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

   https://doi.org/10.5194/acp-22-2553-2022  

   Kulju, Kathryn D.; McNamara, Stephen M.; Chen, Qianjie; Kenagy, Hannah S.; Edebeli, Jacinta; Fuentes, Jose D.; Bertman, Steven B.; Pratt, Kerri A. (January 2022, Atmospheric Chemistry and Physics)

   Abstract. The atmospheric multiphase reaction of dinitrogenpentoxide (N2O5) with chloride-containing aerosol particlesproduces nitryl chloride (ClNO2), which has been observed across theglobe. The photolysis of ClNO2 produces chlorine radicals and nitrogendioxide (NO2), which alter pollutant fates and air quality. However,the effects of local meteorology on near-surface ClNO2 production arenot yet well understood, as most observational and modeling studies focus onperiods of clear conditions. During a field campaign in Kalamazoo, Michigan,from January–February 2018, N2O5 and ClNO2 were measuredusing chemical ionization mass spectrometry, with simultaneous measurementsof atmospheric particulate matter and meteorological parameters. We examinethe impacts of atmospheric turbulence, precipitation (snow, rain) and fog,and ground cover (snow-covered and bare ground) on the abundances ofClNO2 and N2O5. N2O5 mole ratios were lowest duringperiods of lower turbulence and were not statistically significantlydifferent between snow-covered and bare ground. In contrast, ClNO2 moleratios were highest, on average, over snow-covered ground, due to salinesnowpack ClNO2 production. Both N2O5 and ClNO2 moleratios were lowest, on average, during rainfall and fog because ofscavenging, with N2O5 scavenging by fog droplets likelycontributing to observed increased particulate nitrate concentrations. Theseobservations, specifically those during active precipitation and withsnow-covered ground, highlight important processes, including N2O5and ClNO2 wet scavenging, fog nitrate production, and snowpackClNO2 production, that govern the variability in observed atmosphericchlorine and nitrogen chemistry and are missed when considering only clearconditions. 

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2. How can mountaintop CO₂ observations be used to constrain regional carbon fluxes?

   https://doi.org/10.5194/acp-17-5561-2017  

   Lin, John C.; Mallia, Derek V.; Wu, Dien; Stephens, Britton B. (January 2017, Atmospheric Chemistry and Physics)

   Despite the need for researchers to understand terrestrial biospheric carbon fluxes to account for carbon cycle feedbacks and predict future CO₂ concentrations, knowledge of these fluxes at the regional scale remains poor. This is particularly true in mountainous areas, where complex meteorology and lack of observations lead to large uncertainties in carbon fluxes. Yet mountainous regions are often where significant forest cover and biomass are found – i.e., areas that have the potential to serve as carbon sinks. As CO₂ observations are carried out in mountainous areas, it is imperative that they are properly interpreted to yield information about carbon fluxes. In this paper, we present CO₂ observations at three sites in the mountains of the western US, along with atmospheric simulations that attempt to extract information about biospheric carbon fluxes from the CO₂ observations, with emphasis on the observed and simulated diurnal cycles of CO₂. We show that atmospheric models can systematically simulate the wrong diurnal cycle and significantly misinterpret the CO₂ observations, due to erroneous atmospheric flows as a result of terrain that is misrepresented in the model. This problem depends on the selected vertical level in the model and is exacerbated as the spatial resolution is degraded, and our results indicate that a fine grid spacing of ∼ 4 km or less may be needed to simulate a realistic diurnal cycle of CO₂ for sites on top of the steep mountains examined here in the American Rockies. In the absence of higher resolution models, we recommend coarse-scale models to focus on assimilating afternoon CO₂ observations on mountaintop sites over the continent to avoid misrepresentations of nocturnal transport and influence. 

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3. Relationship between meteoric ¹⁰Be and NO₃⁻ concentrations in soils along Shackleton Glacier, Antarctica

   https://doi.org/10.5194/esurf-9-1363-2021  

   Diaz, Melisa A.; Corbett, Lee B.; Bierman, Paul R.; Adams, Byron J.; Wall, Diana H.; Hogg, Ian D.; Fierer, Noah; Lyons, W. Berry (January 2021, Earth Surface Dynamics)

