Elevated production of NH₄NO₃ from the photochemical processing of vehicle exhaust: Implications for air quality in the Seoul Metropolitan Region

A vehicle fleet representative of passenger vehicles driven in the Seoul Metropolitan Region was investigated for primary emissions and secondary chemistry. Exhaust was photochemically oxidized in a flow reactor to determine the ammonium nitrate (NH₄NO₃) aerosol formation potential from vehicles of gasoline, diesel and liquid petroleum gasoline (LPG) fuel types. Secondary formation of aerosol NH₄NO₃, was larger than primary emissions for all vehicle fuel types except diesel, for which negligible secondary NH₄NO₃production was observed. Although diesel vehicles emitted more primary nitrogen oxides than other vehicle types, ammonia emitted from gasoline and liquid petroleum gasoline fuels types limited the secondary production of NH₄NO₃. The results suggest that gasoline and liquid petroleum gasoline vehicles with three-way catalysts could be an important source of ammonia for NH₄NO₃aerosol formation in ammonia-limited environments, including the Seoul Metropolitan Region.