Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv Summertime diurnal variations in the isotopic composition of atmospheric nitrogen dioxide at a small midwestern United States city Wendell W. Walters a,*,1 , Huan Fang a , Greg Michalski a,b a Department of Earth, Atmospheric, and Planetary Sciences Purdue University, 550 Stadium Mall Drive, West Lafayette, IN, 47907, United States b Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN, 47907, United States ARTICLE INFO Keywords: Nitrogen Nitrogen oxides Isotopes Atmospheric emissions Oxidation ABSTRACT The nitrogen and oxygen stable isotopes (δ 15 N& δ 18 O) of nitrogen oxides (NO x = nitric oxide (NO) + nitrogen dioxide (NO 2 )) may be a useful tool for partitioning NO x emission sources and for evaluating NO x photochemical cycling, but few measurements of in situ NO x exist. In this study, we have collected and characterized the diurnal variability in δ 15 N and δ 18 O of NO 2 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 δ 15 N(NO 2 ) and δ 18 O(NO 2 ) that ranged from -31.4 to 0.4‰ and 41.5–112.5‰, re- spectively. 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 δ 15 N(NO 2 ) and δ 18 O(NO 2 ), respectively. The large variability observed in δ 15 N(NO 2 ) is predicted to be driven by changing contributions of local NO x emission sources, as calculated isotope effects predict a minor impact on δ 15 N(NO 2 ) relative to δ 15 N(NO x ) that is generally less than 2.5‰ under the sample collection conditions of high ozone concentration ([O 3 ]) relative to [NO x ]. A statistical δ 15 N mass-balance model suggests that traffic-derived NO x 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 δ 18 O(NO 2 ) is hypothesized to be a result of the photochemical cycling of NO x that elevates δ 18 O(NO 2 ) during the daytime relative to the nighttime. Overall, this data suggests the potential to use δ 15 N(NO 2 ) for NO x source partitioning under en- vironmental conditions of high [O 3 ] relative to [NO x ] and δ 18 O(NO 2 ) for evaluating VOC-NO x -O 3 chemistry. 1. Introduction Nitrogen oxides (NO x = nitric oxide (NO) + nitrogen dioxide (NO 2 )) play a key role in controlling the concentrations of atmospheric oxidants that drive tropospheric chemistry (Crutzen, 1973, 1979; Leighton, 1961; Logan, 1983). Photochemical reactions involving NO x , carbon monoxide, and volatile organic compounds (VOC) lead to the formation of tropospheric ozone (O 3 ), which is a greenhouse gas, an oxidizing pollutant, and influences the lifetimes of other greenhouse gases (Atkinson, 2000; Atkinson and Arey, 2003; Crutzen, 1979). Photochemical cycling involving NO x and reduced hydrogen oxide ra- dicals (HO x = hydroxyl radical (OH) + peroxy radicals (HO 2 and RO 2 )) is terminated when NO 2 is further oxidized to nitric acid (HNO 3 ). Once HNO 3 is formed, it is primarily removed via wet and/or dry deposition leading to degradation of drinking water, soil acidification, eu- trophication, and biodiversity change in terrestrial ecosystems (Galloway et al., 2004). Thus, due to the environmental and human health consequences of NO x and its oxidation products, it is important to understand the relative contributions of NO x emission sources and the oxidation processes responsible for its removal. Sources of NO x are both of natural (e.g. lightning, soil nitrification/ denitrification, and wildfires) and anthropogenic (e.g. fossil fuel com- bustion, industry, and agriculture) origins (Galloway et al., 2004; Jaeglé et al., 2005; Reis et al., 2009), but there are uncertainties in the temporal and spatial contributions of various emission sources that might be resolved by nitrogen (N) stable isotope analysis (δ 15 N). Nu- merous studies have quantified the difference in δ 15 N values of various NO x sources, which indicate relative distinctive values for biogenic NO x (nitrification/denitrification), the transportation sector, and coal-fired power plants (Ammann et al., 1999; Felix et al., 2012; Felix and Elliott, 2013; Fibiger et al., 2014; Heaton, 1987, 1990; Hoering, 1957; Li and Wang, 2008; Miller et al., 2017; Moore, 1977; Snape et al., 2003; Walters et al., 2015a, 2015b). These isotopic “fingerprints” may be a useful tool for constraining the NO x emission budget; however, it is https://doi.org/10.1016/j.atmosenv.2018.01.047 Received 22 May 2017; Received in revised form 12 January 2018; Accepted 27 January 2018 * Corresponding author. 1 Present address: Department of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook Street, Providence, RI, 02912, United States. E-mail address: waltersw@purdue.edu (W.W. Walters). Atmospheric Environment 179 (2018) 1–11 1352-2310/ © 2018 Elsevier Ltd. All rights reserved. T