publications

2024

1. Can we achieve atmospheric chemical environments in the laboratory? An integrated model-measurement approach to chamber SOA studies

   Kenagy, Hannah S., Heald, Colette L., Tahsini, Nadia, Goss, Matthew B., and Kroll, Jesse H.

   Science Advances, under revision 2024

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   Secondary organic aerosol (SOA), atmospheric particulate matter formed from low-volatility products of volatile organic compound (VOC) oxidation, impacts both air quality and climate. Current 3D models, however, cannot reproduce the observed variability in atmospheric organic aerosol. Because many SOA model descriptions are derived from environmental chamber experiments, our ability to represent atmospheric conditions in chambers directly impacts our ability to assess the air quality and climate impacts of SOA. Here, we develop a new approach that leverages global modeling and detailed mechanisms to design chamber experiments that mimic the atmospheric chemistry of organic peroxy radicals (RO₂), a key intermediate in VOC oxidation. Drawing on decades of laboratory experiments, we develop a framework for quantitatively describing RO₂ chemistry and show that no previous experimental approaches to studying SOA formation have accessed the relevant atmospheric RO₂ fate distribution. We show proof-of-concept experiments that demonstrate how SOA experiments can access a range of atmospheric chemical environments and propose several directions for future studies.

2022

1. Urban inland wintertime N₂O₅ and ClNO₂ influenced by snow-covered ground, air turbulence, and precipitation

   Kulju, Kathryn D., McNamara, Stephen M., Chen, Qianjie,  Kenagy, Hannah S., Edebeli, Jacinta, Fuentes, Jose D., Bertman, Steven B., and Pratt, Kerri A.

   Atmos. Chem. Phys. 2022

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   The atmospheric multiphase reaction of dinitrogen pentoxide (N₂O₅) with chloride-containing aerosol particles produces nitryl chloride (ClNO₂), which has been observed across the globe. The photolysis of ClNO₂ produces chlorine radicals and nitrogen dioxide (NO₂), which alter pollutant fates and air quality. However, the effects of local meteorology on near-surface ClNO₂ production are not yet well understood, as most observational and modeling studies focus on periods of clear conditions. During a field campaign in Kalamazoo, Michigan, from January–February 2018, N₂O₅ and ClNO₂ were measured using chemical ionization mass spectrometry, with simultaneous measurements of atmospheric particulate matter and meteorological parameters. We examine the impacts of atmospheric turbulence, precipitation (snow, rain) and fog, and ground cover (snow-covered and bare ground) on the abundances of ClNO₂ and N₂O₅. N₂O₅ mole ratios were lowest during periods of lower turbulence and were not statistically significantly different between snow-covered and bare ground. In contrast, ClNO₂ mole ratios were highest, on average, over snow-covered ground, due to saline snowpack ClNO₂ production. Both N₂O₅ and ClNO₂ mole ratios were lowest, on average, during rainfall and fog because of scavenging, with N₂O₅ scavenging by fog droplets likely contributing to observed increased particulate nitrate concentrations. These observations, specifically those during active precipitation and with snow-covered ground, highlight important processes, including N₂O₅ and ClNO₂ wet scavenging, fog nitrate production, and snowpack ClNO₂ production, that govern the variability in observed atmospheric chlorine and nitrogen chemistry and are missed when considering only clear conditions.

2021

1. Contribution of Organic Nitrates to Organic Aerosol over South Korea during KORUS-AQ

   Kenagy, Hannah S., Romer Present, Paul S., Wooldridge, Paul J., Nault, Benjamin A., Campuzano-Jost, Pedro, Day, Douglas A., Jimenez, Jose L., Zare, Azimeh, Pye, Havala O.T., Yu, Jinhyeok, Song, Chul H., Blake, Donald R., Woo, Jung Hun, Kim, Younha, and Cohen, Ronald C.

