Wintertime urban air pollution poses severe health risks. Yet, few studies have investigated the sources and aging of urban particulate matter at high latitudes, where low temperatures impact emissions and particle chemistry. Atmospheric particles (0.18-3.2 mu m) were collected from January to February 2022 during the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in Fairbanks, Alaska. The morphology and elemental composition of 33,422 individual particles were measured by computer-controlled scanning electron microscopy with energy-dispersive X-ray spectroscopy. The smallest measured particles (0.18-0.32 mu m) were primarily fractal soot from residential wood burning and fossil fuel combustion, and carbon, oxygen, and sulfur (C/O/S)-rich particles, most of which were aqueous/liquid (12 +/- 2% semisolid, by number). The largest particles (1.8-3.2 mu m) were mechanically-generated road dust and road salt, as well as superaggregate soot from inefficient combustion. Sulfur-containing fractal soot particles provide evidence of significant primary sulfur aerosol emissions. C/O/S, road salt, and dust particles showed evidence of secondary sulfur formation. The C/O/S particles reflect the transformation of hygroscopic combustion particles to larger particles containing mainly secondary organic and sulfate aerosol. These results demonstrate the heterogeneity of the high-latitude urban aerosol population and the participation of multiple sources in primary and secondary sulfur chemistry.

Hypoiodous acid (HOI) and molecular iodine (I2) are important precursors of reactive gaseous iodine, which plays an important role in the oxidative capacity of the atmosphere and in aerosol formation in the marine boundary layer. HOI and I2 are emitted from the ocean surface and recycled on atmospheric aerosol via heterogeneous chemistry. Measurements of these molecules, which are typically present in the marine boundary layer at the low-to-sub part per trillion (ppt) level, are sparse, in part due to difficulties in quantification with a lack of appropriate instrumentation and calibration techniques. A novel calibration technique is developed for HOI via generation from I2 hydrolysis and then 1:1 conversion of HOI back to I2 through a NaI trap, allowing the sensitivity of HOI to be calculated relative to I2, which is readily calibrated using a permeation tube system. Using this calibration method, we describe the use of a reduced pressure high resolution chemical ionisation mass spectrometer (CIMS) to characterise the sensitivities of HOI and I2 over a range of humidities representative of the marine boundary layer and to measure these molecules in the field. At humidities of over 50 % RH, the CIMS sensitivity of I2 is humidity independent whereas HOI exhibits a slight negative humidity dependence. The effect of inlet interactions on HOI and I2 signals is investigated, with HOI observed to convert to I2. The implications of these inlet interactions and humidity sensitivities for future ambient measurement configurations are discussed.

Abstract Tropospheric reactive halogens influence the oxidizing capacity of the atmosphere. Although previous studies have shown that iodine exerts the strongest impact on tropospheric ozone concentrations compared to chlorine and bromine, the impact of aerosol iodide recycling back to the gas phase on oxidants has not been estimated prior to this work. Here, we explicitly represent aerosol iodine speciation in a chemical transport model, including soluble organic iodine (SOI), iodate, and iodide, and allow for interconversion among these species. We find that aerosol iodine speciation, interconversion, and recycling substantially affect modeled oxidant abundances. Across model sensitivity studies, tropospheric Ox (the odd‐oxygen family for ozone) and Oz (the odd‐oxygen family for the HOx family, including OH) burdens vary by up to 10% and 5%, respectively. These ranges arise from uncertainties in heterogeneous iodine chemistry, demonstrating a need for future laboratory experiments and field observations in this area.

A potential impact of reactive halogens on climate and air quality is through oxidation of sulfur dioxide by hypohalous acids (HOX with X = Cl, Br, or I) to produce sulfate aerosols. This sulfate formation mechanism is typically not included in climate and air quality models, largely due to the limited observational evidence supporting its significance. In this study, we measured the oxygen isotopic composition of sulfate in aerosol particles as well as the concentrations of hypohalous acids during a field campaign in coastal Hong Kong. The oxygen isotopic signatures suggest that hypohalous acids account for more sulfate production than the traditional oxidant hydrogen peroxide (H2O2) in both clean air masses from the South China Sea and polluted air masses from the southeast coast of China. Throughout the campaign, the contribution of hypohalous acids to sulfate production is estimated to range from 30 to 76%. Among the hypohalous acids, the contribution of HOI to sulfate production is largest in clean air masses, whereas the contribution of HOI and HOCl are comparable in polluted air masses. Our results highlight the need for future measurements of HOI in the coastal marine boundary layer.

