Donald F. Gatz

Relative contributions of different sources of urban aerosols: Application of a new estimation method to multiple sites in Chicago

A new method for estimating the percentage contributions (source coefficients) of various sources to the total atmospheric aerosol content requires detailed data on the chemical composition of the urban aerosol and emissions from the various sources considered. Enough data are now available for Chicago to permit a tentative estimation of source coefficients for the following aerosol sources: automobiles, fuel oil buring, cement manufacturing, iron and steel manufacturing, coal burning, and wind-raised soil dust. Comparison of estimated source coefficients for Chicago and published results for the Los Angeles area indicates that the method gives reasonable results. This paper also examines the possibility of applying the method to multiple stations within a single city or region.

The standard error of a weighted mean concentration—I. Bootstrapping vs other methods

Concentrations of chemical constituents of precipitation are frequently expressed in terms of the precipitation-weighted mean, which has several desirable properties. Unfortunately, the weighted mean has no analytical analog of the standard error of the arithmetic mean for use in characterizing its statistical uncertainty. Several approximate expressions have been used previously in the literature, but there is no consensus as to which is best. This paper compares three methods from the literature with a standard based on bootstrapping. Comparative calculations were carried out for nine major ions measured at 222 sampling sites in the National Atmospheric Deposition/National Trends Network (NADP/NTN). The ratio variance approximation of Cochran (1977) gave results that were not statistically different from those of bootstrapping, and is suggested as the method of choice for routine computing of the standard error of the weighted mean. The bootstrap method has advantages of its own, including the fact that it is nonparametric, but requires additional effort and computation time.

Pollutant aerosol deposition into Southern Lake Michigan

Current estimates of pollutant aerosol input to southern Lake Michigan are based on a single calculated emission inventory and various estimates of the fraction of emissions that enter the Lake. Alternative, but still crude, estimates of urban elemental emissions and their wet and dry deposition in the lake are made here. Observed elemental concentrations in urban air are used to calculate emissions and recently measured wet and dry deposition parameters are used to calculate deposition.All available treatments conclude that atmospheric inputs of at least Fe, Pb, Ti, and V are sizable fractions of total lake input of these metals. This study suggests tentatively that 1) wet and dry inputs from the atmosphere are about equal, 2) between 3 and 15 % of elemental pollutant emissions from Chicago and NW Indiana enter the Lake, and 3) this fraction increases with particle size.

A large silicon-aluminum aerosol plume in Central Illinois: North African desert dust?

Abstract During a summer atmospheric chemistry field project in central Illinois in 1979, unusually high concentrations of Si, Al, and other earths crust elements were observed simultaneously in two series of 2-h aerosol samples collected at sites separated by about 20 km. The SiAl “event” persisted for about 32 h, and was accomparded by winds shifting from east through north as a weak low pressure area moved eastward across southern Illinois, and a nearly stationary front moved back and forth across the study area. Comparison of aerosol Si/Al and Ca/Al ratios during the event with those in possible sources showed that local soils, coal flyash, and Al smelter emissions were unlikely sources of the observed plume, and focused the investigation on distant dust sources. Climatology favors north Africa over the western United States as a source of desert dust in summer. Forty-eight hour back-trajectories from Illinois show flow from the Gulf of Mexico. Temperature and humidity soundings at Key West, Florida, three days before the onset of the dust plume in Illinois, showed a typical “Sahara air” profile, and simultaneous mineral dust measurements at Miami indicated a strong influx of North African dust at the same time. The evidence indicates that North African dust is the most likely source of the observed high concentrations of Si and Al in central Illinois in July 1979, and illustrates that long range transport can influence air quality at great distances from source areas.

Emissions of alkaline elements calcium, magnesium, potassium, and sodium from open sources in the contiguous United States

Models of dust emissions by wind erosion (including winds associated with regional activity as well as dust devils) and vehicular disturbances of unpaved roads were developed, calibrated,and used to estimate alkaline dust emissions from elemental soil and road composition data. Emissions from tillage of soils were estimated from the work of previous researchers. The area of maximum dust production by all of those sources is the area of the old “Dust Bowl” of the 1930s (the panhandles of Texas and Oklahoma, eastern New Mexico and Colorado, and western Kansas). The areas of maximum alkaline dust production are the arid southwest, the “Dust Bowl,” and the midwestern-mideastern states from Iowa to Pennsylvania. Our calculations show that calcium is the dominant alkaline element produced by “open sources” (sources too great in extent to be controlled by enclosure or ducting). Although the largest dust mass source is wind erosion (by winds associated with regional activity and convective activity), the largest producer of the alkaline component is road dust because the abundance of alkaline materials in road coverings (which include crushed limestone) is significantly higher than for soils. Comparing the above estimated sources of alkaline material with inventories of SO2 and NOx emissions by previous investigators gives the rough approximation that alkaline emission rates are of the order of the SO2 + NOx emissions in the western United States and that they are much smaller than SO2 + NOx in the eastern United States. This approximation is substantiated by data on Ca/(SO4 + NO3) for wet deposition for National Atmospheric Deposition Program sites.

