N 2 + O 2 → 2NO (8.6.7)
to produce NO. Combustion of fuels that contain organically bound nitrogen, such as
coal, also produces NO, some of which is converted to NO 2 in the atmosphere.
An extremely important aspect of nitrogen dioxide in the troposphere is that when
it is exposed to electromagnetic radiation of wavelengths below 398 nm it undergoes
photodissociation
NO 2 + hν → NO + O (8.6.8)to produce highly reactive O atoms. The O atoms can participate in a series of chain reactions
through which NO is converted back to NO 2 , which can undergo photodissociation again
to start the cycle over. Nitrogen dioxide is very reactive, undergoing photodissociation
within a minute or two in direct sunlight.
Nitrogen dioxide, NO 2 , is significantly more toxic than NO, although concentrations
of NO 2 in the outdoor atmosphere rarely reach toxic levels. Accidental releases of NO 2
can be sufficient to cause toxic effects or even death. Brief exposures to 50–100 parts
per million (ppm) of NO 2 in air inflames lung tissue for 6-8 weeks followed by recovery.
Exposure to 500 ppm or more of NO 2 causes death within 2–10 days. Exposure to 100-
500 ppm of NO 2 causes a lung condition with the ominous name of bronchiolitis fibrosa
obliterans that is fatal within 3–5 weeks after exposure. Fatal incidents of NO 2 poisoning
have resulted from accidental release of the gas used as an oxidant in rocket fuels and
from burning of nitrogen-containing celluloid and nitrocellulose moving picture film
(long banned because of catastrophic fires that killed numerous people). Plants exposed
to nitrogen dioxide may suffer decreased photosynthesis, leaf spotting, and breakdown
of plant tissue.
The release of NO from combustion sources can be reduced by limiting excess
air so that there is not enough excess oxygen to produce NO according to Reaction
8.6.7. Exhaust catalytic converters reduce NOx emissions from automobile exhausts.
Scrubbing NOx from furnace and power plant stack gases is difficult due to the low
water solubilities of NOx gases.
8.7. Acid Rain
Along with hydrogen chloride, HCl, emitted to the atmosphere by the combustion
of chlorine-containing organic compounds, sulfur dioxide and nitrogen oxides react in
the atmosphere to produce strongly acidic H 2 SO 4 and HNO 3 , respectively. Incorporated
into rainwater, these acids fall to the ground as acid rain. A more general term, acid
deposition, refers to the effects of atmospheric strong acids, acidic gases (SO 2 ), and
acidic salts (NH 4 NO 3 and NH 4 HSO 4 ). Acid deposition is a major air pollution problem.
Figure 8.4 shows a typical distribution of acidic precipitation in the 48 contiguous
U.S. states. This figure illustrates that acidic precipitation is a regional air pollution
problem, not widespread enough to be a global problem, but spreading beyond local
210 Green Chemistry, 2nd ed