Is Improved Vehicular NOx Control Leading to Increased Urban NH₃ Emissions?

Abstract

Over the past several years, it has become apparent that vehicular emissions include significant quantities of ammonia (NH3) arising from the implementation of increasingly stringent nitrogen oxide (NOx) emissions controls on light-duty (LDV) and heavy-duty (HDV) vehicles. (1) In the case of LDVs, the NH3 forms and slips through the catalytic reduction of NOx in the 3-way catalyst systems and the selective catalytic reduction (SCR), although details of the mechanism are not fully understood. Using remote sensing methods, Farren et al. (2) reported that NH3 emissions increased as vehicle mileage increased and that cold start emissions were 1.7 times higher than when the catalyst was fully operational. Emissions from HDVs and new diesel LDVs come from overdosing with urea and maldistribution of the NH3 decomposed from the urea on the catalyst during SCR. (3) As a result of these increased NOx controls, vehicular emissions might now be the dominant urban source of NH3 (see Zhou et al. and references therein as an example). (4) A recent study by Reche et al. (5) has also found that NH3 concentrations were highest at the traffic site in Barcelona compared to a number of other locations. They also found that concentrations have been increasing over the past decade (2011–2020).

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