assumptions better approximate particle phase for high and low altitudes, respectively. Fluctuations in the CRH, which can be induced by other constituents in sulfate particles such as minerals or organic molecules, strongly affect the solids mass fraction in the boundary layer but not at higher altitudes.
A23B-1252
Aerosol Water Uptake in the Caribbean Region: the Impact of Organics Using the EQSAM3
We present first results on how the hygroscopic growth of natural sea-salt aerosols changes with air masses of different origin and with different pollution levels (e.g. organics) in the Caribbean islands. For this purpose we used aerosols samples collected as part of the Rain in Cumulus over the Ocean Experiment (RICO) during December 2004 and January 2005 in two different ground-based marine sites in the Caribbean: Dian Point (DP), Antigua and Cape San Juan (CSJ), Puerto Rico. The thermodynamic model EQSAM3 (Metzger and Lelieveld, 2007) was used to determine the water uptake of the collected aerosol samples. EQSAM3 (EQuilibrium Simplified Aerosol Model) allows a consistent calculation of the aerosol composition and the gas/liquid/solid partitioning of various mixed inorganic/organic multicomponent solutions, due to an explicit treatment of aerosol hygroscopic growth that is based on efflorescence and deliquescence relative humidities (hysteresis effect). The aerosol samples were collected using a 13-stage Dekati low-pressure impactor (Dp 0.1 to 10 μm), a 10-stage micro-orifice uniform deposit impactor (Dp 0.054 to 18 μm), and stacked-filter units (Dp < 1.7 μm). Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO2-, NO3-, SO42-, acetate, formate, malonate, and oxalate were determined using ion chromatography. Thermal- optical analysis was used to determine the concentrations of aerosol total carbon (TC), organic carbon (OC), and elemental carbon (EC). The chemical characterization together with the five-day back trajectories calculated using the NOAA's HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) model allowed the identification of air masses coming from the North Atlantic (maritime air), Northwest Africa (desert dust), and North America (anthropogenic pollution). The measured ion concentrations were then used for aerosol composition calculations with EQSAM3 to determine the neutralization reactions, the water mass associated with inorganic and organic salt compounds, and the associated growth factors (GFs). Results for size-resolved water uptake calculations showed that the GFs of the accumulation mode particles are closer to reference calculations of pure sea salt. Aitken and coarse mode particles showed considerably lower GFs. The magnitude of these factors showed a strong dependence on the air mass origin and the level of air pollution (e.g., sulfates and organics). We further show the humidity effect of various organics compounds on the GF and aerosol water mass.
A23B-1253
Laboratory Investigation of the Relative Humidity Dependence of Light Extinction by Mixed Organic/Sulfate Particles
Light extinction by atmospheric particles is strongly dependent on the size, chemical composition, and water content of the aerosol. Since light extinction by particles directly impacts climate and visibility, direct measurement of extinction at various relative humidity (RH) conditions is needed. In this work, the optical growth factors, fRHext(80%RH, Dry) have been measured using cavity ring-down aerosol extinction spectroscopy for particles of varying organic/sulfate compositions. Specifically, slightly soluble, multifunctional aromatic compounds resulting from biomass burning have been investigated in this work. In general, the organic compounds studied exhibit much smaller optical growth than inorganic compounds such as ammonium sulfate. Also, a linear relationship between mass fraction organic and optical growth has been observed for most organic compounds studied. These results, an exception to the linear relationship, comparisons to growth factor measurements, Gf, refractive index determinations, and implications for climate calculations will be presented.
A23B-1254
Column Water Vapor Retrieval Method and Observations Associated With the SAM Sensor
SAM (Sun and Aureole Measurement) is a sun tracking imaging sensor designed and produced by Visidyne Inc. in Burlington, Massachusetts. SAM has applications in many areas such as climate change monitoring, cloud physics research, and complementing NASA's AERONET (AErosol RObotic NETwork). Recently a spectrometer was added to SAM to measure the column water vapor amount as a function of time and location. These measurements are obtained by the use of a silicon-detector based spectrometer. The ratio of two wavelength bands is used to retrieve the column water vapor amount, one band representing the water vapor absorption band and the other a nearby guard band. The formula for calculating this quantity was derived from the relationships representing the irradiance measured at these two wavelengths. The water absorption is approximated by a least squares fit to MODTRAN (MODerate resolution atmospheric TRANsmission) calculations. The results of this study are presented for several locations. One of which was co-located with AERONET at the SGP (Southern Great Plains) ARM central facility during the CLASIC/CHAPS campaign for a calibration standard. A three constant approach was found best to determine the column water vapor.
