GEORGE D. ROBINSON

Center for Environment and Man, Inc.

Particles are a normal constituent of the atmosphere. If we exclude the larger water droplets in clouds or fog that have formed in slightly supersaturated air, particles can properly be called a trace constituent, with a global average mass mixing ratio near the surface of about 10⁻⁸, or 10 μg⁻³, in the units conventionally used in monitoring. We are concerned here with the proportion of this load that can be attributed to man’s activities and with any effects that a change of this proportion might have on climate.

Particles may be characterized as wind-raised dust; wind-raised sea salt; direct products of combustion, soot, ash, condensed organic materials, etc.; indirect products of combustion, i.e., particles formed by chemical reactions in the atmosphere from the gaseous products of combustion— sulfates, organic nitrates, sulfuric and nitric acid; volcanic particles; and particles formed in the atmosphere from such products of plant and animal life and decay as terpenes, H₂S, and NH₃. Table 3.1 summarizes two attempts to estimate the annual production of particles by these various means. The material is several years old, but more recent work has not reduced the uncertainties indicated, which are certainly not overstated: the “nitrate” entry is particularly suspect. Table 3.1 also attempts to separate “natural” and “anthropogenic” sources; there is very little basis on which to make this separation in some categories, particularly “forest fires” and “soil dust.”

Figure 3.1 shows the range of particle radius with which we are concerned and indicates the general nature of the radius-number distributions that are observed.

In the steady state, production and loss of atmospheric particles balance and a “mean residence time” may be defined as the ratio of loading to production rate. The estimate of mean loading given above was obtained from an assumed production of 1.8 × 10¹⁵ g yr⁻¹ and a residence time of