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Project Status: complete

Report Published December 1989:

Title: Measurements of nitrous acid, nitrate radicals, formaldehyde and nitrogen dioxide for the Southern California Air Quality Study (SCAQS) by differential optical absorbtion spectroscopy

Principal Investigator / Author(s): Winer, Arthur

Contractor: Statewide Air Pollution Research Center, UC Riverside

Contract Number: A6-146-32

Topic Areas: Acid Deposition, Monitoring


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.


For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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