D. Eli Sherman

The chemical composition of fogs and intercepted clouds in the United States

Over the past decade, the chemical compositions of fogs and intercepted clouds have been investigated at more than a dozen locations across the United States. Sampling sites have been located in the northeast, southeast, Rocky Mountain, and west coast regions of the US. They include both pristine and heavily polluted locations. Frontal/orographic clouds (warm and supercooled), intercepted coastal stratiform clouds, and radiation fogs have all been examined. Sample pH values range from below 3 to above 7. Major ions also exhibit a wide concentration range, with clouds at some locations exhibiting high sea salt concentrations, while composition at other locations is dominated by ammonium and sulfate or nitrate.

Ice Formation by Sulfate and Sulfuric Acid Aerosol Particles under Upper-Tropospheric Conditions

Abstract Ice formation in ammoniated sulfate and sulfuric acid aerosol particles under upper-tropospheric conditions was studied using a continuous flow thermal diffusion chamber. This technique allowed for particle exposure to controlled temperatures and relative humidities for known residence times. The phase states of (NH4)2SO4 and NH4HSO4 particles were found to have important impacts on their ice formation capabilities. Dry (NH4)2SO4 particles nucleated ice only at high relative humidity (RH ≥ 94%) with respect to water at temperatures between −40° and −60°C. This result suggested either an impedance or finite time dependence to deliquescence and subsequent homogeneous freezing nucleation. Ammonium sulfate particles that entered the diffusion chamber in a liquid state froze homogeneously at relative humidities that were 10% lower than where ice nucleated on initially dry particles. Likewise, crystalline or partially crystallized (as letovicite) NH4HSO4 particles required higher relative humidities fo...

Aerosol size distributions and visibility estimates during the Big Bend regional aerosol and visibility observational (BRAVO) study

The Big Bend Regional Aerosol and Visibility Observational (BRAVO) study was conducted in Big Bend National Park in 1999. The park is located in a remote region of southwest Texas but has some of the poorest visibility of any Class 1 monitored area in the western US. The park is frequently influenced by air masses carrying emissions from Mexico and eastern Texas. Continuous physical, optical and chemical aerosol measurements were performed in an effort to understand the sources of and contributions to haze in the park. As part of this characterization, dry aerosol size distributions were measured over the size range of 0:05oDpo20mm. Three instruments with different measurement techniques were used to cover this range. Complete size distributions were obtained from all of the instruments in terms of a common measure of geometric size using a new technique. Size parameters for accumulation and coarse particle modes were computed and demonstrate periods when coarse mode volume concentrations were significant, especially during suspected Saharan dust episodes in July and August. Study average (and one standard deviation) geometric volume mean diameters for the accumulation and coarse particle modes were 0.2670.04 and 3.470.8mm, respectively. Dry light scattering coefficients (bsp) were computed using measured size distributions and demonstrated periods when contributions to bsp from coarse particles were significant. The study average computed bsp was 0.02670.016 km � 1 . Computed dry bsp values were highly correlated with measured values (r 2 ¼ 0:97). Real-time sulfate measurements were correlated with accumulation mode volume concentrations (r 2 ¼ 0:89) and computed dry light scattering coefficients (r 2 ¼ 0:86), suggesting sulfate aerosols were the dominant contributor to visibility degradation in the park. r 2002 Elsevier Science Ltd. All rights reserved.

Aerosol Particle Processing and Removal by Fogs: Observations in Chemically Heterogeneous Central California Radiation Fogs

Fog composition and deposition fluxes of fog waterand fog solutes were measured in six radiation fogevents in San Joaquin Valley, California duringwinter 1998/1999. Measurements made at 2 hrintervals with 0.30 m2 and 0.06 m2 Teflondeposition plates yielded excellent reproducibility(relative standard deviations of 3.8–6.0%) forwater, nitrate, sulfate and ammonium. Water fluxesmeasured at 5 min intervals with a recordingbalance agreed well with the deposition platemeasurements before 8:00 AM. After 8:00 AMevaporation proved problematic. The averagedeposition velocity from the study for fog nitrate(3.8 cm s-1) was less than those for fogsulfate (5.1 cm s-1) and ammonium (6.7 cms-1). All three species generally exhibitedsmaller deposition velocities than fog water. Thespecies dependent trend in deposition velocitieswas consistent with preferential enrichment ofthese species in small fog drops (nitrate > sulfate> ammonium).

Development of a multi-stage cloud water collector Part 1: Design and field performance evaluation

Abstract Cloud chemistry can vary as a function of drop size. In order to investigate variations in chemical composition across the drop size spectrum, a new multi-stage cloud water collector was developed. The Colorado State University 5-Stage cloud water collector (CSU 5-Stage) separates drops, based upon the principles of cascade inertial impaction, into five different fractions. Its design incorporates many features to facilitate its use in the field, and maintain both consistent performance between varying atmospheric conditions and the chemical and physical integrity of the collected sample. Limited field tests indicate the CSU 5-Stage works reasonably within field measurement uncertainty, and its results are comparable to those from other cloud collectors and consistent with additional concurrent measurements. Data obtained using the CSU 5-Stage provide additional insight into drop size-dependent chemistry in fogs/clouds. These insights should result in an improved understanding of both the impact of clouds on the fate of atmospheric species, and cloud microphysics and dynamics.

Sulfur dioxide oxidation in clouds at Whiteface Mountain as a function of drop size

In situ oxidation of SO2 has been determined in clouds as a function of droplet size using a trace element technique during July 1998 at Whiteface Mountain, New York. The pH of the cloud water ranged from 2.8 to 4.7 with a mean of 3.4, and therefore SO2 oxidation was dominated by hydrogen peroxide. Size-fractioned cloud samples were collected from six different events at the mountains summit (1.5 km above mean sea level) using a size-fractionating California Institute of Technology Active Strand Cloudwater Collector. Bulk samples were collected using both passive and active collectors. During each event, below-cloud and interstitial aerosols were collected every 2 hours. Cloud water and aerosol samples were analyzed for major ions and selected trace elements. Continuous measurements of gas phase species SO2, H2O2, and O3 were carried out at the summit and below-cloud sites. Concentrations of cloud water SO42−, NO3−, H2O2, and trace elements, as well as pH, were largely independent of droplet size. The component of cloud water SO42− produced from in situ oxidation (SO2−4in) was also largely independent of droplet size. The results are in agreement with calculated relative production rates in the small and large drop sizes based on known laboratory reaction rates.

The influence of chemical heterogeneity among cloud drop populations on processing of chemical species in winter clouds

Abstract Drop size-resolved measurements of winter cloud composition in the Rocky Mountains of northern Colorado revealed significant variations of cloud drop pH, ion (SO 4 2− , NO 3 − , NH 4 + , and Ca 2+ ) concentrations, and concentrations of trace metal catalysts (Fe and Mn) with drop size. Simultaneous measurements of snow chemical composition and the degree of cloud drop capture by snow crystals (riming) revealed a positive correlation between snow composition and the extent of ice crystal riming in two of four cases studied. Observations indicate that the size-dependent chemical composition of the clouds tends to enhance aqueous phase sulfate production rates when the primary oxidant is ozone. Enrichment of accumulation mode aerosol species in small cloud drops, which are inefficiently scavenged by ice crystals, appears to cause small (typically less than 15%) reductions in the efficiency with which these species are scavenged by precipitation during accretional ice crystal growth.