Urban air pollution in cold environments poses a significant public health risk. However, the physicochemical processes determining the concentrations of primary and secondary pollutants remain poorly understood. This is due to fundamentally different conditions compared to warmer environments, such as extremely shallow polluted surface layers (PSLs) and low ultraviolet radiation. We apply an observation-driven chemical transport model to a multi-day persistent PSL event during the 2022 Alaskan Layered Pollution and Chemical Analysis (ALPACA) experiment in Fairbanks, AK, USA. The simulations account for pollutant emissions, multi-phase chemical kinetics, and turbulent and advective exchange of PSL air with the clean background atmosphere. This exchange is continuous, occurs on a time scale of 30 min to 3.5 h, and is essential for an accurate representation of the PSL composition. We find that measured diurnal cycles of particulate nitrate reflect the interplay between photochemical nitric acid formation and the loss of nitrate aerosol from of the PSL through mixing with clean background air. The continuous removal and replenishment of PSL aerosol counteracts self-acidification and prevents coagulation with acidic primary sulfate aerosol, thus sustaining sulfate and hydromethanesulfonate (HMS) formation throughout the pollution event. Sensitivity calculations show that within the coupled chemistry and transport system of a shallow PSL, reductions of nitrogen oxide emissions can change the multi-phase oxidation regime and thereby even increase the levels of secondary nitrate and sulfate in the PSL.

Reactive bromine species in the troposphere affect ozone (O3) as well as the cycling and fate of hydrogen oxide (HO x ) and nitrogen oxide (NO x ) radicals, with implications for the oxidative capacity of the atmosphere, greenhouse gas budgets, air quality, and human health. However, uncertainties remain in the global distribution and chemical mechanism of inorganic bromine with few in situ observations, especially outside of polar regions. We present results of speciated gas-phase reactive bromine observations made in the Marine Boundary Layer (MBL) at a coastal site in the Northwest Atlantic Ocean during January and February 2023, as part of the Bermuda boundary Layer Experiment on the Atmospheric Chemistry of Halogens (BLEACH) campaign. Using an iodide-adduct high resolution time of flight chemical ionization mass spectrometer (HR-ToF-CIMS) equipped with a custom atmospheric pressure transverse ion-molecule reaction region (t-IMR), we observe subppt levels of atmospheric inorganic bromine compounds including Br2, BrCl, HOBr, BrONO2, and BrO in the marine boundary layer. To our knowledge, these are the first reported in situ observations of gaseous bromine nitrate (BrONO2). An experimental calibration for hypobromous acid (HOBr) produces sensitivity values in agreement with previous studies and theoretical calculations involving binding enthalpies, which we use to convert measured ion signals to units of ambient mixing ratios. We characterize the relationship between reactive bromine levels and surrounding environmental factors including meteorological conditions, influences from local pollution sources, and "clean marine" periods of onshore winds. Measured mixing ratios for all bromine species are found to be much lower than predicted by a GEOS-Chem model simulation, which includes fully coupled reactive halogen chemistry. We discuss the implications of this discrepancy on reactive bromine sources and multiphase chemistry, highlighting the need to better constrain heterogeneous uptake rates for key species.

Tropospheric reactive iodine influences the oxidizing capacity of the atmosphere and serves as an important source of ultra-fine particles. However, the paucity of observations of gas-phase and aerosol iodine, combined with incomplete understanding and representation of iodine chemistry in models, leads to substantial uncertainties in understanding iodine abundance, speciation, and impacts. Motivated by known gaps in previous modeling studies, we introduced speciated aerosol iodine and aerosol iodide recycling to the global chemical transport model, GEOS-Chem. Modeled aerosol iodine is speciated into fine and coarse mode soluble organic iodine (SOI), iodate, and iodide. Aerosol iodide is recycled into the gas phase via heterogeneous chemistry involving halogen nitrates and hypohalous acids to form I2, ICl, and IBr, which represents an additional source of gas-phase iodine to the atmosphere. Iodide dehalogenation doubles the tropospheric burden of reactive iodine (Iy) while reducing model-measurement bias for IO and aerosol iodine. The rate of aerosol iodine conversion to Iy is more than twice as fast as the combined rates of inorganic ocean emissions and the photolysis of organic iodine gases, suggesting that aerosols are important in mediating the abundance and lifetime of tropospheric Iy. The incorporation of SOI and iodate into the model prevents iodide dehalogenation by partitioning iodide into less reactive reservoirs, which has a stabilizing effect for reactive iodine chemistry. These findings have implications for reactive halogen abundances and global oxidant budgets in the troposphere.