The role of alkaline materials in precipitation chemistry: A brief review of the issues

The purpose of this paper is to provide a brief overview of the relatively unexplored role of alkaline materials in precipitation chemistry. Clearly, they can play fully as important a role as acidic materials in determining pH. The first major issue concerns sources. Comparison of Ca/K and Ca/Mg ratios in precipitation and dry deposition with those in likely sources indicates that both unpaved roads and soils make important contributions. Elemental emissions fluxes have been derived from literature estimates of mass emissions fluxes and element abundances in the important sources, but are subject to large uncertainties owing to a lack of adequate data. It is quite clear, however, that conventional (smokestack) sources are minor compared to open sources such as those already identified. Another major issue concerns interactions between alkaline aerosols and water in the atmosphere. Reactions involving suspended solids that lead to removal of H-ions from solution include ion exchange and mineral weathering. A simplified model of acid buffering indicates that NH4, Ca, Mg, K, and Na buffer between 25 and 50% of the potential acids in U.S. precipitation everywhere east of the Mississippi River. The third major issue concerns wet and dry deposition fluxes of alkaline materials. Wet deposition fluxes are currently being measured adequately by a nationwide network of weekly samplers. There is no agreed-upon method for monitoring dry deposition, but available information suggests that dry deposition accounts for somewhat more than half of the Ca deposition. A list of research and data needs is also provided.

The standard error of a weighted mean concentration—II. Estimating confidence intervals

One motivation for estimating the standard error, SEMw, of a weighted mean concentration, Mw, of an ion in precipitation is to use it to compute a confidence interval for Mw. Typically this is done by multiplying the standard error by a factor that depends on the degree of confidence one wishes to express, on the assumption that the weighted mean has a normal distribution. This paper compares confidence intervals of Mw concentrations of ions in precipitation, as computed using the assumption of a normal distribution, with those estimated from distributions produced by bootstrapping. The hypothesis that Mw was normally distributed was rejected about half the time (at the 5% significance level) in tests involving nine major ions measured at ten diverse sites in the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). Most of these rejections occurred at sites with fewer than 100 samples, in agreement with previous results. Nevertheless, the hypothesis was often rejected at sites with more than 100 samples as well. The maximum error (relative to Mw) in the 95% confidence limits made by assuming a normal distribution of the Mw at the ten sites examined was about 27%. Most such errors were less than 10%, and errors were smaller at sampling sites with > 100 samples than at those with < 100 samples.

A review of chemical tracer experiments on precipitation systems

Abstract Experiments involving the introduction of materials into precipitation systems for the purpose of tracing the motions of air and/or hydrometeors within them began in the U.S.S.R. in 1964. Since then approximately 100 such experiments have been performed. Most of these experiments have been carried out on convective clouds, but the frontal layer-type cloud systems of the northwest U.S. coast have also been the object of tracer experiments. The tracers have been released mostly from aircraft, but ground generators and rockets have also been used. At least 13 different elements or isotopes have been employed as tracers. Present results generally claim nearly complete removal of tracer by the precipitation system and very complex mesoscale transport before deposition. However, the validity of these claims depends on the experimenters ability to identify and quantify his tracer in the presence of natural background and to estimate deposition accurately. Neither of these subjects has received adequate attention in most of the available literature. However, it is shown that accuracy of deposition estimates should not be a problem in heavy rain events measured by square grid networks with gage spacing of 8 km or less, if tracer background has been adequately assessed.

Source apportionment of rain water impurities in central Illinois

Abstract The identity and relative contributions of various sources of impurities in precipitation are needed to understand the acidic precipitation phenomenon. This paper reports results of using factor analysis and a chemical element balance (CEB) to apportion sources of impurities in a series of 191 wet-only event precipitation samples collected near Champaign, IL. Factor analysis showed that four major groups of constituents accounted for 86% of the variance in the data: crustal dust, pollutants with gaseous precursors, sea salt, and possibly strong acids. Apportionment of plausible sources by CEB yielded estimates of a 2% contribution of sea salt to the total mass of impurities, a 32% contribution by ammonium and sulfates (including sulfuric acid) and a 16% contribution from nitrates. Calculated results were more sensitive to spatial variations in crustal source composition than to differences in composition between bulk materials and ‘source aerosols.’ Without taking account of insoluble materials, the probable contributions of soil dust and road dust were 7 and 12%, respectively, with outside limits of 3–8% and 8–24%, respectively. A reasonable assumption regarding Ca and K solubility increased the likely soil contribution from 7 to 15%, but had little effect on the road dust contribution. The use of extracted source material compositions as input to the CEB analyses to compensate for analysis of only the soluble portion of the precipitation samples was not successful.

Water solubility of atmospheric aerosols

Abstract The water solubility of large aerosol particles was measured. Two eight-stage Andersen impactors were used to collect aerosol for a 10-day period. Laboratory analysis determined that approx 30% of the material on the first two stages (≥ 4.6 μm radius) of the impactor was soluble. These particles would have been effective cloud condensation nuclei.