The Atmosphere

On a mesoscale (Figure 3.2) as temperature varies with altitude, so does density. In general, the air grows progressively less dense as we move upward from the troposphere through the stratosphere and the chemosphere to ionosphere. In the upper reaches of the ionosphere, the gaseous molecules are few and far between as compared with the troposphere.

The ionosphere and chemosphere are of interest to space scientists because they must be traversed by space vehicles en route to or from the moon or the planets, and they are regions in which satellites travel in the Earth’s orbit. These regions are of interest to communications scientists because of their influence on radio communications and they are of interest to air pollution scientists primarily because of their absorption and scattering of solar energy, which influences the amount and spectral distribution of solar energy and cosmic rays reaching the stratosphere and troposphere.

The stratosphere is of interest to aeronautical scientists because it is traversed by airplanes; to communications scientists because of radio and television communications; and to air pollution scientists because global transport of pollution, particularly the debris of aboveground atomic bomb tests and volcanic eruptions occur in this region and because absorption and scattering of solar energy also occur there. The lower portion of this region contains the stratospheric ozone layer which absorbs harmful UV solar radiation. Global change scientists are interested in modifications of this layer by long‐term accumulation of chlorofluorocarbons and other gases released at the Earth’s surface or by high‐altitude aircraft.

The troposphere is the region in which we live and is the primary focus of this book.

Unpolluted Air

The gaseous composition of unpolluted tropospheric air is given in Table 3.7. Unpolluted air is a concept, i.e., what the composition of the air would be if humans and their works were not on Earth. We will never know the precise composition of unpolluted air because by the time we had the means and the desire to determine its composition, humans had been polluting the air for thousands of years. Now even at the most remote locations at sea, at the poles, and in the deserts and mountains, the air may be best described as dilute polluted air. It closely approximates unpolluted air, but differs from it to the extent that it contains vestiges of diffused and aged human‐made pollution.

The real atmosphere is more than a dry mixture of permanent gases. It has other constituents‐vapor of both water and organic liquids and particulate matter held in suspension. Above their temperature of condensation, vapor molecules act just like permanent gas molecules in the air. The predominant vapor in the air is water vapor. Below its condensation temperature, if the air is saturated, water changes from vapor to liquid. We are all familiar with this phenomenon because it appears as fog or mist in the air and as condensed liquid water on windows and other cold surfaces exposed to air. The quantity of water vapor in the air varies greatly from almost complete dryness to super‐saturation, i.e., between 0 and 4% by weight. Gaseous composition in Table 3.7 is expressed as parts per million by volume – ppm (vol) (when a concentration is expressed simply as ppm).

Mobile Sources and Emission Inventory

Generally, mobile sources imply transportation, but sources such as construction, equipment, gasoline‐powered lawn mowers, and gasoline‐powered tools are included in the category. Mobile sources, therefore, consists of many different types of vehicles powered by engines using different cycles, fueled by a variety of products and emitting varying amounts of both simple and complex pollutants. The emissions from a gasoline‐powered vehicle come from many sources. With most of today’s automobiles using unleaded gasoline, lead emissions are no longer a major concern.

An emission inventory is a list of the amount of pollutants from all sources entering the air in a given time period. The boundaries of the area are fixed. The emission inventories are very useful to control agencies as well as planning and zoning agencies. They can point out the major sources whose control can lead to a considerable reduction of pollution in the area. They can be used with appropriate mathematical models to determine the degree of overall control necessary to meet ambient air quality standards. They can be used to indicate the type of sampling network and the locations of individual sampling stations if the areas chosen are small enough. For example, if an area uses very small amounts of sulfur‐bearing fuels, establishing an extensive SO₂ monitoring network in the area would not be an optimum use of public funds. Emission inventories can be used for publicity and political purposes: “If natural gas cannot meet the demands of our area, we will have to burn more high‐sulfur fuel, and the SO₂ emissions will increase by 8 tons per year.”