The Arctic climate is sensitive to aerosol abundance, making accurate historical records of atmospheric aerosols essential for understanding the recent rapid Arctic warming. We present 235-year ice-core records of sulfate, nitrate, chloride and other impurities from central Greenland, and compare them with emission inventories in the surrounding continents. The comparisons reveal a high correlation between the ice-core records and North American emissions, consistent with North America being the dominant pollution source region to Greenland. Since around 1970, sulfate, nitrate and chloride exhibit divergent patterns relative to their pre-1970 increasing trends, in parallel with anthropogenic emissions. In particular, sulfate declined in step with North American SO2 emissions but decreased more rapidly after ∼1990, probably due to combined effects from the enhanced sulfate loss in the source regions due to intensified in-cloud sulfur oxidation and the reduced transport efficiency. Nitrate tracked NOx emissions since ∼1900 until 1990, but remained high after 1990 when anthropogenic emissions in all source regions decreased. This post-1990 pattern may arise from the increasing natural NOx emissions in the Arctic, while feedbacks of atmospheric chemistry to a changing atmospheric acidity during this period may also contribute through affecting the phase partitioning and then long-range transport of nitrate. Chloride also paralleled with anthropogenic emissions since the 1960s but was further modulated by acid displacement processes, as indicated by the covariation of Cl− excess with reconstructed snow acidity. Our results demonstrate that Arctic aerosols reflect both emission controls and their modulation by atmospheric chemistry and transport.

We investigated how various sources contributed to observations of over 40 trace gas and particulate species in a typical Fairbanks residential neighborhood during the Alaskan Layered Pollution and Chemical Analysis campaign in January–February 2022. Aromatic volatile organic compounds (VOCs) accounted for ∼50% of measured VOCs (molar ratio), while methanol and ethanol accounted for ∼34%. The total wintertime VOC burden and contribution from aromatics were much higher than other US urban areas. Based on diel cycles and positive matrix factorization (PMF) analyses, we find traffic was the largest source of NO, CO, black carbon, and aromatic VOCs. Formic and acetic acid, hydroxyacetone, furanoids, and other VOCs were primarily attributed to residential wood combustion (RWC). Formaldehyde was one of several VOCs featuring significant contributions from multiple sources: RWC (∼35%), aging (∼30%), traffic (∼21%), and heating oil combustion (HO, ∼14%). PMF solutions assigned primary fine particulate matter to RWC (10%–30%), traffic (25%–40%), and HO (30%–60%), the latter likely reflecting high sulfur emissions from older furnaces and fast secondary chemistry. Despite cold and dark conditions, secondary processes impacted many trace gas and particle species' budget by ±10%–20% and more in some cases. Transport of O 3 ‐rich regional air into Fairbanks contributed to aging, specifically NO 3 radical formation. This work highlights a long‐term trend observed in Fairbanks: increasing traffic and decreasing RWC relative contributions as total pollution decreases. Fairbanks exports a relatively fresh pollutant mixture to the regional arctic, the fate of which warrants future study.

Ice core nitrate can serve as a valuable tracer for past atmospheric nitrogen oxide (NOx) and oxidant concentrations. However, photolysis-driven postdepositional processing can alter ice core nitrate signals and complicate their interpretation. We present a new nitrogen isotope (delta 15N) record of nitrate measured in the West Antarctic Ice Sheet (WAIS) Divide ice core covering the last glacial period and Holocene. The glacial delta 15N(NO3-) is substantially higher than the Holocene delta 15N(NO3-), with a glacial-interglacial difference of (26.6 +/- 5.7) parts per thousand. All samples exhibit a strong negative correlation between delta 15N(NO3-) and the snow accumulation rate, suggesting that postdepositional processing is the primary driver of the delta 15N(NO3-) variability. Photochemical model calculations indicate that changes in the degree of postdepositional processing can fully explain the observed glacial-interglacial delta 15N(NO3-) difference, with 12.7% and 31.8% of nitrate mass loss during the Holocene and glacial climate, respectively. Comparison with the Greenland GISP2 delta 15N(NO3-) record indicates that the glacial level of postdepositional modification was higher at WAIS Divide despite its snow accumulation rate was twice that at Summit, Greenland. This is due to the higher light-absorbing impurity contents in the GISP2 ice core that reduced postdepositional losses. We also assess the ability of using delta 15N(NO3-) for past surface mass balance (SMB) reconstruction at WAIS Divide. We find this proxy is reliable for regions/periods with snow accumulation rates lower than 150 kg m-2 a-1. While exceeding this threshold, the assumption of a constant delta 15N of initially deposited nitrate is no longer valid, leading to significant bias in SMB reconstruction such as WAIS Divide.

