Atmospheric Concentrations of Photochemical Oxidants

The main purpose of this chapter is to survi atmospheric concentrations of photochemical oxidants, with emphasis on surface concentrations and the distribution patterns associated with them. The reason for that em phasis is that the photochemical oxidants that affect public health and welfare are largely concentrated in this region. The whole subject of stratospheric ozone (and its filtering of ultraviolet light and interactions with supersonic-transport exhaust products), nuclear weapon reaction products, and halogenated hydrocarbon decomposition pr ucts is not treated here. 

As in Air Quality Criteria for Photochemical Oxidants,our concern will be with the broad subject of oxidant pollution in the atmosphere and the net oxidizing ability of the contaminants in an air sample. The standard corrections or adjustments to remove interferences from the data have already been implied in the primary references in most cases. Therefore, we need not refer to the early distinction between oxidant or total oxidants and corrected or adjusted oxidant.  

It should be noted that there are still unresolved discrepancies in oxidant data owing to differences in primary standards. lodometric calibration techniques for ozone monitors were compared by an ad hoc committee appointed by the California Air Resources Board (carb). The committee set out to find an accurate method for measuring ozone, to relate the recommended method to earlier data, and to recommend 

This review begins with a summary of the sources of monitoring data operated primarily by public agencies. The spatial and temporal patterns of oxidant concentrations are then discussed—urban versus rural and indoor versus outdoor relationships, diurnal and seasonal patterns, and long-term trends. The chapter includes brief discussions of photochemical oxidants other than ozone and of data quality and concludes with a set of recommendations for guidelines in future monitoring of atmospheric concentrations of ozone and other photochemical oxidants. 

One of the earliest organized efforts to acquire data on photochemical oxidants was that of the Los Angeles County Air Pollution Control District, which began in the middle 1950 s and has produced the largest data base now available for these studies. In 1%1, the California Department of Public Health set up a 16-station Statewide Cooperative Air Monitoring Network (scan). 

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In comparison with previously available material on atmospheric concentrations of photochemical oxidants, we now have a far richer data base and a deeper understanding of how to interpret the reported concentrations. The recent information on hydrogen peroxide and the broader geographic coverage of measurements abroad are examples of new data that have come to light. 

Regional scale episodes of elevated concentrations of photochemical oxidants occur every summer in Europe. During summertime, anticyclonic weather conditions, ozone concentrations steadily build up over several days and may exceed internationally-accepted criteria values (ref) set to protect human health, crops and trees [22]. There are no emissions of ozone into the atmosphere and all the ozone found close to the ground in pollution episodes has been formed there by chemical reactions involving the precursors, hydrocarbons and the nitrogen oxides, in the presence of sunlight. 

A critical question concerning atmospheric concentrations of ozone and other photochemical oxidants is What fraction of the observed values in each locale can be controlled by reduction of emissions Some contend that natural background concentrations exceed the federal ambient air quality standard (0.08 ppm). Another point of view is that background ozone concentrations rarely exceed about 0.05-0.06 ppm at the surface and that higher concentrations are caused by man-made sources. 

In this Chapter we discuss the distribution of DMS and H S in marine air. The discussion focuses on 1) analytical techniques used to obtain the existing data base, 2) the measurements of DMS and H2S over the oceans, and 3) modelling efforts to test current concepts of tropospheric cycling of these compounds. Results from simple box model of the marine boundary layer are presented for comparison of estimated rates of sea/air exchange and photochemical oxidation with atmospheric concentrations of DMS and H2S in the marine boundaiy layer. 

In this contribution the re-evaluated yields from the OH-radical initiated oxidation of benzene, toluene, p-xylene, and initial results of new simulation chamber experiments on prompt glyoxal formation from isoprene oxidation are presented. A detailed discussion of sources, sinks and their uncertainties to model atmospheric concentrations of glyoxal is presented, and exemplifies how basic research in environmental simulation chambers besides giving input for photochemical models also triggers advancements with measurement techniques for field observations. The integration of laboratory and field observations by models in turn will guide future research on atmospheric chemical processes. 

A reaction of ozone provides an example of concentration effects. Ozone in the atmosphere near the Earth s surface is a serious pollutant that damages soft tissues such as the lungs. In major urban areas, smog alerts are issued whenever there are elevated concentrations of ozone in the lower atmosphere. Nitmgen oxide, another component of photochemical smog, is a colorless gas produced in a side reaction in automobile engines. One reaction that links these species is the reaction of NO and O3 to produce O2 and NO2 ... 

Approximately the first third of this report is concerned with the origins and measurement of ozone and other photochemical oxidants and the relationship of atmospheric concentrations to emissions. The middle third deals with toxicologic studies and effects on humans, and the last with effects on plants, ecosystems, and materials. 

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