   Environ. Sci. Technol. 2021

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   The role of anthropogenic NO_x emissions in secondary organic aerosol (SOA) production is not fully understood but is important for understanding the contribution of emissions to air quality. Here, we examine the role of organic nitrates (RONO₂) in SOA formation over the Korean Peninsula during the Korea–United States Air Quality field study in Spring 2016 as a model for RONO₂ aerosol in cities worldwide. We use aircraft-based measurements of the particle phase and total (gas + particle) RONO₂ to explore RONO₂ phase partitioning. These measurements show that, on average, one-fourth of RONO₂ are in the condensed phase, and we estimate that ≈15% of the organic aerosol (OA) mass can be attributed to RONO₂. Furthermore, we observe that the fraction of RONO₂ in the condensed phase increases with OA concentration, evidencing that equilibrium absorptive partitioning controls the RONO₂ phase distribution. Lastly, we model RONO₂ chemistry and phase partitioning in the Community Multiscale Air Quality modeling system. We find that known chemistry can account for one-third of the observed RONO₂, but there is a large missing source of semivolatile, anthropogenically derived RONO₂. We propose that this missing source may result from the oxidation of semi- and intermediate-volatility organic compounds and/or from anthropogenic molecules that undergo autoxidation or multiple generations of OH-initiated oxidation.

2020

1. Evidence of Nighttime Production of Organic Nitrates During SEAC⁴RS, FRAPPÉ and KORUS-AQ

   Kenagy, Hannah S., Sparks, Tamara L, Wooldrige, Paul J., Weinheimer, Andrew J., Ryerson, Thomas B., Blake, Donald R., Hornbrook, Rebecca S., Apel, Eric C., and Cohen, Ronald C.

   Geophys. Res. Lett. 2020

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   Organic nitrates (RONO₂) are an important NO_x sink. In warm, rural environments dominated by biogenic emissions, nocturnal NO₃-initiated production of RONO₂ is competitive with daytime OH-initiated RONO₂ production. However, in urban areas, OH-initiated production of RONO₂ has been assumed dominant and NO₃-initiated production considered negligible. We show evidence for nighttime RONO₂ production similar in magnitude to daytime production during three aircraft campaigns in chemically distinct summertime environments: Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC⁴RS) in the rural Southeastern United States, Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) in the Colorado Front Range, and Korea-United States Air Quality Study (KORUS-AQ) around the megacity of Seoul. During each campaign, morning observations show RONO₂ enhancements at constant, near-background O_x (≡ O₃+NO₂) concentrations, indicating that the RONO₂ are from a non-photochemical source, whereas afternoon observations show a strong correlation between RONO₂ and O_x resulting from photochemical production. We show that there are sufficient precursors for nighttime RONO₂ formation during all three campaigns. This evidence impacts our understanding of nighttime NO_x chemistry.

2018

1. NO_x Lifetime and NO_y Partitioning During WINTER

   Kenagy, Hannah S., Sparks, Tamara L., Ebben, Carlena J., Wooldrige, Paul J., Lopez-Hilfiker, Felipe D., Lee, Ben H., Thornton, Joel A., McDuffie, Erin E., Fibiger, Dorothy L., Brown, Steven S., Montzka, Denise D., Weinheimer, Andrew J., Schroder, Jason C., Campuzano-Jost, Pedro, Day, Douglas A., Jimenez, Jose L., Dibb, Jack E., Campos, Teresa, Shah, Viral, Jaeglé, Lyatt, and Cohen, Ronald C.

   J. Geophys. Res. Atmos. 2018

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   Although urban NO_x lifetimes have been examined extensively during summertime conditions, wintertime NO_x chemistry has been comparatively less studied. We use measurements of NO_x and its oxidation products from the aircraft-based WINTER (Wintertime INvestigation of Transport, Emissions, and Reactivity) experiment over the northeastern United States during February–March 2015 to describe the NO_x lifetime during conditions when days are shorter, actinic flux is reduced, and temperatures are colder. By analyzing regional outflow from the East Coast, we show that NO_x is long lived during the winter, with a longer daytime lifetime (29 hr) than nighttime lifetime (6.3 hr). We demonstrate that wintertime NO_x emissions have an overall lifetime controlled by the nighttime conversion of NO_x to nitric acid (HNO₃) via N₂O₅ heterogeneous chemistry, and we discuss constraints on the rates of NO_x conversion to HNO₃. Additionally, analysis of the nighttime O_x budget suggests that approximately 15% of O₃ is lost overnight through N₂O₅ production and subsequent reaction with aerosol to form HNO₃.