The method used to develop the emission inventory does have some elements of error, but the other two alternatives are expensive and subject to their own errors. The first alternative would be to monitor continually every major source in the area. The second method would be to monitor continually the pollutants in the ambient air at many points and apply appropriate diffusion equations to calculate the emissions. In practice, the most informative system would be a combination of all three, knowledgeably applied.

The US Clean Air Act Amendments of 1990 (CAAA) strengthened the emission inventory requirements for plans and permits in non‐attainment areas. The amendments state:

INVENTORY – Such plan provisions shall include a comprehensive, accurate, current inventory of actual emissions from all sources of the relevant pollutant or pollutants in such area, including such periodic revisions as the Administrator may determine necessary to assure that the requirements of this part are met.

IDENTIFICATION AND QUANTIFICATION – Such plan provisions shall expressly identify and quantify the emissions, if any, of any such pollutant or pollutants which will be allowed, from the construction and operation of major new or modified stationary sources in each such area. The plan shall demonstrate to the satisfaction of the Administrator that the emissions quantified for this purpose will be consistent with the achievement of reasonable further progress and will not interfere with the attainment of the applicable national ambient air quality standard by the applicable attainment date.

Inventory Techniques

To develop an emission inventory for an area, one must (i) list the types of sources for the area, such as furnaces, automobiles, and home fireplaces; (ii) determine the type of air pollutant emission from each of the listed sources, such as particulates and SO₂; (iii) examine the literature to find valid emission factors for each of the pollutants of concern (e.g. “particulate emissions for open burning of waste wood or sawdust are 10 kg per ton of residue consumed”); (iv) through an actual count, or by means of some estimating technique, determine the number and size of specific sources in the area (the number of steelmaking furnaces can be counted, but the number of home fireplaces will probably have to be estimated); and (v) multiply the appropriate numbers from (iii) and (iv) to obtain the total emissions and then sum the similar emissions to obtain the total for the area.

A typical example will illustrate the procedure. Suppose we wish to determine the amount of carbon monoxide from oil furnaces emitted per day, during the heating season, in a small city of 50 000 population:

Table 3.7 The gaseous composition of unpolluted air (dry basis).

	ppm (vol)	μg/m3
Nitrogen	780 000	8.95 × 108
Oxygen	209 400	2.74 × 108
Water	—	—
Argon	9 300	1.52 × 107
Carbon dioxide	315	5.67 × 105
Neon	18	1.49 × 104
Helium	5.2	8.50 × 102
Methane	1.0–1.2	6.56–7.87 × 102
Krypton	1.0	3.43 × 103
Nitrous oxide	0.5	9.00 × 102
Hydrogen	0.5	4.13 × 101
Xenon	0.08	4.29 × 102
Organic vapors	0.02	—

1. The source is oil furnaces within the boundary area of the city.
2. The pollutant of concern is carbon monoxide.
3. Emission factors for carbon monoxide are listed in various ways (240 gm/1000 l of fuel oil, 50 gm/day/burner, 1½% by volume of exhaust gas, etc.). For this example, use 240 gm/1000 l of fuel oil.Fuel oil sales figures, obtained from the local dealers association, average 40 000 l/day.

Data Reduction and Compilation

The final emission inventory can be prepared on a computer. This will enable the information to be stored on magnetic tape or disk so that it can be updated rapidly and economically as new data or new sources appear. The computer program can be written so that changes can easily be made. There will be times when major changes occur and the inventory must be completely changed. Imagine the change that would take place when natural gas first becomes available in a commercial‐residential area which previously used oil and coal for heating.

To determine emission data, as well as the effect that fuel changes would produce, it is necessary to use the appropriate thermal conversion factor from one fuel to another. Table 3.2 lists these factors for fuels in common use.

A major change in the emissions for an area will occur if control equipment is installed. This can be shown in the emission inventory to illustrate the effect on the community.

By keeping the emission inventory current and updating it at least yearly as fuel uses change, industrial and population changes occur and control equipment is added, a realistic record for the area is obtained.