Abstract. Fine aerosol liquid water content (ALWC) and acidity (pH) are co-determined, pH primarily reflects the ratio of hydrogen-ion concentrations in air (H+air) to ALWC. Inorganic ions dominate H+air and often ALWC, whereas organic aerosol (OA) mainly adds water. Added OA water, however, shifts gas–particle partitioning of semi-volatile species, altering H+air itself, and thus pH and the aqueous-phase processes it governs. We characterize PM1 ALWC and pH over four North American cities using airborne AEROMMA measurements (June–August 2023), including periods influenced by aged wildfire smoke with high OA but little effect on inorganic species. ALWC and pH were predicted with ISORROPIA-Lite, which includes OA water, and evaluated against measured partitioning of NH3–NH4+ and HNO3–NO3-. Predicted ammonia partitioning agreed with observations (R2 > 0.75, within ~±10 %), whereas nitrate was systematically over-predicted by ~27 %. Outside smoke, inorganic ions dominated ALWC despite being a minor mass fraction; within smoke, organic water dominated (45–65 %). Across all cities, pH remained low and varied little (1.5–2.5, 10th–90th percentile). Particle-phase fractions were 0.17–0.56 for NH4+ but only 0.1–0.22 for NO3-, giving nitrate less redistribution and pH-buffering capacity than NH4+. Including OA water raised pH during smoke by at most 0.62 units and improved HNO3–NO3- agreement. Summertime PM1 acidity thus remains persistently low and thermodynamically stabilized across diverse composition regimes, with wide-ranging implications for regulatory, environmental, and human-health impacts.

Abstract. Aerosols play an important role for atmospheric radiative transfer in biomass burning (BB) plumes, where they control photochemistry, direct radiative forcing, and radiation-induced atmospheric dynamics. The optical properties of BB aerosol, however, remain poorly constrained, with respect to their absorptive properties at ultraviolet and visible wavelengths. In-situ observations show considerable variability due to heterogeneity in BB plumes, and different measurement methods do not agree with each other. To overcome this challenge, we have developed an algorithm based on the VLIDORT for photochemistry (VPC) radiative transfer model to retrieve the imaginary refractive index k(λ) from airborne actinic flux observations at wavelengths λ of 310–440 nm. Using three flights from NASA/NOAA’s Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) airborne experiment, we obtain values k(387 nm) between 0.02 and 0.03 for different transects, while the absorption Angstrom exponent αk is 4±1. Volume absorption cross section (VAC) and single scattering albedo generally agree with in situ observations, but show less variability, most likely because of the inherent spatial averaging of our observations. k(λ), VAC and single-scattering co-albedo decrease with physical plume age, with half-lives τ1/2 of 13±3, 16±4 and 17±4 hours, respectively. Based on our observations, we present a parameterization of the absorptive properties of BB aerosol from western US wildfires as a function of wavelength and plume age, which will help to improve the representation of BB aerosol in models.

Declining nitrogen oxide (NOx = NO + NO2) emissions have transformed oxidation pathways in urban atmospheres, with implications for air quality. Organic peroxy radicals (RO2), key intermediates in volatile organic compound oxidation, typically react with NO to form ozone (O3). Under lower-NO conditions, alternative RO2 fates, including isomerization forming highly oxidized organic molecules (HOMs), can enhance secondary organic aerosol (SOA) production. We combine aircraft observations over four major North American cities with geostationary satellite data to characterize isoprene-derived RO2 fate across urban environments. We infer RO2 bimolecular lifetimes (τbi) as a proxy for isomerization potential, finding longer τbi (17 ± 11 seconds) in New York, Chicago, and Toronto compared to Los Angeles (7 ± 6 seconds). Satellite measurements reveal that long τbi is widespread across urban North America, suggesting that declining NOx is likely to lead to greater HOM formation in urban regions. These findings indicate that atmospheric models omitting RO2 isomerization chemistry may incorrectly simulate organic oxidation and the subsequent oxidation state of volatile organic compounds and SOA.

