Katharine S. Law

Aromatic hydrocarbons are hazardous air pollutants with high toxicity, but sources in cold urban Arctic winter environments are poorly understood. Surface observations in Fairbanks, Interior Alaska, during the ALPACA field campaign in winter 2022, recorded very high concentrations of benzene, toluene, ethylbenzene, and xylene (BTEX), reaching 40–100 ppb during pollution episodes. Lagrangian BTEX tracer simulations capture observed variability but underestimate observations during pollution episodes. Inclusion of previously overlooked oil space heating emissions and accounting for enhanced gasoline vehicle emissions at cold temperatures and at higher relative humidities (warmer temperatures) substantially reduce model biases during polluted periods, except in very stable conditions when results are sensitive to vertical mixing. In the improved model, traffic, then oil, and wood space heating are the main emission sectors contributing to simulated BTEX. Near-surface modeled BTEX are largely confined below surface-based temperature inversions (SBIs) and concentrated over downtown and surrounding residential areas. Intermittent upward transport, even under strongly stable conditions, contributes to regional pollution and potentially to background Arctic haze. Replacing residential wood burning with oil heating during pollution episodes reduces near-surface benzene by up to 20%, highlighting synergies with mitigation strategies designed to reduce particulate matter concentrations. The study findings have implications for an improved understanding of sources of toxic aromatic hydrocarbons in other cold urban winter environments.

The role of volatile organic compound (VOC) emissions in secondary aerosol chemistry in urban subarctic environments remains poorly understood. Previous studies in wintertime Fairbanks, Alaska suggest that aldehydes may serve as precursors of sulfur(IV) adducts in aerosols, contributing to the high concentrations of wintertime PM _2.5 pollution Fairbanks often experiences. It is unknown to what extent VOCs participate in secondary aerosol chemistry, or what the sources of largely secondary VOCs (like formaldehyde) are in such a cold and dark environment. Here, we explore the sources of VOCs and examine their role in secondary chemistry. We use measurements from an online proton transfer reaction time of flight mass spectrometer, combined with complementary gas and aerosol measurements from the Alaskan Layered Pollution and Chemical Analysis (ALPACA) wintertime field campaign in 2022, to examine VOC sources and their roles in aerosol chemistry in downtown Fairbanks. We find that alcohols, aromatics and carbonyls together account for ∼ 70 % of measured VOCs, with methanol, ethanol, formaldehyde, benzene and toluene as dominant species. Positive matrix factorization (PMF) analysis indicates up to 56 % of VOCs are vehicle-associated, with 29 %–36 % from direct emissions and 21 % from windshield wiper fluid. Wood heating and heating oil together contribute 14 %. Formaldehyde is primarily linked to diesel emissions, as well as primary and secondary sources associated with local aged air masses. By comparing PMF factors with measured PM _2.5 S(IV) species, we find that vehicle-related emissions of ammonia (which modulates aerosol pH) and formaldehyde could promote the formation of hydroxymethanesulfonate (HMS) in Fairbanks.

Wintertime PM_2.5 pollution is a longstanding issue in the urban subarctic environment such as Fairbanks, Alaska. While previous studies suggest that aldehydes may serve as precursors of S(IV) species in aerosols, the role of volatile organic compound (VOC) emissions in secondary aerosol chemistry remains poorly understood. Here, we use measurements from an online proton transfer reaction time of flight mass spectrometer (PTR-ToF-MS), combined with complementary gas and aerosol measurements from the the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in 2022, to examine VOC sources and their roles in aerosol chemistry in downtown Fairbanks. We find that alcohols, aromatics and carbonyls together account for ~70% of measured VOCs, with methanol, ethanol, formaldehyde, benzene and toluene as dominant species. Positive matrix factorization (PMF) analysis indicate that approximately 56% of VOCs are associated with vehicle emissions, while wood heating and heating oil together contribute about 14%. Formaldehyde is primarily linked to diesel emissions, as well as primary and secondary sources associated with aged air masses. By comparing PMF factors with measured PM_2.5 S(IV) species, we find that vehicle-related emissions of ammonia and formaldehyde likely play a key role in the formation of hydroxymethanesulfonate (HMS) in Fairbanks.