A23B-1255
Deliquescence Measurements of Potassium Salts
Potassium compounds such as KCl, K2SO4, and KNO3 are salts resulting from biomass burning. With time the number of aerosol particles containing KCl decreases, and the number of particles containing KNO3 and K2SO4 increases. The transformation of KCl to K2SO4 and KNO3 with aging of the smoke could lead to changes in the hygroscopic properties of the smoke particles and thus their cloud-nucleating potential. Similar reaction mechanisms are likely to be involved in the conversion of KCl in smoke particles as occur for NaCl in sea salt. Little experimental work has been published on the hygroscopic properties of potassium salts because of their high DRH values. Instruments that are commonly used to measure hygroscopic properties such as differential mobility analyzers or electrodynamic balances do not operate accurately at RH > 90%. Here we present data describing the hygroscopic properties of several fresh potassium salts, as well as laboratory generated mixed salts, using transmission and scanning electron microscopes (TEM and SEM). Both microscopes have environmental chambers that enable study of the interaction of water with single particles. DRH values for KCl, KNO3 and K2SO4 were found to be 86%, 92%, and 97%, respectively. KNO3 particles formed by atomization appear rounded and undergo continuous hygroscopic growth without a distinct deliquescence point. Similar results have been published for NaNO3. In contrast, when KNO3 powder is ground in a mortar and pestle and placed in the SEM, the grains appear euhedral and have a DRH at 92%, in agreement with literature values. It appears that KNO3 particles formed by atomization will readily take up water at RH values below their DRH. Our results indicate that the hygroscopic properties of KNO3 particles are influenced by their histories. Water associated with aged or mixed particles at RH's less than their DRH will affect how these particles uptake and react with gases.
A23B-1256
A comparison of water uptake by aerosols using the thermodynamic models of Metzger and Jacobson
The water uptake by hygroscopic aerosols can significantly alter aerosol size, optical properties and direct radiative forcing. Two methods to determine the uptake of water by aerosols are investigated here. Thermodynamic aerosol models determine aerosol water uptake by accounting for a mixing rule, also known as the Zdanovskii-Stokes-Robsin (ZSR) relationship, which is usually solved iteratively for a given equilibrium condition. The Metzger method (Metzger and Lelieveld, ACP, 2007) determines water uptake analytically (without any iteration). This yields a significant reduction of computing time. They assume the water activity to equal the ambient relative humidity at equilibrium, and hence only use relative-humidity-dependent activity coefficients for volatile aerosol compounds. Here, we examine this new method of uptake of water by aerosols by comparing the results with those of the thermodynamic model currently used in our global chemical transport model (Feng and Penner, JGR, 2007) under typical ambient conditions.
A23B-1257
Cloud Condensation Nucleus Activity of calcite and calcite coated with model humic and fulvic acids
Many recent studies have shown that organics can alter the water adsorption and cloud condensation nuclei (CCN) activity of common deliquescent species in the Earth's atmosphere. However, very little is known about the effect of organics on water adsorption and CCN activity of common inactive cloud nuclei, such as mineral aerosol. As many studies have shown that a large fraction of unidentified organic material in aerosol particles is composed of polycarboxylic acids resembling humic substances, the presence of these large molecular weight Humic-Like Substances (HULIS) may also alter the water adsorption and CCN activity of mineral aerosol. Thus, we have measured the water adsorption and CCN activity of model humic and fulvic acids. Additionally, the water adsorption and CCN activity of mineral aerosol particles coated with humic and fulvic acids have been studied. We find that humic and fulvic acids show continual multilayer water adsorption as the relative humidity is raised. Additionally, we find that calcite particles mixed with humic and fulvic acids take up more water by mass, by a factor of two, compared to the uncoated calcite particles at approximately 70% RH. CCN measurements also indicate that internally mixed calcite-humic or fulvic acid aerosols are more CCN active than the otherwise inactive, uncoated calcite particles. Our results suggest that mineral aerosol particles coated with high molecular weight organic materials will take up more water and become more efficient CCN in the Earth's atmosphere than single-component mineral aerosol.
