Article pubs.acs.org/JPCA
High-Resolution Absorption Cross Sections of Formaldehyde in the 30285−32890 cm−1 (304−330 nm) Spectral Region 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: Absolute room temperature (294 ± 2 K) absorption cross sections for the à 1A2−X̃ 1A1 electronic transition of formaldehyde have been measured over the spectral range 30285−32890 cm−1 (304−330 nm) using ultraviolet (UV) laser absorption spectroscopy. Accurate high-resolution absorption cross sections are essential for atmospheric monitoring and understanding the photochemistry of this important atmospheric compound. Absorption cross sections were obtained at an instrumental resolution better than 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 so we were able to resolve all but the most closely spaced lines. Comparisons with previous data as well as with computer simulations have been made. Pressure broadening was studied for the collision partners He, O2, N2, and H2O and the resulting broadening parameters have been measured and increase with the strength of intermolecular interaction between formaldehyde and the collision partner. The pressure broadening coefficient for H2O is an order of magnitude larger than the coefficients for O2 and N2 and will contribute significantly to spectral line broadening in the lower atmosphere. Spectral data are made available as Supporting Information.
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INTRODUCTION As an intermediate in the oxidation of hydrocarbons to carbon monoxide, formaldehyde (HCHO) plays a primary role in tropospheric chemistry.1 HCHO is the most abundant and most important organic carbonyl compound in the atmosphere. Levels range from ∼50 pptv (parts per trillion by volume) in clean tropospheric air2 to ≤70 ppbv (parts per billion by volume) in urban areas.3,4 The main sources of HCHO are secondary formation by the oxidation of methane, isoprene, acetone, and other volatile organic compounds (VOCs); and primary emission through fossil fuel combustion and biomass burning.5,6 The dominant loss mechanism for HCHO is photolysis which occurs via two pathways, with threshold energies7−9 indicated in parentheses: HCHO + hv → HCO + H
(30328.5 cm−1)
(R1)
HCHO + hv → H2 + CO
(27720 cm−1)
(R2)
(R4). CO is also produced by (R3) and oxidizes to produce CO2. (R3)
H + O2 + M → HO2 + M
(R4)
Thus, for every photon absorbed, the photolysis of HCHO can contribute one CO2 molecule to the global greenhouse budget via (R2) or one CO2 molecule to the global greenhouse budget and two HO2 radicals to the tropospheric HOx (OH + HO2) cycle via (R1) followed by (R3) and (R4). The HO2 radicals produced during HCHO photolysis have been implicated in the formation of photochemical smog.10,11 The HO2 radicals act as radical chain carriers and convert NO to NO2, which produces OH radicals and ultimately results in the catalytic production of O3. Results from the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project indicate that in urban areas, the reaction of O(1D) with H2O via (R5) accounted for