Wintertime PM_2.5 pollution is a longstanding issue in the urban subarctic environment such as Fairbanks, Alaska. While previous studies suggest that aldehydes may serve as precursors of S(IV) species in aerosols, the role of volatile organic compound (VOC) emissions in secondary aerosol chemistry remains poorly understood. Here, we use measurements from an online proton transfer reaction time of flight mass spectrometer (PTR-ToF-MS), combined with complementary gas and aerosol measurements from the the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in 2022, to examine VOC sources and their roles in aerosol chemistry in downtown Fairbanks. We find that alcohols, aromatics and carbonyls together account for ~70% of measured VOCs, with methanol, ethanol, formaldehyde, benzene and toluene as dominant species. Positive matrix factorization (PMF) analysis indicate that approximately 56% of VOCs are associated with vehicle emissions, while wood heating and heating oil together contribute about 14%. Formaldehyde is primarily linked to diesel emissions, as well as primary and secondary sources associated with aged air masses. By comparing PMF factors with measured PM_2.5 S(IV) species, we find that vehicle-related emissions of ammonia and formaldehyde likely play a key role in the formation of hydroxymethanesulfonate (HMS) in Fairbanks.

Wintertime PM_2.5 pollution is a longstanding issue in the urban subarctic environment such as Fairbanks, Alaska. While previous studies suggest that aldehydes may serve as precursors of S(IV) species in aerosols, the role of volatile organic compound (VOC) emissions in secondary aerosol chemistry remains poorly understood. Here, we use measurements from an online proton transfer reaction time of flight mass spectrometer (PTR-ToF-MS), combined with complementary gas and aerosol measurements from the the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in 2022, to examine VOC sources and their roles in aerosol chemistry in downtown Fairbanks. We find that alcohols, aromatics and carbonyls together account for ~70% of measured VOCs, with methanol, ethanol, formaldehyde, benzene and toluene as dominant species. Positive matrix factorization (PMF) analysis indicate that approximately 56% of VOCs are associated with vehicle emissions, while wood heating and heating oil together contribute about 14%. Formaldehyde is primarily linked to diesel emissions, as well as primary and secondary sources associated with aged air masses. By comparing PMF factors with measured PM_2.5 S(IV) species, we find that vehicle-related emissions of ammonia and formaldehyde likely play a key role in the formation of hydroxymethanesulfonate (HMS) in Fairbanks.

Wintertime PM2.5 pollution is a longstanding issue in the urban subarctic environment such as Fairbanks, Alaska. While previous studies suggest that aldehydes may serve as precursors of S(IV) species in aerosols, the role of volatile organic compound (VOC) emissions in secondary aerosol chemistry remains poorly understood. Here, we use measurements from an online proton transfer reaction time of flight mass spectrometer (PTR-ToF-MS), combined with complementary gas and aerosol measurements from the the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in 2022, to examine VOC sources and their roles in aerosol chemistry in downtown Fairbanks. We find that alcohols, aromatics and carbonyls together account for ~70% of measured VOCs, with methanol, ethanol, formaldehyde, benzene and toluene as dominant species. Positive matrix factorization (PMF) analysis indicate that approximately 56% of VOCs are associated with vehicle emissions, while wood heating and heating oil together contribute about 14%. Formaldehyde is primarily linked to diesel emissions, as well as primary and secondary sources associated with aged air masses. By comparing PMF factors with measured PM2.5 S(IV) species, we find that vehicle-related emissions of ammonia and formaldehyde likely play a key role in the formation of hydroxymethanesulfonate (HMS) in Fairbanks.

Fairbanks, Alaska is an example of a subarctic city that experiences severe wintertime air quality episodes characterized by high concentrations of fine particulate matter (PM _2.5 ) and nitrogen oxides (NO and NO ₂ = NO _x ). Previous research into these air quality episodes has focused on the impact of surface‐based inversions that trap local emissions. However, the availability of ozone is a limiting factor in local wintertime pollutant transformation, and sensitivity of Fairbanks air quality to ozone availability has not been investigated on a regional scale. Here, we assess the impact of regional transport on ozone, NO _x , and particulate matter concentrations in Fairbanks, Alaska using three boundary conditions in the Community Multiscale Air Quality (CMAQ) model v5.4. We compared model results with measurements made during the ALPACA field campaign (model‐observation comparison), and we assessed the differences between pairs of models (model‐model comparison) using Euclidean distances of NO ₂ , SO ₂ , PM _2.5 , and O ₃ . We find that modeling of NO _x in the Fairbanks area depends on, amongst other factors, accurate boundary conditions of background ozone because NO + O ₃ titration is the dominant loss reaction of NO. PM _2.5 in Fairbanks is affected by transport of existing background particulate matter rather than the transport of precursors. The boundary condition transport impacts metrics of air pollutants used for regulatory purposes throughout the model domain. Our results have important implications for accurate modeling of air quality in Arctic regions subject to strong local emissions and surface‐based inversions, and have implications for assessment of air quality attainment status.

