Article pubs.acs.org/JPCA
Radical Quantum Yields from Formaldehyde Photolysis in the 30 400−32 890 cm−1 (304−329 nm) Spectral Region: Detection of Radical Photoproducts Using Pulsed Laser Photolysis−Pulsed Laser Induced Fluorescence Cheryl Tatum Ernest, Dieter Bauer, and Anthony J. Hynes* Division of Marine and Atmospheric Chemistry, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, United States S Supporting Information *
ABSTRACT: The relative quantum yield for the production of radical products, H + HCO, from the UV photolysis of formaldehyde (HCHO) has been measured using a pulsed laser photolysis−pulsed laser induced fluorescence (PLP− PLIF) technique across the 30 400−32 890 cm−1 (304−329 nm) spectral region of the à 1A2−X̃ 1A1 electronic transition. The photolysis laser had a bandwidth of ∼0.09 cm−1, which is slightly broader than the Doppler width of a rotational line of formaldehyde at 300 K (∼0.07 cm−1), and the yield spectrum shows detailed rotational structure. The H and HCO photofragments were monitored using LIF of the OH radical as a spectroscopic marker. The OH radicals were produced by rapid reaction of the H and HCO photofragments with NO2. This technique produced an “action” spectrum that at any photolysis wavelength is the product of the H + HCO radical quantum yield and HCHO absorption cross section at the photolysis wavelength and is a relative measurement. Using the HCHO absorption cross section previously obtained in this laboratory, the relative quantum yield was determined two different ways. One produced band specific yields, and the other produced yields averaged over each 100 cm−1. Yields were normalized to a value of 0.69 at 31 750 cm−1 based on the current recommendation of Sander et al. (Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; et al. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation No. 17; Jet Propulsion Laboratory: Pasadena, CA, USA, 2011). The resulting radical quantum yields agree well with previous experimental studies and the current JPL recommendation but show greater wavelength dependent structure. A significant decrease in the quantum yield was observed for the 510 + 110410 combination band centered at 31 125 cm−1. This band has a low absorption cross section and has little impact on the calculated atmospheric photodissociation rate but is a further indication of the complexity of HCHO photodissociation dynamics.
1. INTRODUCTION Formaldehyde (HCHO) is ubiquitous in the troposphere and is one of the most important carbonyl molecules in the lower atmosphere.2−4 It is produced by the oxidation of methane, isoprene, acetone, and other volatile organic compounds (VOCs), fossil fuel combustion, and biomass burning.5,6 With an atmospheric lifetime on the order of hours,7 levels range from ∼50 pptv (parts per trillion by volume) in clean tropospheric air8 to ≤70 ppbv (parts per billion by volume) in urban areas,9,10 HCHO has the potential to significantly influence both local and regional scale atmospheric chemistry.2 HCHO is one of the indicators of poor air quality because essentially every organic species in the atmosphere degrades with a reaction pathway that includes HCHO.11,12 The dominant loss mechanism for HCHO is photolysis which occurs via two pathways (R1 and R2) with threshold energies13−15 indicated in parentheses: © 2012 American Chemical Society
HCHO + hν → HCO + H
HCHO + hν → H 2 + CO
(30 328.5 cm−1)
(27 720 cm−1)
(R1) (R2)
The first pathway (R1) is referred to as the radical channel, while the second pathway (R2) is referred to as the molecular channel. The products of both pathways play a significant role in atmospheric chemistry. The CO that is produced by the molecular channel undergoes further oxidation to produce CO2. Under atmospheric conditions, the H atom and formyl radical that are produced by the radical channel undergo rapid reactions with O2 to produce the hydroperoxyl radical (HO2) via reactions R3 and R4. CO is also produced by reaction R3 and oxidizes to produce CO2. Received: December 6, 2011 Revised: May 21, 2012 Published: May 25, 2012 6983
dx.doi.org/10.1021/jp2117399 | J. Phys. Chem. A 2012, 116, 6983−6995
The Journal of Physical Chemistry A
Article
HCO + O2 → HO2 + CO
(R3)
H + O2 + M → HO2 + M
(R4)
The HO2 radicals act as radical chain carriers and convert NO to NO2. The photolysis of NO2 leads to the production of O3, a key component of photochemical smog.16,17 Through the indirect production of HOx (OH + HO2), HCHO can influence the oxidizing capacity of the atmosphere, because OH is the principal daytime tropospheric oxidant.4 Groundbased HOx field measurements in polluted environments illustrated that HCHO photolysis can be a more significant radical source in the early morning and late evening than the primary OH production route of O3 photolysis followed by reaction of the resultant O(1D) with H2O.18−21 Production by HCHO is favored because the longer wavelength threshold for HCHO photolysis compared to that for O3 photolysis results in greater rates of HCHO photolysis at high solar zenith angles (SZA).4 Additionally, convective injection of HCHO from the boundary layer into the upper troposphere followed by photodissociation serves as an additional source of HO2 radicals in this dry region of the atmosphere where the production of OH resulting from the reaction of O(1D) with H2O becomes less significant. This mechanism may help reconcile the discrepancies between observed and modeled HOx concentrations in the upper troposphere.22,23 An accurate understanding of HCHO’s photochemistry is thus essential for understanding and accurately modeling the overall tropospheric chemistry associated with hydrocarbon oxidation, the mechanisms involving the cycling among odd hydrogen species (HOx) and odd nitrogen species (NOx), and the global budgets of OH and CO.3 There are two components that need to be determined in order to accurately characterize the role of HCHO photochemistry in the troposphere. Accurate values of high-resolution wavelength dependent HCHO absorption cross sections, σHCHO(λ), and absolute radical (H + HCO) quantum yields, ΦH+HCO(λ), are necessary to combine with solar flux data to calculate the photolysis rates of HCHO via the radical channel. Numerous measurements of HCHO absorption cross sections have been made under a wide range of different experimental parameters including spectral range, resolution, temperature, pressure, and method of HCHO generation.24−36 As a necessary first step in our laboratory investigations of HCHO photochemistry, we have measured room temperature absolute absorption cross sections obtained at an instrumental resolution better than 0.09 cm−1 measured under conditions of low partial pressure of HCHO (
