Comparison of a 4000-reaction chemical mechanism with the carbon bond IV and an adjusted carbon bond IV-EX mechanism using SMVGEAR II

The well-known carbon bond IV (CBIV) chemical mechanism (33 species, 81 rea ctions) is compared with an adjusted carbon bond mechanism (ACBM) (109 spec ies, 233 reactions) and a more explicit master chemical mechanism (MCM) (14 27 species, 3911 reactions) in tests of their predictions of O-3, NOx( = NO + NO2), HCHO, HNO3, H2O2, and peroxyacetlynitrate (PAN). The ACBM was deve loped from a fourth mechanism, the expanded carbon bond mechanism (CBM-EX), by explicitly including the decomposition of C2H6, C3H8, and C3H6 All thre e mechanisms tested were updated with the inorganic chemistry from the ACBM and implemented into the sparse-matrix, ordinary differential equation sol ver, SMVGEAR II. Sparse-matrix treatment in SMVGEAR II reduced the number o f calculations during matrix decomposition for the MCM by a factor of 15,00 0 (99.995%), or from an estimated 154 h to 37 s of cpu time per simulation day in one grid cell on an SGI origin 2000, in comparison with a full-matri x solution. Computer time for each mechanism was linearly proportional to t he number of species in the mechanism. It is shown that the three mechanism s agreed closely when aromatic concentrations were initially low in compari son with alkane, alkene, and aldehyde initial concentrations. When aromatic concentrations were initially high (higher than that observed in urban air ), the yields of O-3, HCHO, and PAN differed significantly among the three mechanisms although the daily maximum concentrations of these species agree d better. The aromatic representation in MCM appears to lead to systematic overprediction of ozone, according to a comparison with smog chamber data. For initial conditions taken from measurements at nine sites in Los Angeles , the daily maximum concentrations of O-3, HCHO, PAN, and H2O2 predicted by the three mechanisms differed by 30-50%, 10-40%, 15-40%, and 60-80%, respe ctively. The relative differences between the daytime series of O-3, HCHO, H2O2, and PAN predicted by the three mechanisms were 7-68%, 7-46%, 35-150%, and 10-64%, respectively. The use of the aromatic scheme of ACBM in MCM si gnificantly reduced the disagreement with respect to ozone. The measurement of H2O2 in smog chamber experiments would be useful in validating chemical mechanisms. (C) 2000 Elsevier Science Ltd. All rights reserved.