Photolysis rate calculation

These equations identify the dominant source and loss processes for HO and H02 when NMHC reactions are unimportant. Imprecisions inherent in the laboratory measured rate coefficients used in atmospheric mechanisms (for instance, the rate constants in Equation E6) can, themselves, add considerable uncertainty to computed concentrations of atmospheric constituents. A Monte-Carlo technique was used to propagate rate coefficient uncertainties to calculated concentrations (179,180). For hydroxyl radical, uncertainties in published rate constants propagate to modelled [HO ] uncertainties that range from 25% under low-latitude marine conditions to 72% under urban mid-latitude conditions. A large part of this uncertainty is due to the uncertainty (la=40%) in the photolysis rate of 0(3) to form O D, /j. 

From the prevailing NO and HONO levels occurring during this period of the irradiation, the HONO photolysis rate (11,14), and the rate constant for the OH + NO reaction (15), we estimate that steady state OH levels of "2 x 1Q7 molecule cm" were present. From this OH radical concentration and assuming an UDMH + OH rate constant similar to those observed for N2Hi and MMH (, j ) we calculate a UDMH decay rate which is in reasonable agreement with what is observed. Thus, the HONO level measured during the initial period is entirely consistent with our assumed mechanism. 

Most commercial spectrometers report absorbance, as defined in Eq. (Q), versus wavelength. This is very important to recognize, since as we will see later, calculations of the rate of light absorption in the atmosphere require the use of absorption coefficients to the base e rather than to the base 10. While the recent atmospheric chemistry literature reports absorption cross sections to the base e, most measurements of absorption coefficients reported in the general chemical literature are to the base 10. If these are to be used in calculating photolysis rates in the atmosphere, the factor of 2.303 must be taken into account. 

It is these contrasting effects of aerosol particles, combined with uncertainties in the contribution of absorption due to 03, that provide the largest uncertainties in calculations of actinic fluxes and photolysis rates in the boundary layer (e.g., Schwander et al., 1997). As a result, it is important to use the appropriate input... 

Figure 3.29, for example, shows measurements of the photolysis rate of 03, J(03), made at the Mauna Loa Observatory on two different days, compared to model calculations of the photolysis rate constant (Shetter et al., 1996). The two model calculations use different assumptions regarding the quantum yield for 03 photolysis in the absorption tail beyond 310 nm (see Chapter 4.B). The measurements are in excellent agreement for the second day but somewhat smaller than the model calculations on the first. 

The reasons for discrepancies between various measurements and between the measured and model calculated values are not clear. Lantz et al. (1996) suggest that one factor that will affect instantaneous photolysis rates is cloud cover and that under some circumstances, the instantaneous photolysis rates may exceed... 

FIGURE 3.30 Values of 7(N02) at 7- to 7.5-km altitude as a function of solar zenith angle (0) measured using 2ir radiometers (circles) compared to a model calculated photolysis rate (solid line). (Adapted from Volz-Thomas et al., 1996.)... 

Only reaction (7) leads to the removal of NOz via photochemistry and hence the quantum yield for reaction (7) is needed to calculate the photolysis rate. Data on both primary quantum yields and absorption cross section [Pg.80]

It must again be stressed that the absorption cross sections, a (A), used to calculate photolysis rates are to the base e, not base 10, even though the latter is what has often been measured and reported in the literature in the past. 

TABLE 3.19 3. SPECTROSCOPY AND PHOTOCHEMISTRY FUNDAMENTALS Calculated Photolysis Rate Constants for CH CHO Photolysis at 30°N Latitude Six Hours after Noon on July 1 ... 

Once the actinic fluxes, quantum yields, and absorption cross sections have been summarized as in Table 3.19, the individual products < .,v(A)wavelength interval can be calculated and summed to give kp. Note that the individual reaction channels (9a) and (9b) are calculated separately and then added to get the total photolysis rate constant for the photolysis of acetaldehyde. However, the rate constants for the individual channels are also useful in that (9a) produces free radicals that will participate directly in the NO to N02 conversion and hence in the formation of 03, etc., while (9b) produces relatively unreactive stable products. 

Calculate the photolysis rate constants, kp, for each of the two photolysis paths as well as the overall... 

While the relative importance of the various paths is not well established, it is expected that dissociation to the alkoxy radical, RO, and N02 will predominate. Luke et al. (1989) experimentally measured rates of photolysis of simple alkyl nitrates and compared them to rates calculated using the procedures outlined in Chapter 3.C.2. Figure 4.22 compares the experimentally determined values of the photolysis rate constants (kp) for ethyl and n-propyl nitrate with the values calculated assuming a quantum yield for photodissociation of unity. The good agreement suggests that the quantum yield for photodissociation of the alkyl nitrates indeed approaches 1.0. 

FIGURE 4.22 Experimental values of the photolysis rate constant, kp, for (a) ethyl nitrate and (b) n-propyl nitrate as a function of zenith angle compared to calculated values shown by the solid lines. Different symbols represent different measurement days (adapted from Luke et al., 1989). 

The absolute values of the absorption cross sections of HCHO have been somewhat controversial. This appears to be due to a lack of sufficient resolution in some studies as discussed in Chapter 3.B.2, if the spectral resolution is too low relative to the bandwidth, nonlinear Beer-Lambert plots result. The strongly banded structure means that calculations of the photolysis rate constant require actinic flux data that have much finer resolution than the 2- to 5-nm intervals for which these flux data are given in Chapter 3 or, alternatively, that the measured absorption cross sections must be appropriately averaged. One significant advantage of the highly structured absorption of HCHO is that it can be used to measure low concentrations of this important aldehyde in the atmosphere by UV absorption (see Sections A.ld and A.4f in Chapter 11.). 

Sasha Madronich generously not only reviewed the section on atmospheric radiation, but provided his unpublished calculations of actinic fluxes at different altitudes in a form useful to the atmospheric chemistry community for estimates of photolysis rates from the troposphere through the stratosphere. A number of colleagues reviewed chapters or portions of chapters, and their insightful comments and suggestions are... 

From measurements of the concentration C, of the compound i as a function of exposure time, the first-order photolysis rate constant, kp(/1), is then determined by calculating the slope of a plot of In C, /C,0 versus time (see Section 12.3). Since the... 

Enumerate all factors that determine the direct photolysis rate of a given compound in surface waters. Explain how you can determine and estimate the various parameters required to calculate the direct photolysis rate constant for a given situation. 

The chemistry code [4, 9] is applied between 6 and 55 km altitude. It includes 55 components and considers 104 thermal reactions, 47 photolysis reactions and 7 heterogeneous reactions. Photolysis rates are calculated interactively by the model of Kylling etal. [5]. 

To assess the efficiency of the reactions listed in Table 7.2, Bolton et al. (1996) proposed a generally applicable standard for a given photochemical process. The proposed standard provides a direct link to the electrical efficiency. In this model, electrical energy per unit mass is calculated according to the quantum yield of the direct photolysis rate. Braun et al. (1997) calculated the quantum yield (O) according to Equation (7.13) ... 

Thus, in the calculation of the photolysis rate constant, the increase in the baseline absorption has been taken into account and the kinetic rate constant has been determined using the absorbance read at 217-220 nm. 

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