LONG PATHLENGTH DIFFERENTIAL OPTICAL-ABSORPTION SPECTROSCOPY (DOAS) MEASUREMENTS OF GASEOUS HONO, NO2 AND HCHO IN THE CALIFORNIA SOUTH COAST AIR BASIN

A differential optic-al absorption spectrometer (DOAS) system was oper ated at Long Beach, CA during the 1987 SCAQS Fall episodes to measure atmospheric concentrations of nitrous acid (HONO), as well as ambient levels of nitrogen dioxide (NO2) and formaldehyde (HCHO). The rapid sc anning (approximately 3000 spectra per min) spectrometer was interface d to a 25 m basepath open, multiple reflection system operated routine ly at a total optical path of 800 m. During several of the Fall episod es at Long Beach, levels of gaseous HONO were the highest (>15 ppb) re ported to date by the DOAS technique. Although approximately half, to all, of the measured nighttime HONO concentrations could be accounted for by proposed heterogeneous formation pathways involving NO2, HONO c oncentrations correlated well with primary pollutants such as CO and N O, suggesting that elevated nighttime HONO concentrations in the weste rn end of the Los Angeles 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. Estimates of hydroxyl (OH) radical production show that photolysis of HONO short ly after sunrise on these episode days produces a large pulse of OH ra dicals at a time of the day when OH production from photolysis of O3 a nd HCHO is low. In terms of integrated OH radical production, HONO is of comparable importance to HCHO and much more important than O3 durin g these Fall periods.