Photolysis frequencies ,

Junkermann, W., U. Platt, and A. Volz-Thomas, A Photoelectric Detector for the Measurement of Photolysis Frequencies of Ozone and Other Atmospheric Molecules, . /. Atmos. Chem., 8, 203-227 (1989). 

Ruggaber, A., R. Dlugi, A. Bott, R. Forkel, H. Herrmann, and H.-W. Jacobi, Modelling of Radiation Quantities and Photolysis Frequencies in the Aqueous Phase in the Troposphere, Atmos. Environ., 31, 3137-3150 (1997). 

Thorough analysis of the data from the campaign, including final QA/QC procedures as well as UV and CTM modelling studies are currently under way. In Fig. 2, a preliminary analysis of the photolysis frequency data is presented. 

Fig. 2. Observed correlations between the photolysis frequency of ozone and total ozone (upper panel) and the photolysis frequency of NO, and the TOMS index of aerosol load (lower panel) during PAURII (May 1999).

Fieure8 One hour averages of (a) JN02 and (b) JO( D) as a function of solar zenith angle under cloudless conditions. The solid line represents a best fit of the upper limit of measured photolysis frequencies, corresponding to a clear atmosphere with background aerosol load. The broken lines denote the results of model simulations based on different model aerosol scenarios (from figure 6 of Reuder and Schwander 1999). 

Ruggaber, A., Dlugi, R., and Nakajima, T. (1994) Modelling radiation quantities and photolysis frequencies in the troposphere, J. Atmos. Chem. 18, 171-210. 

In the following we will call the a u,n,j) partial photodissociation cross sections.t They are the cross sections for absorbing a photon with frequency u and producing the diatomic fragment in a particular vibrational-rotational state (n,j). Partial dissociation cross sections for several photolysis frequencies constitute the main body of experimental data and the comparison with theoretical results is based mainly on them. Summation over all product channels (n,j) yields the total photodissociation cross section or absorption cross section ... 

Two options are available to calculate photolysis frequencies for the photochemical reactions of the gas-phase chemistry model. These are based on Madroiuch (1987) and Wild et al. (2000) and are also calculated on-line. 

Photolysis frequencies in MOZART-3 are based on tabulated values of the Tropospheric Ultraviolet and Visible radiation model ((TUV) version 3.0) (Madronich and Flocke 1998) for clear sky conditions. The adjustment for cloudiness is described in Brasseur et al. (1998). 

Volkamer, R., U. Platt and K. Wirtz OH reaction rate constants and photolysis frequencies of a series of aromatic aldehydes and phenols in The European Photoreactor, EUPHORE, 3" report, (2000) 1. 

About 580 actinic flux spectra recorded under different meteorological conditions (clear sky, partially cloudy or overcast) in the EUPHORE smog chambers, have been used to calculate the photolysis frequencies for various small carbonyl compounds that are considered to be important from the atmospheric chemistry point of view. The results are presented here. 

Determination of Photolysis Frequencies for Selected Carbonyl Compounds... 

To obtain a clear dependency a statistical treatment of all the calculated photolysis frequencies, derived from all the actinic spectra recorded in the EUPHORE chamber has been performed. Figure 3 shows an example of the complete dataset for acetaldehyde. The statistical treatment has allowed a clear dependency between the calculated photolysis frequency and the solar zenith angle to be established. The result obtained for acetaldehyde statistical treatment after is presented graphically in Figure 4. The error bars represent the statistical (la) error only. Photolysis frequencies have been calculated in the range of 19 to 71.5 solar zenith angles for 17 carbonyl compounds. The calculated photolysis frequencies obtained for the different zenith angles as derived from all the EUPHORE actinic flux spectra measurements are presented in Table 1. 

The empirical parameters m, n and obtained from fits of the calculated photolysis frequencies as a function of solar zenith angle are listed in Table 2. 

Figure 2. Plot of the calculated photolysis frequencies of acetaldehyde from the entire data set against solar zenith angle.

Figure 3. Calculated photolysis frequency profile for one year derived from data recorded using the EUPHORE outdoor chamber.

