Study of different Carbon Bond 6 (CB6) mechanisms by using a concentration sensitivity analysis

• Main Authors: L. Cao, S. Li, L. Sun
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-08-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/12687/2021/acp-21-12687-2021.pdf

<p>Since the year 2010, different versions of the Carbon Bond 6 (CB6) mechanism have been developed to accurately estimate the contribution to air pollution by the chemistry. In order to better understand the differences in simulation results brought about by the modifications between different versions of the CB6 mechanism, in the present study, we investigated the behavior of three different CB6 mechanisms (CB6r1, CB6r2 and CB6r3) in simulating ozone (<span class="inline-formula">O₃</span>), nitrogen oxides (<span class="inline-formula">NO_<i>x</i></span>) and formaldehyde (<span class="inline-formula">HCHO</span>) under two different emission conditions by applying a concentration sensitivity analysis in a box model. The results show that when the surface emission is weak, the <span class="inline-formula">O₃</span> level predicted by CB6r1 is approximately 7 <span class="inline-formula">ppb</span> higher than that predicted by CB6r2 and CB6r3, specifically due to the change in the sink of acyl peroxy radicals with high-order carbons (i.e., species CXO3) in the mechanism and the difference in the ozone dependence on the isoprene emission. In contrast, although CB6r1 estimates higher values of <span class="inline-formula">NO_<i>x</i></span> and <span class="inline-formula">HCHO</span> than the other two mechanisms at an early stage of the simulation, the levels of <span class="inline-formula">NO_<i>x</i></span> and <span class="inline-formula">HCHO</span> estimated by these three CB6 mechanisms at the end of the 7 <span class="inline-formula">d</span> simulation are mostly similar, when the surface emission is weak. After the increase in the surface emission, the simulated profiles of <span class="inline-formula">O₃</span>, <span class="inline-formula">NO_<i>x</i></span> and <span class="inline-formula">HCHO</span> obtained by CB6r2 and CB6r3 were found to be nearly the same during the simulation period, but CB6r1 tends to estimate substantially higher values than CB6r2 and CB6r3. The deviation between the <span class="inline-formula">O₃</span> levels provided by CB6r1 and the other two CB6 mechanisms (i.e., CB6r2 and CB6r3) was found to be enlarged compared with the weak-emission scenario because of the weaker dependence of ozone on the emission of isoprene in CB6r1 than those in CB6r2 and CB6r3 in this scenario. Moreover, <span class="inline-formula">HCHO</span> predicted by CB6r1 was found to be larger than those predicted by CB6r2 and CB6r3, which is caused by an enhanced dependence of <span class="inline-formula">HCHO</span> on the emission of isoprene in CB6r1. Regarding <span class="inline-formula">NO_<i>x</i></span>, it was found that CB6r1 gives a higher value than the other two mechanisms, which is caused by the relatively stronger connection between the <span class="inline-formula">NO_<i>x</i></span> prediction and the release of <span class="inline-formula">NO</span> and <span class="inline-formula">NO₂</span> in CB6r1 due to the change in the product of the reaction between isoprene and <span class="inline-formula">NO₃</span> in CB6r1. Consequently, more emitted <span class="inline-formula">NO_<i>x</i></span> is involved in the reaction system denoted by CB6r1, which enables a following <span class="inline-formula">NO_<i>x</i></span> formation and thus a higher <span class="inline-formula">NO_<i>x</i></span> prediction of CB6r1.</p>