Handbook of Solvents - George Wypych - ChemTech - Ventech!

1188 Michelle Bergin, Armistead Russell
17.4 ORGANIC SOLVENT IMPACTS ON TROPOSPHERIC AIR
POLLUTION
Michelle Bergin and Armistead Russell
Georgia Institute of Technology, Atlanta, Georgia, USA
17.4.1 SOURCES AND IMPACTS OF VOLATILE SOLVENTS
Solvents, either by design or default, are often emitted in to the air, and the total mass of
emissions of solvents is not small. In a typical city in the United States, solvents can rival
automobile exhaust as the largest source category of volatilized organic compound (VOC)
emissions into the atmosphere. 1 In the United Kingdom, solvent usage accounted for 36%
of the estimated total VOC mass emissions in 1995. 2 Such widespread emissions leads to increased
concentrations of many different compounds in the ambient environment, and their
release has diverse impacts on air quality.
A large variety of solvent-associated compounds are emitted, many of which are hydrocarbons,
oxygenates. Those solvents may have multiple atmospheric impacts. For example,
toluene is potentially toxic and can reach relatively high concentrations at small spatial
scales, such as in a workplace. Toluene also contributes to the formation of tropospheric
ozone at urban scales, while at regional scales toluene can lower the rate of tropospheric
ozone formation. Other solvents likewise can have a range of impacts, ranging from local
contamination to modification of the global climate system.
This diversity of potential impacts is due, in part, to differences in the chemical properties
and reactions that a compound may undergo in the atmosphere, differences in emissions
patterns, and differences in the spatial and temporal scales of atmospheric
phenomena. Transport and fate of chemical species is closely tied to the speed at which the
compound degrades (from seconds to centuries, depending on the compound) as well as to
the environmental conditions in which the compound is emitted. If a compound degrades
very quickly, it may still have toxic effects near a source where concentrations can be high.
In contrast, extremely stable compounds (such as chlorofluorocarbons; CFCs) are able to
circumvent the globe, gradually accumulating to non-negligible concentrations. 3
Of the myriad of solvents emitted into the air, the ones of primary concern are those
with the greatest emissions rates, and/or those to which the environment has a high sensitivity.
Compounds with very large emissions rates include tri- and tetrachloroethylene (e.g.,
from dry-cleaning), aromatics (benzene, toluene and xylenes, e.g., from coatings), alcohols,
acetone and, historically, CFCs. While those compounds are often emitted from solvent use,
other applications lead to their emission as well. For example, gasoline is rich in aromatics
and alkanes, and in many cases fuel use dominates emissions of those compounds. CFCs
have been used as refrigerants and as blowing agents. This diversity of originating sources
makes identifying the relative contribution of solvents to air quality somewhat difficult
since there are large uncertainties in our ability to quantify emissions rates from various
source categories.
Solvents with a high environmental sensitivity include benzene (a potent carcinogen),
xylenes (which are very effective at producing ozone), formaldehyde (both toxic and a
strong ozone precursor), and CFCs (ozone depleters and potential greenhouse gases). Most
of the solvents of concern in terms of impacting ambient air are organic, either hydrocarbons,
oxygenated organics (e.g., ethers, alcohols and ketones) or halogenated organics (e.g.,
dichlorobenzene). Some roles of these compounds in the atmosphere are discussed below.