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During July and August of 1990, a differential optical
absorption spectrometer (DOAS) made by OPSIS Inc. was used to measure
gaseous air pollutants over three separate open paths in Atlanta, GA.
Over path 1 (1099 m) and path 2 (1824 m), ozone (03), sulfur dioxide
(SO2) nitrogen dioxide (NO2), nitrous acid (HNO2) formaldehyde
(HCHO), benzene, toluene, and o-xylene were measured. Nitric oxide
(NO) and ammonia (NH3) were monitored over path 3 (143 m). The data
quality and data capture depended on the compound being measured and
the path over which it was measured. Data quality criteria for each
compound were chosen such that the average relative standard
deviation would be less than 25%. Data capture ranged from 43% for
o-xylene for path 1 to 95% for ozone for path 2. Benzene, toluene,
and o-xylene concentrations measured over path 2, which crossed over
an interstate highway, were higher than concentrations measured over
path 1, implicating emissions from vehicles on the highway as a
significant source of these compounds. Federal Reference Method
(FRN) instruments were located near the DOAS light receivers and
measurements of 03, NO2, and NO were made concurrently with the DOAS.
Correlation coefficients greater than 0.85 were obtained between the
DOAS and FRM's; however, there was a difference between the mean
values obtained by the two methods for 03 and NO. A gas
chromatograph for measuring volatile organic compounds was operated
next to the FRN's. Correlation coefficients of about 0.66 were
obtained between the DOAS and GC measurements of benzene and o-
xylene. However, the correlation coefficient between the DOAS and GC
measurements of toluene averaged only 0.15 for the two DOAS
measurement paths. The lack of correlation and other factors
indicate the possibility of a localized source of toluene near the
GC. In general, disagreements between the two measurement methods
could be caused by atmospheric inhomogeneities or interferences in
the DOAS and other methods.
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