TESTS OF SOME REDUCTION HYPOTHESES MADE IN PHOTOCHEMICAL MECHANISMS

The purpose of this paper is to evaluate the errors induced by differe nt hypotheses used to elaborate reduced kinetic mechanisms of troposph eric chemistry. To do that, a reference chemical kinetic scheme was de veloped: it includes a limited number of representative primary specie s (6 alkanes, 4 alkenes, 2 aromatics) for which up to date kinetic con stants and mechanisms were used. This chemical scheme constitutes a re ference against which the various reduction methods were tested. The t ests were performed for three different scenarios characterised by var ious VOC/NOx ratios (5,10,20). These scenarios are assumed to correspo nd to the chemical state of the atmosphere in urban areas and in rural situations. In a first step, the high NOx approximation (i.e. a chemi cal scheme without treatment of peroxy + HO2 and peroxy + peroxy react ions) was tested. The results show that this scheme does not produce s ignificant error on the simulated concentrations for NO concentrations above 2 ppb. In a second step, three successive reduction methods wer e applied to the reference mechanism: (1) use of the chemical operator concept to treat the organic peroxy chemistry, (2) loss of informatio n on the organic peroxy class, (3) lumping of secondary organic specie s into surrogate species. The use of chemical operators provides a sat isfactory representation of the organic peroxy chemistry for NO concen tration down to 100 ppt. The scheme obtained after the loss of informa tion on the organic peroxy class increases only slightly the errors co mpared to the reference scheme. The lumping of secondary VOCs into sur rogate species does not generate significant errors on most of the key species (O-3, NOx, OH, etc.) but induces a significant overestimation of PAN. At this step, the final reduced scheme involves 71 species an d 150 reactions. These reduction methods seem to be relevant for most of the diurnal situations encountered in the lower continental troposp here. Copyright (C) 1996 Elsevier Science Ltd