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NASA NTRS1994

Fast two-stream method for computing diurnal-mean actinic flux in vertically inhomogeneous atmospheres

Based on a derivation of the two-stream daytime-mean equations of radiative flux transfer, a method for computing the daytime-mean actinic fluxes in the absorbing and scattering vertically inhomogeneous atmosphere is suggested. The method applies direct daytime integration of the particular solutions of the two-stream approximations or the source functions. It is valid for any duration of period of averaging. The merit of the method is that the multiple scattering computation is carried out only once for the whole averaging period. It can be implemented with a number of widely used two-stream approximations. The method agrees with the results obtained with 200-point multiple scattering calculations. The method was also tested in runs with a 1-km cloud layer with optical depth of 10, as well as with aerosol background. Comparison of the results obtained for a cloud subdivided into 20 layers with those obtained for a one-layer cloud with the same optical parameters showed that direct integration of particular solutions possesses an 'analytical' accuracy. In the case of the source function interpolation, the actinic fluxes calculated above the one-layer and 20-layer clouds agreed within 1%-1.5%, while below the cloud they may differ up to 5% (in the worst case). The ways of enhancing the accuracy (in a 'two-stream sense') and computational efficiency of the method are discussed.

Filyushkin, V. V.↗

NASA NTRS1993

A photostationary state analysis of the NO2-NO system based on airborne observations from the subtropical/tropical North and South Atlantic

The Chemical Instrumentation Test and Evaluation 3 (CITE 3) NO-NO2 database has provided a unique opportunity to examine important aspects of tropospheric photochemistry as related to the rapid cycling between NO and NO2. Our results suggest that when quantitative testing of this photochemical system is based on airborne field data, extra precautions may need to be taken in the analysis. This was particularly true in the CITE 3 data analysis where different regional environments produced quite different results when evaluating the photochemical test ratio (NO2)(sub expt)/(NO2)(sub calc), designated here as R(sub E)/R(sub C). The quantity (NO2)(sub Calc) was evaluated using the following photostationary state expression: (NO2)(sub Calc) = k(sub 1)(O3) + k(sub 4)(HO2) + k(sub 5)(CH3O2) + k(sub 6)(RO2))(NO)(sub Expt)/J(sub 2). The four most prominent regional environmental data sets identified in this analysis were those labeled here as free-tropospheric northern hemisphere (FTNH), free-tropospheric tropical northern hemisphere (FTTNH), free-tropospheric southern hemisphere (FTSH), and tropical-marine boundary layer (plume) (TMBL(P)). The respective R(sub E)/R(sub C) mean and median values for these four data subsets were 1.74, 1.69; 3.00, 2.79; 1.01, 0.97; and 0.99, 0.94. Of the four data subsets listed, the two that were statistically the most robust were FTNH and FTSH; for these the respective R(sub E)/R(sub C) mean and standard deviation of the mean values were 1.74 +/- 0.07 and 1.01 +/- 0.04. The FTSH observations were in good agreement with theory, whereas those from the FTNH data set were in significant disagreement. An examination of the critical photochemical parameters O3, UV(zenith), NO, NO2, and non-methane hydrocarbons (NMHCs) for these two databases indicated that the most likely source of the R(sub E)/R(sub C) bias in the FTNH results was the presence of a systematic error in the observational data rather than a shortening in our understanding of fundamental photochemical processes. Although neither a chemical nor meteorological analyses of these data identified this error with complete certainty, they did point to the three most likely possibilities: (1) an NO2 interference from a yet unidentified NO(y) species: (2) the presence of unmeasured hydrocarbons, the integrated reactivity of which would be equivalent to approximately 2.7 parts per billion by volume (ppbv) of toluene; or (3) some combination of points (1) and (2). Details concerning hypotheses (1) and (2) as well as possible ways to minimize these problems in future airborne missions are discussed.

Davis, D. D.↗

NASA NTRS1988

Visible-ultraviolet absorption cross sections for NO2 as a function of temperature

A redetermination of the temperature dependence of the absorption cross-section (sigma) of NO2 in the visible-ultraviolet region was made in order to provide a more reliable data base for the calculation of NO2 photolysis rates in the atmosphere. Experiments over a wide range of temperatures and NO2 concentrations were conducted. The integral of a plot of sigma versus the inverse of the wavelength was essentially independent of temperature. Increasing temperature produced a shift of the spectrum toward longer wavelengths, resulting in a small negative temperature dependence of sigma over the 264-400 nm range and a small positive dependence over the 450-649 nm range. Increasing temperature produced broadening of individual spectral features, resulting in a systematic lowering of peaks and filling of valleys. Recommended cross sections are presented for use in tropospheric NO2 photolysis rate calculations.

Davidson, J. A.↗