Evidence links air pollution to dementia, yet its role in Lewy body dementia (LBD) remains unclear. In this work, we showed in a cohort of 56.5 million individuals across the United States that fine particulate matter (PM2.5) exposure raises LBD risk. Mechanistically, we found that PM2.5 exposure led to brain atrophy in wild-type mice, an effect not seen in α-synuclein (αSyn)-deficient mice. PM2.5 exposure generated a highly pathogenic αSyn strain, PM2.5-induced preformed fibril (PM-PFF), with enhanced proteinase K resistance and neurotoxicity, resembling αSyn LBD strains. PM2.5 samples from China, the United States, and Europe consistently induced proteinase-resistant αSyn strains and in vivo pathology. Transcriptomic analyses revealed shared responses between PM2.5-exposed mice and LBD patients, underscoring PM2.5's role in LBD and stressing the need for interventions to reduce air pollution and its associated neurological disease burden.

This paper presents the findings from a collaborative interlaboratory comparison exercise designed to assess oxidative potential (OP) measurements conducted by 20 laboratories worldwide. This study represents an innovative effort as the first exercise specifically aimed at harmonising this type of OP assay, setting a new benchmark in the field. Over the last decade, there has been a noticeable increase in OP studies, with numerous research groups investigating the effects of exposure to air pollution particles through the evaluation of OP levels. However, the absence of standardised methods for OP measurements has resulted in variability in results across different groups, rendering meaningful comparisons challenging. To address this issue, this study engages in an international effort to compare OP measurements using a simplified method (with a dithiothreitol (DTT) assay). Here, we quantify the OP in liquid samples to focus on the protocol measurement itself, while future international OP interlaboratory comparisons (ILCs) should aim to assess the whole chain process, including the sample extraction. We analyse the similarities and discrepancies observed in the results, identifying the critical parameters (such as the instrument used, the use of a simplified protocol, the delivery and analysis time) that could influence OP measurements and provide recommendations for future studies and interlaboratory comparisons even if other crucial aspects, such as sampling PM methods, sample storage, extraction methods and conditions, and the evaluation of other OP assays, still need to be standardised. This collaborative approach enhances the robustness of the OP DTT assay and paves the way for future studies to build on a unified framework. This pioneering work concludes that interlaboratory comparisons provide essential insights into the OP metric and are crucial to move toward the harmonisation of OP measurements.

Biomass burning (BB) affects air quality and climate by releasing large quantities of gaseous and particulate pollutants into the atmosphere. Photochemical processing during daylight transforms these emissions, influencing their overall environmental impact. Accurately quantifying the photochemical drivers, namely actinic flux and photolysis frequencies, is crucial to constraining this chemistry. However, the complex radiative transfer within BB plumes presents a significant challenge for both direct observations and numerical models. This study introduces an expanded version of the 1D VLIDORT-QS radiative transfer (RT) model, named VLIDORT for photochemistry (VPC). VPC is designed for photochemical and remote sensing applications, particularly in BB plumes and other complex scenarios. To validate VPC and investigate photochemical conditions within BB plumes, the model was used to simulate spatial distributions of actinic fluxes and photolysis frequencies for the Shady wildfire (Idaho, US, 2019) based on plume composition data from the NOAA/NASA FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality) campaign. Comparison between modeling results and observations by the CAFS (charged-coupled device actinic flux spectroradiometer) yields a modeling accuracy of 10 %–20 %. Systematic biases between the model and observations are within 2 %, indicating that the uncertainties are most likely due to variability in the input data caused by the inhomogeneity of the plume as well as 3D RT effects not captured in the model. Random uncertainties are largest in the ultraviolet (UV) spectral range, where they are dominated by uncertainties in the plume particle size distribution and brown carbon (BrC) absorptive properties. The modeled actinic fluxes show a decrease from the plume top to the bottom of the plume with a strong spectral dependence caused by BrC absorption, which darkens the plume towards shorter wavelengths. In the visible (Vis) spectral range, actinic fluxes above the plume are enhanced by up to 60 %. In contrast, in the UV, actinic fluxes above the plume are not affected or even reduced by up to 10 %. Strong reductions exceeding an order of magnitude in and below the plume occur for both spectral ranges but are more pronounced in the UV.

