Measurement of Nitrous Acid in Motor Vehicle Exhaust

Abstract

Nitrous acid (HONO) is an important tropospheric air pollutant. Photolysis of HONO produces hydroxyl 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 (NO x ). The relative contribution of these sources to ambient HONO concentrations is not well known. In this study, HONO and NO x 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; NO x 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/NO x ratio in motor vehicle exhaust was (2.9 ( 0.5) × 10 -3 . The HONO/NO x ratio in vehicle exhaust measured at the Caldecott Tunnel was higher than that reported previously for wellmaintained, catalyst-equipped vehicles, but was lower than that for older vehicles with limited emission controls. Nighttime ambient HONO/NO x ratios are typically much larger than the HONO/NO x ratio measured at the Caldecott Tunnel, which suggests that ambient HONO concentrations are governed mainly by secondary formation.

Source Identification and Trends in Concentrations of Gaseous and Fine Particulate Principal Species in Seoul, South Korea

“…33 Several heterogeneous reactions have been proposed for atmospheric HONO formation, and motor vehicle exhausts can also be considered as another source in urban areas. 26,[33][34][35] SO 2 concentrations were the highest (10.5 g/m Ϫ3 ) in winter, followed by spring (9.14 g/m Ϫ3 ), fall (6.57 g/m Ϫ3 ), and summer (the lowest, 3.10 g/m Ϫ3 ). Seasonal SO 2 pattern was consistent with the amount of fuel used in Seoul; the general trend of fuel consumption is in order (winter Ͼ spring Ͼ fall Ͼ summer).…”

Quantification of the unknown HONO daytime source and its relation to NO<sub>2</sub>

“…Measured HONO/NO x ratios were always higher than those reported for direct emissions (max. reported 0.8 %) (Pitts et al, 1984;Kirchstetter et al, 1996;Kleffmann et al, 2003). Thus, no pure direct emissions were measured.…”

Measurements of nitrous acid (HONO) in urban area of Shanghai, China

“…Direct emissions from combustion processes (i.e. vehicles exhaust, biomass burning) also contribute, which may account for a large proportion of observed HONO in heavy polluted areas (Kirchstetter et al 1996;Kurtenbach et al 2001).…”