Becky Alexander’s research while affiliated with University of Washington and other places

The accurate representation of cloud droplet number concentration (Nd) is crucial for predicting future climate. However, models often underestimate Nd 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 Nd 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 Nd 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.

Naturally occurring chlorate (ClO3⁻) 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 ¹⁷O‐excess (∆¹⁷O) and the ³⁶Cl‐to‐total‐Cl ratio (³⁶Cl/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 (HClO3), 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 ∆¹⁷O 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 ³⁶Cl/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.

Subarctic cities notoriously experience severe winter pollution episodes with PM2.5 concentrations above 35 µg m-3, the US EPA’s 24-hour 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 2022 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 2x10-12 M and 3x10-11 M, respectively, with moderate hydroxyl radical concentrations up to 5x10-15 M. Although our modeling predicts 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 highest rates in late winter when Fairbanks experiences severe pollution and in summer when wildfires generate BrC.

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 ALPACA (Alaska Layered Pollution And Chemical Analysis) 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 (normalized to Fairbanks winter sunlight) range from 6 to 71 µM hr-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 hr-1. Corresponding rates of sulfate formation from particle-produced HOOH are 0.05 – 0.5 µg m-3 hr-1, accounting for a significant portion of the secondary sulfate production rate. Our results show that HOOH formed in particles is 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.

Nitrogen (N) isotopic fractionation during nitrogen oxides (NOx) cycling and conversion into atmospheric nitrate alters the original N isotopic composition (δ¹⁵N) of NOx emissions. Limited quantification of these isotopic effects in urban settings hampers the δ¹⁵N‐based identification and apportionment of NOx sources. δ¹⁵N of nitrogen dioxide (NO2) measured during winter in downtown Fairbanks, Alaska, displayed a large temporal variability, from −10.2 to 24.1‰. δ¹⁵N(NO2) records are found to be driven by equilibrium isotopic fractionation, at a rate in very close agreement with theoretical predictions. This result confirms that N isotopic partitioning between NO and NO2 can be accurately predicted over a wide range of conditions. This represents an important step for inferring NOx emission sources from isotopic composition measurement of reactive nitrogen species. After correcting our δ¹⁵N(NO2) measurements for N fractionation effects, a δ¹⁵N‐based source apportionment analysis identifies vehicle and space heating oil emissions as the dominant sources of breathing‐level NOx at this urban site. Despite their large NOx emissions, coal‐fired power plants with elevated chimney stacks (>26 m) appear to make a small contribution to surface NOx levels in downtown Fairbanks (likely less than 18% on average). The combined uncertainties of the δ¹⁵N of NOx from heating oil combustion and of the influence of low temperatures on the δ¹⁵N of NOx emitted by vehicle exhaust prevent a more detailed partitioning of surface NOx sources in Fairbanks.

Dimethyl sulfide (DMS) is primarily emitted by marine phytoplankton and oxidized in the atmosphere to form methanesulfonic acid (MSA) and sulfate aerosols, which affect climate by influencing radiation and cloud properties. Ice cores in regions affected by pollution show an industrial-era decline in MSA, which has previously been interpreted to indicate 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, over three time periods (preindustrial, peak North Atlantic NOx pollution, and 21st century) to investigate the chemical drivers of the industrial-era changes in MSA and bioSO4, and examine whether four DMS oxidation mechanisms reproduce trends and seasonality in DMS, MSA, and bioSO4 observations. We find that GEOS-Chem and box model simulations can reproduce ice core trends in MSA and bioSO4, but model results are sensitive to both DMS oxidation mechanism and oxidant concentrations. Our simulations support the hypothesized nitrate-radical driven decline in MSA over the industrial era, but none of the GEOS-Chem simulations can capture the seasonality of in situ DMS observations while also reproducing ice core trends in MSA and bioSO4. To reduce uncertainty in modeling DMS-derived aerosols, 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.

An industrial-era decline in Greenland ice-core methanesulfonic acid is thought to herald a collapse in North Atlantic marine phytoplankton stocks related to a weakening of the Atlantic meridional overturning circulation. By contrast, stable levels of total marine biogenic sulfur contradict this interpretation and point to changes in atmospheric oxidation as a potential cause of the methanesulfonic acid decline. However, the impact of oxidation on methanesulfonic acid production has not been quantified, nor has this hypothesis been rigorously tested. Here we present a multi-century methanesulfonic acid record from the Denali, Alaska, ice core, which shows a methanesulfonic acid decline similar in magnitude but delayed by 93 years relative to the Greenland record. Box-model results using updated dimethyl sulfide oxidation pathways indicate that oxidation by pollution-driven nitrate radicals has suppressed atmospheric methanesulfonic acid production, explaining most, if not all, of Denali’s and Greenland’s methanesulfonic acid declines without requiring a change in phytoplankton production. The delayed timing of the North Pacific methanesulfonic acid decline, relative to the North Atlantic, reflects the distinct history of industrialization in upwind regions and is consistent with the Denali and Greenland ice-core nitrate records. These results demonstrate that multidecadal trends in industrial-era Arctic ice-core methanesulfonic acid reflect rising anthropogenic pollution rather than declining marine primary production.