Summertime diurnal variations in the isotopic composition of atmospheric nitrogen dioxide at a small midwestern United States city

Publication date: April 2018
Source:Atmospheric Environment, Volume 179
Author(s): Wendell W. Walters, Huan Fang, Greg Michalski
The nitrogen and oxygen stable isotopes (δ¹⁵N & δ¹⁸O) of nitrogen oxides (NOx = nitric oxide (NO) + nitrogen dioxide (NO2)) may be a useful tool for partitioning NOx emission sources and for evaluating NOx photochemical cycling, but few measurements of in situ NOx exist. In this study, we have collected and characterized the diurnal variability in δ¹⁵N and δ¹⁸O of NO2 from ambient air at a small Midwestern city (West Lafayette, IN, USA, 40.426° N, 86.908° W) between July 7 to August 5, 2016, using an active sampling technique. Large variations were observed in both δ¹⁵N(NO2) and δ¹⁸O(NO2) that ranged from −31.4 to 0.4‰ and 41.5–112.5‰, respectively. Daytime averages were −9.2 ± 5.7‰ (x̅ ± 1σ) and 86.5 ± 14.1‰ (n = 11), while nighttime averages were −13.4 ± 7.3‰ and 56.3 ± 7.1‰ (n = 12) for δ¹⁵N(NO2) and δ¹⁸O(NO2), respectively. The large variability observed in δ¹⁵N(NO2) is predicted to be driven by changing contributions of local NOx emission sources, as calculated isotope effects predict a minor impact on δ¹⁵N(NO2) relative to δ¹⁵N(NOx) that is generally less than 2.5‰ under the sample collection conditions of high ozone concentration ([O3]) relative to [NOx]. A statistical δ¹⁵N mass-balance model suggests that traffic-derived NOx is the main contributor to the sampling site (0.52 ± 0.22) with higher relative contribution during the daytime (0.58 ± 0.19) likely due to higher traffic volume than during the nighttime (0.47 ± 0.22). The diurnal cycle observed in δ¹⁸O(NO2) is hypothesized to be a result of the photochemical cycling of NOx that elevates δ¹⁸O(NO2) during the daytime relative to the nighttime. Overall, this data suggests the potential to use δ¹⁵N(NO2) for NOx source partitioning under environmental conditions of high [O3] relative to [NOx] and δ¹⁸O(NO2) for evaluating VOC-NOx-O3 chemistry.



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