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1. Controls on the photochemical production of hydrogen peroxide in Lake Erie

   https://doi.org/10.1039/d2em00327a  

   Pandey, Dhurba Raj; Polik, Catherine; Cory, Rose M. (November 2022, Environmental Science: Processes & Impacts)

   In Lake Erie, toxin-forming harmful algal blooms (HABs) occur following high concentrations of hydrogen peroxide (H 2 O 2 ). Correlation between H 2 O 2 concentrations and HABs revealed knowledge gaps on the controls of H 2 O 2 production in Lake Erie. One way H 2 O 2 is produced is upon absorption of sunlight by the chromophoric fraction of dissolved organic matter (CDOM). Rates of this photochemical production of H 2 O 2 may increase in proportion to the apparent quantum yield of H 2 O 2 ( Φ H 2 O 2 ,λ ) from CDOM. However, the Φ H 2 O 2 ,λ for H 2 O 2 production from CDOM remains too poorly constrained to predict the magnitude and range of photochemically produced H 2 O 2 , particularly in freshwaters like Lake Erie. To address this knowledge gap, the Φ H 2 O 2 ,λ was measured approximately biweekly from June–September 2019 in the western basin of Lake Erie along with supporting analyses ( e.g. , CDOM concentration and composition). The average Φ H 2 O 2 ,λ in Lake Erie was within previously reported ranges. However, the Φ H 2 O 2 ,λ varied 5-fold in space and time. The highest Φ H 2 O 2 ,λ was observed in the Maumee River, a tributary of Lake Erie. In nearshore waters of Lake Erie, the Φ H 2 O 2 ,λ decreased about five-fold from June through September. Integration of the controls of photochemical production of H 2 O 2 in Lake Erie show that the variability in rates of photochemical H 2 O 2 production was predominantly due to the Φ H 2 O 2 ,λ . In offshore waters, CDOM concentration also strongly influenced photochemical H 2 O 2 production. Together, the results confirm prior work suggesting that photochemical production of H 2 O 2 contributes but likely cannot account for all the H 2 O 2 associated with HABs in Lake Erie. 

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2. Depth-resolved photochemical production of hydrogen peroxide in the global ocean using remotely sensed ocean color

   https://doi.org/10.3389/frsen.2022.1009398  

   Zhu, Yuting; Powers, Leanne C.; Kieber, David J.; Miller, William L. (October 2022, Frontiers in Remote Sensing)

   Hydrogen peroxide (H 2 O 2 ) is an important reactive oxygen species (ROS) in natural waters, affecting water quality via participation in metal redox reactions and causing oxidative stress for marine ecosystems. While attempts have been made to better understand H 2 O 2 dynamics in the global ocean, the relative importance of various H 2 O 2 sources and losses remains uncertain. Our model improves previous estimates of photochemical H 2 O 2 production rates by using remotely sensed ocean color to characterize the ultraviolet (UV) radiation field in surface water along with quantitative chemical data for the photochemical efficiency of H 2 O 2 formation. Wavelength- and temperature-dependent efficiency (i.e., apparent quantum yield, AQY) spectra previously reported for a variety of seawater sources, including coastal and oligotrophic stations in Antarctica, the Pacific Ocean at Station ALOHA, the Gulf of Mexico, and several sites along the eastern coast of the United States were compiled to obtain a “marine-average” AQY spectrum. To evaluate our predictions of H 2 O 2 photoproduction in surface waters using this single AQY spectrum, we compared modeled rates to new measured rates from Gulf Stream, coastal, and nearshore river-outflow stations in the South Atlantic Bight, GA, United States; obtaining comparative differences of 33% or less. In our global model, the “marine-average” AQY spectrum was used with modeled solar irradiance, together with satellite-derived surface seawater temperature and UV optical properties, including diffuse attenuation coefficients and dissolved organic matter absorption coefficients estimated with remote sensing-based algorithms. The final product of the model, a monthly climatology of depth-resolved H 2 O 2 photoproduction rates in the surface mixed layer, is reported for the first time and provides an integrated global estimate of ∼21.1 Tmol yr −1 for photochemical H 2 O 2 production. This work has important implications for photo-redox reactions in seawater and improves our understanding of the role of solar irradiation on ROS cycling and the overall oxidation state in the oceans. 

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3. Volatile Abundances, Extended Coma Sources, and Nucleus Ice Associations in Comet C/2014 Q2 (Lovejoy)

   https://doi.org/10.3847/PSJ/ac323c  

   Dello Russo, Neil; Vervack, Ronald J.; Kawakita, Hideyo; Bonev, Boncho P.; DiSanti, Michael A.; Gibb, Erika L.; McKay, Adam J.; Cochran, Anita L.; Weaver, Harold A.; Biver, Nicolas; et al (January 2022, The Planetary Science Journal)

