Handbook of Solvents - George Wypych - ChemTech - Ventech!

1190 Michelle Bergin, Armistead Russell
17.4.2.2 Formation of secondary compounds
In addition to transport, organic compounds emitted into the air may also participate in complex
sets of chemical reactions. While many of these reactions “cleanse” the atmosphere
(most organic compounds ultimately react to form carbon dioxide), a number of undesirable
side effects may also occur. Such adverse impacts include the formation of respiratory irritants
and the destruction of protective components of the atmosphere. Ozone is a classic example
of the complexity of secondary atmospheric impacts. Ozone is a highly reactive
molecule consisting of three oxygen atoms (O3). In one part of the atmosphere ozone is beneficial,
in another, it is a pollutant of major concern. Solvents and other organic emissions
may either increase or decrease ozone concentrations, depending on the compound, location
of reaction, and background chemistry. The mechanisms of some adverse secondary responses
are discussed below.
17.4.2.3 Spatial scales of secondary effects
Two layers of the Earth’s atmosphere are known to be adversely impacted by solvents - the
troposphere and the stratosphere. These two layers are closest to Earth, and have distinct
chemical and physical properties. The troposphere (our breathable atmosphere) is the closest
layer, extending from the Earth to a height of between 10 to 15 km. The rate of chemical
reaction generally determines the spatial scale over which emissions have an impact in the
troposphere. Most non-halogenated solvents have lifetimes of a week or less, and elevated
concentrations will only be found near the sources. 4 Compounds that do not react rapidly in
the troposphere (e.g., CFCs) are relatively uniformly distributed, and may eventually reach
the stratosphere. The stratosphere is the next vertical layer of the atmosphere, extending
from the tropopause (the top of the troposphere) to about 50 km in altitude. Little vertical
mixing occurs in the stratosphere, and mixing between the troposphere and the stratosphere
is slow.
Impacts on the stratosphere can be considered global in scale, while impacts on the
troposphere are generally urban or regional in scale. Distinct chemical systems of interest
concerning solvents in the atmosphere are stratospheric ozone depletion, global climate
change, and tropospheric photochemistry leading to enhanced production of ozone, particulate
matter, and other secondary pollutants such as organonitrates.
17.4.2.3.1 Global impacts
Because some solvent compounds are nearly inert, they can eventually reach the stratosphere
where they participate in global scale atmospheric dynamics such as the destruction
of stratospheric ozone and unnatural forcing of the climate system. Stratospheric ozone depletion
by chlorofluorocarbons (CFCs) is a well-known example of global scale impacts.
CFCs were initially viewed as environmentally superior to organic solvents. They are generally
less toxic than other similarly acting compounds, less flammable and are virtually inert
in the troposphere. Replacing solvents using volatile organic compounds (VOCs) with
CFCs was hoped to reduce the formation of tropospheric ozone and other secondary pollutants.
Because of their inert properties, there are no effective routes for the troposphere to remove
CFCs, and, over the decades, emissions of CFCs have caused their accumulation,
enabling them to slowly leak into the stratosphere. In the stratosphere, the strong ultraviolet
(UV) light photodissociates CFCs, releasing chlorine, which then catalytically attacks
ozone. CFC use has been largely eliminated for that reason. Partially halogenated organic
solvents do not contribute as seriously to this problem since they react faster in the tropo-