Tests of some reduction hypotheses made in photochemical mechanisms

The purpose of this paper is to evaluate the errors induced by different hypotheses used to elaborate reduced kinetic mechanisms of tropospheric chemistry. To do that, a reference chemical kinetic scheme was developed: it includes a limited number of representative primary species (6 alkanes, 4 alkenes, 2 aromatics) for which up to date kinetic constants and mechanisms were used. This chemical scheme constitutes a reference against which the various reduction methods were tested. The tests were performed for three different scenarios characterised by various VOC/NOx ratios (5,10, 20). These scenarios are assumed to correspond 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 chemical scheme without treatment of peroxy + HO2 and peroxy + peroxy reactions) was tested. The results show that this scheme does not produce significant error on the simulated concentrations for NO concentrations above 2 ppb. In a second step, three successive reduction methods were applied to the reference mechanism: (1) use of the chemical operator concept to treat the organic peroxy chemistry, (2) loss of information on the organic peroxy class, (3) lumping of secondary organic species into surrogate species. The use of chemical operators provides a satisfactory representation of the organic peroxy chemistry for NO concentration down to 100 ppt. The scheme obtained after the loss of information on the organic peroxy class increases only slightly the errors compared to the reference scheme. The lumping of secondary VOCs into surrogate species does not generate significant errors on most of the key species (O3, NOx, OH, etc.) but induces a significant overestimation of PAN. At this step, the final reduced scheme involves 71 species and 150 reactions. These reduction methods seem to be relevant for most of the diurnal situations encountered in the lower continental troposphere.