Primary Sulfate Is the Dominant Source of Particulate Sulfate during Winter in Fairbanks, Alaska

Abstract

Within and surrounding high-latitude cities, poor air quality disturbs Arctic ecosystems, influences the climate, and harms human health. The Fairbanks North Star Borough has wintertime particulate matter (PM) concentrations that exceed the Environmental Protection Agency's (EPA) threshold for public health. Particulate sulfate (SO 4 2− ) is the most abundant inorganic species and contributes approximately 20% of the total PM mass in Fairbanks, but air quality models underestimate observed sulfate concentrations. Here we quantify sulfate sources using sizeresolved δ 34 S(SO 4 2− ), δ 18 O(SO 4 2− ), and Δ 17 O(SO 4 2−) of particulate sulfate in Fairbanks from January 18th to February 25th, 2022 using a Bayesian isotope mixing model. Primary sulfate contributes 62 ± 12% of the total sulfate mass on average. Most primary sulfate is found in the size bin with a particle diameter < 0.7 μm, which contains 90 ±5% of total sulfate mass and poses the greatest risk to human health. Oxidation by all secondary formation pathways combined contributes 38 ± 12% of total sulfate mass on average, indicating that secondary sulfate formation is inefficient in this cold, dark environment. On average, the dominant secondary sulfate formation pathways are oxidation by H 2 O 2 (13 ± 6%), O 3 (8 ± 4%), and NO 2 (8 ± 3%). These findings will inform mitigation strategies to improve air quality and public health in Fairbanks and possibly other high-latitude urban areas during winter.

Assessing the Oxidative Potential of Outdoor PM2.5 in Wintertime Fairbanks, Alaska

“…Fairbanks is a subarctic city (64.84°N latitude) in Alaska’s interior with unique wintertime meteorology and emissions that contribute to high fine particle (PM 2.5 ) mass concentrations that often exceed air quality standards (e.g., 24-h average of 35 μg/m 3 ). During the November to March winter (cold) season, low solar insolation leads to extremely low temperatures (January average low of ∼ −25 °C) and strong near-surface temperature inversions that limit the dispersion of surface-emitted pollutants. , A main source for the high PM 2.5 concentrations is residential heating with wood, which is estimated to contribute 19–80% to overall outdoor PM 2.5 mass, although it has been decreasing in recent years. − The sulfate mass fraction ranges from 8 to 33%, which is mainly from the combustion of sulfur-containing fuels, such as home heating oil. ,, Other sources of PM 2.5 are vehicle-related emissions that contribute 0–31% ,,, as well as minerals and salts for road traction at <5% of the mass fraction. , Emissions and PM 2.5 mass concentrations in Fairbanks and surrounding communities are not spatially homogeneous. The downtown area is influenced more by vehicles, while the residential neighborhoods (especially the nearby town of North Pole) have higher biomass burning emissions from residential wood heating …”

“… 3 − 7 The sulfate mass fraction ranges from 8 to 33%, which is mainly from the combustion of sulfur-containing fuels, such as home heating oil. 2 , 5 , 8 Other sources of PM 2.5 are vehicle-related emissions that contribute 0–31% 4 , 5 , 7 , 9 as well as minerals and salts for road traction at <5% of the mass fraction. 5 , 7 Emissions and PM 2.5 mass concentrations in Fairbanks and surrounding communities are not spatially homogeneous.…”

Overview of the Alaskan Layered Pollution and Chemical Analysis (ALPACA) Field Experiment

“…During poor air quality events, such as the “cold pollution event” highlighted in Figure , sulfate, shown on Figure , on average was 26% of PM 2.5 mass but exceeded 40% for individual spikes. Analysis of sulfate isotopes in Fairbanks during the ALPACA field study showed that 62% ± 12% of sulfate came from primary sources, with a smaller influence of secondary chemistry . Besides increasing PM mass concentration, sulfate also affects the uptake of ammonia into particles, which indirectly further increases PM mass and can affect pH .…”

“…Analysis of sulfate isotopes in Fairbanks during the ALPACA field study showed that 62% ± 12% of sulfate came from primary sources, with a smaller influence of secondary chemistry. 121 Besides increasing PM mass concentration, sulfate also affects the uptake of ammonia into particles, which indirectly further increases PM mass 122 and can affect pH. 123 In the past, sulfate was the only sulfur-containing species in PM that was quantified and reported by ADEC; however, it was noted that PM total sulfur by X-ray fluorescence analysis was on average about 10% larger than sulfur in the sulfate detected by ion chromatography, indicating a missing sulfur species.…”

Investigating processes influencing simulation of local Arctic wintertime anthropogenic pollution in Fairbanks, Alaska during ALPACA-2022

“…However, SO 2 can be oxidised and form secondary sulfate species through other reactions e.g. by oxidation with hydrogen peroxide (H 2 O 2 ) (Alexander et al, 2012;Moon et al, 2023). Based on isotope observations used in Moon et al…”