Page 238 - Fundamentals of Air Pollution

I. Analysis and Measurement of Gaseous Pollutants 201

rate (TCM). Absorbed SO 2 forms a stable complex with TCM. This enhanced
stability permits the collection, transport, and short-term storage of samples
at a central laboratory. The analysis proceeds by adding bleached pararosan-
iline dye and formaldehyde to form red-purple pararosaniline methylsul-
fonic acid. Optical absorption at 548 nm is linearly proportional to the SO 2
concentration. Procedures are followed to minimize interference by O 3,
oxides of nitrogen, and heavy metals.
The continuous methods combine sample collection and the measure-
ment technique in one automated process. The measurement methods used
for continuous analyzers include conductometric, colorimetric, coulometric,
and amperometric techniques for the determination of SO 2 collected in
a liquid medium (7). Other continuous methods utilize physicochemical
techniques for detection of SO 2 in a gas stream. These include flame photo-
metric detection (described earlier) and fluorescence spectroscopy (8). In-
struments based on all of these principles are available which meet standard
performance specifications.

E. Nonmethane Volatile Organic Compounds
The large number of individual hydrocarbons in the atmosphere and the
many different hydrocarbon classes make ambient air monitoring a very
difficult task. The ambient atmosphere contains an ubiquitous concentra-
tion of methane (CH 4) at approximately 1.6 ppm worldwide (9). The concen-
tration of all other hydrocarbons in ambient air can range from 100 times
less to 10 times greater than the methane concentration for a rural versus
an urban location. The terminology of the concentration of hydrocarbon
compounds is potentially confusing. Hydrocarbon concentrations are re-
ferred to by two units—parts per million by volume (ppmV) and parts per
million by carbon (ppmC). Thus, 1 /ul of gas in 1 liter of air is 1 ppmV, so
the following is true:

The unit parts per million by carbon takes into account the number of
carbon atoms contained in a specific hydrocarbon and is the generally
accepted way to report ambient hydrocarbons. This unit is used for three
reasons: (1) the number of carbons atoms is a very crude indicator of
the total reactivity of a group of hydrocarbon compounds, (2) historically,