Article pubs.acs.org/JPCA
Cavity-Enhanced Measurements of Hydrogen Peroxide Absorption Cross Sections from 353 to 410 nm Tara F. Kahan,† Rebecca A. Washenfelder,‡,§ Veronica Vaida,†,‡ and Steven S. Brown*,§ †
Department of Chemistry and Biochemistry, University of Colorado, Campus Box 215, Boulder, Colorado 80309, United States Cooperative Institute for Research in Environmental Sciences, University of Colorado, Box 216 UCB, Boulder, Colorado 80309, United States § Chemical Sciences Division, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado 80305, United States ‡
S Supporting Information *
ABSTRACT: We report near-ultraviolet and visible absorption cross sections of hydrogen peroxide (H2O2) using incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS), a recently developed, high-sensitivity technique. The measurements reported here span the range of 353−410 nm and extend published electronic absorption cross sections by 60 nm to absorption cross sections below 1 × 10−23 cm2 molecule−1. We have calculated photolysis rate constants for H2O2 in the lower troposphere at a range of solar zenith angles by combining the new measurements with previously reported data at wavelengths shorter than 350 nm. We predict that photolysis at wavelengths longer than those included in the current JPL recommendation may account for up to 28% of the total hydroxyl radical (OH) production from H2O2 photolysis under some conditions. Loss of H2O2 via photolysis may be of the same order of magnitude as reaction with OH and dry deposition in the lower atmosphere; these processes have very different impacts on HOx loss and regeneration.
1. INTRODUCTION Hydrogen peroxide is an important trace species in the atmosphere due to its role in atmospheric HOx cycling and as an oxidant in aqueous environments. Its ambient concentrations near the ground level range from a few parts per trillion (ppt) to a few parts per billion (ppb) (see, for example, refs 1 and 2). It is formed in the atmosphere primarily through reaction 1 (e.g., ref 3). k1
HO2 + HO2 → H2O2 + O2
(1)
Hydrogen peroxide is lost primarily through photolysis (reaction 2), reaction with OH (reaction 3), and by deposition (e.g., refs 2 and 4−6.). Reaction 2 regenerates all of the HOx lost in reaction 1, while reaction 3 and deposition both result in a net loss of two HOx. J2
H2O2 + hν → 2OH
Figure 1. Literature room-temperature absorption cross sections for H2O2 from 190 to 350 nm. The JPL recommendation is the mean of the displayed measurements. The data displayed here were taken from the Mainz Spectral Database.36
(2)
k3
H2O2 + OH → H2O + HO2
1 × 10−20 cm2 molecule−1, and the cross section at 350 nm is less than 4 × 10−22 cm2 molecule−1. Due to the technical challenges of measuring small absorption cross sections, there is a high degree of uncertainty associated with reported cross sections at longer wavelengths; at 350 nm, the longest wavelength considered in the
(3)
Models currently underpredict measured atmospheric HOx levels, especially in low-NOx environments (e.g., refs 7−9). The photolysis of multifunctional hydroperoxides has been suggested as a potential HOx source.10 Inaccuracies in the efficiency of reaction 2 may lead to a small but non-negligible bias in models as well. Figure 1 shows measured absorption cross sections of the 1A′−1A″ transition of H2O2.5,11−14 The cross sections are small at atmospherically relevant wavelengths. At 290 nm, the absorption cross section is less than © 2012 American Chemical Society
Special Issue: A. R. Ravishankara Festschrift Received: October 31, 2011 Revised: January 5, 2012 Published: January 6, 2012 5941
dx.doi.org/10.1021/jp2104616 | J. Phys. Chem. A 2012, 116, 5941−5947
The Journal of Physical Chemistry A
Article
Jet Propulsion Laboratory (JPL) recommendation, reported that cross sections differ by up to 39%.5,11−14 Although weak absorptions are difficult to measure, they can be atmospherically important (e.g., ref 15−19). In the case of H2O2, weak absorptions at wavelengths longer than 350 nm could increase the importance of reaction 2 beyond the level currently estimated in models that truncate H2O2 photolysis where the database for the experimental cross sections currently ends. Extrapolation of measured cross sections to longer wavelengths suggests that absorption at wavelengths longer than 350 nm could contribute to approximately 20% of the overall H2O2 photolysis.12 Accurate absorption cross sections at long wavelengths will also increase the accuracy of pernitric acid (PNA, HO2NO2) absorption cross sections, photolysis rate constants, and quantum yields. Hydrogen peroxide and nitric acid are impurities in PNA samples, and weak absorption at long wavelengths from these two species contributes to uncertainty in measured PNA absorption coefficients.20
In the past decade, cavity-enhanced spectroscopic techniques have been developed that are far more sensitive than traditional absorption spectroscopy. We have recently developed an incoherent broad-band cavity-enhanced absorption spectrometer (IBBCEAS)21,22 capable of sensitive absorption measurements between 350 and 475 nm. We have used it to measure absorption cross sections of ozone down to 10−24 cm2 molecule−1 in the absorption minimum between the Huggins and Chappuis bands between 375 and 390 nm.22 In the current study, we use this technique to extend measurements of H2O2 absorption cross sections to longer wavelengths than those reported in current databases.
2. EXPERIMENTAL SECTION 2.1. Description of the IBBCEAS Instrument and Spectra Acquisition. The IBBCEAS apparatus used in this work, shown in Figure 2, is similar to the apparatus described previously for ozone measurements.22 Two channels, centered
Figure 2. (a) Schematic of the two-channel incoherent broad-band cavity-enhanced absorption spectrometer (IBBCEAS) and the single-pass absorption cell. (b) Block diagram showing gas delivery of H2O2 to the IBBCEAS and the single-pass absorption cell. 5942
dx.doi.org/10.1021/jp2104616 | J. Phys. Chem. A 2012, 116, 5941−5947
The Journal of Physical Chemistry A
Article
samples of H2O2 have been concentrated to approximately 95% (v/v).23 Approximately 3−6 mL of concentrated H2O2 was transferred to a bubbler kept under a constant helium purge for use in our experiments. The saturated flow of H2O2 in helium was further diluted by a factor of 1.2−4.0 by a separate flow of He prior to measurement. Total flow rates ranged from 800 to 1800 standard cubic cm per minute (sccm). Purge volumes prior to cavity mirrors may be used to maintain mirror cleanliness during atmospheric sampling but were not used in these experiments. Hydrogen peroxide concentrations were determined from the optical extinction of the 213.9 nm line of a zinc Pen-Ray lamp. These measurements were performed in a 1 m single-pass absorption cell placed in series with the IBBCEAS. Band-pass filters (214 nm) were placed at both ends of the absorption cell, and the intensity at the cell output was measured by a phototube connected to a picoammeter. The H2O2 concentration was calculated based on Beer’s Law, using the measured absorption cross section at 213.9 nm of 3.304 ± 0.217 × 10−19 cm2. 13 Concentrations ranged from 0.6 to 5 × 10 15 molecules cm−3. The precision of the concentration measurement was 1.4%, based on the stability of the measurement during the 5 min acquisition period. 2.3. Optimization and Validation of Measurements. We addressed two potential sources of systematic error through design and testing of the apparatus. Hydrogen peroxide is a sticky and reactive gas that may be lost on surfaces of the measurement and sample delivery system. At low flow rates (
