Long pathlength differential optical absorption spectroscopy (D O A S) measurements of gaseous HONO, NO2 and HCNO in the California South Coast Air Basin

Abstract

A differential optical absorption spectrometer (DOAS) system was operated 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 scanning (-3000 spectra per min) spectrometer was interfaced to a 25 m basepath open, multiple reflection system operated routinely at a total optical path of 800 m. During several of the Fall episodes at Long Beach, levels of gaseous HONO were the highest (>15 ppb) reported 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 concentrations correlated well with primary pollutants such as CO and NO, suggesting that elevated nighttime HONO concentrations in the western 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 shortly after sunrise on these episode days produces a large pulse of OH radicals at a time of the day when OH production from photolysis of O3 and HCHO is low. In terms of integrated OH radical production, HONO is of comparable importance to HCHO and much more important than O3 during these Fall periods.