Predicting tropospheric degradation of chemicals: from estimation to computation.

For the majority of commercial chemicals present in the troposphere, the reaction with OH radicals during the day and with NO3 radicals at night are the most important abiotic pathways for their degradation and removal from the troposphere. Today, there are only a couple of methods available for estimating the reactivity of commercial chemicals with tropospheric radicals, which are not class specific, he Atkinson fragment contribution method and QSAR models, based on a linear correlation of OH (NO3) radical reactivity with the corresponding ionization energies, allow a rapid estimation of the rate constants of OH or NO3 radicals for various classes of organic compounds. Both methods are described and their limitations are discussed. A lot of work has been done to develop QSAR models for tropospheric degradation of commercial chemicals that will be based on calculated quantum chemical descriptors. This fast expanding area of QSAR research is presented and evaluated. Particular emphasis is given to the precision of various methods as well as to the latest results from our laboratory. The recent dramatic development in computing technology enables to precisely calculate energy profiles of tropospheric reactions with OH radicals. The semiempirical and ab initio molecular orbital calculations have been performed for hydrogen abstraction reactions for several classes of tropospheric pollutants. The best results of the high-level ab initio molecular orbital calculations are presented and discussed.

Ozone formation in the greater Cairo area.

Ozone formation in the greater Cairo area was studied in 1990 in a 3-week measurement period performed at three sites (Shoubra El-Kheima, Mokattam Hill, Helwan), covering a north-south direction of 27 km, and in 1991, from the beginning of April until the end of October, by measurement of the seasonal variation of ozone at one site at El-Kobba. The sinusoidal shape in the diurnal volume fraction plots with peak values of 120 ppb and daily mean value of 50 ppb throughout the year indicate a substantial contribution of photochemistry to the ozone content of the atmosphere. Ozone is produced predominantly over the industrial area in the north and in the centre of Cairo and transported southward by the prevailing northerly winds. Contrary to many urban areas in Europe and in North America, fairly high average ozone levels of 40 ppb are observed during the night throughout the spring and the summer. This may imply that health hazards and crop damage are higher in the greater Cairo area than in Central Europe.