A single parameter representation of hygroscopic growth and cloud condensation nucleus activity – Part 3: Including surfactant partitioning

Atmospheric particles can serve as cloud condensation nuclei in the atmosphere. The presence of surface active compounds in the particle may affect the critical supersaturation that is required to activate a particle. Modelling surfactants in the context of Köhler theory, however, is difficult because surfactant enrichment at the surface implies that a stable radial concentration gradient must exist in the droplet. In this study, we introduce a hybrid model that accounts for partitioning between the bulk and surface phases in the context of single parameter representations of cloud condensation nucleus activity. The presented formulation incorporates analytical approximations of surfactant partitioning to yield a set of equations that maintain the conceptual and mathematical simplicity of the single parameter framework. The resulting set of equations allows users of the single parameter model to account for surfactant partitioning by applying minor modifications to already existing code

Size corrections based on refractive index for particle measuring systems active scattering aerosol spectrometer probe (ASASP-X)

January 1996.Includes bibliographical references.The response function for the ASASP-X is affected by the optical properties of atmospheric aerosols. The manufacturer calibration is based on polystyrene latex spheres (m=l.588-0i), therefore the size distributions derived from measurements taken with the ASASP-X should be corrected for particles of different refractive index. Corrections based on the manufacturer calibration and Mie theory are used to derive size corrections for different refractive indices. These corrections are applied to data and demonstrate the significant over and underestimation of aerosol volume distributions possible if no corrections to diameter are applied.Funding agency: National Park Service #1443-CA0001-92-006 96.5

A single parameter representation of hygroscopic growth and cloud condensation nucleus activity – Part 2: Including solubility

The ability of a particle to serve as a cloud condensation nucleus in the atmosphere is determined by its size, hygroscopicity and its solubility in water. Usually size and hygroscopicity alone are sufficient to predict CCN activity. Single parameter representations for hygroscopicity have been shown to successfully model complex, multicomponent particles types. Under the assumption of either complete solubility, or complete insolubility of a component, it is not necessary to explicitly include that component's solubility into the single parameter framework. This is not the case if sparingly soluble materials are present. In this work we explicitly account for solubility by modifying the single parameter equations. We demonstrate that sensitivity to the actual value of solubility emerges only in the regime of 2×10<sup>−1</sup>–5×10<sup>−4</sup>, where the solubility values are expressed as volume of solute per unit volume of water present in a saturated solution. Compounds that do not fall inside this sparingly soluble envelope can be adequately modeled assuming they are either infinitely soluble in water or completely insoluble

A single parameter representation of hygroscopic growth and cloud condensation nucleus activity

International audienceWe present a method to describe the relationship between dry particle diameter and cloud condensation nuclei (CCN) activity using a single hygroscopicity parameter. Values of the hygroscopicity parameter are between 0.5 and 2 for highly-CCN-active salts such as sodium chloride, between 0.01 and 0.5 for slightly to very hygroscopic organic species, and 0 for nonhygroscopic components. If compositional data are available and if the hygroscopicity parameter of each component is known, a multicomponent hygroscopicity parameter can be computed by weighting component hygroscopicity parameters by their volume fractions in the mixture. In the absence of information on chemical composition, experimental data for complex, multicomponent particles can be fitted to obtain the hygroscopicity parameter. The hygroscopicity parameter can thus also be used to conveniently model the CCN activity of atmospheric particles, including those containing insoluble components. We confirm the general applicability of the hygroscopicity parameter and its mixing rule by applying it to published hygroscopic diameter growth factor and CCN-activation data for single- and multi-component particles