Fairbanks, Alaska, is a sub-Arctic city that frequently suffers from the non-attainment of national air quality standards in the wintertime due to the coincidence of weak atmospheric dispersion and increased local emissions. As part of the Alaskan Layered Pollution and Chemical Analysis (ALPACA) campaign, we deployed a Chemical Analysis of Aerosol Online (CHARON) inlet coupled with a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF MS) and an Aerodyne high-resolution aerosol mass spectrometer (AMS) to measure organic aerosol (OA) and non-refractory submicron particulate matter (NR-PM1), respectively. We deployed a positive matrix factorization (PMF) analysis for the source identification of NR-PM1. The AMS analysis identified three primary factors: biomass burning, hydrocarbon-like, and cooking factors, which together accounted for 28 %, 38 %, and 11 % of the total OA, respectively. Additionally, a combined organic and inorganic PMF analysis revealed two further factors: one enriched in nitrates and another rich in sulfates of organic and inorganic origin. The PTRCHARON factorization could identify four primary sources from residential heating: one from oil combustion and three from wood combustion, categorized as low temperature, softwood, and hardwood. Collectively, all residential heating factors accounted for 79 % of the total OA. Cooking and road transport were also recognized as primary contributors to the overall emission profile provided by PTRCHARON. All PMF analyses could apportion a single oxygenated secondary organic factor. These results demonstrate the complementarity of the two instruments and their ability to describe the complex chemical composition of PM1 and related sources. This work further demonstrates the capability of PTRCHARON to provide both qualitative and quantitative information, offering a comprehensive understanding of the OA sources. Such insights into the sources of submicron aerosols can ultimately assist environmental regulators and citizens in improving the air quality in Fairbanks and in rapidly urbanizing regional sub-Arctic areas.

NO2) initiates complex photochemical processes that produce organic nitrates (RONO2) and influence atmospheric oxidation capacity, air quality, and the fate of reactive nitrogen. However, tracking the chemical fate of RONO2 remains challenging as it includes pathways such as renoxification, aerosol partitioning, deposition, and/or hydrolysis to nitric acid (HNO3). Stable oxygen (Delta O-17, delta O-18) and nitrogen (delta N-15) isotope measurements can provide a unique tool to probe these processes, as NOy species can exhibit distinct isotopic signatures due to characteristic oxygen-transfer dynamics and isotope fractionation. Here, we present chamber experiments of alpha-pinene oxidation in the presence of NOx under a range of oxidant and photochemical conditions, reporting the Delta O-17, delta O-18, and delta N-15 values of simultaneously collected NO2, HNO3, and particulate nitrate (pNO(3)), the latter of which derived predominantly from RONO2 in the conducted experiments. A strong linear relationship between delta O-18 and Delta O-17 across all NOy species (r=0.992; p<0.01) supports a two-endmember mixing model, in which oxygen atoms are transferred from isotopically distinct sources that include ozone (O-3) with high delta O-18 and Delta O-17 as well as peroxy and hydroxyl radicals (RO2, HO2, OH) with lower values. Nitrogen isotope fractionation, quantified as the difference in delta N-15 values (Delta delta N-15), revealed consistently positive Delta delta N-15(HNO3-NO2) values (+28.9 +/- 13.4 parts per thousand in daytime experiments; +22.2 +/- 1.4 parts per thousand at night) and negative Delta delta N-15(pNO(3)-NO2) values (-13.6 +/- 5.8 parts per thousand in daytime experiments). This reflected distinct formation pathways and isotope effects including NOx photochemical cycling, thermal dinitrogen pentoxide (N2O5)-nitrate radical (NO3)-NO2 equilibrium, and HNO3 production mechanisms. Box-model simulations based on Delta O-17 values as a constraint were conducted using a newly developed gas-phase mechanism, which reproduced Delta O-17(NO2) and Delta O-17(pNO(3)) (compared to simulated Delta O-17(RONO2)) accurately, with an average model bias of 0.9 +/- 2.4 parts per thousand (R-2=0.98) and -1.4 +/- 2.4 parts per thousand (R-2= 0.55 and R-2= 0.97 when excluding one outlier), respectively. We further empirically derived important isotopic parameters such as the Delta O-17 value transferred from O-3 through comparison of model-simulated oxygen atom source contributions with observed Delta O-17 values for NO2 and pNO(3) across experiments. This yielded best-fit slopes of 39.4 +/- 0.6 parts per thousand for NOx photochemical cycling and 41.7 +/- 1.2 parts per thousand for RONO2 formation, consistent with near-surface observations of Delta O-17 in the terminal oxygen atom of O-3. Despite the agreement with NO2 and RONO2, accurately simulating Delta O-17(HNO3) proved challenging. Sensitivity tests revealed that model biases likely stemmed from a combination of factors including background HNO3 chamber blanks affecting low-NOx experiments, missing N2O5 heterogeneous hydrolysis under nighttime conditions, and an overestimation in the Delta O-17(HNO3) mass balance resulting from the NO2+ OH reaction, which was improved by adjusting the contribution from (2/3)Delta O-17(NO2) to (1/2)Delta O-17(NO2). These adjustments reduced the average model bias in Delta O-17(HNO3) from 6.7 +/- 3.3 parts per thousand (R-2= 0.39) in the base mechanism to 1.6 +/- 1.3 parts per thousand (R-2= 0.48) in the modified mechanism. These findings demonstrate the utility of Delta O-17 and delta N-15 for disentangling nitrate formation mechanisms, while also highlighting critical gaps in our understanding of the isotope dynamics involving HNO3 formation. Future experimental work targeting isolated HNO3 pathways is essential to refine isotopic mass balance assumptions and nitrogen isotope fractionation.

