First direct measurements of the nitrogen isotopic composition of vehicle tailpipe-emitted NH and implications for the source apportionment of NH in urban atmosphere.

Sci Total Environ

Institute at Brown for Environment and Society and Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI 02912, USA.

Published: December 2024

The introduction of three-way catalytic converter (TWC) to meet stringent vehicular NO emission standards worldwide has led to an unintended consequence of vehicle-derived ammonia (NH) emission, which might degrade air quality and affect human health, especially in urban areas. The nitrogen stable isotope composition of NH (δN-NH) may be a useful tool to trace NH sources, but the isotopic signature of vehicle-emitted NH is lacking. Here we report the δN-NH measured from tailpipe exhausts collected directly from 19 different vehicles equipped with TWC using "grab" sample technique optimized to avoid isotopic fractionation. We found a large of range of NH concentration (from 1 to 37 ppm, x̄ ± 1σ = 10 ± 8 ppm) and δN value (from -38.1 ‰ to 49.0 ‰, x̄ ± 1σ = 1.2 ± 20.9 ‰) for our samples (n = 57). Our results indicate a correlation between δN-NH and NH concentrations (i.e. δN-NH (‰) = 16.9(± 2.2)In(NH concentration; ppm) - 31.7(± 4.7) (R = 0.53, P < 0.01) that is associated with the catalyst temperature, which is generally agree with the results of theoretical calculation. After consideration of the vehicle emission evolution, dominant driving condition and tunnel δN-NH variation, a representative δN-NH value (mean) of vehicle source is identified as 0.8000, which is distinct from (larger than) other NH volatilization-related emission sources. Using our newly measured δN values and Bayesian mixing model, we approximate the overall contribution of vehicles to ambient NH before, during, and after an international event in Beijing was 14.1 %, 7.9 %, and 21.5 %, respectively, which agree well with the course of traffic restriction. Collectively, our findings demonstrate the importance of using vehicle-emitted δN-NH to quantify vehicular NH contribution in urban atmospheres.

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http://dx.doi.org/10.1016/j.scitotenv.2024.177415DOI Listing

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