Benzene, toluene, ethylbenzene, and xylene (BTEX) are hazardous air pollutants with high toxicity and a strong potential for secondary pollutant formation. However, their occurrence and behavior in the Arctic remain poorly understood. During the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in Fairbanks, Alaska in January-February 2022. Surface observations in downtown Fairbanks revealed two major pollution periods, with extremely cold (down to -35°C) and warmer temperatures (around 0°C), respectively. BTEX concentrations reached 4–12 times higher than those reported in the US and European countries under dark, cold Arctic winter conditions at breathing level, posing a significant health risk. We simulated BTEX atmospheric distributions in the Fairbanks region using the FLEXible PARTicle-Weather Research and Forecasting (FLEXPART-WRF) Lagrangian particle dispersion model and anthropogenic emissions at the surface and aloft. Due to limited photochemical loss in to the dark polar winter conditions, we treat BTEX as an unreactive tracer in the model. The control run with the emission inventory developed by Alaska Department of Environmental Conservation (ADEC) substantially underestimates BTEX concentrations compared to observations during both polluted periods, indicating deficiencies in winter emissions and near-surface mixing. Enhancing cold-start gasoline vehicle emissions by a factor of 2 during very low-temperatures substantially improved model results during the cold polluted period, while introducing a relative humidity dependence for mobile emissions improved simulated BTEX during the warm, humid pollution period. Addition of emissions of residential heating oil aromatics, not taken into account in the ADEC inventory, also reduced normalized mean biases by 5-10%.The improved model simulation was used to investigate contributing source sectors. While mobile traffic emissions were identified as the dominant source of BTEX across the Fairbanks North Star Borough, residential heating and non-point sources contributed substantially in downtown Fairbanks. Replacing residential wood burning in the inventory with oil heating during severe pollution periods, in line with air quality control guidelines, was found to effectively reduce BTEX concentrations, particularly benzene by up to 30%. While persistent surface-based temperature inversions largely confined BTEX below ~20 m, upward transport, induced by wind shear, during severe episodes, sometimes lofted near-surface pollutants to higher altitudes, potentially contributing to regional pollution and background Arctic haze.The findings of this study emphasise the need to accurately account for temperature and humidity dependent vehicle emissions, residential oil heating emissions, and winter boundary-layer dynamics for improved simulations of air quality in cold wintertime environments, not only in the Arctic but also in mid-latitudes.

The Arctic is warming at a rate several times faster than the global mean, a phenomenon commonly referred to as Arctic amplification. Short-lived climate forcers, particularly aerosols acting as ice-nucleating particles (INPs), may influence this amplification through aerosol-cloud indirect effects. During the polar night, INPs modulate the ratio of liquid-to-ice in mixed-phase clouds, altering their capacity to trap outgoing longwave radiation and warm the surface. Despite their importance, the sources and transport pathways of INPs in high-latitude regions remain poorly constrained. While truly pristine Arctic environments are rare, cold, polluted sub-Arctic regions such as interior Alaska provide natural laboratories for investigating INP populations under conditions that combine low temperatures with enhanced anthropogenic and regional aerosol influences. Such environments may be particularly relevant to Arctic locations experiencing episodic pollution, long-range aerosol transport, or increasing local emissions. While chemical fingerprinting provides critical insights into particle composition and local abundance, it cannot inherently resolve the geographic origins or transport history of air masses bringing INPs to a given region.To address this limitation, we apply backward trajectory-based modelling in an attempt to link observed INPs to their potential source regions. We build on recent work investigating the sources of wintertime INPs in the sub-Arctic urban environment of Fairbanks, Alaska, using observations from the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign conducted in January and February 2022. During the campaign, Fairbanks experienced persistent surface-based temperature inversions and extreme cold events that favored the accumulation of locally emitted anthropogenic aerosols. Analysis of ALPACA-2022 data has reported INP concentrations significantly higher at relatively cold freezing temperatures than those typically observed at other high-latitude sites, consistent with three dominant INP classes: heat-labile biological particles, potentially associated with local vegetation such as lichens; organic particles linked to residential wood combustion, supported by correlations with levoglucosan; and a source attributed to road dust, possibly generated by the application of traction gravel on icy roads.Using a backward trajectory modeling framework, we investigate the spatial origins and atmospheric transport of INP sourced from the Fairbanks region. Backward transport simulations are conducted using the FLEXible PARTicle dispersion model (FLEXPART), driven by 1.33 km resolution wind fields from the Weather Research and Forecasting (WRF) model, including assimilation of meteorological data from the ALPACA campaign. The surface influence and residence time of air masses arriving at the ALPACA measurement site in downtown Fairbanks are quantified. Potential Emission Sensitivity (PES) footprints are calculated by combining with high resolution emissions fields of potential INP sources, based on downscaling emissions using vegetation, road and building datasets. Interpreting PES fields, in conjunction with the observed INP analysis, allows characterization of both the INP sources and their transport pathways in Fairbanks. The results have broad implications for INP sources and aerosol-cloud indirect effects over the wider sub-Arctic and potentially Arctic region.