In-situ laser induced fluorescence detection and modeling of formaldehyde above a forest canopy

TitleIn-situ laser induced fluorescence detection and modeling of formaldehyde above a forest canopy
Publication TypeThesis
Year of Publication2009
AuthorsHottle JRobert
AdvisorKeutsch FN
Academic DepartmentDepartment of Chemistry
DegreeDoctor of Philosophy
Number of Pages190 pp.
UniversityUniversity of Wisconsin
CityMadison, WI
Thesis TypePhD Dissertation
KeywordsVOLATILE ORGANIC COMPOUNDS
Abstract

Formaldehyde (HCHO) is ubiquitous throughout the atmosphere. Its main source within the troposphere is the oxidation of volatile organic compounds (VOCs). The photolysis and oxidation of HCHO leads to elevated levels of ozone (O 3 ) and secondary organic aerosols (SOA), which play critical roles in human health and climate change. The design, development and implementation of an in-situ laser induced fluorescence (LIF) based instrument for atmospheric detection of HCHO is described. To the best of the author's knowledge, the first published reports using LIF for the measurement of atmospheric HCHO are presented. The instrument was field tested during the PROPHET, 2008 field intensive, at the University of Michigan Biological Station, where it continuously measured HCHO for one week with a detection limit (3σ) of 0.120 ppb v /min. HCHO levels ranged from ∼0.5 - 4.5 ppb v at a height of ∼10 m above the forest canopy from August 1-8, 2008, in good agreement with previously reported values at the site. Two photochemical box models were constructed to evaluate the understanding of the photochemistry controlling HCHO in a forested region dominated by isoprene emissions. An explicit photochemical box model, as defined by the Master Chemical Mechanism ver3.1, was compared to an implicit chemical model previously developed in the literature. These two models were evaluated to determine the accuracy of the assumptions within the implicit model, in addition to further examining our understanding of HCHO chemistry within an isoprene dominated forest. These two models were found to agree within ∼5% throughout the daytime (1000-1800 h, EST) and both calculations over-predict HCHO levels by a factor of ∼2.8. In an effort to estimate the influence of vertical advection, a vertical dilution term was added to both models and it was found to reduce the over-prediction of both from a factor of ∼2.8 to ∼1.6. These results imply that the photochemistry occurring within an isoprene impacted forested area is well represented while the effects of horizontal and vertical advection require further investigation to fully constrain the HCHO budget.

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