   Abstract High-resolution infrared spectra of comet C/2014 Q2 Lovejoy were acquired with NIRSPEC at the W. M. Keck Observatory on two post-perihelion dates (UT 2015 February 2 and 3). H 2 O was measured simultaneously with CO, CH 3 OH, H 2 CO, CH 4 , C 2 H 6 , C 2 H 4 , C 2 H 2 , HCN, and NH 3 on both dates, and rotational temperatures, production rates, relative abundances, H 2 O ortho-to-para ratios, and spatial distributions in the coma were determined. The first detection of C 2 H 4 in a comet from ground-based observations is reported. Abundances relative to H 2 O for all species were found to be in the typical range compared with values for other comets in the overall population to date. There is evidence of variability in rotational temperatures and production rates on timescales that are small compared with the rotational period of the comet. Spatial distributions of volatiles in the coma suggest complex outgassing behavior. CH 3 OH, HCN, C 2 H 6 , and CH 4 spatial distributions in the coma are consistent with direct release from associated ices in the nucleus and are peaked in a more sunward direction compared with co-measured dust. H 2 O spatial profiles are clearly distinct from these other four species, likely due to a sizable coma contribution from icy grain sublimation. Spatial distributions for C 2 H 2 , H 2 CO, and NH 3 suggest substantial contributions from extended coma sources, providing further evidence for distinct origins and associations for these species in comets. CO shows a different spatial distribution compared with other volatiles, consistent with jet activity from discrete nucleus ice sources. 

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4. Gas Sources from the Coma and Nucleus of Comet 46P/Wirtanen Observed Using ALMA

   https://doi.org/10.3847/1538-4357/ace0bc  

   Cordiner, M. A.; Roth, N. X.; Milam, S. N.; Villanueva, G. L.; Bockelée-Morvan, D.; Remijan, A. J.; Charnley, S. B.; Biver, N.; Lis, D. C.; Qi, C.; et al (August 2023, The Astrophysical Journal)

   Abstract Gas-phase molecules in cometary atmospheres (comae) originate primarily from (1) outgassing by the nucleus, (2) sublimation of icy grains in the near-nucleus coma, and (3) coma (photo)chemical processes. However, the majority of cometary gases observed at radio wavelengths have yet to be mapped, so their production/release mechanisms remain uncertain. Here we present observations of six molecular species toward comet 46P/Wirtanen, obtained using the Atacama Large Millimeter/submillimeter Array during the comet’s unusually close (∼0.1 au) approach to Earth in 2018 December. Interferometric maps of HCN, CH₃OH, CH₃CN, H₂CO, CS, and HNC were obtained at an unprecedented sky-projected spatial resolution of up to 25 km, enabling the nucleus and coma sources of these molecules to be accurately quantified. The HCN, CH₃OH, and CH₃CN spatial distributions are consistent with production by direct outgassing from (or very close to) the nucleus, with a significant proportion of the observed CH₃OH originating from sublimation of icy grains in the near-nucleus coma (at a scale lengthL_p= 36 ± 7 km). On the other hand, H₂CO, CS, and HNC originate primarily from distributed coma sources (withL_pvalues in the range 550–16,000 km), the identities of which remain to be established. The HCN, CH₃OH, and HNC abundances in 46P are consistent with the average values previously observed in comets, whereas the H₂CO, CH₃CN, and CS abundances are relatively low. 

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5. A SUBLIME 3D Model for Cometary Coma Emission: The Hypervolatile-rich Comet C/2016 R2 (PanSTARRS)

   https://doi.org/10.3847/1538-4357/ac5893  

   Cordiner, M. A.; Coulson, I. M.; Garcia-Berrios, E.; Qi, C.; Lique, F.; Zołtowski, M.; de Val-Borro, M.; Kuan, Y. -J.; Ip, W. -H.; Mairs, S.; et al (April 2022, The Astrophysical Journal)

   Abstract The coma of comet C/2016 R2 (PanSTARRS) is one of the most chemically peculiar ever observed, in particular due to its extremely high CO/H₂O and ${\mathrm{N}}_2^{+}$/H₂O ratios, and unusual trace volatile abundances. However, the complex shape of its CO emission lines, as well as uncertainties in the coma structure and excitation, has lead to ambiguities in the total CO production rate. We performed high-resolution, spatially, spectrally, and temporally resolved CO observations using the James Clerk Maxwell Telescope and Submillimeter Array to elucidate the outgassing behavior of C/2016 R2. Results are analyzed using a new, time-dependent, three-dimensional radiative transfer code (SUBlimating gases in LIME; SUBLIME, based on the open-source version of the LIne Modeling Engine), incorporating for the first time, accurate state-to-state collisional rate coefficients for the CO–CO system. The total CO production rate was found to be in the range of (3.8 − 7.6) × 10²⁸s⁻¹between 2018 January 13 and February 1 (atr_H= 2.8–2.9 au), with a mean value of (5.3 ± 0.6) × 10²⁸s⁻¹. The emission is concentrated in a near-sunward jet, with a half-opening angle of ∼62° and an outflow velocity of 0.51 ± 0.01 km s⁻¹, compared to 0.25 ± 0.01 km s⁻¹in the ambient (and nightside) coma. Evidence was also found for an extended source of CO emission, possibly due to icy grain sublimation around 1.2 × 10⁵km from the nucleus. Based on the coma molecular abundances, we propose that the nucleus ices of C/2016 R2 can be divided into a rapidly sublimating apolar phase, rich in CO, CO₂, N₂, and CH₃OH, and a predominantly frozen (or less abundant), polar phase containing more H₂O, CH₄, H₂CO, and HCN. 

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