|
Principal Investigator: Arthur M. Winer
University of California , Riverside
December 1989
ARB Contract No. A6-146-32 (Full Report)
ABSTRACT
Two differential optical absorption spectrometer (DOAS) systems were operated during the 1987 Southern California
Air Quality Study (SCAQS) for the primary purpose of measuring atmospheric concentrations of nitrous acid (HONO)
and the nitrate (NO) radical, as well as ambient levels of nitrogen dioxide (NO2) and formalde ayde (HCHO). Measurements
were conducted at the "Type A" sites at Long Beach and Claremont, California, during the summer phase
of the program and at Long Beach during the fall episodes. In both DOAS systems, rapid scanning spectrometers
(-3000 spectra per min) were interfaced to 25 m basepath open, multiple reflection systems operated at total optical
paths of 800 m. An extensive set of time-resolved (i.e., 15 min) measurements of HONO, NO2 and HCHO concentrations,
as well as NO3 radical concentrations, were obtained during the SCAQS program by the DOAS technique under a wide
range of pollutant and meteorological conditions.
Pronounced diurnal profiles in the atmospheric concentrations of NO2, HONO, HCHO and NO3 radicals were observed
for most of the episodes studied, with clear evidence for the influence of mobile emission sources on ambient
levels of NO2 and HCHO at both Claremont and Long Beach.
During several of the fall episodes at Long Beach, levels of gaseous HONO were the highest observed to date by
the DOAS technique (>15 ppb). Although ~50%, to as much as 100%, of the measured nighttime HONO could be accounted
for by heterogeneous formation pathways involving NO2 and water vapor, HONO concentrations correlated well with
primary pollutants such as CO and NO, suggesting that elevated nighttime HONO concentrations in the western end
of the basin may be influenced by emissions of HONO from combustion sources. This has significant implications
for models which assume HONO arises only from secondary formation, rather than a combination of direct emissions
and atmospheric reactions.
Calculations showed that OH radical production at Long Beach from photolysis of elevated HONO concentrations after
sunrise produced a large pulse of OH radicals at a time when there was little or no contribution from photolysis
of HCHO and 03. For the fall episodes, the total OH radical photolysis under these conditions was comparable
to total OH radical production from HCHO photolysis through the rest of the day.
As expected, elevated NO3 concentrations (~50-175 ppt) were observed in the summer at Claremont only under conditions
involving the co-occurrence of significant 03 and NO2 concentrations and the absence of NO. Levels of HONO observed
at Claremont during the summer episodes fell in the range -1-7 ppb, characteristic of HONO concentrations previously
observed in downwind receptor areas of California's South Coast Air Basin.
With further analysis and correlation with other SCAQS data, the DOAS results for HCHO should be useful in exploring
the partitioning of ambient concentrations of HCHO between direct emissions and formations from secondary reactions
in the atmosphere.
Back to the Main Menu
|