A23B-1258
Aerosol Chemistry and Humidity Effect on Visibility Impairment in Guangzhou During the 2006 PRD Campaign
In order to characterize the cause of visibility impairment in the metropolitan area of Guangzhou, China, aerosol optical and chemical measurements were conducted from 2 to 31 July 2006 at a Guangzhou urban site as part of the Observation Experiment for Regional Air Quality in Pearl River Delta of China. Light extinction, scattering, and absorption coefficients were measured simultaneously with a transmissometer, a nephelometer and an aethalometer, respectively. Continuous aerosol chemical measurements were also made with Sunset elemental carbon/organic carbon (EC/OC) analyzers and on-line ion monitors. Humidograph coupling with a nephelometry was used to estimate relative humidity scattering enhancement factor, f(RH) of each aerosol component including sulfate, nitrate, and sea salt. During the intensive monitoring period, the average visual range and light extinction coefficient were 11.4 ¢®¨ú 4.0 km and 419.2 ¢®¨ú 188.1 Mm-1, respectively. The latter was found to be composed of 210.9 ¢®¨ú 145.8 Mm-1 (46.9 %) by NHSO, 33.0 ¢®¨ú 32.9 Mm-1 (7.3 %) by NHNO, 65.4 ¢®¨ú 37.9 Mm-1 (14.6 %) by OMC, 51.6 ¢®¨ú 24.9 Mm-1 (11.5 %) by EC, 40.2 ¢®¨ú 48.8 Mm-1 (8.9 %) by sea salt, 10.2 ¢®¨ú 7.7 Mm-1 (2.3 %) by fine soil, and 12.0 ¢®¨ú 6.5 Mm-1 (2.7 %) by coarse particles. Average humidity effect on visibility impairment was estimated to be 38 % of total light extinction coefficient, which consists of 25.7 % (115.4 Mm-1) by NHSO, 4.1 % (18.6 Mm-1) by NHNO, and 8.2 % (36.8 Mm-1) by sea salt. Two episodes of severe visibility impairment were observed during the intensive measurement period. The 1st one was classified as a local accumulation case based on the Hysplit back trajectory analyses while the 2nd one as a transport case from mainland China. During the 1st haze episode, concentration of each aerosol component increased up to 30 % ~ 70 % compared to the relatively clean days with an average increase in light extinction coefficient of 152.2 Mm-1. During the 2nd haze episode, sulfate and nitrate dominantly increased up to 280 % ~ 360 %, resulting in 150 % increase in light extinction coefficient of 409.1 Mm-1.
A23B-1259
Parameterisation of detailed aerosol processes
Modelling the equilibrium composition and phase state of aqueous aerosols is a complex challenge requiring the use of fundamental frameworks that can deal with the composition variability of mixed inorganic/organic aerosol where appropriate laboratory data is lacking. Such a framework was developed at the University of Manchester which is often used as a closure and prognostic tool for studies of aerosol particle hygroscopicity and gas/particle phase partitioning (ADDEM – Aerosol Diameter Dependent Equilibrium Model). However, large scale models, which are used in impacts and effects studies, are computationally expensive and cannot treat all the relevant physical/chemical processes taking place in the atmosphere which much complexity. Thus, there is still a need for developing links across the hierarchy of model frameworks between so called ‘bottom up' and ‘top down' modelling approaches. With this in mind, key variables have been parameterised for use in predicting both the hygroscopic properties and gas/particle partitioning in large scale models. Developments of appropriate parameterisations are challenging mathematically as one ideally requires a generic model that retains variables with physical/chemical significance. Similarly, the nature of aqueous atmospheric aerosol particles is such that processes taking place within a liquid phase often require iterative steps depending on the chemical composition, further increasing the computational complexity of any ‘simplified' framework. Whilst challenging mathematically, the framework developed here is such that any number of dimensions (chemical components) can be added for future developments and the nature of parameter fitting has allowed one to bypass the above mentioned iterative steps. These modules provide a direct output of detailed processes studies continually carried out within the area of aerosol dynamics. Indeed, this development means we can now directly exploit important future fundamental process studies when attempting to asses large scale impacts by incorporation of appropriate data into the above parameterisations.
A23B-1260
Parameterization of the CCN Activity of Organic, Nitrate and Mixed Aerosol During Pollution and Biogenic Episodes at a Rural Field Campaign in Ontario
An intensive field campaign was conducted at a rural site near Egbert, Ontario, Canada (70 km north of Toronto) in May and June of 2007. Periods of specific interest include: (1) Northwest winds resulting in exceedingly clean air; (2) Northerly winds corresponding with terpene oxidation; and (3) Southerly winds carrying the urban outflow from Toronto. A range of aerosol chemical compositions were measured including organic-rich, nitrate-rich or mixed sulfate/organic aerosol. An objective of the study is to develop a physically accurate understanding of the role of organic aerosol constituents on hygroscopic growth that is sufficiently simple to be included in climate models. Because it can be difficult to characterize the water activity of organic aerosols due to their complexity and the lack of detailed chemical composition information, we determine a single parameter that describes both hygroscopic and cloud condensation nucleus activity (kappa: Petters and Kreidenweis [2007]) for selected air masses. Size-resolved chemical composition measurements, a continuous flow cloud condensation nucleus counter (CCNc) and a static flow CCNc were used to determine kappa values. These kappa values were implemented into a simplified version of a detailed microphysical adiabatic cloud parcel (ACP) model representing the water activity via kappa and tested against the detailed ACP model.
