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1. Measurements and model comparisons suggest that HONO is not an important source of OH radical concentrations below a rural forest canopy

   https://doi.org/10.1039/D5EA00122F  

   Hoover, Bode; Reidy, Emily; Spink, Ian; Hamachek, Amanda; Bottorff, Brandon; Stevens, Philip S (May 2026, Environmental Science: Atmospheres)

   The hydroxyl radical (OH) initiates the oxidation of volatile organic compounds (VOCs) in the atmosphere, leading to the production of ozone and secondary organic aerosols (SOA). While nitrous acid (HONO) can be an important OH source during the daytime, the mechanisms controlling HONO concentrations remain poorly understood, with unknown sources commonly reported. To improve our understanding of their sources and sinks, HONO and OH concentrations were measured below a forest canopy using a Laser-Photofragmentation/Laser-Induced Fluorescence instrument and were modeled using a zero-dimensional model based on the Master Chemical Mechanism. Heterogeneous conversion of NO2 on ground surfaces dominated nighttime HONO production (95%) while the reaction between OH and NO was the dominant daytime source (77%). Deposition was a primary HONO removal process at night (99%) and during the day (64%) followed by photolysis (34%). Overall, the model was able to reproduce the HONO measurements, although it could not reproduce observed increases in HONO during individual rain events, which were likely due to elevated heterogeneous production. Measured concentrations of OH were similar to previous observations above the canopy at this site, suggesting that significant chemical oxidation was occurring in the canopy that could impact the concentration of oxygenated VOCs and SOA above the canopy. Ozonolysis of monoterpenes was a dominant source of radicals below the canopy, accounting for 70% of total radical initiation. Photolysis of HONO contributed approximately 1% to total radical initiation, suggesting that at this site HONO is not an important source of OH radicals beneath the canopy. 

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2. Relative humidity effect on the formation of highly oxidized molecules and new particles during monoterpene oxidation

   https://doi.org/10.5194/acp-19-1555-2019  

   Li, Xiaoxiao; Chee, Sabrina; Hao, Jiming; Abbatt, Jonathan P.; Jiang, Jingkun; Smith, James N. (January 2019, Atmospheric Chemistry and Physics)

   Abstract. It has been widely observed around the world that the frequency and intensityof new particle formation (NPF) events are reduced during periods of highrelative humidity (RH). The current study focuses on how RH affects theformation of highly oxidized molecules (HOMs), which are key components ofNPF and initial growth caused by oxidized organics. The ozonolysis ofα-pinene, limonene, and Δ³-carene, with and without OHscavengers, were carried out under low NO_x conditions undera range of RH (from ∼3 % to ∼92 %) in atemperature-controlled flow tube to generate secondary organic aerosol (SOA).A Scanning Mobility Particle Sizer (SMPS) was used to measure the sizedistribution of generated particles, and a novel transverse ionizationchemical ionization inlet with a high-resolution time-of-fight massspectrometer detected HOMs. A major finding from this work is that neitherthe detected HOMs nor their abundance changed significantly with RH, whichindicates that the detected HOMs must be formed from water-independentpathways. In fact, the distinguished OH- and O₃-derived peroxyradicals (RO₂), HOM monomers, and HOM dimers could mostly beexplained by the autoxidation of RO₂ followed by bimolecularreactions with other RO₂ or hydroperoxy radicals (HO₂),rather than from a water-influenced pathway like through the formation of astabilized Criegee intermediate (sCI). However, as RH increased from ∼3 % to ∼92 %, the total SOA number concentrations decreased bya factor of 2–3 while SOA mass concentrations remained relatively constant. These observations show that, whilehigh RH appears to inhibit NPF as evident by the decreasing numberconcentration, this reduction is not caused by a decrease inRO₂-derived HOM formation. Possible explanations for these phenomenawere discussed. 

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3. Observations of biogenic volatile organic compounds over a mixed temperate forest during the summer to autumn transition

   https://doi.org/10.5194/acp-23-4123-2023  

   Vermeuel, Michael P.; Novak, Gordon A.; Kilgour, Delaney B.; Claflin, Megan S.; Lerner, Brian M.; Trowbridge, Amy M.; Thom, Jonathan; Cleary, Patricia A.; Desai, Ankur R.; Bertram, Timothy H. (January 2023, Atmospheric Chemistry and Physics)