Sulfate comprises an average of 20% of the ambient PM2.5 mass during the winter months in Fairbanks, as indicated by 24-hour average filter measurements. During ALPACA 2022 field campaign (Jan 15th-Feb28th of 2022), we deployed two aerosol mass spectrometers (AMS) and one aerosol chemical speciation monitor (ACSM) at three urban sites, combined with Scanning Mobility Particle Sizer (SMPS), to examine the evolution of aerosol composition and size distribution at a sub-hourly time scale. During an intense pollution episode (ambient temperature is between -25 and -35 °C), all three instruments (two AMS and one ACSM) exhibit a sharp increase in sulfate mass within a matter of hours, while organic aerosols, black carbon and SO2 concentrations remain relatively stable. This notable increase in sulfate mass contributes to approximately half of the observed change in ambient PM2.5. The abrupt rise in sulfate mass is concurrent with a substantial increase in particle number density within the accumulation mode (100-1000 nm), suggesting the secondary formation of sulfate onto pre-existing aerosols. We further investigate possible mechanisms and have ruled out the possible role of cloud chemistry and transition metal ion. The rapid formation of sulfate seems to be linked to the ambient level of nitrogen oxides and, possibly, sunlight. Further investigation is underway to elucidate the intricate connections underlying this rapid sulfate formation.

Atmospheric brown carbon (BrC) from wildfires is a key component of light-absorbing carbon that significantly contributes to global radiative forcing, but its atmospheric evolution and lifetime remain poorly understood. In this study, we investigate BrC evolution by synthesizing data from one laboratory campaign and four aircraft campaigns spanning diverse spatial scales across North America. To estimate initial conditions for evaluating plume evolution, we develop a method to parametrize the emission ratios of BrC and other species using commonly measured inert tracers, acetonitrile and hydrogen cyanide. The evolution of BrC absorption in the free troposphere is characterized as a function of hydroxyl radical (OH) exposure, yielding an effective photochemical rate constant of 9.7-1.6+4.8 × 10-12 cm3 molecule-1 s-1. The relatively slow reaction rate results in small BrC decay within the first few hours after emission, making it difficult to distinguish from source variability. This helps explain the absence of clear evolutionary trends in near-field studies. Assuming an OH concentration of 1.26 × 106 molecules cm-3, this rate constant corresponds to an e-folding lifetime of approximately 23 h. After extensive photooxidation (OH exposure ∼1012 molecules cm-3 s), 4 ± 2% of the emitted BrC persists, representing a recalcitrant fraction with potential long-term climate impacts. These results improve our understanding of BrC variability and photochemical processing and provide critical constraints for modeling its impacts on climate.