A23B-1261
The cloud forming potential of secondary organic aerosol under near atmospheric conditions
Cloud formation occurs through droplet nucleation by ever-present atmospheric aerosol particles. Atmospheric aerosol populations invariably contain organic material, and photo-oxidation of biogenically emitted volatile organic compounds (VOCs) to form secondary organic aerosol (SOA) is widely considered a major contribution to the organic fraction. Smog chambers are used to investigate the formation and evolution of SOA particles and the potential effect on cloud formation of changes in particle size, composition, and surface tension. High concentrations of VOCs are commonly used to improve measurements statistics in chamber studies. Here we demonstrate for the first time that lower precursor concentrations are necessary for a lower aerosol mass yield, with atmospherically representative particle composition, hygroscopicity, and cloud formation potential. Furthermore, by probing cloud activation behaviour, we show that the supersaturation required to activate particles decreases with particle age at a given size. This demonstrates that composition plays a role in cloud droplet formation as well as size. We also demonstrate that a semi-empirical approach can be accurately used to calculate critical supersaturation of SOA particles from the measured hygroscopic growth below 100% RH and, based on an extensive sensitivity study, conclude that the surface tension of the SOA solution at the droplet size of activation differs minimally from that of pure water. Thus, at critical supersaturation SOA does not significantly contribute to the reduction in surface tension attributed to the organic aerosol, with important implications for cloud formation and climate.
A23B-1262
Observations of hygroscopic and optical properties of biogenic secondary organic aerosol generated using a simple continuous flow reaction chamber
Biogenic secondary organic aerosols (BSOA) are a major component of atmospheric particles that impact air quality and climate via radiative and hygroscopic processes. Direct measurements of BSOA properties in the atmosphere are usually complicated by the presence of other aerosol species. Here we examine BSOA properties using a simple continuous flow reaction chamber. We present measurements of the optical and hygroscopic properties of BSOA generated through the reaction of alpha-pinene and ozone at two controlled relative humidity values (<5 and 90%) and selected concentrations and ratios of alpha-pinene to ozone. We measured aerosol size distributions using a differential mobility analyzer and optical particle counter sizing system. Based on optical and mobility size we retrieved a refractive index, n ~ 1.55, which is within the range of values assumed by models, and similar to values retrieved previously using the same method for other oxygenated carbonaceous aerosols. BSOA hygroscopicity was determined using a humidified tandem differential mobility analyzer (60 < RH < 90%) and a cloud condensation nuclei (CCN) counter (0.3 < s < 1.0%). The results are interpreted using the hygroscopicity parameter kappa, which compares the two measurements on a common scale. Hygroscopicity inferred from growth factor data (0.008 < kappa < 0.045) was considerably lower than required to explain CCN activity (0.08 < kappa < 0.16), consistent with previous studies of BSOA generated in smog chambers. To test various proposed hypotheses that may explain this discrepancy we will present water uptake data measured at high RH (80 < RH < 100%) using the Leipzig Aerosol Cloud Interaction Simulator (LACIS) and using a high humidity tandem differential mobility analyzer (HH- TDMA).
A23B-1263
Distinct Cloud Droplet Growth Kinetics Observed Above the Marine Boundary Layer
The influence of aerosols on cloud properties is an important modulator of the climate system, and remains one of the most uncertain components of the anthropogenic influence on the radiative budget of the atmosphere. Traditional Kohler theory predicts the ability of an atmospheric particle of known size and composition to act as a cloud condensation nucleus (CCN) at equilibrium. However, it is not known to what extent particles exist in the atmosphere that may be prevented from acting as CCN by kinetic limitations. We measured the rate of cloud droplet formation at various sites across the United States during the summer of 2006. Our results suggested that kinetically-limited drops, with apparent mass accommodation coefficients (α) up to 10× lower than those observed for lab-generated ammonium sulfate particles, may be prevalent in the free troposphere. Here we report the results of similar measurements made in 2007 at a high-elevation site near the California coast that is typically above the marine boundary layer. We found that the ambient aerosol at this site was an external mixture of particles with distinct growth kinetics: approximately 50% of the time, we observed two or more modes in the α spectrum. The slower mode typically contained 25% to 50% of the total CCN at ~0.4% supersaturation. These results cannot be explained without incorporating kinetic effects into Kohler theory. By varying the chamber parameters (supersaturation ratio, residence time), we determine whether these kinetic limitations are more consistent with a model of lower α or a delay to activation.