Reactions of dissolved sulfur dioxide (SO2) with aldehydes form particulate hydroxyalkylsulfonates, including hydroxymethanesulfonate (HMS), hydroxyethanesulfonate (HES), dihydroxyethanesulfonate (DHES), and hydroxyacetylmethanesulfonate (HAMS). Recent work has shown enhanced production of HMS within aqueous particles at low temperatures; however, little is known about the presence of HES, DHES, and HAMS. During Jan.-Feb. 2022 in Fairbanks, Alaska, these hydroxyalkylsulfonates were identified and quantified within 340,877 individual aerosol particles using single-particle mass spectrometry. HMS was identified within 27 +/- 3% of the particle number concentration in the size range of 0.1-1.0 mu m. The primary source of the majority (similar to 94%, by number) of individual HMS-containing particles was identified as residential heating (wood and oil combustion). The average HMS mass fraction within these HMS-containing particles was 7.0%. The number fraction of HMS-containing particles increased with particle diameter and plateaued at similar to 500 nm, consistent with aqueous-phase processing. Therefore, locally emitted residential heating combustion particles accumulated water, promoting aqueous-phase reactions and secondary organosulfur formation. Notably, HES, DHES, and HAMS accounted for 48 +/- 14% of the average 0.1-1.0 mu m S(IV) mass. Together, the identified hydroxyalkylsulfonates, including HMS, comprised similar to 90% of the measured S(IV) mass, with inorganic S(IV) estimated at similar to 10%. Therefore, focusing measurements and modeling solely on sulfate and/or HMS may underestimate organic S(IV) and misrepresent the sulfur budget. We utilize single-particle and bulk aerosol measurements to identify and quantify often misidentified organosulfur species formed in low-temperature, aqueous aerosol during urban winter.

The Brewer-Dobson circulation is the global atmospheric overturning circulation and regulates global mass and energy redistributions in the stratosphere as well as mass exchange between the troposphere and the stratosphere. Through transporting O3 to the troposphere and redistributing O3 in the stratosphere that influence surface UV-B radiation, the Brewer-Dobson circulation also plays an critical role in atmospheric oxidation capacity which matters for air quality, greenhouse gas removal and climate change. While many studies have assessed changes in the Brewer-Dobson circulation in the past few decades and in the future in response to greenhouse gas warming, that how Brewer-Dobson circulation has changed in the past climate is poorly constrained and limited modelling studies reached opposite results on the strength of the Brewer-Dobson circulation in the last glacial maximum (LGM). Atmospheric nitrate O-17 excess signal is sensitive to O3 abundance, and the latter is influenced by the strength of the Brewer-Dobson circulation especially during climate transitions (e.g., the glacial-interglacial cycle and the abrupt climate events). By comparing the nitrate O-17 excess records from the GISP2 and WAIS divide ice cores, we find a coherent changes in the O-17 excess signal from the LGM to the Holocene, suggesting a stronger Brewer-Dobson circulation in the LGM. In addition, during abrupt climate changes (i.e., the D-O events in Greenland and the corresponding AIM events in Antarctica), there is an apparent difference in the changes of O-17 excess in response to temperature in Greenland and Antarctica, a result implies that the Brewer-Dobson circulation should be enhanced when Greenland cools. These results are consistent with earlier model studies, but are in conflict with recent model work which predicts a weakened Brewer-Dobson circulation in the LGM. Specific mechanisms driving the response of the Brewer-Dobson circulation to past climate changes remains to be explored.