The indoor air quality of a residential home during winter in Fairbanks, Alaska, was investigated and contrasted with outdoor levels. Twenty-four-hour average indoor and outdoor filter samples were collected from January 17 to February 25, 2022, in a residential area with high outdoor PM2.5 concentrations. The oxidative potential of PM2.5 was determined using the dithiothreitol-depletion assay (OPDTT). For the unoccupied house, the background indoor-to-outdoor (I/O) ratio of mass-normalized OP (OPmDTT), a measure of the intrinsic health-relevant properties of the aerosol, was less than 1 (0.53 ± 0.37), implying a loss of aerosol toxicity as air was transported indoors. This may result from transport and volatility losses driven by the large gradients in temperature (average outdoor temperature of -19°C/average indoor temperature of 21 °C) or relative humidity (average outdoor RH of 78%/average indoor RH of 11%), or both. Various indoor activities, including pellet stove use, simple cooking experiments, incense burning, and mixtures of these activities, were conducted. The experiments produced PM2.5 with a highly variable OPmDTT. PM2.5 from cooking emissions had the lowest OP values, while pellet stove PM2.5 had the highest. Correlations between volume-normalized OPDTT (OPvDTT), relevant to exposure, and indoor PM2.5 mass concentration during experiments were much lower compared to those in outdoor environments. This suggests that mass concentration alone can be a poor indicator of possible adverse effects of various indoor emissions. These findings highlight the importance of considering both the quantity of particles and sources (chemical composition), as health metrics for indoor air quality.

Hydroxymethanesulfonate (HMS) in fine aerosol particles has been reported at significant concentrations along with sulfate under extreme cold conditions (-35 °C) in Fairbanks, Alaska, a high latitude city. HMS, a component of S(IV) and an adduct of formaldehyde and sulfur dioxide, forms in liquid water. Previous studies may have overestimated HMS concentrations by grouping it with other S(IV) species. In this work, we further investigate HMS and the speciation of S(IV) through the Alaskan Layered Pollution and Chemical Analysis (ALPACA) intensive study in Fairbanks. We developed a method utilizing hydrogen peroxide to isolate HMS and found that approximately 50% of S(IV) is HMS for total suspended particulates and 70% for PM2.5. The remaining unidentified S(IV) species are closely linked to HMS during cold polluted periods, showing strong increases in concentration relative to sulfate with decreasing temperature, a weak dependence on particle water, and similar particle size distributions, suggesting a common aqueous formation process. A portion of the unidentified S(IV) may originate from additional aldehyde-S(IV) adducts that are unstable in the water-based chemical analysis process, but further chemical characterization is needed. These results show the importance of organic S(IV) species in extreme cold environments that promote unique aqueous chemistry in supercooled liquid particles.

Abstract Furans are a major class of volatile organic compounds emitted from biomass burning. Their high reactivity with atmospheric oxidants leads to the formation of secondary organic aerosol (SOA), including secondary brown carbon (BrC) that can affect global climate via interactions with solar radiation. Here, we investigate the optical properties and chemical composition of SOA generated via photooxidation of furfural, 2‐methylfuran, and 3‐methylfuran under dry (RH < 5%) and humid (RH ∼ 50%) conditions in the presence of nitrogen oxides (NOx) and ammonium sulfate seed aerosol. Dry furfural oxidation has the greatest BrC formation, including reduced nitrogen‐containing organic compounds (NOCs) in SOA, which are dominated by amines and amides formed from reactions between carbonyls and ammonia/ammonium. Based on the products detected, we propose novel formation pathways of NOCs in furfural photooxidation, which can contribute to BrC via accretion reactions during the photochemical aging of biomass burning plumes.

