Measurement of nitrous acid in motor vehicle exhaust

Nitrous acid (HONO) is an important tropospheric air pollutant. Photolysis of HONO produces hydroxy] radicals that promote ozone formation. HONO may also adversely affect human health. Sources of HONO include both direct emission from combustion processes and secondary atmospheric formation from nitrogen oxides (NOx). The relative contribution of these sources to ambient HONO concentrations is not well known. In this study, HONO and NOx emissions from on-road vehicles were measured at the Caldecott Tunnel during summer 1995. The Caldecott Tunnel is located on a heavily used highway in the San Francisco Bay area. The mean and median model years of vehicles observed during this study were 1989.3 and 1990, respectively. Nitrous acid was collected on sodium carbonate-coated glass annular denuders; NOx concentrations were measured using chemiluminescent analyzers. Average HONO concentrations in the tunnel exhaust and background air were 6.9 +/- 1.4 and 0.7 +/- 0.3 ppb, respectively. The average HONO/NOx ratio in motor vehicle exhaust was (2.9 +/- 0.5) x 10(-3). The HONO/NOx ratio in vehicle exhaust measured at the Caldecott Tunnel was higher than that reported previously for well-maintained, catalyst-equipped vehicles, but was lower than that for older vehicles with limited emission controls. Nighttime ambient HONO/NOx ratios are typically much larger than the HONO/NOx ratio measured at the Caldecott Tunnel, which suggests that ambient HONO concentrations are governed mainly by secondary formation.