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1. Head in the clouds

   https://doi.org/10.1016/S0262-4079(23)00580-8  

   Ponsford, Matthew (April 2023, New Scientist)
2. The acidity of atmospheric particles and clouds

   https://doi.org/10.5194/acp-20-4809-2020  

   Pye, Havala O.; Nenes, Athanasios; Alexander, Becky; Ault, Andrew P.; Barth, Mary C.; Clegg, Simon L.; Collett Jr., Jeffrey L.; Fahey, Kathleen M.; Hennigan, Christopher J.; Herrmann, Hartmut; et al (January 2020, Atmospheric Chemistry and Physics)

   Abstract. Acidity, defined as pH, is a central component of aqueouschemistry. In the atmosphere, the acidity of condensed phases (aerosolparticles, cloud water, and fog droplets) governs the phase partitioning ofsemivolatile gases such as HNO3, NH3, HCl, and organic acids andbases as well as chemical reaction rates. It has implications for theatmospheric lifetime of pollutants, deposition, and human health. Despiteits fundamental role in atmospheric processes, only recently has this fieldseen a growth in the number of studies on particle acidity. Even with thisgrowth, many fine-particle pH estimates must be based on thermodynamic modelcalculations since no operational techniques exist for direct measurements.Current information indicates acidic fine particles are ubiquitous, butobservationally constrained pH estimates are limited in spatial and temporalcoverage. Clouds and fogs are also generally acidic, but to a lesser degreethan particles, and have a range of pH that is quite sensitive toanthropogenic emissions of sulfur and nitrogen oxides, as well as ambientammonia. Historical measurements indicate that cloud and fog droplet pH haschanged in recent decades in response to controls on anthropogenicemissions, while the limited trend data for aerosol particles indicateacidity may be relatively constant due to the semivolatile nature of thekey acids and bases and buffering in particles. This paper reviews andsynthesizes the current state of knowledge on the acidity of atmosphericcondensed phases, specifically particles and cloud droplets. It includesrecommendations for estimating acidity and pH, standard nomenclature, asynthesis of current pH estimates based on observations, and new modelcalculations on the local and global scale. 

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3. Seasonality of the QBO Impact on Equatorial Clouds

   https://doi.org/10.1029/2022JD037737  

   Sweeney, Aodhan; Fu, Qiang; Pahlavan, Hamid A; Haynes, Peter (April 2023, Journal of Geophysical Research: Atmospheres)

   Abstract The Quasi‐Biennial Oscillation (QBO) dominates the interannual variability in the tropical lower stratosphere and is characterized by the descent of alternating easterly and westerly zonal winds. The QBO impact on tropical clouds and convection has received great attention in recent years due to its implications for weather and climate. In this study, a 15‐year record of high vertical resolution cloud observations from CALIPSO and a 50 hPa zonal wind QBO index from ERA5 are used to document the QBO impact on equatorial (10°S–10°N) clouds. Observations from radio occultations, the CERES instrument, and the ERA5 reanalysis are also used to document the QBO impact on temperature, cloud radiative effect (CRE), and zonal wind, respectively. It is shown that the QBO impact on zonal mean equatorial cloud fraction has a strong seasonality. The strongest cloud fraction response to the QBO occurs in boreal spring and early summer, which extends from above the mean tropopause to ∼12.5 km and results in a significant longwave CRE anomaly of 1 W/m². The seasonality of the QBO impact on cloud fraction is synchronized with the QBO impacts on temperature and zonal wind in the tropical upper troposphere. 

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4. Revisiting the Vertical Distribution of HI Absorbing Clouds

   Wenger, Trey; Rybarczyk, Daniel; Stanimirovic, Snezana (February 2024, Bulletin American Astronomical Society)

   The three-dimensional distribution of neutral hydrogen in the Milky Way disk is a key constraint on models of Galactic spiral structure, galaxy evolution, and star formation. In particular, the vertical distributions of the different phases of hydrogen (ionized, warm neutral, cold neutral, and molecular) inform our understanding of the evolution of gas between these phases. Although the scale height of the HI emission disk has been well-characterized across the Galaxy, the vertical distribution of the cold HI component is significantly more challenging to constrain due to the sensitive absorption observations required to characterize this phase. Almost four decades ago, Crovisier (1987) pioneered a kinematic method to estimate the vertical distribution of cold HI clouds in the solar neighborhood using the latest results from the Nancay 21-cm absorption survey. This method was subsequently used in other studies to constrain the vertical distribution of neutral and molecular clouds. We have discovered an error in Crovisier's method that can lead to a factor of two inaccuracy in the inferred scale height. We will discuss the mistake and, using the original Nancay data and a corrected method based on Crovisier's technique, demonstrate the magnitude of the error in the inferred scale height of the local cold HI disk. Furthermore, we will introduce a new Monte Carlo Markov Chain method to infer the vertical distribution of HI absorbing clouds with fewer assumptions and better accuracy. This method will be used with the latest HI absorption data from the Galactic ASKAP HI survey of the Milky Way disk to provide an unprecedented view of the 3D distribution of the cold neutral medium in the solar neighborhood. 

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5. Seeing Through the Clouds With DeepWaterMap

   https://doi.org/10.1109/LGRS.2019.2953261  

   Isikdogan, Leo F.; Bovik, Alan; Passalacqua, Paola (November 2019, IEEE Geoscience and Remote Sensing Letters)