The oxidative potential (OP) of outdoor PM2.5 in wintertime Fairbanks, Alaska, is investigated and compared to those in wintertime Atlanta and Los Angeles. Approximately 40 filter samples collected in January-February 2022 at a Fairbanks residential site were analyzed for OP utilizing dithiothreitol-depletion (OPDTT) and hydroxyl-generation (OPOH) assays. The study-average PM2.5 mass concentration was 12.8 μg/m3, with a 1 h average maximum of 89.0 μg/m3. Regression analysis, correlations with source tracers, and contrast between cold and warmer events indicated that OPDTT was mainly sensitive to copper, elemental carbon, and organic aerosol from residential wood burning, and OPOH to iron and organic aerosol from vehicles. Despite low photochemically-driven oxidation rates, the water-soluble fraction of OPDTT was unusually high at 77%, mainly from wood burning emissions. In contrast to other locations, the Fairbanks average PM2.5 mass concentration was higher than Atlanta and Los Angeles, whereas OPDTT in Fairbanks and Atlanta were similar, and Los Angeles had the highest OPDTT and OPOH. Site differences were observed in OP when normalized by both the volume of air sampled and the particle mass concentration, corresponding to exposure and the intrinsic health-related properties of PM2.5, respectively. The sensitivity of OP assays to specific aerosol components and sources can provide insights beyond the PM2.5 mass concentration when assessing air quality.

Abstract. The chemical interaction between nitrogen oxides (NOx = NO + NO2) and α-pinene plays a critical role in air quality and climate. However, uncertainties remain regarding their coupling in NOx loss, renoxification, and oxidation chemistry. To address these gaps, we conducted controlled chamber experiments, analyzing nitric acid (HNO3), NO2, and particulate nitrate (pNO3) for their oxygen and nitrogen stable isotope variations (Δ17O, δ18O, and δ15N). A strong linear relationship between δ18O and Δ17O across experiments revealed contributions of oxygen from ozone (O3) and atmospheric oxygen (O2) in forming reactive radicals. The δ15N values followed the order δ15N(pNO3) < NO2 < HNO3, reflecting isotope fractionation during NOx oxidation. A new chemical mechanism accurately predicted aerosol precursor decay and simulated Δ17O and δ15N values. Simulations showed NOx photochemical cycling and pNO3 formation, primarily from organic nitrate, with Δ17O(NO2) simulations achieving a root mean square error (RMSE) of 1.7 ‰. Improved δ15N(NO2) and pNO3 simulations used a nitrogen isotope fractionation factor (15α) of 0.997 for NO2 + OH reactions. However, modeling Δ17O and δ15N of HNO3 proved challenging, likely due to sampling artifacts. This study provides insights into Δ17O transfer dynamics, nitrogen isotope fractionation, and the role of NOx-BVOC chemistry in air quality, highlighting the potential of Δ17O and δ15N as tools for evaluating complex atmospheric processes.

The Alaskan Layered Pollution And Chemical Analysis (ALPACA) field experiment was a collaborative study designed to improve understanding of pollution sources and chemical processes during winter (cold climate and low-photochemical activity), to investigate indoor pollution, and to study dispersion of pollution as affected by frequent temperature inversions. A number of the research goals were motivated by questions raised by residents of Fairbanks, Alaska, where the study was held. This paper describes the measurement strategies and the conditions encountered during the January and February 2022 field experiment, and reports early examples of how the measurements addressed research goals, particularly those of interest to the residents. Outdoor air measurements showed high concentrations of particulate matter and pollutant gases including volatile organic carbon species. During pollution events, low winds and extremely stable atmospheric conditions trapped pollution below 73 m, an extremely shallow vertical scale. Tethered-balloon-based measurements intercepted plumes aloft, which were associated with power plant point sources through transport modeling. Because cold climate residents spend much of their time indoors, the study included an indoor air quality component, where measurements were made inside and outside a house to study infiltration and indoor sources. In the absence of indoor activities such as cooking and/or heating with a pellet stove, indoor particulate matter concentrations were lower than outdoors; however, cooking and pellet stove burns often caused higher indoor particulate matter concentrations than outdoors. The mass-normalized particulate matter oxidative potential, a health-relevant property measured here by the reactivity with dithiothreiol, of indoor particles varied by source, with cooking particles having less oxidative potential per mass than pellet stove particles.

