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Abstract. 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. 

Fairbanks-North Star Borough, Alaska (FNSB) regularly experiences some of the worst wintertime air quality in the United States. 

Brown carbon (BrC) plays an important role in global radiative budget but there have been few studies on BrC in Arctic despite rapid warming and increasing wildfires in this region. Here we investigate the optical properties of BrC from boreal fires in Alaska summer, with two sets of measurements from PILS-LWCC-TOC (Particle-Into-Liquid-Sampler – Liquid-Waveguide Capillary flow-through optical Cell - Total-Organic-Carbon analyzer) and filter measurements. We show that during intense wildfires, the mass absorption coefficient at 365 nm (MAC365) from water soluble organic carbon (WSOC) remained stable at ∼1 m2 g−1. With all plumes sampled and derived transport time, we show a decrease of MAC365 with plume age, with a shorter photobleaching lifetime (∼11 h) at 365 nm compared to 405 nm (∼20 h). The total absorption by organic aerosols measured from filters at 365 nm is higher than the absorption by WSOC by a factor 2–3, suggesting a dominant role of insoluble organic carbon. Overall BrC dominates absorption in the near-ultraviolet and visible radiation during wildfire season in Alaska summer. 

The prevailing view for aqueous secondary aerosol formation is that it occurs in clouds and fogs, owing to the large liquid water content compared to minute levels in fine particles. Our research indicates that this view may need reevaluation due to enhancements in aqueous reactions in highly concentrated small particles. Here, we show that low temperature can play a role through a unique effect on particle pH that can substantially modulate secondary aerosol formation. Marked increases in hydroxymethanesulfonate observed under extreme cold in Fairbanks, Alaska, demonstrate the effect. These findings provide insight on aqueous chemistry in fine particles under cold conditions expanding possible regions of secondary aerosol formation that are pH dependent beyond conditions of high liquid water. 

Fairbanks-North Star Borough (FNSB), Alaska perennially experiences some of the worst wintertime air quality in the United States. FNSB was designated as a “serious” nonattainment area by the U.S. Environmental Protection Agency in 2017 for excessive fine particulate matter (PM 2.5 ) concentrations. The ALPACA (Alaskan Layered Pollution And Chemical Analysis) field campaign was established to understand the sources of air pollution, pollutant transformations, and the meteorological conditions contributing to FNSB's air quality problem. We performed on-road mobile sampling during ALPACA to identify and understand the spatial patterns of PM across the study domain, which contained multiple stationary field sites and regulatory measurement sites. Our measurements demonstrate the following: (1) both the between-neighborhood and within-neighborhood variations in PM 2.5 concentrations and composition are large (>10 μg m −3 ). (2) Spatial variations of PM in Fairbanks are tightly connected to meteorological conditions; dramatic between-neighborhood differences exist during strong temperature inversion conditions, but are significantly reduced during weaker temperature inversions, where atmospheric conditions are more well mixed. (3) During strong inversion conditions, total PM 2.5 and black carbon (BC) are tightly spatially correlated and have high absorption Ångstrom exponent values (AAE > 1.4), but are relatively uncorrelated during weak inversion conditions and have lower AAE. (4) PM 2.5 , BC, and total particle number (PN) concentrations decreased with increasing elevation, with the fall-off being more dramatic during strong temperature inversion conditions. (5) Mobile sampling reveals important air pollutant concentration differences between the multiple fixed sites of the ALPACA study, and demonstrates the utility of adding mobile sampling for understanding the spatial context of large urban air quality field campaigns. These results are important for understanding both the PM exposure for residents of FNSB and the spatial context of the ALPACA study. 

Atmospheric electrical discharges are now known to generate unexpectedly large amounts of the atmosphere’s primary oxidant, hydroxyl (OH), in thunderstorm anvils, where electrical discharges are caused by atmospheric charge separation. The question is “Do other electrical discharges also generate large amounts of oxidants?” In this paper, we demonstrate that corona formed on grounded metal objects under thunderstorms produce extreme amounts of OH, hydroperoxyl (HO 2 ), and ozone (O 3 ). Hundreds of parts per trillion to parts per billion of OH and HO 2 were measured during seven thunderstorms that passed over the rooftop site during an air quality study in Houston, TX in summer 2006. A combination of analysis of these field results and laboratory experiments shows that these extreme oxidant amounts were generated by corona on the inlet of the OH-measuring instrument and that corona are easier to generate on lightning rods than on the inlet. In the laboratory, increasing the electric field increased OH, HO 2 , and O 3 , with 14 times more O 3 generated than OH and HO 2 , which were equal. Calculations show that corona on lightning rods can annually generate OH that is 10–100 times ambient amounts within centimeters of the lightning rod and on high-voltage electrical power lines can generate OH that is 500 times ambient a meter away from the corona. Contrary to current thinking, previously unrecognized corona-generated OH, not corona-generated UV radiation, mostly likely initiates premature degradation of high-voltage polymer insulators. 

Abstract. Organic aerosol (OA), with a large biogenic fraction in the summertime southeast US, adversely impacts air quality and human health. Stringent airquality controls have recently reduced anthropogenic pollutants including sulfate, whose impact on OA remains unclear. Three filter measurementnetworks provide long-term constraints on the sensitivity of OA to changes in inorganic species, including sulfate and ammonia. The 2000–2013summertime OA decreases by 1.7 % yr−1–1.9 % yr−1 with little month-to-month variability, while sulfatedeclines rapidly with significant monthly difference in the early 2000s. In contrast, modeled OA from a chemical-transport model (GEOS-Chem) decreasesby 4.9 % yr−1 with much larger monthly variability, largely due to the predominant role of acid-catalyzed reactive uptake ofepoxydiols (IEPOX) onto sulfate. The overestimated modeled OA dependence on sulfate can be improved by implementing a coating effect and assumingconstant aerosol acidity, suggesting the needs to revisit IEPOX reactive uptake in current models. Our work highlights the importance of secondaryOA formation pathways that are weakly dependent on inorganic aerosol in a region that is heavily influenced by both biogenic and anthropogenicemissions.