Wintertime ammonium nitrate aerosol pollution is a severe air quality issue affecting both developed and rapidly urbanizing regions from Europe to East Asia. In the United States, it is acute in western basins subject to inversions that confine pollutants near the surface. Measurements and modeling of a wintertime pollution episode in Salt Lake Valley, Utah, demonstrate that ammonium nitrate is closely related to photochemical ozone through a common parameter, total odd oxygen, O_x,total. We show that the traditional nitrogen oxide and volatile organic compound (NOx‐VOC) framework for evaluating ozone mitigation strategies also applies to ammonium nitrate. Despite being nitrate‐limited, ammonium nitrate aerosol pollution in Salt Lake Valley is responsive to VOCs control and, counterintuitively, not initially responsive to NO_x control. We demonstrate simultaneous nitrate limitation and NO_x saturation and suggest this phenomenon may be general. This finding may identify an unrecognized control strategy to address a global public health issue in regions with severe winter aerosol pollution.

The Korea–United States Air Quality Study (KORUS-AQ) conducted during May–June 2016 offered the first opportunity to evaluate direct-sun observations of formaldehyde (HCHO) total column densities with improved Pandora spectrometer instruments. The measurements highlighted in this work were conducted both in the Seoul megacity area at the Olympic Park site (37.5232^∘ N, 27.1260^∘ E; 26 m a.s.l.) and at a nearby rural site downwind of the city at the Mount Taehwa research forest site (37.3123^∘ N, 127.3106^∘ E; 160 m a.s.l.). Evaluation of these measurements was made possible by concurrent ground-based in situ observations of HCHO at both sites as well as overflight by the NASA DC-8 research aircraft. The flights provided in situ measurements of HCHO to characterize its vertical distribution in the lower troposphere (0–5 km). Diurnal variation in HCHO total column densities followed the same pattern at both sites, with the minimum daily values typically observed between 6:00 and 7:00 local time, gradually increasing to a maximum between 13:00 and 17:00 before decreasing into the evening. Pandora vertical column densities were compared with those derived from the DC-8 HCHO in situ measured profiles augmented with in situ surface concentrations below the lowest altitude of the DC-8 in proximity to the ground sites. A comparison between 49 column densities measured by Pandora vs. aircraft-integrated in situ data showed that Pandora values were larger by 16 % with a constant offset of 0.22 DU (Dobson units; R²=0.68). Pandora HCHO columns were also compared with columns calculated from the surface in situ measurements over Olympic Park by assuming a well-mixed lower atmosphere up to a ceilometer-measured mixed-layer height (MLH) and various assumptions about the small residual HCHO amounts in the free troposphere up to the tropopause. The best comparison (slope $=\mathrm{1.03}\pm \mathrm{0.03}$ ; intercept $=\mathrm{0.29}\pm \mathrm{0.02}$ DU; and $R^{\mathrm{2}}=\mathrm{0.78}\pm \mathrm{0.02}$ ) was achieved assuming equal mixing within ceilometer-measured MLH combined with an exponential profile shape. These results suggest that diurnal changes in HCHO surface concentrations can be reasonably estimated from the Pandora total column and information on the mixed-layer height. More work is needed to understand the bias in the intercept and the slope relative to columns derived from the in situ aircraft and surface measurements.

Signatures of mass-independent isotope fractionation (MIF) are found in the oxygen (¹⁶O,¹⁷O,¹⁸O) and sulfur (³²S,³³S,³⁴S,³⁶S) isotope systems and serve as important tracers of past and present atmospheric processes. These unique isotope signatures signify the breakdown of the traditional theory of isotope fractionation, but the physical chemistry of these isotope effects remains poorly understood. We report the production of large sulfur isotope MIF, with Δ³³S up to 78‰ and Δ³⁶S up to 110‰, from the broadband excitation of SO₂in the 250–350-nm absorption region. Acetylene is used to selectively trap the triplet-state SO₂( ³B₁), which results from intersystem crossing from the excited singlet ( ¹A₂/ ¹B₁) states. The observed MIF signature differs considerably from that predicted by isotopologue-specific absorption cross-sections of SO₂and is insensitive to the wavelength region of excitation (above or below 300 nm), suggesting that the MIF originates not from the initial excitation of SO₂to the singlet states but from an isotope selective spin–orbit interaction between the singlet ( ¹A₂/ ¹B₁) and triplet ( ³B₁) manifolds. Calculations based on high-level potential energy surfaces of the multiple excited states show a considerable lifetime anomaly for³³SO₂and³⁶SO₂for the low vibrational levels of the ¹A₂state. These results demonstrate that the isotope selectivity of accidental near-resonance interactions between states is of critical importance in understanding the origin of MIF in photochemical systems.