We conducted a 2-year study utilizing a network of fixed sites with sampling throughout an extended prescribed burning period to characterize the emissions and evolution of smoke from silvicultural prescribed burning at a military base in the southeastern USA. The measurement approach and an assessment of the instrument performance are described. Smoke sources, including those within and off the base, are identified, and plume ages are determined to quantify emissions and study the evolution of smoke PM2.5 (particulate matter with aerodynamic diameters 2.5 mu m or smaller) mass, black carbon (BC), and brown carbon (BrC). Over the 2021 and 2022 prescribed burning seasons (nominally January to May), we identified 64 smoke events based on high levels of PM2.5 mass, BC, BrC, and carbon monoxide (CO), of which 61 were linked to a specific burning area. Smoke transport times were estimated in two ways: using the mean wind speed and the distance between the fire and the measurement site, and from Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back-trajectories. PM2.5 emission ratios based on Delta PM2.5 mass / Delta CO for fresh smoke (age <= 1 h) ranged between 0.04 and 0.18 mu g m-3 ppb-1 with a mean of 0.117 mu g m-3 ppb-1 (median of 0.121 mu g m-3 ppb-1). Both the mean emission ratio and the variability were similar to findings from other prescribed fire studies but were lower than those from wildfires. The mean emission ratios of BC and BrC were 0.014 mu g m-3 ppb-1 and 0.442 Mm-1 ppb-1, respectively. Ozone enhancements (Delta O3) were always observed in plumes detected in the afternoon. Delta PM2.5 mass / Delta CO was observed to increase with plume age in all of the ozone-enhanced plumes, suggesting photochemical secondary aerosol formation. In contrast, Delta BrC/Delta CO was not found to vary with plume ages less than 8 h during photochemically active periods.

Sulfate comprises an average of 20% of the ambient PM2.5 mass during the winter months in Fairbanks, based on 24-hour filter measurements. During the ALPACA 2022 field campaign (Jan 15th-Feb 28th of 2022), we deployed two aerosol mass spectrometers (AMS) and one aerosol chemical speciation monitor (ACSM) at three urban sites, combined with Scanning Mobility Particle Sizers (SMPS), to examine the evolution of aerosol composition and size distribution at a sub-hourly time scale. During an intense pollution episode with ambient temperature between -25 and -35 degrees C, all three instruments (two AMS and one ACSM) recorded a sharp increase in sulfate mass, ranging from 5 to 40 mu g m(-3) within a few hours. This increase contributed up to half of the observed rise in ambient PM2.5 mass concentration and coincided with a substantial shift in the number distribution from particle sizes less than 100 nm diameter (D-p < 100 nm) to larger particles (D-p > 100 nm) with little increase in number concentration. The corresponding increase in the volume concentration and distribution shift to larger particle size suggests the secondary formation of sulfate and organic aerosol onto pre-existing aerosols. Comparing AMS-sulfate (all sulfur species) to inorganic sulfate measured by online particle-into-liquid sampler-ion chromatography (PILS-IC), we find roughly 80% of sulfate increase was due to organic sulfur, consistent with the observation of mass spectral signatures in the AMS of organosulfur compounds. The rapid formation of sulfate appears to coincide with spikes in ambient aldehyde concentrations (formaldehyde and acetaldehyde) and an increase in S(iv) in ambient PM2.5. This likely results from multiphase chemistry, where hydroxymethanesulfonate (HMS) and other aldehyde-S(iv) adducts are formed through reactions between aldehydes and SO2 in deliquesced aerosols. We estimate that all S(iv) species, including HMS, contribute an average of 30% to aerosol sulfur, with a dominant fraction occurring during rapid sulfate increase events. Our work highlights the crucial role of controlling aldehydes to mitigate severe air pollution events in Fairbanks and may apply to other urban areas. It also emphasizes the significance of multiphase chemistry in driving particle growth from Aitken mode to accumulation mode, a key step for aerosol-cloud interactions.