Large particle characteristics over the southern ocean during ACE 1

December 1998.Also issued as Janel T. Davis's thesis (M.S.) -- Colorado State University, 1998.Includes bibliographical references.The Aerosol Characterization Experiment (ACE-1) in November and December 1995 was designed to characterize aerosol physical, chemical, and optical properties in remote marine regions in the Southern Hemisphere. Data from six ACE-1 research flights were used to examine concentrations of large particles in two size ranges: those having diameters, Dp, 0.5 Dp 50 µm (N1) and those with 2.0 Dp 50 µm (N2). Reported here are observations of vertical profiles of N1 and N2 for heights, z, from ~30 to 7000 mover the ocean surface. Number concentrations near the surface (z 900 m) varied from 0.8 to ~30 cm-3, while maximum N2 concentrations were ~ 2.0 cm-3. Above altitudes of 2400 m, N1 concentrations were found to vary from greater than 0.07 to 1.2 cm-3. Significant concentrations (> 0.02 cm-3) of N2 particles aloft were usually associated with regions of deep convection, cloud outflow, and cloud dissipation. Calculated dry mass concentrations for N1 particles near the surface (z 100 m) assumed to be primarily sea salt, showed dependence on wind speed. Computed dry sea salt mass concentrations varied from 2.0 to 30.0 µg m-3 and varied with wind speed similarly to previously proposed relationships. Aerosol size distributions were used to compute particle light scattering coefficients and aerosol visible optical depths. The light scattering coefficient for N1 particles ranged from 0.002 to 0.08 1an-1 at altitudes less than 900 m, and from 0.00005 to 0.05 km·1 at higher altitudes. For N2 particles, the light scattering coefficient ranged from 0.001 to 0.05 km-1 for z 900 m. The large particles are a significant contribution to the total aerosol light scattering coefficient. Optical depths for these particles ranged from 0.043 to 0.085 for N1 and from 0.019 to 0.039 for N2.Sponsored by the National Science Foundation (NSF), Significant Opportunities in Atmospheric Research and Science (SOARS)

Aerosol hygroscopicity and visibility estimates in the Great Smoky Mountains National Park

National Park Service.Includes bibliographical references (pages 79-82).Summertime visibility in the National Parks in the Eastern United States is often very poor, due to high particulate mass concentrations and high relative humidities. As a part of the Southeastern Aerosol and Visibility Study (SEAVS) in the Great Smoky Mountains National Park during the summer of 1995, aerosol size distributions (Dp = 0.1-3 µm) were measured with an Active Scattering Aerosol Spectrometer (ASASP-X). A relative humidity (RH) controlled inlet allowed for both dry and humidified measurements. The objective of this experiment was to examine the aerosol size distribution and its variation with RH to characterize its effect on visibility in the region. The ASASP-X was calibrated with polystyrene latex spheres (PSL) (m = 1.588), however, the instrument response was sensitive to the refractive index of the measured particles, which was typically much lower than that of PSL. An inversion technique accounting for varying particle real refractive index was developed to invert ASASP-X data to particle size. Dry (RH < 15%) particle refractive indices were calculated using the partial molar refractive index method and 12-hour fine aerosol (<2.5 µm) chemical compositions from the National Park Service Interagency Monitoring of Protected Visual Environments (IMPROVE) filter samples. A study average dry refractive index of m = 1.49 ± 0.02 was determined. The dry aerosol number distributions inverted using the scaling method were fit with single mode lognormal curves, resulting in dry accumulation mode size parameters. A study average total volume concentration of 7 ± 5 µm3 cm-3 was determined, with a maximum value of 26 µm3 cm-3. The large variability was due to extremes in meteorological situations occurring during the study. The study average volume median diameter was 0.18 ± 0.03 µm, with an average geometric standard deviation of 1.45 ± 0.06. A newly-developed iteration method was used to determine wet refractive indices, wet accumulation mode volume concentrations and water mass concentrations as a function of relative humidity. Theoretical predictions of water mass concentrations were determined using a chemical equilibrium model assuming only ammonium and sulfate were hygroscopic. Comparisons of predicted and experimental water mass showed agreement within experimental uncertainties. To examine the effects of particles on visibility, particle light scattering coefficients, bsp, were calculated with derived size parameters, refractive index and Mie theory. Dry scattering agreed well with nephelometer measurements made at SEAVS, with an average bsp of 0.0406-km-1. Estimates of particle light scattering growth (b/b0) were determined from ratios of wet and dry light scattering coefficients, and also agreed with nephelometer results. The new inversion techniques were compared to earlier, simpler methods which ignored variations in aerosol chemical composition. The simpler method yielded smaller mean diameters, however, hygroscopicity estimates were comparable to those derived using daily varying chemical composition. This suggests that although the aerosol chemical composition is needed to determine aerosol size parameters, it may not be critical for deriving hygroscopicity (or other ratios of size parameters). This result may be specific to this study, as the variation in refractive index with RH assumed by previous models appears to be a good estimate for that observed during SEAVS.Funding agency: National Park Service # 1443-CA0001-920006, AMD#5/SUB#3

