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.