The Journal of
Physical Chemistry
0 Copyright, 1984, by the American Chemical Society
VOLUME 88, NUMBER 7
MARCH 29, 1984
LETTERS Ultraviolet Absorption Spectrum of Peroxyacetyl Nitrate and Peroxypropionyl Nitrate G. I. Senurn,* Y.-N. Lee, and J. S. Gaffney Environmental Chemistry Division, Department of Applied Science, Brookhaven National Laboratory, Upton, New York 1 1 973 (Received: July 21, 1983; In Final Form: February 7 , 1984)
The UV absorption spectrum was measured for peroxyacetyl nitrate (PAN) and peroxypropionyi nitrate (PPN) at 298 f 1 K in the 200-400-nm wavelength region. From this, the absorption cross sections for PAN and PPN have been calculated. Solar photolysis rates for PAN and PPN are estimated on the basis of the measured and extrapolated cross sections.
Introduction Peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) have now come to be regarded as ubiquitous components of the lower atmosphere.'-' Both are produced from the photochemical reaction of hydrocarbons and nitrogen oxides in the atmosphere. Background PAN concentrations are estimated to range from 20 to 40 ppt and PAN concentrations up to several hundred ppt have been measured in rural areas.*s3 P P N concentrations have been measured at concentrations of 10-40% of that for PAN in several urban location^.^,^ (1) H. B. Singh and P. L. Hanst. Geophys. Res. Lett., 8, 941 (1981). (2) H. B. Singh and L. J. Salas, Nature (London), 302, 326 (1983). (3) T. Nielsen, U.Samuelsson, P. Grennfelt, and E. L. Thomsen, Nature (London) 293,553 (1981). (4) S. A. Penkett, F. J. Sandalls, and J. E. Lovelock, Atmos. Enuiron., 9, 139 (1975). (5) H.B. Singh, L. J. Salas, A. J. Smith, and H. Shigeishi, Atmos. Enuiron. 15,601 (1981). (6) C. W. Spicer, W. Holdren, and G. W. Keigley, Atmos.,Environ., 17, I055 (1983). (7) T. Nielsen, A. M. Hansen, and E. L. Thomsen, Atmos. Enuiron., 16, 2447 (1982). (8) J. S. Gaffney, R. Fajer, and G. I. Senum, Atmos. Enuiron., 18, 215 (1984).
0022-3654/84/2088-1269$01.50/0
Theoretical models are now suggesting that PAN and, to a lesser degree, PPN, are important reservoirs for nitrogen oxides in the unpolluted troposphere and consequently can act as transporting agent for nitrogen oxides into other regions of the atmosphere.' Qne aspect of these models that determines the lifetimes of PAN and PPN in the atmosphere is their ultraviolet photolysis rates. Previously, only the UV absorption spectrum of P A N had been measured by Stephensg and on the basis of this datum Baulch et al.1° have estimated provisionally the absorption cross sections for PAN in their review. Similarly, Singh and Hanst' have estimated the solar photolysis rate constant for PAN on the basis of Stephens' data. Recent advances in the syntheses for peroxyacyl have allowed gas samples of PAN and PPN to be prepared with a high degree of purity (>98%) at concentrations as high as their equilibrium vapor pressures. As a result of this availability of pure gaseous peroxyacyl nitrates and the recognition of the importance of these compounds in atmospheric chemistry, the ul(9) E. R. Stephens, Adv. Enuiron. Sci. Technol., 1, 119 (1969). (10) D. L. Baulch, R. A. Cox, P. J. Crutzen, R. F. Hampson, Jr., J. A. Kerr, J. Troe, and R. T. Watson, J . Phys. Chem. Re5 Data, 11, 327 (1982). ( 1 1 ) K. L.Demerjian, K. L. Schere, and J. T. Peterson, Adu. Enuiron. Sci. Technol., 10, 369 (1980). '.
