Choi W

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Publications (2)0 Total impact

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    Article: Observations of elevated formaldehyde over a forest canopy suggest missing sources from rapid oxidation of arboreal hydrocarbons
    Choi W, I. C. Faloona, N. C. Bouvier-Brown, McKay M, A. H. Goldstein, Mao J, W. H. Brune, B. W. LaFranchi, R. C. Cohen, G. M. Wolfe, J. A. Thornton, D. M. Sonnenfroh, D. B. Millet
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    ABSTRACT: To better understand the processing of biogenic VOCs (BVOCs) in the pine forests of the US Sierra Nevada, we measured HCHO at Blodgett Research Station using Quantum Cascade Laser Spectroscopy (QCLS) during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) of late summer 2007. Four days of the experiment exhibited particularly copious HCHO, with midday peaks between 15–20 ppbv, while the other days developed delayed maxima between 8–14 ppbv in the early evening. From the expansive photochemical data set, we attempt to explain the observed HCHO concentrations by quantifying the various known photochemical production and loss terms in its chemical budget. Overall, known chemistry predicts a factor of 3–5 times less HCHO than observed. By examining diurnal patterns of the various budget terms we conclude that, during the high HCHO period, local, highly reactive oxidation chemistry produces an abundance of formaldehyde at the site. The results support the hypothesis of previous work at Blodgett Forest suggesting that large quantities of oxidation products, observed directly above the ponderosa pine canopy, are evidence of profuse emissions of very reactive volatile organic compounds (VR-VOCs) from the forest. However, on the majority of days, under generally cooler and more moist conditions, lower levels of HCHO develop primarily influenced by the influx of precursors transported into the region along with the Sacramento plume.
    Atmospheric Chemistry and Physics. 01/2010;
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    Article: Estimating the atmospheric boundary layer height over sloped, forested terrain from surface spectral analysis during BEARPEX
    Choi W, I. C. Faloona, McKay M, A. H. Goldstein, Baker B
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    ABSTRACT: In this study the atmospheric boundary layer (ABL) height (zi) over complex, forested terrain is estimated based on the power spectra and the integral length scale of horizontal winds obtained from a three-axis sonic anemometer during the BEARPEX (Biosphere Effects on Aerosol and Photochemistry) Experiment. The zi values estimated with this technique showed very good agreement with observations obtained from balloon tether sonde (2007) and rawinsonde (2009) measurements under unstable conditions (z/L < 0) at the coniferous forest in the California Sierra Nevada. The behavior of the nocturnal boundary layer height (h) and power spectra of lateral winds and temperature under stable conditions (z/L > 0) is also presented. The nocturnal boundary layer height is found to be fairly well predicted by a recent interpolation formula proposed by Zilitinkevich et al. (2007), although it was observed to only vary from 60–80 m during the experiment. Finally, significant directional wind shear was observed during both day and night with winds backing from the prevailing west-southwesterlies in the ABL (anabatic cross-valley circulation) to consistent southerlies in a layer ~1 km thick just above the ABL before veering to the prevailing westerlies further aloft. We show that this is consistent with the forcing of a thermal wind driven by the regional temperature gradient directed due east in the lower troposphere.
    Atmospheric Chemistry and Physics Discussions. 01/2010;
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