No abstract

No abstract

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.

Concern about the apparent increase in the acidity of rainfall from the 1950s to the 1970s prompted reexamination of data from the intermittent, short-term sampling networks that are the basis of the trend estimates. A reassessment of precipitation chemistry data for the mid-1950s reveals excessively high values of calcium and magnesium in comparison with current measurements. The most likely explanation is the severe drought and duststorms that much of the United States experienced in the 1950s. When these excess soil loadings are adjusted within reason to nondrought conditions, newly calculated pH values for this period are not much different from those in recent years. These results suggest that the downward pH trend due to the increase in acid-forming emissions since the mid-1950s is much smaller than previously estimated. must be compared with data obtained at another time. Obviously, the individual measurements must be comparable in quality for a valid comparison. Preferably, identical instrumentation for sample collection and identical chemical analytical methods should be employed throughout the period over which the trend is to be calculated. A number of advances have been made in precipitation sampling, handling, and analysis methods since the 1953-56 data were acquired. A factor of major importance, because of the relatively short existence of sampling networks and stations, is the climatic representativeness of the periods of sampling. All of these variables will be considered to present an interpretation of changing precipitation quality in the eastern United States over the past 25 years.

No abstract