Allen Williams

Influence of Soluble Surfactant Properties on the Activation of Aerosol Particles Containing Inorganic Solute

Abstract Atmospheric aerosol particles consisting of ammonium sulfate [(NH4)2SO4] or sodium chloride (NaCl) have reasonably well-defined hygroscopic properties compared to other materials in aerosol particles, such as organic material. The effect of internally mixing organic compounds with these salts is not clear when considering the hygroscopic properties of the resulting particles, including activation of particles in clouds. This research describes the activation of aerosol particles consisting of sodium dodecyl sulfate (SDS) and NaCl solute. SDS is used as a surrogate for soluble atmospheric surfactants. Kohler theory is used to model droplet activation while considering droplet properties such as surface tension (σ), surface excess surfactant concentration, and critical micelle concentration (CMC). Reduction in critical supersaturation (Sc) caused by the reduction in σ (Kelvin effect) associated with the surfactant is dominated by the increase in Sc with the decreasing number of moles of solute in t...

A preliminary synthesis of modeled climate change impacts on U.S. regional ozone concentrations.

This paper provides a synthesis of results that have emerged from recent modeling studies of the potential sensitivity of U.S. regional ozone (O3) concentrations to global climate change (ca. 2050). This research has been carried out under the auspices of an ongoing U.S. Environmental Protection Agency (EPA) assessment effort to increase scientific understanding of the multiple complex interactions among climate, emissions, atmospheric chemistry, and air quality. The ultimate goal is to enhance the ability of air quality managers to consider global change in their decisions through improved characterization of the potential effects of global change on air quality, including O3 The results discussed here are interim, representing the first phase of the EPA assessment. The aim in this first phase was to consider the effects of climate change alone on air quality, without accompanying changes in anthropogenic emissions of precursor pollutants. Across all of the modeling experiments carried out by the differe...

Volcanic aerosols and interannual variation of high clouds

Interannual variability of high-level cloudiness (HC) is examined using global outgoing longwave radiation (OLR). Variations of HC are analyzed versus a measure of global stratospheric aerosol amount and an El Nino index. Volcanic aerosols are apparently associated with widespread increases of up to 10% in an OLR-based HC index. The most significant effects occurred in middle latitudes and persisted for several years after major eruptions. El Nino is found to be associated with decreased cloud activity in the subtropics. This study suggests that volcanic aerosols can significantly modify global cloudiness, and that stratospheric aerosol loading can be an important variable controlling the interannual variations of high level clouds and climate.

Theory of Droplet Growth in Clouds: II. Diffusional Interaction Between Two Growing Droplets

Abstract The interaction of two growing droplets in a supersaturated atmosphere has been examined, and the temperature and vapor density profiles have been determined. It is found that the smaller droplet tends to “catch up” with the larger at a slower rate than predicted by conventional diffusion theory. Consideration of droplet fallspeeds leads to the conclusion that, under atmospheric conditions, growth interaction becomes significant only for droplet “pairs” having equal or nearly equal radii. The number of such pairs is generally small enough so that the effect on the size distribution is quite small. Of a much greater importance is the possibility of a resulting attractive diffusio-phoretic force between two growing drops which, in turn, gives rise to a net velocity of one drop toward the other. If this diffusion force of attraction becomes sufficiently strong to overcome the hydrodynamic and thermo-phoretic forces acting in the opposite direction, both collision efficiencies and coagulation of smal...

Seasonal Simulation of Tropospheric Ozone over the Midwestern and Northeastern United States: An Application of a Coupled Regional Climate and Air Quality Modeling System

Abstract The impacts of air pollution on the environment and human health could increase as a result of potential climate change. To assess such possible changes, model simulations of pollutant concentrations need to be performed at climatic (seasonal) rather than episodic (days) time scales, using future climate projections from a general circulation model. Such a modeling system was employed here, consisting of a regional climate model (RCM), an emissions model, and an air quality model. To assess overall model performance with one-way coupling, this system was used to simulate tropospheric ozone concentrations in the midwestern and northeastern United States for summer seasons between 1995 and 2000. The RCM meteorological conditions were driven by the National Centers for Environmental Prediction/Department of Energy global reanalysis (R-2) using the same procedure that integrates future climate model projections. Based on analyses for several urban and rural areas and regional domains, fairly good agr...