2. Nitrogen Oxides Emissions, Chemistry, Deposition, and Export Over the Northeast United States During the WINTER Aircraft Campaign

   Jaeglé, L., Shah, V., Thornton, J. A., Lopez-Hilfiker, F. D., Lee, B. H., McDuffie, E. E., Fibiger, D., Brown, S. S., Veres, P., Sparks, T. L., Ebben, C. J., Wooldridge, P. J.,  Kenagy, Hannah S., Cohen, R. C., Weinheimer, A. J., Campos, T. L., Montzka, D. D., Digangi, J. P., Wolfe, G. M., Hanisco, T., Schroder, J. C., Campuzano-Jost, P., Day, D. A., Jimenez, J. L., Sullivan, A. P., Guo, H., and Weber, R. J.

   J. Geophys. Res. Atmos. 2018

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   We examine the distribution and fate of nitrogen oxides (NO_x) in the lower troposphere over the Northeast United States (NE US) using aircraft observations from the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign in February–March 2015, as well as the GEOS-Chem chemical transport model and concurrent ground-based observations. We find that the National Emission Inventory from the U.S. Environmental Protection Agency is consistent with WINTER observations of total reactive nitrogen (TNO_y) to within 10% on average, in contrast to the significant overestimate reported in past studies under warmer conditions. Updates to the dry deposition scheme and dinitrogen pentoxide (N₂O₅) reactive uptake probability, ɣ(N₂O₅), result in an improved simulation of gas-phase nitric acid (HNO₃) and submicron particulate nitrate (pNO₃⁻), reducing the longstanding factor of 2–3 overestimate in wintertime HNO₃ + pNO₃⁻ to a 50% positive bias. We find a NO_x lifetime against chemical loss and deposition of 22 hr in the lower troposphere over the NE US. Chemical loss of NO_x is dominated by N₂O₅ hydrolysis (58% of loss) and reaction with OH (33%), while 7% of NO_x leads to the production of organic nitrates. Wet and dry deposition account for 55% and 45% of TNO_y deposition over land, respectively. We estimate that 42% of the NO_x emitted is exported from the NE US boundary layer during winter, mostly in the form of HNO₃ + pNO₃⁻ (40%) and NO_x (38%).

2016

1. Greater nitrogen dioxide concentrations at child versus adult breathing heights close to urban main road kerbside

   Kenagy, Hannah S., Lin, C., Wu, H., and Heal, M.R.

   Air Qual. Atmos. Heal. 2016

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   Nitrogen dioxide (NO₂) is a ubiquitous air pollutant with high concentrations particularly close to main roads. The focus of this study was on possible differences in NO₂ concentrations between adult and child heights as a function of different distances from heavily trafficked roads in urban areas. Passive diffusion tubes were used to measure NO₂ concentrations at heights of 0.8 m (approximate inhalation height of children and closer to vehicle exhaust height) and 2.0 m (approximate inhalation height of adults) above the ground at a number of locations and over several weeks in the city of Edinburgh, UK. Evidence for significant differences in NO₂ between heights was observed up to at least 1.2 m from kerbside of busy roads, with tubes at 0.8 m measuring concentrations 5–15 % (a few μg m⁻³) greater than at 2.0 m. The vertical NO₂ concentration difference was not observable at distances 2.5 m or greater from the kerbside. Fitting of horizontal transects of NO₂ concentrations away from main roads demonstrated the strong influence of wind speed in yielding faster fall-off in NO₂ concentration from the roadside, and in near-ground vertical gradient in NO₂, and lower background NO₂ concentrations. These observations have potential public health implications for differential NO₂ exposures between children walking, or in buggies, close to heavily trafficked urban roads compared with adults.