Further development and testing of a bimodal aerosol dynamics model

April 1994.Also issued as Debra A. Youngblood's thesis (M.S.)- Colorado State University, 1994.Includes bibliographical references.A previously reported bimodal monodisperse aerosol model is further developed and tested. The starting point is the BImodal MOnoDisperse Aerosol Model (BIMODAM I) which was developed to model the formation of ammonium sulfate ((NH4) 2SO4) particles from sulfuric acid (H2SO4) vapor. The model follows the evolution of two monodisperse modes where each mode, i, is characterized by a unique mean diameter and the number of particles with that mean diameter. The aerosol distribution is assumed to undergo typical atmospheric processes such as condensational growth, coagulation, nucleation, and deposition. In BIMODAM I, the effect of each process on the aerosol distribution is represented as a rate equation. The prognostic equations are coupled, so a variable time step differential equation solver is utilized to simultaneously solve the system of equations to predict the mass and number concentration in each mode. The diameter of each mode is diagnosed from the mass and number concentrations. In the first part of this work, two new parameterizations were developed for BIMODAM I. First, a condensation rate factor was developed to account for the lack of polydispersity in the model. Second, a criterion was developed which dictates when the two modes may be merged without generating large errors. In the second part of this work, a new version of the model (BIMODAM II) was developed to give the same accurate results as BIMODAM I without using the variable time step differential equation solver. A key development in BIMODAM II is a parameterization for the process of homogeneous nucleation. This parameterization is based on the approximation of the time-dependent nucleation rate with a triangular function; using this approach, only two parameters are needed to predict the total number of particles resulting from a nucleation event The two parameters are correlated to chemical source rate and relative humidity. Therefore, prediction of the number concentration of particles resulting from a nucleation burst depends on knowing the relative humidity and determining the chemical source rate. This development has been shown to perform well in the presence and absence of preexisting particles and over short and long time scale simulations. Further developments in BIMODAM II include simple analytical solutions of the differential equations for coagulation and deposition. Using a mass balance equation, a simple solution was also derived to predict the amount of sulfuric acid in the vapor phase at any time during the simulation. From this calculation, the amount of mass in the aerosol phase is calculated by subtracting the amount in the vapor phase from the total amount of sulfuric acid produced during any given time step. By using the simplifications and parameterizations mentioned above, computational time is saved by eliminating the variable time stepping differential equation solver. This model is shown to perform well when compared against a simulation which uses a more detailed description of the aerosol size distribution.Sponsored by the Western Regional Center of the National Institute for Global Change W/GEC91-114, and Colorado State University Graduate School

Optical measurements of aerosol size distributions in Great Smoky Mountains National Park: particle hygroscopicity and its impact on visibility

National Park Service.Includes bibliographical references (pages 70-73).Aerosol size distributions were measured during the 1995 Southeastern Aerosol and Visibility Study (SEAVS) in Great Smoky Mountains National Park using a PMS ASASP-X optical aerosol spectrometer. Ambient aerosol was conditioned in a relative humidity (RH) controlled inlet before sampling. 130 dry (RH ~ 15%) and 112 humidified aerosol size distributions, plus 24 distributions at ambient RH, were recorded during daylight hours for aerosol in the size range 0.1 < Dp <2.5 µ. Particle light scattering from the ASASP-X was inverted to particle sizes using Mie theory and applying a refractive index of either 1.530-0i or 1.501-0i for dry conditions, depending on the ambient aerosol chemical composition. A dry aerosol volume concentration time line from this work, when compared with a similar time line of aerosol mass concentration from IMPROVE samplers, indicates the ASASP-X provided a reliable representation of temporal trends in the ambient aerosol loading. The median dry aerosol geometric mass mean diameter measured during SEAVS was 0.28 µm, with a range from 0.24 to 0.38 µm, and median geometric standard deviation of 1.64. Sequential dry and humidified aerosol size distributions were corrected for refractive index dependence on RH and used to derive ambient aerosol hygroscopicity as a function of RH. This work demonstrates that experimentally derived water absorption is equivalent to or less than predicted by theory, assuming ambient aerosol water uptake is dictated by ionic compounds that have a chemical composition consistent with the particle fine mass measured during SEAVS. In this work, special consideration is given to the uncertainty in derived aerosol water contents and the degree to which this uncertainty propagates to estimates of light scattering. An ultimate goal of this project is to augment visibility and radiative transfer models through a better understanding of how RH affects the ambient aerosol size distribution in the southeastern U.S.Funding agency: National Park Service # 1443-CA0001-92-0006 96.5