0 1984 American Chemical Society
Letters
1270 The Journal of Physical Chemistry, Vol. 88, No. 7, 1984
TABLE I: UV Absorption Cross Sections for PAN and PPN 1020a,cm2 PAN A,
a
nm
200 205 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 krom ref
this work
lit.a
PPN this work
317 I 23 26gb 211 i 4 237 i 22 155 i 6 165 i 14 101 i 6 115 ?- 9 77t6 100 69i5 55+4 70 47.9 i 3.4 34.7 i 1.4 39.9 i. 3.1 50 24.2i 1.8 37 29.0 i 2.1 27 20.9 i 1.6 17.3 i 1.3 15.0 i 1.2 12.4 i 0.8 20 15 8.9 t 0.6 10.9 i 0.9 7.9 i 0.6 11 6.5 i 0.4 5.7 t 0.4 8 4.6 i 0.4 3.24 i 0.33 6 4.04 i 0.30 4 2.29 i 0.22 2.79 i 0.17 1.82 i- 0.12 1.51 i 0.15 1.14 i 0.08 0.99 i 0.15 0.716 i 0.023 0.60 i 0.11 0.414 i 0.025 0.33 L 0.04 0.221 i 0.016 0.16 i 0.04 0.105 + 0.023 0.097 f 0.009 10. Based on a single measurement.
traviolet absorption spectra of PAN and PPN have been measured and the UV absorption cross sections have been calculated.
Experimental Section Samples of PAN and PPN were prepared according to the method of Gaffney et al.* and gaseous samples were introduced into a 10.1-cm cell for measurement by using a greaseless vacuum system. The cell was fitted with sapphire windows so us to allow both infrared and UV measurements to be performed on the same sample. Gas pressures were measured by using a MKS Baratron capacitance manometer (Model PDR A) with 0-1000-mtorr and 0-1000 mbar ranges. Prior to recording the UV absorption spectra, we examined each PAN or PPN sample with a Nicolet 7199 Fourier transform infrared (FT-IR) spectrometer to verify the absence of any impurities, e.g., peracetic or perpropionic acid, or any decomposition products, e.g., nfethyl nitrate, nitromethane, ethyl nitrate, nitroethane, or NOz, which would interfere with the UV absorption spectrum. Consequently, each gas sample had a purity greater than 98% before the UV spectra were recorded. The ultraviolet spectra were recorded with a Beckman DU-7 microprocessor controlled single-beam UV/visible spectrometer with a resolution of 2 nm. Each sample was scanned at a rate of 300 nm/min from 400 to 200 nm. Both the sample and the cell blank were referenced against air. The stated photometric accuracy was *OS% of the absorbance reading and the wavelength accuracy was f0.5 nm. Several PAN and PPN samples were examined by FT-IR afterward and indicated a negligible degree (98% PAN, no observable NOz). The contribution of impurities to the ultraviolete spectrum resulting from the PAN and PPN syntheses or their decomposition products (Le., the alkyl nitrates and nitrttes) based on their reported UV absorption spectra (methyl nitrate and methyl nitrite,I2 ethyl nitrate and ethyl nitriteI3) are within the experimental error reported here. The solar photolysis rates of PAN and PPN are calculated by using the presently measured cross sections up to 300 nm. Estimates of the cross sections to 330 nm were made by linear extrapolation of logarithmic plots of absorption cross section vs. wavelength. The actinic fluxes at the earth's surface as a function of wavelength and solar zenith angle were taken from Table 4 of Demerjian et al." The resulting solar photolysis rate constants are given in Table I1 as a funuion of solar zenith angle. The solar photolysis rates for PPN are slightly larger than those for PAN, which is due to the slight red shifting of the PPN UV spectrum. The solar photolysis rate constqnts calculated here are approximately a factor of 10 larger than those calculated by Singh and Hanst' for PAN based on the absorption spectrum given by step hen^.^ However, this reevaluation of the solar photolysis rate constant is not significant with respect to the tropospheric chemistry of PAN since the thermal decomposition rate of PAN is faster and consequently predominantes.'
Acknowledgment. We thank R. Fajer for preparation of the PAN and PPN samples used in this work. This research was performed under the auspices of the U S . Department of Energy under contract no. DE-AC02-76CH00016. (12) W. D. Taylor, T. D. Allston, M. J. Moscato, G. B. Fazebas, R. Kozlowski, and G. A. Takacs, I n t . J . Chem. Kine?., 12, 231 (1980). (13) P. A. Leighton, 'Photochemistry of Air Pollutants", Academic Press, New York, 1961, pp 62, 65.