   Abstract. The exchange of trace gases between the biosphere and the atmosphere is an important process that controls both chemical and physical properties of the atmosphere with implications for air quality and climate change. The terrestrial biosphere is a major source of reactive biogenic volatile organic compounds (BVOCs) that govern atmospheric concentrations of the hydroxy radical (OH) and ozone (O3) and control the formation andgrowth of secondary organic aerosol (SOA). Common simulations of BVOCsurface–atmosphere exchange in chemical transport models use parameterizations derived from the growing season and do not considerpotential changes in emissions during seasonal transitions. Here, we useobservations of BVOCs over a mixed temperate forest in northern Wisconsinduring broadleaf senescence to better understand the effects of the seasonal changes in canopy conditions (e.g., temperature, sunlight, leaf area, and leaf stage) on net BVOC exchange. The BVOCs investigated here include the terpenoids isoprene (C5H8), monoterpenes (MTs; C10H16), a monoterpene oxide (C10H16O), and sesquiterpenes (SQTs; C15H24), as well as a subset of other monoterpene oxides and dimethyl sulfide (DMS). During this period, MTs were primarily composed of α-pinene, β-pinene, and camphene, with α-pinene and camphene dominant during the first half of September and β-pinene thereafter. We observed enhanced MT and monoterpene oxide emissions following the onset of leaf senescence and suggest that senescence has the potential to be a significant control on late-season MT emissions in this ecosystem. We show that common parameterizations of BVOC emissions cannot reproduce the fluxes of MT, C10H16O, and SQT during the onset and continuation of senescence but can correctly simulate isoprene flux. We also describe the impact of the MT emission enhancement on the potential to form highly oxygenated organic molecules (HOMs). The calculated production rates of HOMs and H2SO4, constrained by terpene and DMS concentrations, suggest that biogenic aerosol formation and growth in this region should be dominated by secondary organics rather than sulfate. Further, we show that models using parameterized MT emissions likely underestimate HOM production, and thus aerosol growth and formation, during early autumn in this region. Further measurements of forest–atmosphere BVOC exchange during seasonal transitions as well as measurements of DMS in temperate regions are needed to effectively predict the effects of canopy changes on reactive carbon cycling and aerosol production. 

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4. Boundary Layer Observations and Near‐Surface Wind Estimation During the Landfalls of Hurricanes Ida (2021) and Zeta (2020)

   https://doi.org/10.1029/2025GL117487  

   Leffler, Zebulon W; Chen, Xiaomin; Carey, Lawrence D; Knupp, Kevin; Lee, Wen‐Chau (September 2025, Geophysical Research Letters)

   This study examines the boundary layer wind profile and turbulence variables during the landfalls of Hurricanes Ida (2021) and Zeta (2020) using ground‐based Doppler radar observations and a nearby anemometer's wind measurements. While the radar sampled different parts of the hurricane circulation of the two cases, the observed maximum near‐surface wind and frictional velocity were similar. Radar‐retrieved wind profiles in both hurricanes revealed a boundary‐layer jet generally >1 km AGL, descending toward smaller radii as the hurricanes moved inland. A “knee‐like” structure in most wind profiles below the jet suggests an internal boundary layer (IBL) below 200 m and a log layer above it. Among the three methods for estimating near‐surface sustained winds from radar‐retrieved winds, leveraging low‐level IBL winds improves estimation accuracy and reduces the uncertainty to the selection of upstream surface roughness length. These findings offer valuable guidance for developing future probabilistic near‐surface wind products. 

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5. Identifying Canopy Snow in Subalpine Forests: A Comparative Study of Methods

   https://doi.org/10.1029/2023WR036996  

   Harvey, Natasha; Burns, Sean P; Musselman, Keith N; Barnard, Holly; Blanken, Peter D (January 2025, Water Resources Research)

   Abstract The interception of snow by the canopy is an important process in the water and energy balance in cold‐region coniferous forests. Direct measurements of canopy snow interception are difficult at scales larger than individual trees, requiring indirect methods such as eddy covariance, time‐lapse photography, or modeling. At the Niwot Ridge Subalpine Forest AmeriFlux site in the Colorado Front Range, USA, we compared methods that estimate or simulate the presence of snow interception. Timelapse photography images were analyzed using thresholding analysis and used to train a Convolutional Neural Network (CNN) model to estimate canopy snow presence. Interception was also estimated from eddy covariance measurements above and below the canopy, as well as from model simulations. These methods were applied over January 2019, with binarized results compared to a “ground truth” of human labeled images to calculate the Balanced Accuracy Score. The highest accuracy was achieved by the CNN predictions. Based on the Balanced Accuracy Scores, select methods were extended to estimate the presence of canopy snow for the 2018/2019 winter. All methods provided insight into the process of interception in a subalpine forest but presented challenges, including differing flux footprints of the above‐ and below‐canopy eddy covariance measurements and the inability of red‐green‐blue imagery to monitor snow interception at night, during sunrise, and during sunset. 

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