Naturally occurring chlorate (ClO 3 − ) has been observed on Earth and potentially plays important roles in hydrology and mineralogy on Mars. However, natural sources of chlorate are uncertain. Here, we quantify the importance of atmospheric sources of chlorate. We use GEOS‐Chem, a global three‐dimensional chemical transport model, to simulate the formation, photochemical loss, transport, and deposition of atmospheric chlorate on present‐day Earth. We also develop a method to estimate the 17 O‐excess (∆ 17 O) and the 36 Cl‐to‐total‐Cl ratio ( 36 Cl/Cl) of atmospheric chlorate to interpret the observed isotopic composition of chlorate accumulated in desert soils. The model predicts that gas‐phase chemistry can produce 15 Gg Cl year −1 of chloric acid (HClO 3 ), which predominantly is taken up by aerosols to form particulate chlorate. Comparing the model with observations suggests that particulate chlorate undergoes chemical loss in the atmosphere, which controls the amount reaching Earth's surface. We show that the initial ∆ 17 O that atmospheric chlorate acquires during formation would be erased rapidly in acidic aerosols due to the exchange of oxygen atoms with water. The analysis of 36 Cl/Cl does not preclude a partial stratospheric origin for chlorate deposits in the Atacama Desert. In Death Valley, aqueous‐phase oxidation of oxychlorine species and anthropogenic activities potentially have greater influence. Our findings highlight the need for more observations of atmospheric chlorate and laboratory measurements of its reactivity in acidic conditions. Atmospheric chemistry should be considered in the future studies of the origin of chlorate on Mars.

Lagrangian tracer simulations are deployed to investigate processes influencing vertical and horizontal dispersion of anthropogenic pollution in Fairbanks, Alaska, during the Alaskan Layered Pollution and Chemical Analysis (ALPACA) 2022 field campaign. Simulated concentrations of carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen oxides (NOx), including surface and elevated sources, are the highest at the surface under very cold stable conditions. Pollution enhancements above the surface (50–300 m) are mainly attributed to elevated power plant emissions. Both surface and elevated sources contribute to Fairbanks' regional pollution that is transported downwind, primarily to the south-west, and may contribute to wintertime Arctic haze. Inclusion of a novel power plant plume rise treatment that considers the presence of surface and elevated temperature inversion layers leads to improved agreement with observed CO and NOx plumes, with discrepancies attributed to, for example, displacement of plumes by modelled winds. At the surface, model results show that observed CO variability is largely driven by meteorology and, to a lesser extent, by emissions, although simulated tracers are sensitive to modelled vertical dispersion. Modelled underestimation of surface NOx during very cold polluted conditions is considerably improved following the inclusion of substantial increases in diesel vehicle NOx emissions at cold temperatures (e.g. a factor of 6 at −30 °C). In contrast, overestimation of surface SO2 is attributed mainly to model deficiencies in vertical dispersion of elevated (5–18 m) space heating emissions. This study highlights the need for improvements to local wintertime Arctic anthropogenic surface and elevated emissions and improved simulation of Arctic stable boundary layers.

The high levels of sulfate in wintertime particles in Fairbanks, Alaska, are a subject of keen research interest and regulatory concern. Recent results from the 2022 Alaska Layered Pollution And Chemical Analysis (ALPACA) field campaign indicate that roughly 40 % of wintertime sulfate in Fairbanks is secondary, with hydrogen peroxide (HOOH) the dominant oxidant. Since formation of HOOH in the gas phase should be negligible during ALPACA because of high levels of NOx, we examined whether reactions within particles could be a significant source of HOOH. To test this, we collected particulate matter (PM) samples during the ALPACA campaign, extracted them, illuminated them with simulated sunlight, and measured HOOH production. Aqueous extracts showed significant light absorption, a result of brown carbon (BrC) from sources such as residential wood combustion. Photoformation rates of HOOH in the PM extracts (PMEs; normalized to Fairbanks winter sunlight) range from 6 to 71 µM h−1. While light absorption is nearly independent of pH, HOOH formation rates decrease with increasing pH. Extrapolating to the concentrated conditions of aerosol liquid water (ALW) gives an average rate of in-particle HOOH formation of ∼ 0.1 M h−1. Corresponding rates of sulfate formation from particle-produced HOOH are 0.05–0.5 µgm-3h-1, accounting for a significant portion of the secondary sulfate production rate. Our results show that HOOH formed in particles makes an important contribution to sulfate formation in ambient wintertime particles, even under the low actinic flux conditions typical of winter in subarctic locations like Fairbanks.

