# Simulation of Aerosol Evolution within Background Pollution for Nucleated Vehicle Exhaust via TEMOM
## 5 Conclusions  
The coupling of the NS equation and the PBE is performed to study the effect of background aerosol on its evolution. The flow field simulation is implemented with SIMPLEC; PBE is solved by TEMOM. Coagulation, condensation, and nucleation are considered. Three years of data were used in the simulation to ensure the generality of the conclusions. According to the observed phenomenon, the number and concentration of particulate matter in the pollution condition are smaller. The reason and mechanism behind it are studied. The following conclusions can be drawn:  
1. Compared with nonpolluted conditions (low concentration of background particles), the nucleation rate in polluted conditions will be reduced. In addition, the number concentration of particles will also be reduced. This phenomenon makes number concentration unable to reflect the degree of pollution. The decrease in nucleation rate makes the concentration of critical sulfuric acid molecular clusters higher in the flow field (this is limited to a relatively small area). This is a disadvantage in certain operations that remove contaminants;
2. The evolution of aerosols mainly depends on the nucleation rate. In addition, the nucleation rate has a strong regional distribution (that is, the spatial distribution of the nucleation rate is limited to a small range). Therefore, the source of controlling the pollutant is the key. Compared with the core area of nucleation, other regions have little impact on the generation and evolution of particulate matter;
3. The presence of background particulate matter will slow down the nucleation process of aerosol. The mass concentration of particulate matter in the polluted condition is higher than that in the nonpolluted condition. Therefore, the slowing effect of particulate matter on the nucleation process of aerosol will be more obvious in pollution conditions;
4. In the process of aerosol evolution, the independence of mass concentration and number concentration has guiding significance in various occasions involving the control of aerosol concentration. In this paper, the evolution mechanism behind this phenomenon is obtained by the simulation method. This is conducive to the use of this phenomenon. For example, it is necessary to confirm which factors effectively control the pollution situation; filtering to reduce the mass concentration of pollutants in order to reduce the number concentration is invalid;
5. The significant change regions of nucleation, condensation, and coagulation are different. The nucleation process is spatially closer to the source of pollution. The nucleation process also provides more particles for the condensation and coagulation processes. Condensation comes next, and coagulation comes last.  
Author Contributions: [PERSON] devised the project. M.Y. the main conceptual ideas and proof outline. [PERSON] worked out the technical details, and performed the numerical calculations for the suggested simulation. [PERSON] verified the numerical results. Conceptualization, [PERSON]; methodology, [PERSON]; software, [PERSON]; writing original draft, [PERSON]; writing review and editing, [PERSON]; visualization, T.W. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.