Atmospheric Environment (1967)最新文献

Major ion and trace metal (Zn, Cu, Cd, Pb, Fe, Mn) concentrations have been determined in 32 rainfall events at a North Sea coastal site over a period of 14 months (June 1987–July 1988). Precautions have been taken to avoid trace metal contamination. Trace metal depositions are positively correlated to excess sulphate deposition; the exception is Cd for which no clear relationship is evident. This is probably due to contamination of the samples with respect to Cd.

The highest trace metal concentrations are associated with small rainfall events and with the occurrence of fog and mist. Trace metal concentrations (volume weighted average) are in general agreement with earlier data. For Cd and Zn, however, they are lower and consequently it is suggested that estimates of annual deposition be revised downwards.

Interception of cloudwater by forests in the Sierra Nevada Mountains may contribute significantly to acidic deposition in the region. Cloudwater sampled in Sequoia National Park had pH values ranging from 4.4 to 5.7. The advance of cold fronts into the Park appears to lead to higher aerosol and gas phase concentrations than are seen under normal mountain-valley circulations, producing higher cloud-water concentrations than might otherwise be expected. Estimates of annual deposition rates of NO₃⁻, SO₄²⁻, NH₄⁺ and H⁺ due to cloudwater impaction are comparable to those measured in precipitation.

A photochemical model has been used to quantify the sensitivity of the tropospheric oxidants O₃ and OH to changes in CH₄, CO and NO emissions and to perturbations in climate and stratospheric chemistry. Coefficients of the form ∂1n[O₃]/∂1n[X] and ∂1n[OH]/∂1n[X], where [X] = flux of CH₄, CO, NO; stratospheric O₃ and H₂O have been calculated for a number of “chemically coherent” regions (e.g. nonpolluted continental, nonpolluted marine, urban) at low and middle latitudes. Sensitivities in O₃ and OH vary with regional emissions patterns and are nonlinear within a given region as [X] changes. In most cases increasing CH₄ and CO emissions will suppress OH (negative coefficients) and increase O₃ (positive coefficients) except in areas where NO and O₃ influenced by pollution are sufficient to increase OH. Stratospheric O₃ depletion will tend to decrease O₃ (except in high NO_x areas) and increase OH through enhanced u.v. photolysis. Increased levels of water vapor (one possible outcome of a global warming) will also decrease O₃ and increase OH. We conclude that in most regions, NO, CO and CH₄ emission increases will suppress OH and increase O₃, but these trends may be opposed by stratospheric O₃ depletion and climate change. A regional survey of OH and O₃ levels suggests that the tropics have a pivotal role in determining the earth's future oxidizing capacity.

A simple General Finite Line Source Model (GFLSM), based on the Gaussian diffusion equation is formulated so that it could be used for any orientation of wind direction with roadway and also does not have the infinite line source constraint of the General Motors (GM) model. The GFLSM is also modified to predict particulate concentrations by incorporating some simple corrections. The GFLSM, along with some other models viz. GM, CALINE-3 and HIWAY-2, has been applied to predict carbon monoxide (CO) concentrations near two traffic junctions in Bombay city. Staistical analysis of the predicted and observed (coilected by Municipal Corporation of Greater Bombay) CO levels reveals that the GFLSM performs better than other models when the line source is relatively finite in length. The modified GFLSM for particulates along with the GM model has been evaluated against the data base prepared by the New York State Department of Environmental Conservation and it is observed that the new model gives satisfactory results.

Oxidation reactions of SO₂ and NO₂ in clouds are considered important atmospheric acid formation processes. This paper evaluates the feasibility of detecting the occurrence of these oxidation reactions in natural clouds by means of field measurements. This evaluation is performed by calculating the changes expected in reagent and product concentrations resulting from these reactions in representative cloud types, and comparing those changes with concentration differences detectable by available analytical methods in the context of typical atmospheric variability. Four in-cloud oxidation reactions are considered: aqueous-phase reactions of SO₂ with O₃ and with H₂O₂, and gas-phase reactions of NO₂ with OH radical and with O₃, the latter leading to acid formation by reaction of N₂O₅ with cloud liquid water. The cloud types considered are fog, stratus, cumulus and mountain lee wave. This evaluation indicates that oxidation of SO₂ by H₂O₂ should be detectable in a wide variety of cloud conditions, but that oxidation of SO₂ by O₃ is unlikely to be detected by field measurements. Oxidation of NO₂ may be detectable in fog and stratus clouds, which provide long in-cloud residence times. The paper includes discussion of factors which favor or hinder detection of acid producing reactions in clouds, and reviews evidence from published field studies on the occurrence of these reactions.

The General Motors mobile Atmospheric Research Laboratory was situated in rural western Massachusetts in the Berkshire Mountains for 62 days during the summer of 1984. One purpose of this study was to determine the source regions of wet acid deposition for this northeastern U.S. location. First, to apportion the precursors (sulfate, sulfur dioxide and nitrate) to source regions, daily ambient air samples were analyzed for the precursors as well as for tracer species that are associated with particular sources. Factor and trajectory analyses were then used to deduce the contributions of the Midwest and the Northeast to these precursors. Finally, the contribution of the precursors to precipitation acidity was estimated by analyzing the chemical constituents in the rain during seven precipitation events. Averaged over the entire duration of the study, the data show that Northeast sources accounted for about 60% of the precipitation sulfate and nitrate, while Midwest sources accounted for about 30%. The balance (~ 10%) was accounted for by background sulfate. A more useful way of examining the data is according to the type of storm that caused the wet deposition. The site was affected by two basic types of storms: coastal low-pressure systems that traveled up the Atlantic Coast, and cold fronts that approached from the west. During the coastal lowpressure events, the Midwestern contribution to precipitation acidity was zero, as easterly flows from the Atlantic Ocean dominated. The cold front events, however, were all associated with southwesterly flows, and the Midwest contributions exceeded the Northeast contributions. During these events, the average Midwest contribution to precipitation acidity was about 50%. For all events, the ratio of sulfate to nitrate was approximately 2:1 on an equivalent basis. During the coastal lows, the relative nitrate contributions were the highest. It was estimated that particulate sulfate scavenging was responsible for about half of the sulfate in the rain, while the other half was due to in-cloud oxidation of gaseous sulfur dioxide.

This paper describes the development of a severe SO² smog episode that occurred over central Europe during mid-January 1985. Data presented here summarize the knowledge about this episode as was available at an OECD workshop in October 1985. Spatial and temporal extent of the episode are assessed, emission reduction measures that were taken are described and their effectiveness is evaluated.