Measured and Modeled Light Scattering Values for Dry and Hydrated Laboratory Aerosols

Abstract Closure experiments were completed to compare measured and modeled aerosol optical properties and their dependence on controlled relative humidity (RH) and wavelength of light. NaCl, (NH4)2SO4, and NH4NO3 aerosol particles with approximate geometric mass mean diameters of 0.2 μm and geometric standard deviations of 1.7 were tested as part of this study. High evaporative losses (up to 40%) were observed for NH4NO3 aerosol at this particle size range due to heating, and the results from these tests have been excluded from the closure analysis. Aerosol optical properties were measured with a RH-scanning nephelometry system (humidograph) and modeled with a Mie–Lorentz light scattering model. Particle size distributions were measured with a scanning differential mobility analyzer. Closure between the measured and modeled values of the total light scattering coefficient (σsp), backscatter ratio (b), and Angstrom exponent (a) for dry (low RH) aerosols was achieved within 0.0%–5%, 4%–15%, and 3%–17%, res...

Sensitivity of Surface Ozone Simulation to Cumulus Parameterization

Abstract Different cumulus schemes cause significant discrepancies in simulated precipitation, cloud cover, and temperature, which in turn lead to remarkable differences in simulated biogenic volatile organic compound (BVOC) emissions and surface ozone concentrations. As part of an effort to investigate the impact (and its uncertainty) of climate changes on U.S. air quality, this study evaluates the sensitivity of BVOC emissions and surface ozone concentrations to the Grell (GR) and Kain–Fritsch (KF) cumulus parameterizations. Overall, using the KF scheme yields less cloud cover, larger incident solar radiation, warmer surface temperature, and higher boundary layer height and hence generates more BVOC emissions than those using the GR scheme. As a result, the KF (versus GR) scheme produces more than 10 ppb of summer mean daily maximum 8-h ozone concentration over broad regions, resulting in a doubling of the number of high-ozone occurrences. The contributions of meteorological conditions versus BVOC emiss...

Automated Low-Pressure Carbonate Eluent Ion Chromatography System with Postsuppressor Carbon Dioxide Removal for the Analysis of Atmospheric Gases and Particles

We present a low-pressure, automated, semi-continuous Gas-Particle Ion Chromatograph to measure soluble ionogenic gases and soluble ionic constituents of PM2.5. The system utilizes a short separation column, an isocratic carbonate eluent and post suppressor CO2 removal. Measured constituents include ammonium, nitrate, and sulfate in the particle fraction, and nitric acid, sulfur dioxide, and ammonia among soluble gases. Two independent sampling channels are used. In one channel, a wet denuder collects soluble gases. In the second channel, following removal of large particles by a cyclone and soluble gases by a wet denuder, a hydrophobic filter-based particle collector collects and extracts the soluble components of PM2.5. The aqueous particle extract is aspirated by a peristaltic pump onto serial cation and anion preconcentrator columns. Gas samples are similarly loaded onto another set of serial cation and anion preconcentrator columns. The cation preconcentrator is eluted with NaOH and the evolved NH3 is passed across a membrane device whence it diffuses substantially into a deionized water receptor stream; the conductivity of the latter provides a measure of NH3 (NH4 +). The anion preconcentrator column(s) are subjected to automated periodic analysis by ion chromatography. This system provides data every 30 min for both particles (NO3 −, SO4 2 − and NH4 +) and gases (HNO3, SO2 and NH3). Gas and particle extract samples are each collected for 15 min. The analyses of the gas and particle samples are staggered 15 min apart. The limit of detection (S/N = 3) for NO3 −, SO4 2− and NH4 + are 2.6, 5.3, and 2.1 ng/m3, respectively.