Atmospheric nitrate (NO₃⁻ = particulate NO₃⁻ + gas‐phase nitric acid [HNO₃]) and sulfate (SO₄²⁻) are key molecules that play important roles in numerous atmospheric processes. Here, the seasonal cycles of NO₃⁻ and total suspended particulate sulfate (SO₄²⁻_(TSP)) were evaluated at the South Pole from aerosol samples collected weekly for approximately 10 months (26 January to 25 October) in 2002 and analyzed for their concentration and isotopic compositions. Aerosol NO₃⁻ was largely affected by snowpack emissions in which [NO₃⁻] and δ¹⁵N(NO₃⁻) were highest (49.3 ± 21.4 ng/m³, n = 8) and lowest (−47.0 ± 11.7‰, n = 5), respectively, during periods of sunlight in the interior of Antarctica. The seasonal cycle of Δ¹⁷O(NO₃⁻) reflected tropospheric chemistry year‐round with lower values observed during sunlight periods and higher values observed during dark periods, reflecting shifts from HO_x‐ to O₃‐dominated oxidation chemistry. SO₄²⁻_(TSP) concentrations were highest during austral summer and fall (86.7 ± 73.7 ng/m³, n = 18) and are indicated to be derived from dimethyl sulfide (DMS) emissions, as δ³⁴S(SO₄²⁻)_(TSP) values (18.5 ± 1.0‰, n = 10) were similar to literature δ³⁴S(DMS) values. The seasonal cycle of Δ¹⁷O(SO₄²⁻)_(TSP) exhibited minima during austral summer (0.9 ± 0.1‰, n = 5) and maxima during austral fall (1.3 ± 0.3‰, n = 6) and austral spring (1.6 ± 0.1‰, n = 5), indicating a shift from HO_x‐ to O₃‐dominated chemistry in the atmospheric derived SO₄²⁻ component. Overall, the budgets of NO₃⁻ and SO₄²⁻_(TSP) at the South Pole were complex functions of transport, localized chemistry, biological activity, and meteorological conditions, and these results will be important for interpretations of oxyanions in ice core records in the interior of Antarctica.

Vehicle emissions have been identified as an important urban source of ammonia (NH₃). However, there are large uncertainties regarding the contribution of vehicle emissions to urban NH₃ budgets, as well as the role of NH₃ in spatiotemporal fine particulate matter (PM_2.5) formation and nitrogen (N) deposition patterns. The N stable isotopic composition (δ¹⁵N) may be a useful observational constraint to track NH₃ emission sources and chemical processing, but previously reported vehicle δ¹⁵N(NH₃) emission signatures have reported a wide range of values, indicating the need for further refinement. Here we have characterized δ¹⁵N(NH₃) spatiotemporal variabilities from vehicle plumes in stationary and on-road measurements in the USA and China using an active NH₃ collection technique demonstrated to accurately characterize δ¹⁵N(NH₃) on the order of hourly time resolution. Significant spatial and temporal δ¹⁵N(NH₃) variabilities were observed and suggested to be driven by vehicle fleet composition and influences from NH₃ dry deposition on tunnel surfaces. Overall, a consistent δ¹⁵N(NH₃) signature of 6.6±2.1 ‰ ($\stackrel{\mathrm{‾}}{x}\pm \mathrm{1}\mathit{\sigma }$ ; n=80) was found in fresh vehicle plumes with fleet compositions typical of urban regions. Our recommended vehicle δ¹⁵N(NH₃) signature is significantly different from previous reports. This difference is due to a large and consistent δ¹⁵N(NH₃) bias of approximately −15.5 ‰ between commonly employed passive NH₃ collection techniques and the laboratory-tested active NH₃ collection technique. This work constrains the δ¹⁵N(NH₃) urban traffic plume signature, which has important implications for tracking vehicle NH₃ in urban-affected areas and highlights the importance of utilizing verified collection methods for accurately characterizing δ¹⁵N(NH₃) values.

Atmospheric ammonia (NH₃) is a critical component of our atmosphere that contributes to air quality degradation and reactive nitrogen deposition; however, our knowledge of NH₃ in urban environments remains limited. Year-long ambient NH₃ and related species were measured for concentrations and the nitrogen isotopic compositions (δ¹⁵N) of NH₃ and particulate ammonium ($p{\mathrm{NH}}_{\mathrm{4}}^{+}$ ) were measured to understand the temporal sources and chemistry of NH₃ in a northeastern US urban environment. We found that urban NH₃ and $p{\mathrm{NH}}_{\mathrm{4}}^{+}$ concentrations were elevated compared to regional rural background monitoring stations, with seasonally significant variations. Local and transported sources of NH_x (NH₃+ $p{\mathrm{NH}}_{\mathrm{4}}^{+}$ ) were identified using polar bivariate and statistical back trajectory analysis, which suggested the importance of vehicles, volatilization, industry, and stationary fuel combustion emissions. Utilizing a uniquely positive δ¹⁵N(NH₃) emission source signature from vehicles, a Bayesian stable isotope mixing model (SIMMR) indicates that vehicles contribute 46.8±3.5 % (mean ±1σ) to the annual background level of urban NH_x, with a strong seasonal pattern with higher relative contribution during winter (56.4±7.6 %) compared to summer (34.1±5.5 %). The decrease in the relative importance of vehicle emissions during the summer was suggested to be driven by temperature-dependent NH₃ emissions from volatilization sources, seasonal fuel-combustion emissions related to energy generation, and change in seasonal transport patterns based on wind direction, back trajectory, and NH₃ emission inventory analysis. This work highlights that reducing vehicle NH₃ emissions should be considered to improve wintertime air quality in this region.