Grantee Research Project Results
Final Report: Effects of Non-Uniform Cloud Drop Composition on Pollutant Transformation and Removal in Winter Clouds
EPA Grant Number: R823979Title: Effects of Non-Uniform Cloud Drop Composition on Pollutant Transformation and Removal in Winter Clouds
Investigators: Collett Jr., Jeffrey L.
Institution: Colorado State University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1995 through September 30, 1998
Project Amount: $339,597
RFA: Exploratory Research - Chemistry and Physics of Air (1995) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Safer Chemicals
Objective:
Clouds are important processors of atmospheric aerosol particles. New particle mass can be formed by reactions of volatile species within cloud drops. For example, dissolved sulfur dioxide can be efficiently oxidized to sulfate in cloud drops. The sulfate is nonvolatile and remains as a particle residue when the cloud drop evaporates. Removal of aerosol particles can also be enhanced by clouds. Particles which are scavenged by cloud drops can be incorporated with the drops into precipitation which falls to the ground.
Traditionally most studies of cloud processing of aerosols have assumed that cloud drops possess a uniform composition. Over the past decade' however, increasing evidence has been provided of the presence of chemical heterogeneity among cloud drop populations. In particular, the composition of cloud drops has been shown to vary with drop size. A few studies have shown the potential influence of chemical heterogeneity on rates of in-cloud sulfate production and on the efficiency with which individual chemical species are scavenged and removed by precipitation. For oxidation pathways which depend nonlinearly on droplet composition, including oxidation by ozone and trace metal catalyzed autooxidation, sulfate production rates in chemically heterogeneous cloud drop populations can significantly exceed rates expected from the average drop composition. In the case of precipitation scavenging, the most effective route for incorporation of aerosol material into precipitation is through impaction scavenging of cloud drops by ice crystals or rain drops. Since the efficiency of drop scavenging increases with drop size, those chemical species found at relatively higher concentrations in large drops will be incorporated into precipitation more efficiently than chemical species which are enriched in smaller cloud drops.
Although there has been a significant increase over the past decade in our knowledge about size-dependent cloud chemistry in warm clouds, very little is known about bulk or drop size-dependent cloud chemistry in supercooled winter clouds. This study was conducted in order to increase our understanding of chemical heterogeneity present among winter cloud drop populations and its effects on aerosol processing by winter clouds.
The objectives of the project were to: ( I ) develop a new cloud sampler capable of collecting three independent cloud drop size fractions from supercooled winter clouds; (2) calibrate the performance of the new sampler; (3) field test the new cloud sampler and use it to collect drop size-resolved cloudwater samples from winter clouds; (4) develop semi-automated methods for characterizing size distributions of rime drops on ice crystal surfaces; (S) characterize variations in the chemical composition of collected cloudwater as a function of drop size; and (6) examine the impacts of the observed chemical heterogeneity present among winter cloud drop populations on aerosol processing in winter clouds. The final objective included looking at effects on new aerosol formation through oxidation of dissolved SO2 to sulfate and effects on aerosol incorporation into precipitation in mixed-phase (ice plus supercooled water) winter clouds.
Summary/Accomplishments (Outputs/Outcomes):
A new cloud collector was developed for use in this project, based upon the principles of cascade impaction using jet impactors. The new collector, known as FROSTY, features three impaction stages, arranged in series, to permit simultaneous collection of three independent drop size fractions. Each stage of the impactor consists of a jet and downstream impaction surface. Droplets are pumped through progressively narrower jets permitting collection of increasingly smaller droplets as the air moves through the collector. Since the supercooled cloud droplets freeze upon contact with an impaction surface, a vertical rectangular jet orientation is used with removable impaction surfaces. At the end of each sampling period, impaction surfaces are easily and quickly removed for sample retrieval.
The performance of the new cloud sampler was evaluated through computational fluid dynamics simulations and laboratory calibration using fluorescein tagged monodisperse oleic acid drops. Good agreement was found between the numerical and experimental results. 50% cut sizes of 4, 10, and 17,um drop diameter were found for the three impaction stages for collection of supercooled cloud drops at 3000 m elevation. Operation of the FROSTY supercooled cloud collector in the field proved its suitability to operation in harsh winter storm environments characteristic of high elevation Rocky Mountain locations.
Semi-automated methods were developed for determining characteristics of rimed snow crystals. Plastic replicas of ice crystals were created using the Formvar technique and later viewed under a stereo zoom microscope coupled with an image analysis system. The image analysis system was used to construct size distributions of cloud drops on ice crystal surfaces.
The primary field campaigns of this project were conducted at the Desert Research Institute's Storm Peak Laboratory (SPL, 3210m elevation) near Steamboat Springs, Colorado. In addition to the cloud chemistry measurements, the field campaigns included measurements of cloud and precipitation micro-physical properties (including cloud drop size distribution, liquid water content, ice crystal habit and riming characteristics), precipitation chemistry, aerosol size distributions, and gaseous hydrogen peroxide concentrations.