Signatures of sulfur mass‐independent fractionation (S‐MIF) are observed for sulfur minerals in Archean rocks, and for modern stratospheric sulfate aerosols (SSA) deposited in polar ice. Ultraviolet light photolysis of SO₂ is thought to be the most likely source for these S‐MIF signatures, although several hypotheses have been proposed for the underlying mechanism(s) of S‐MIF production. Laboratory SO₂ photolysis experiments are carried out with a flow‐through photochemical reactor with a broadband (Xe arc lamp) light source at 0.1 to 5 mbar SO₂ in 0.25 to 1 bar N₂ bath gas, in order to test the effect of SO₂ pressure on the production of S‐MIF. Elemental sulfur products yield high δ³⁴S values up to 140 ‰, with δ³³S/δ³⁴S of 0.59 ± 0.04 and Δ³⁶S/Δ³³S ratios of −4.6 ± 1.3 with respect to initial SO₂. The magnitude of the isotope effect strongly depends on SO₂ partial pressure, with larger fractionations at higher SO₂ pressures, but saturates at an SO₂ column density of 10¹⁸ molecules cm⁻². The observed pressure dependence and δ³³S/δ³⁴S and Δ³⁶S/Δ³³S ratios are consistent with model calculations based on synthesized SO₂ isotopologue cross sections, suggesting a significant contribution of isotopologue self‐shielding to S‐MIF for high SO₂ pressure (>0.1 mbar) experiments. Results of dual‐cell experiments further support this conclusion. The measured isotopic patterns, in particular the Δ³⁶S/Δ³³S relationships, closely match those measured for modern SSA from explosive volcanic eruptions. These isotope systematics could be used to trace the chemistry of SSA after large Plinian volcanic eruptions.

The Korea – United States Air Quality Study (May – June 2016) deployed instrumented aircraft and ground-based measurements to elucidate causes of poor air quality related to high ozone and aerosol concentrations in South Korea. This work synthesizes data pertaining to aerosols (specifically, particulate matter with aerodynamic diameters <2.5 micrometers, PM_2.5) and conditions leading to violations of South Korean air quality standards (24-hr mean PM_2.5 < 35 µg m^–3). PM_2.5 variability from AirKorea monitors across South Korea is evaluated. Detailed data from the Seoul vicinity are used to interpret factors that contribute to elevated PM_2.5. The interplay between meteorology and surface aerosols, contrasting synoptic-scale behavior vs. local influences, is presented. Transboundary transport from upwind sources, vertical mixing and containment of aerosols, and local production of secondary aerosols are discussed. Two meteorological periods are probed for drivers of elevated PM_2.5. Clear, dry conditions, with limited transport (Stagnant period), promoted photochemical production of secondary organic aerosol from locally emitted precursors. Cloudy humid conditions fostered rapid heterogeneous secondary inorganic aerosol production from local and transported emissions (Transport/Haze period), likely driven by a positive feedback mechanism where water uptake by aerosols increased gas-to-particle partitioning that increased water uptake. Further, clouds reduced solar insolation, suppressing mixing, exacerbating PM_2.5 accumulation in a shallow boundary layer. The combination of factors contributing to enhanced PM_2.5 is challenging to model, complicating quantification of contributions to PM_2.5 from local versus upwind precursors and production. We recommend co-locating additional continuous measurements at a few AirKorea sites across South Korea to help resolve this and other outstanding questions: carbon monoxide/carbon dioxide (transboundary transport tracer), boundary layer height (surface PM_2.5 mixing depth), and aerosol composition with aerosol liquid water (meteorologically-dependent secondary production). These data would aid future research to refine emissions targets to further improve South Korean PM_2.5 air quality.