The accurate representation of cloud droplet number concentration (N d ) is crucial for predicting future climate. However, models often underestimate N d over the Southern Ocean (SO), where natural sources dominate, and aerosols are composed primarily of marine biogenic sulfate and sea spray. This study uses a range of diverse data sets to evaluate and untangle biases in Energy Exascale Earth System Model version 2 (E3SMv2) simulated clouds, aerosols, and sulfur species. The default E3SMv2 underestimates N d over SO by a factor of 2 when compared with observations in 3 km‐resolution simulations. Updating the dimethyl sulfide (DMS) emission and chemistry leads to a better agreement between the model and the observations in N d and boundary layer aerosols, but low biases persist in the free tropospheric aerosol concentrations larger than 70 nm, possibly attributable to insufficient particle growth. Furthermore, updates to DMS emissions and chemistry resulted in reduced vertical DMS concentrations and improved the overall agreement between simulated and observed DMS vertical profiles. Preliminary evaluation also reveals remaining biases in simulated sulfur species, including overestimation in DMS at high latitudes, and in simulated sulfate mass concentration, highlighting the necessity for further efforts to improve the model treatment of relevant processes.

Dimethyl sulfide (DMS) is primarily emitted by marine phytoplankton and oxidized in the atmosphere to form methanesulfonic acid (MSA) and sulfate aerosols. Ice cores in regions affected by anthropogenic pollution show an industrial-era decline in MSA, which has previously been interpreted as indicating a decline in phytoplankton abundance. However, a simultaneous increase in DMS-derived sulfate (bioSO4) in a Greenland ice core suggests that pollution-driven oxidant changes caused the decline in MSA by influencing the relative production of MSA versus bioSO4. Here we use GEOS-Chem, a global chemical transport model, and a zero-dimensional box model over three time periods (preindustrial era, peak North Atlantic NOx pollution, and 21st century) to investigate the chemical drivers of industrial-era changes in MSA and bioSO4, and we examine whether four DMS oxidation mechanisms reproduce trends and seasonality in observations. We find that box model and GEOS-Chem simulations can only partially reproduce ice core trends in MSA and bioSO4 and that wide variation in model results reflects sensitivity to DMS oxidation mechanism and oxidant concentrations. Our simulations support the hypothesized increase in DMS oxidation by the nitrate radical over the industrial era, which increases bioSO4 production, but competing factors such as oxidation by BrO result in increased MSA production in some simulations, which is inconsistent with observations. To improve understanding of DMS oxidation, future work should investigate aqueous-phase chemistry, which produces 82 %–99 % of MSA and bioSO4 in our simulations, and constrain atmospheric oxidant concentrations, including the nitrate radical, hydroxyl radical, and reactive halogens.

Subarctic cities notoriously experience severe winter pollution episodes with fine particle (PM2.5) concentrations above 35 µg m−3, the US Environmental Protection Agency (EPA) 24 h standard. While winter sources of primary particles in Fairbanks, Alaska, have been studied, the chemistry driving secondary particle formation is elusive. Biomass burning is a major source of wintertime primary particles, making the PM2.5 rich in light-absorbing brown carbon (BrC). When BrC absorbs sunlight, it produces photooxidants – reactive species potentially important for secondary sulfate and secondary organic aerosol formation – yet photooxidant measurements in high-latitude PM2.5 remain scarce. During the winter of 2022 Alaskan Layered Pollution And Chemical Analysis (ALPACA) field campaign in Fairbanks, we collected PM filters, extracted the filters into water, and exposed the extracts to simulated sunlight to characterize the production of three photooxidants: oxidizing triplet excited states of BrC, singlet molecular oxygen, and hydroxyl radical. Next, we used our measurements to model photooxidant production in highly concentrated aerosol liquid water. While conventional wisdom indicates photochemistry is limited during high-latitude winters, we find that BrC photochemistry is significant: we predict high triplet and singlet oxygen daytime particle concentrations up to 2×10-12 and 3×10-11 M, respectively, with moderate hydroxyl radical concentrations up to 5×10-15 M. Although our modeling predicts that triplets account for 0.4 %–10 % of daytime secondary sulfate formation, particle photochemistry cumulatively dominates, generating 76 % of daytime secondary sulfate formation, largely due to in-particle hydrogen peroxide, which contributes 25 %–54 %. Finally, we estimate triplet production rates year-round, revealing the highest rates in late winter when Fairbanks experiences severe pollution and in summer when wildfires generate BrC.