Measurements of Gas phase Acids in Diesel Exhaust:
A Relevant Source of HNCO?
- Publication date
- Publisher
Abstract
Gas-phase
acids in light duty diesel (LDD) vehicle exhaust were
measured using chemical ionization mass spectrometry (CIMS). Fuel
based emission factors (EF) and NO<sub><i>x</i></sub> ratios
for these species were determined under differing steady state engine
operating conditions. The derived HONO and HNO<sub>3</sub> EFs agree
well with literature values, with HONO being the single most important
acidic emission. Of particular importance is the quantification of
the EF for the toxic species, isocyanic acid (HNCO). The emission
factors for HNCO ranged from 0.69 to 3.96 mg kg<sub>fuel</sub><sup>–1</sup>, and were significantly higher than previous biomass
burning emission estimates. Further ambient urban measurements of
HNCO demonstrated a clear relationship with the known traffic markers
of benzene and toluene, demonstrating for the first time that urban
commuter traffic is a source of HNCO. Estimates based upon the HNCO-benzene
relationship indicate that upward of 23 tonnes of HNCO are released
annually from commuter traffic in the Greater Toronto Area, far exceeding
the amount possible from LDD alone. Nationally, 250 to 770 tonnes
of HNCO may be emitted annually from on-road vehicles, likely representing
the dominant source of exposure in urban areas, and with emissions
comparable to that of biomass burning