Table 1. Calculated photolysis frequencies for carbonyl compounds (in s ) at different zenith angles derived from the EUPHORE chamber measurements.

In Table 3 the mean of the all calculated photolysis frequencies for the various compounds obtained from the analysis of the actinic flux measurements performed using EUPHORE chamber facilities is compared with results from other similar studies. In the estimates a quantum yield of unity has been assumed. 

The errors are a combination of the statistical errors and the errors associated with uncertainties in the molecular parameters. The effective quantum yields, < ) in Table 3 were determined by division of the measured individual photolysis frequencies (J,) by the photolysis frequencies calculated in this study (J,) assuming a quantum yield of unity. For some of the carbonyl compounds the determined ( ), is significandy below unity. The photolydc lifetimes obtained using the present photolysis frequencies are also presented in Table 3. 

Table 3. Photolysis frequencies and effective quantum yield < ) g for some carbonyl compounds as derived from the EUPHORE facilities and found in the literature.

In the present work a large set of actinic spectra recorded in the EUPHORE chamber under various atmospheric conditions has been obtained and used for the calculation of photolysis frequencies of 17 organic carbonyl compounds. From a statistical analysis of the photolysis frequencies calculated for the compounds an analytical form for Jfd) has been derived. For unsaturated compounds (methyl vinyl ketone, methacrolein, acrolein and crotonaldehyde) < ) g is negligible, although those cxompounds possess absorption spectra reaching the near visible. 

IR speetra were recorded at a spectral resolution of 1 cm . Between 8 and 12 spectra were recorded in each kinetic experiment, every spectrum comprised of 128 co-added interferograms. Before the kinetic experiment a series of five experiments were performed for every nitrocresol compound in order to obtain the photolysis and wall loss rates. These latter two constants were used in corrections of the OH kinetic rate data. Formation of OH radicals was observed during the photolysis of nitrocresols. Using isoprene as scavenger for OH radicals (kon = 1.0 x lO cm s, Atkinson and Arey, 2003) it was possible to calculate the eoneentration of OH radicals produced by the photolysis and flius calculate a correct rate eonstant for the photolysis frequency. 

The magnitude of the photolysis rates for methylated nitrophenols under atmospheric conditions can be roughly estimated from the experiments using the superactinic lamps. This is accomplished by adjusting the values obtained in the reactor and presented in Table 2, with a factor comprised of the ratio of NO2 photolysis measured in the atmosphere and that measured in the reactor. For example, a factor of Jno2 (atmosphere)/ Jncu (in the reactor) = (8.5 0.5) X 10 s V (2.0 0.2) x 10 s = 4.25 is obtained when using an atmospheric noontime photolysis frequency of NO2 typical for clear sky conditions at a latitude of 40 N for July 1 (Klotz et al, 1997). 

It is remarkable that for all the reactions displayed in Figure 5, the corresponding sensitivities for the diesel exhaust runs are more or less equal, demonstrating that the different but significant higher ozone formation rates observed in comparison to the reference experiment are not specific for the diesel fuel formulation. They are clearly the result of more or less different initial parameters such as start concentrations or photolysis frequencies during the single experiments. 

Figure 4.37 shows the relative contribution of each band to the ozone photolysis frequency as a function of altitude. It should be noted that the value of the ozone photodissociation coefficient depends critically on the absorption by ozone itself, introducing a nonlinear coupling as a function of altitude. The calculation of Jo3 must include the effects of molecular scattering and albedo (see Figure 4.38). 

In the lower mesosphere and upper stratosphere, the continuum absorption of water vapor occurs in the domain of the O2 Schumann Runge bands. The photolysis frequency at zero optical depth varies from 4.0 to 6.5 x 10-6s-1 depending on the level of solar activity. 

The absorption spectrum of nitric oxide was observed more than a century ago, but the atmospheric importance of this chemical species has been stressed only in recent decades. Cieslik and Nicolet (1973) and Cieslik (1977), for example, used the measurements of Miescher and colleagues at the University of Basel to calculate the photolysis frequency of NO in the middle atmosphere. High resolution measurements of... 

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