Concentrations of major ions in the cloud drops were usually observed to vary with drop size. Ammonium, nitrate and sulfate concentrations were all observed to decrease with increasing drop size; calcium concentrations showed a variable dependence on drop size. Cloud drop pH typically increased with increasing drop size. Differences between large and small drop fractions as large as two pH units were observed during the study. Gaseous soluble peroxide concentrations were typically observed to be a few tenths of a ppbv or less.
The effects of the observed cloud chemical heterogeneity on aqueous phase S(IV) oxidation were calculated and expressed in terms of oxidation enhancement factors (calculated as the average rate of sulfur oxidation in a cloud with three distinct drop compositions divided by the rate of oxidation predicted using the average cloud drop composition). For oxidation by hydrogen peroxide, no enhancement is predicted. For oxidation by ozone, approximately two -thirds of the samples experience enhancement greater than 20%, with one-third of the samples predicted to experience oxidation rate enhancement of at least 500%. For autooxidation catalyzed by Fe(III) and Mn(II), over half of the samples experience enhancement greater than 20%; a small fraction of the samples are predicted to experience metal-catalyzed S(IV) autooxidation rates slower than those expected from the average cloud drop composition. The oxidation rate suppression results from positive correlation of drop size-dependent catalyst and hydrogen ion concentrations. When the rates of all three oxidation paths are summed, approximately 40% of the samples are predicted to experience oxidation rate enhancements deriving from the chemical heterogeneity present among the compositions of the cloud drop population in excess of 20%. Since autooxidation was determined never to dominate the total rate of S(IV) oxidation in this data set, enhancements in aqueous phase sulfur oxidation rates arise primarily from the nonlinear dependence of the oxidation rate for the ozone pathway on the hydrogen ion concentration.
In order to evaluate the effects of drop size-dependent cloud chemistry on precipitation scavenging of individual chemical species during riming, we used observed ice crystal characteristics, observed cloud drop size distributions, and theoretical formulas for cloud drop scavenging by ice crystals to determine the size distribution of cloud drops expected to be scavenged by falling ice crystals at SPL. The composition of this rime drop distribution was then computed using simultaneous observations of size-dependent cloud drop composition. By comparing the composition of the rime drops (Cr) to the average composition of all cloud drops (Cc) we can determine whether scavenging of individual chemical species is enhanced or diminished by the distribution of that species across the drop size spectrum. The ratio Cr/Cc for sulfate, nitrate and ammonium generally fell between 0.75 and 0.99, due to the fact that concentrations of these species increase toward smaller drop sizes while drop collection efficiencies increase toward larger drop sizes. The ratio Cr/Cc for calcium was often greater than one, since calcium concentrations often increased with drop size.
Conclusions:
A new cloud collector (FROSTY) was developed and tested for use in supercooled clouds. FROSTY is a 3-stage cascade impactor capable of simultaneously obtaining three independent drop size fractions of cloud water for chemical analysis. Numerical simulations of FROSTY's collection characteristics, confirmed by laboratory calibrations, reveal 50% size cuts for the three impaction stages corresponding to drop diameters of 4, 10 and 17 um diameter for the operating conditions in the current study.
Significant chemical heterogeneity was observed among winter cloud drop populations in the Rocky Mountains of Colorado. The presence of this heterogeneity tends to enhance aqueous phase sulfate production rates in the clouds. Non-uniform distributions of individual solutes across the drop size spectrum also influence precipitation scavenging of those solutes, with incorporation of accumulation mode aerosol species (sulfate, nitrate, and ammonium) predicted to be up to 25% less efficient than expected from the average cloud drop composition.
Although these findings come from a limited number of studies in one region of the U.S., they suggest the importance of representing chemical heterogeneity among cloud drop populations in model simulations of aerosol processing by winter clouds. More field studies are needed to test how applicable the conclusions of this research are to other time periods and locations. Efforts are also needed to model chemical heterogeneity among winter cloud drop populations in conjunction with experimental measurements that can be used for model verification. This is a challenging problem, given the complex processes occurring in supercooled clouds containing ice crystals.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 18 publications | 3 publications in selected types | All 2 journal articles |
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Collett Jr JL, Bator A, Sherman DE, Moore KF, Hoag KJ, Demoz BB, Rao X, Reilly JE. The chemical composition of fogs and intercepted clouds in the United States. Atmospheric Research 2002;64:29-40. |
R823979 (Final) |
Exit Exit |
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Xu G, Sherman DE, Andrews E, Moore K, Straub D, Hoag K, Collett Jr J. The influence of chemical heterogeneity among cloud drop populations on processing of chemical species in winter clouds. Atmospheric research 1999;51(2):119-140. |
R823979 (Final) |
not available |
Supplemental Keywords:
acid deposition, ambient air, environmental chemistry, measurement methods., RFA, Scientific Discipline, Air, Geographic Area, particulate matter, Environmental Chemistry, Physics, State, Chemistry, Engineering, Chemistry, & Physics, EPA Region, ambient aerosol, particle size, particulates, supercooled cloud, cloud drop composition, precipitation scavenging, chemical composition, pollutant transport, analytical chemistry, chemical kinetics, Region 8, pollution dispersion models, sulfer oxide, Colorado (CO)Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.