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1. Updated temperature correction for computing seawater nitrate with in situ ultraviolet spectrophotometer and submersible ultraviolet nitrate analyzer nitrate sensors

   https://doi.org/10.1002/lom3.10566  

   Plant, Joshua N.; Sakamoto, Carole M.; Johnson, Kenneth S.; Maurer, Tanya L.; Bif, Mariana B. (July 2023, Limnology and Oceanography: Methods)

   Abstract Sensors that use ultraviolet (UV) light absorption to measure nitrate in seawater at in situ temperatures require a correction to the calibration coefficients if the calibration and sample temperatures are not identical. This is mostly due to the bromide molecule, which absorbs more UV light as temperature increases. The current correction applied to in situ ultraviolet spectrophotometer (ISUS) and submersible ultraviolet nitrate analyzer (SUNA) nitrate sensors generally follows Sakamoto et al. (2009, Limnol. Oceanogr. Methods 7, 132–143). For waters warmer than the calibration temperature, this correction model can lead to a 1–2 μmol kg⁻¹positive bias in nitrate concentration. Here we present an updated correction model, which reduces this small but noticeable bias by at least 50%. This improved model is based on additional laboratory data and describes the temperature correction as an exponential function of wavelength and temperature difference from the calibration temperature. It is a better fit to the experimental data than the current model and the improvement is validated using two populations of nitrate profiles from Biogeochemical Argo floats navigating through tropical waters. One population is from floats equipped with ISUS sensors while the other arises from floats with SUNA sensors on board. Although this model can be applied to both ISUS and SUNA nitrate sensors, it should not be used for OPUS UV nitrate sensors at this time. This new approach is similar to that used for OPUS sensors (Nehir et al., 2021, Front. Mar. Sci. 8, 663800) with differing model coefficients. This difference suggests that there is an instrumental component to the temperature correction or that there are slight differences in experimental methodologies. 

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2. Nitrate radical oxidation of <i>γ</i>-terpinene: hydroxy nitrate, total organic nitrate, and secondary organic aerosol yields

   https://doi.org/10.5194/acp-17-8635-2017  

   Slade, Jonathan H.; de Perre, Chloé; Lee, Linda; Shepson, Paul B. (January 2017, Atmospheric Chemistry and Physics)

   Polyolefinic monoterpenes represent a potentially important but understudied source of organic nitrates (ONs) and secondary organic aerosol (SOA) following oxidation due to their high reactivity and propensity for multi-stage chemistry. Recent modeling work suggests that the oxidation of polyolefinic γ-terpinene can be the dominant source of nighttime ON in a mixed forest environment. However, the ON yields, aerosol partitioning behavior, and SOA yields from γ-terpinene oxidation by the nitrate radical (NO₃), an important nighttime oxidant, have not been determined experimentally. In this work, we present a comprehensive experimental investigation of the total (gas + particle) ON, hydroxy nitrate, and SOA yields following γ-terpinene oxidation by NO₃. Under dry conditions, the hydroxy nitrate yield  =  4(+1/−3) %, total ON yield  =  14(+3/−2) %, and SOA yield  ≤  10 % under atmospherically relevant particle mass loadings, similar to those for α-pinene + NO₃. Using a chemical box model, we show that the measured concentrations of NO₂ and γ-terpinene hydroxy nitrates can be reliably simulated from α-pinene + NO₃ chemistry. This suggests that NO₃ addition to either of the two internal double bonds of γ-terpinene primarily decomposes forming a relatively volatile keto-aldehyde, reconciling the small SOA yield observed here and for other internal olefinic terpenes. Based on aerosol partitioning analysis and identification of speciated particle-phase ON applying high-resolution liquid chromatography–mass spectrometry, we estimate that a significant fraction of the particle-phase ON has the hydroxy nitrate moiety. This work greatly contributes to our understanding of ON and SOA formation from polyolefin monoterpene oxidation, which could be important in the northern continental US and the Midwest, where polyolefinic monoterpene emissions are greatest. 

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3. Global inorganic nitrate production mechanisms: comparison of a global model with nitrate isotope observations

   https://doi.org/10.5194/acp-20-3859-2020  

   Alexander, Becky; Sherwen, Tomás; Holmes, Christopher D.; Fisher, Jenny A.; Chen, Qianjie; Evans, Mat J.; Kasibhatla, Prasad (January 2020, Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. The formation of inorganic nitrate is the main sink for nitrogenoxides (NOx = NO + NO2). Due to the importance of NOx forthe formation of tropospheric oxidants such as the hydroxyl radical (OH) andozone, understanding the mechanisms and rates of nitrate formation isparamount for our ability to predict the atmospheric lifetimes of mostreduced trace gases in the atmosphere. The oxygen isotopic composition ofnitrate (Δ17O(nitrate)) is determined by the relativeimportance of NOx sinks and thus can provide an observationalconstraint for NOx chemistry. Until recently, the ability to utilizeΔ17O(nitrate) observations for this purpose was hindered by ourlack of knowledge about the oxygen isotopic composition of ozone (Δ17O(O3)). Recent and spatially widespread observations of Δ17O(O3) motivate an updated comparison of modeled andobserved Δ17O(nitrate) and a reassessment of modeled nitrateformation pathways. Model updates based on recent laboratory studies ofheterogeneous reactions render dinitrogen pentoxide (N2O5)hydrolysis as important as NO2 + OH (both 41 %) for globalinorganic nitrate production near the surface (below 1 km altitude). Allother nitrate production mechanisms individually represent less than 6 %of global nitrate production near the surface but can be dominant locally.Updated reaction rates for aerosol uptake of NO2 result in significantreduction of nitrate and nitrous acid (HONO) formed through this pathway inthe model and render NO2 hydrolysis a negligible pathway for nitrateformation globally. Although photolysis of aerosol nitrate may haveimplications for NOx, HONO, and oxidant abundances, it does notsignificantly impact the relative importance of nitrate formation pathways.Modeled Δ17O(nitrate) (28.6±4.5 ‰)compares well with the average of a global compilation of observations (27.6±5.0 ‰) when assuming Δ17O(O3) = 26 ‰, giving confidence in the model'srepresentation of the relative importance of ozone versus HOx (= OH + HO2 + RO2) in NOx cycling and nitrate formation on theglobal scale. 