   Abstract. Outlet glaciers that flow through the Transantarctic Mountains (TAM) experienced changes in ice thickness greater than other coastal regions of Antarctica during glacial maxima. As a result, ice-free areas that are currently exposed may have been covered by ice at various points during the Cenozoic, complicating our understanding of ecological succession in TAM soils. Our knowledge of glacial extent on small spatial scales is limited for the TAM, and studies of soil exposure duration and disturbance, in particular, are rare. We collected surface soil samples and, in some places, depth profiles every 5 cm to refusal (up to 30 cm) from 11ice-free areas along Shackleton Glacier, a major outlet glacier of the EastAntarctic Ice Sheet. We explored the relationship between meteoric 10Be and NO3- in these soils as a tool for understanding landscape disturbance and wetting history and as exposure proxies. Concentrations of meteoric 10Be spanned more than an order of magnitude across the region (2.9×108 to 73×108 atoms g−1) and are among the highest measured in polar regions. The concentrations of NO3- were similarly variable and ranged from ∼1 µg g−1 to 15 mg g−1. In examining differences and similarities in the concentrations of 10Be and NO3- with depth, we suggest that much of the southern portion of the Shackleton Glacier region has likely developed under a hyper-arid climate regime with minimal disturbance. Finally, we inferred exposure time using 10Be concentrations. This analysis indicates that the soils we analyzed likelyrange from recent exposure (following the Last Glacial Maximum) to possibly>6 Myr. We suggest that further testing and interrogation of meteoric 10Be and NO3- concentrations and relationships in soils can provide important information regarding landscape development, soil evolution processes, and inferred exposure durations of surfaces in the TAM. 

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4. Spatially and temporally resolved measurements of NO_<i>x</i> fluxes by airborne eddy covariance over Greater London

   https://doi.org/10.5194/acp-21-15283-2021  

   Vaughan, Adam R.; Lee, James D.; Metzger, Stefan; Durden, David; Lewis, Alastair C.; Shaw, Marvin D.; Drysdale, Will S.; Purvis, Ruth M.; Davison, Brian; Hewitt, C. Nicholas (January 2021, Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. Flux measurements of nitrogen oxides (NOx) were made over London usingairborne eddy covariance from a low-flying aircraft. Seven low-altitude flights were conducted over Greater London, performing multiple overpasses across the city during eight days in July 2014. NOx fluxes across theGreater London region (GLR) exhibited high heterogeneity and strong diurnalvariability, with central areas responsible for the highest emission rates(20–30 mg m−2 h−1). Other high-emission areas included the M25 orbital motorway. The complexity of London's emission characteristics makes it challenging to pinpoint single emissions sources definitively usingairborne measurements. Multiple sources, including road transport andresidential, commercial and industrial combustion sources, are all likely to contribute to measured fluxes. Measured flux estimates were compared toscaled National Atmospheric Emissions Inventory (NAEI) estimates, accountingfor monthly, daily and hourly variability. Significant differences were found between the flux-driven emissions and the NAEI estimates acrossGreater London, with measured values up to 2 times higher in Central London than those predicted by the inventory. To overcome the limitations ofusing the national inventory to contextualise measured fluxes, we usedphysics-guided flux data fusion to train environmental response functions(ERFs) between measured flux and environmental drivers (meteorological and surface). The aim was to generate time-of-day emission surfaces usingcalculated ERF relationships for the entire GLR; 98 % spatial coverage was achieved across the GLR at 400 m2 spatial resolution. All flight legprojections showed substantial heterogeneity across the domain, with highemissions emanating from Central London and major road infrastructure. Thediurnal emission structure of the GLR was also investigated, through ERF,with the morning rush hour distinguished from lower emissions during the early afternoon. Overall, the integration of airborne fluxes with anERF-driven strategy enabled the first independent generation of surfaceNOx emissions, at high resolution using an eddy-covariance approach,for an entire city region. 

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5. Chlorine-initiated oxidation of <i>n</i>-alkanes under high-NO_<i>x</i> conditions: insights into secondary organic aerosol composition and volatility using a FIGAERO–CIMS

   https://doi.org/10.5194/acp-18-15535-2018  

   Wang, Dongyu S.; Hildebrandt Ruiz, Lea (January 2018, Atmospheric Chemistry and Physics)

   Abstract. Chlorine-initiated oxidation of n-alkanes (C₈₋₁₂) under high-nitrogen oxide conditions was investigated. Observed secondary organic aerosol yields (0.16 to 1.65) are higher than those for OH-initiated oxidation of C₈₋₁₂ alkanes (0.04 to 0.35). A high-resolution time-of-flight chemical ionization mass spectrometer coupled to a Filter Inlet for Gases and AEROsols (FIGAERO–CIMS) was used to characterize the gas- and particle-phase molecular composition. Chlorinated organics were observed, which likely originated from chlorine addition to the double bond present on the heterogeneously produced dihydrofurans. A two-dimensional thermogram representation was developed to visualize the composition and relative volatility of organic aerosol components using unit-mass resolution data. Evidence of oligomer formation and thermal decomposition was observed. Aerosol yield and oligomer formation were suppressed under humid conditions (35% to 67% RH) relative to dry conditions (under 5% RH). The temperature at peak desorption signal, T_max, a proxy for aerosol volatility, was shown to change with aerosol filter loading, which should be constrained when evaluating aerosol volatilities using the FIGAERO–CIMS. Results suggest that long-chain anthropogenic alkanes could contribute significantly to ambient aerosol loading over their atmospheric lifetime. 

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