Photochemical processing of diesel fuel emissions as a large secondary source of isocyanic acid (HNCO)

Abstract

Isocyanic acid (HNCO) is a well‐known air pollutant that affects human health. Biomass burning, smoking, and combustion engines are known HNCO sources, but recent studies suggest that secondary production in the atmosphere may also occur. We directly observed photochemical production of HNCO from the oxidative aging of diesel exhaust during the Diesel Exhaust Fuel and Control experiments at Colorado State University using acetate ionization time‐of‐flight mass spectrometry. Emission ratios of HNCO were enhanced, after 1.5 days of simulated atmospheric aging, from 50 to 230 mg HNCO/kg fuel at idle engine operating conditions. Engines operated at higher loads resulted in less primary and secondary HNCO formation, with emission ratios increasing from 20 to 40 mg HNCO/kg fuel under 50% load engine operating conditions. These results suggest that photochemical sources of HNCO could be more significant than primary sources in urban areas.

Tropospheric sources and sinks of gas-phase acids in the Colorado Front Range

“…HNCO mixing ratios were possibly influenced by additional sources, including traffic, ONG wells, and industrial activity. Traffic exhaust is a primary emission source of HNCO (Brady et al, 2014;Link et al, 2016), but the lack of a morning rush-hour peak or correlation with CO suggests that it was not a strong primary source of HNCO at the site (Fig. 2).…”

“…2). Link et al (2016) found that diesel exhaust was a precursor for photochemical HNCO production, but Jathar et al (2017) suggested that the kinetics do not substantially outcompete dilution, and that urban HNCO is not strongly enhanced by diesel exhaust photochemistry.…”

“…Primary emission and secondary photochemical production sources of gas-phase HNCO have been identified and reported (Borduas et al, 2013;Roberts et al, 2014), but the magnitudes of these sources remain highly uncertain (Young et al, 2012). Combustion processes, including biomass burning, gasoline/diesel fuel combustion, and tobacco smoke are a primary source of HNCO (Roberts et al, , 2014Link et al, 2016). Secondary sources of HNCO include OH oxidation of amine and amide precursors, which are particularly important in urban environments (Link et al, 2016;Roberts et al, 2014;Borduas et al, 2013).…”

“…Combustion processes, including biomass burning, gasoline/diesel fuel combustion, and tobacco smoke are a primary source of HNCO (Roberts et al, , 2014Link et al, 2016). Secondary sources of HNCO include OH oxidation of amine and amide precursors, which are particularly important in urban environments (Link et al, 2016;Roberts et al, 2014;Borduas et al, 2013). HNCO readily partitions into the aqueous-phase given its high solubility at atmospherically relevant pH values, and can hydrolyze to NH 3 .…”

Modeling of the chemistry in oxidation flow reactors with high initial NO

“…The former study reported evidence for the incorporation of nitrogen into SOA. OFRs have also been increasingly employed to process emissions of vehicles, biomass burning, and other combustion sources (Table 1) in which NO can often be hundreds of ppm Martinsson et al, 2015;Karjalainen et al, 2016;Link et al, 2016;Schill et al, 2016;Alanen et al, 2017;Simonen et al, 2017). It can be expected that such a high NO input together with very high VOC concentrations would cause a substantial deviation from the good OFR operation conditions identified in Peng et al (2016).…”