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4. Technical note: Gas-phase nitrate radical generation via irradiation of aerated ceric ammonium nitrate mixtures

   https://doi.org/10.5194/acp-23-13869-2023  

   Lambe, Andrew T.; Bai, Bin; Takeuchi, Masayuki; Orwat, Nicole; Zimmerman, Paul M.; Alton, Mitchell W.; Ng, Nga L.; Freedman, Andrew; Claflin, Megan S.; Gentner, Drew R.; et al (November 2023, Atmospheric Chemistry and Physics)

   Abstract. We present a novel photolytic source of gas-phase NO3 suitable for use in atmospheric chemistry studies that has several advantages over traditional sources that utilize NO2 + O3 reactions and/or thermal dissociation of dinitrogen pentoxide (N2O5). The method generates NO3 via irradiation of aerated aqueous solutions of ceric ammonium nitrate (CAN, (NH4)2Ce(NO3)6) and nitric acid (HNO3) or sodium nitrate (NaNO3). We present experimental and model characterization of the NO3 formation potential of irradiated CAN / HNO3 and CAN / NaNO3 mixtures containing [CAN] = 10−3 to 1.0 M, [HNO3] = 1.0 to 6.0 M, [NaNO3] = 1.0 to 4.8 M, photon fluxes (I) ranging from 6.9 × 1014 to 1.0 × 1016 photons cm−2 s−1, and irradiation wavelengths ranging from 254 to 421 nm. NO3 mixing ratios ranging from parts per billion to parts per million by volume were achieved using this method. At the CAN solubility limit, maximum [NO3] was achieved using [HNO3] ≈ 3.0 to 6.0 M and UVA radiation (λmax⁡ = 369 nm) in CAN / HNO3 mixtures or [NaNO3] ≥ 1.0 M and UVC radiation (λmax⁡ = 254 nm) in CAN / NaNO3 mixtures. Other reactive nitrogen (NO2, N2O4, N2O5, N2O6, HNO2, HNO3, HNO4) and reactive oxygen (HO2, H2O2) species obtained from the irradiation of ceric nitrate mixtures were measured using a NOx analyzer and an iodide-adduct high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS). To assess the applicability of the method for studies of NO3-initiated oxidative aging processes, we generated and measured the chemical composition of oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) from the β-pinene + NO3 reaction using a Filter Inlet for Gases and AEROsols (FIGAERO) coupled to the HR-ToF-CIMS. 

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5. Linking Water Age, Nitrate Export Regime, and Nitrate Isotope Biogeochemistry in a Tile‐Drained Agricultural Field

   https://doi.org/10.1029/2023WR034948  

   Yu, Zhongjie; Hu, Yinchao; Gentry, Lowell E.; Yang, Wendy H.; Margenot, Andrew J.; Guan, Kaiyu; Mitchell, Corey A.; Hu, Minpeng (December 2023, Water Resources Research)

   Abstract Accurately quantifying and predicting the reactive transport of nitrate () in hydrologic systems continues to be a challenge, due to the complex hydrological and biogeochemical interactions that underlie this transport. Recent advances related to time‐variant water age have led to a new method that probes water mixing and selection behaviors using StorAge Selection (SAS) functions. In this study, SAS functions were applied to investigate storage, water selection behaviors, and export regimes in a tile‐drained corn‐soybean field. The natural abundance stable nitrogen and oxygen isotopes of tile drainage were also measured to provide constraints on biogeochemical transformations. The SAS functions, calibrated using chloride measurements at tile drain outlets, revealed a strong young water preference during tile discharge generation. The use of a time‐variant SAS function for tile discharge generated unique water age dynamics that reveal an inverse storage effect driven by the activation of preferential flow paths and mechanically explain the observed variations in isotopes. Combining the water age estimates with isotope fingerprinting shed new light on export dynamics at the tile‐drain scale, where a large mixing volume and the lack of a strong vertical contrast in concentration resulted in chemostatic export regimes. For the first time, isotopes were embedded into a water age‐based transport model to model reactive transport under transient conditions. The results of this modeling study provided a proof‐of‐concept for the potential of coupling water age modeling with isotope analysis to elucidate the mechanisms driving reactive transport. 

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