Aerosol ammonium (NH₄⁺) can be a major component of fine particles, especially during severe haze episodes. The abatement of ammonia (NH₃) emissions is important for reducing fine particles, but NH₃ sources remain poorly constrained and are largely unregulated in China and most other regions. This study uses stable isotopes to interpret the role NH₃ sources play in generating different sized NH₄⁺ aerosols in Beijing between 21 June and 4 July 2013 with fine particle concentrations of 20–242 μg/m³. The concentrations and nitrogen stable isotope composition of aerosol NH₄⁺ (δ¹⁵N‐NH₄⁺) were both elevated during the five haze episodes that were sampled. These increases were driven by enhancements in the fine mode as opposed to substantial increases in the coarse mode aerosol. After accounting for the isotope fractionation that occurs during gas‐to‐particle partitioning (17.7‰ to 28.2‰), the “initial” (prepartitioning) δ¹⁵N‐NH₃ values were estimated to be −35‰ for a clean period (i.e., a nonhazy day) and ranged from −14.3‰ to −22.8‰ for hazy days. Source apportionment using the “IsoSources” isotopic mixing model indicated that the dominant contribution to NH₃ shifted from agricultural sources during the clean period (86%) to fossil fuel emissions (54%–81%) during hazy days and when back trajectories rotate from the northwest to the west and/or south. These results together suggest that even in summer, fossil fuel‐related sources from Beijing and the surrounding areas are the major source of NH₃ during haze events and that controlling these sources may be important for alleviating particulate matter pollution.

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.

The oxygen isotope composition of nitrogen oxides (NOx) in the atmosphere is a useful tool for understanding the oxidation of NOx into nitric acid / nitrate in the atmosphere. A set of experiments was conducted to examine change in isotopic composition of NOx due to NOx–O2–O3 photochemical cycling. At low NOx / O2 mixing ratios, NOx became progressively and nearly equally enriched in 17O and 18O over time until it reached a steady state with Δ17O values of 39.3 ± 1.9‰ and δ18O values of 84.2 ± 4‰, relative to the isotopic composition of the initial O2 gas. As the mixing ratios were increased, the isotopic enrichments were suppressed by isotopic exchange between O atoms, O2, and NOx. A kinetic model was developed to simulate the observed data and it showed that the isotope effects occurring during O3 formation play a dominant role in controlling NOx isotopes and, in addition, secondary kinetic isotope effects or isotope exchange reactions are also important during NOx cycling. The data and model were consistent with previous studies which showed that the NO + O3 reactions occur mainly via the transfer of the terminal atoms of O3. The model predicts that under tropospheric concentrations of NOx and O3, the timescale of NOx–O3 isotopic equilibrium ranges from hours (for ppbv NOx / O2 mixing ratios) to days (for pptv mixing ratios) and yields steady state Δ17O and δ18O values of 45‰ and 117‰ respectively (relative to Vienna Standard Mean Ocean Water (VSMOW)) in both cases. Under atmospheric conditions when O3 has high concentrations, the equilibrium between NOx and O3 should occur rapidly (h) but this equilibrium cannot be reached during polar winters and/or nights if the NOx conversion to HNO3 is faster. The experimentally derived rate coefficients can be used to model the major NOx–O3 isotopologue reactions at various pressures and in isotope modeling of tropospheric nitrate.