Explanation of Atmospheric Persistence

This page was last revised on October 22, 1997

There are a number of factors which influence the atmospheric behavior of chemical compounds in the atmosphere, as briefly discussed below:

Gas/Particle Partitioning: Chemical compounds emitted into the atmosphere, or produced in situ in the atmosphere by chemical reaction, can exist in the gas phase, in the particle phase

(i.e., be associated with particulate matter by absorption into, adsorption onto, or inclusion in particles), or be partitioned between the gas- and particle-phases. The major factor determining partitioning of a chemical between the gas phase and particles is the liquid-phase vapor pressure P_L of the chemical (Bidleman, 1988). The gas/particle partitioning ratio depends on temperature, with the chemical becoming more particle-associated at lower temperatures. While the gas/particle partitioning ratio is a function of P_L, it also depends on the particle loading in the atmosphere (Pankow, 1987; Bidleman, 1988). As a useful approximation, a chemical will exist mainly in the particle phase for liquid-phase vapor pressures <10^-6 Torr at the temperature of the air-mass and will exist, at least partially, in the gas phase for values of P_L >10^-6 Torr at the temperature of the air-mass.

Lifetimes and Half-lives of Chemicals in the Atmosphere: Chemical compounds (whether in the gas- or particle-phase) are removed from the atmosphere by physical processes (wet and dry deposition, see below) and chemical processes. For a gas-phase chemical, the major chemical loss process include photolysis, reaction with the hydroxyl (OH) radical during daylight hours, reaction with the nitrate (NO₃) radical during nighttime hours, and reaction with ozone (O₃) [generally throughout the entire 24-hr period].

The atmospheric lifetime, r, of a gaseous chemical is the time for the concentration of the chemical to decrease to 37% (1/e) of its initial concentration, and the overall lifetime t _overall is given by

1/r _overall = 1/r _physical + 1/r _chemical

where

1/r _physical = 1/r _{wet deposition} + 1/r _{dry deposition}

and

1/r _chemical = 1/r _OH + 1/r _NO3 + 1/r _O3 + 1/r _Photolysis

and, for example,

r _OH = (k _OH[OH])^-1

where k_OH is the rate constant for the chemical reaction with the OH radical and [OH] is the OH radical concentration in the atmosphere, and similarly for the NO₃ radical and O₃ reactions.

The half-life, t _½, is the time for the concentration of the chemical to decrease by 50% (i.e., to 50% of its initial concentration) and is related to the lifetime, r, by

t _½ = 0.693r

Therefore, to calculate the overall lifetime or half-life of a chemical, or the lifetime or half-life due to each of the various loss processes, the reaction rate constants need to be known together with the ambient atmospheric concentrations of OH radicals, NO₃ radicals, and O₃.

All gaseous organic compounds except the chlorofluorocarbons (CFCs) react with the hydroxyl radical and for the majority of organic compounds reaction with the hydroxyl radical is the dominant chemical loss process in the atmosphere. To date, there are few direct measurements of the concentrations of OH radicals in the troposphere, and the apparently reliable measurements available are all at close to ground level. A diurnally, seasonally and annually averaged global tropospheric OH radical concentration has been derived by Prinn et al. (1992) from the emissions and atmospheric concentrations of methyl chloroform, of 8 x 10⁵ molecules/cm^-3 as a 24-hr average. The OH radical concentration exhibits a diurnal cycle because a major production route involves the photolysis of ozone in the presence of water vapor, and OH radical concentrations peak during daylight hours, with peak concentrations of several x 10⁶ molecules/cm^-3. To date, few measurements have been made in polluted urban areas, but the limited information available indicate that OH radical concentrations in urban areas are similar to those in cleaner, more remote areas. This is consistent with the fact that in polluted urban areas, while the formation rates of OH radicals may be increased, so are the loss rates. Therefore, use of the global tropospheric average OH radical concentration is probably reasonably applicable to urban areas and will not underestimate urban airmass OH radical concentrations by more than a factor of about 2-3.

Wet and Dry Deposition of Particles and Particle-Associated Chemicals: Wet deposition refers to the removal of particles and particle-associated chemicals from the atmosphere by precipitation events (i.e., rain- and snow-falls and precipitating fogs). Wet deposition of particles, and hence of particle-associated chemicals, is efficient. Dry deposition refers to the transport of gases and particles from the atmosphere to the Earth's surface, including to soil, vegetation, water surfaces (lakes, rivers and oceans) and to snow-covered ground. Dry deposition of particles and particle-associated chemicals depends on the particle size, and the efficiency of dry deposition of particles is a minimum for particles of diameter about 0.1 to 2 um. Therefore, the atmospheric residence time of particles depends on particle size, and is a maximum for particles of this 0.1-2 um diameter size range.

From atmospheric observations and modeling of particle-associated ²¹⁰Pb, Balkanski et al. (1993) derive an average residence time (lifetime) for 0.1-1 um size particles in the troposphere of 5-15 days due to wet and dry deposition. The lifetimes depend on latitude and altitude in the troposphere, with particles at 0.5 km altitude having a significantly shorter (by a factor of about 4) lifetime than those at 9 km altitude (Balkanski et al, 1993).