Role of black carbon in the formation of primary organic aerosols: Insights from molecular dynamics simulations

1 Many studies on the mixing state of suspended particulate matters (PM) have pointed to the role of carbon particles as 2 nucleation seeds in the formation of atmospheric aerosols. However, the underlying physicochemical mechanisms remain 3 unclear, particularly concerning the involvement of volatile organic compounds (VOCs) at the primary stage of clustering. Here 4 we gain insights into those microscopic formation mechanisms through molecular dynamics simulations of the physisorption 5 of gaseous organic molecules on the surface of a carbon nanoparticle (NP). Six different organic species are selected among the 6 VOCs dominating the atmospheric pollutants of several megacities, to interact with an onion-shell nanostructure that mimics 7 the primary soot particle. We consider organic molecules at various densities on the surface of a NP, as well as the same 8 molecules in the gas phase without any NP. 9 The pollutant molecules are found to cluster in clearly different ways in the presence of the NP than in the gas phase. 10 The contrast in the binding energy of molecular clusters confirms the catalytic role of black carbon in the primary formation 11 of aerosols from VOCs. Morphology analysis reveals different clustering behaviors of aromatic and aliphatic compounds, 12 leading to differences in the thermal stability of the formed PMs. Our simulations also suggest a layer-by-layer formation 13 process of aerosol PM, consistent with the onion-like nanostructures of aerosol particles previously observed in transmission 14 electron microscopy experiments. These results shed light on the microscopic mechanisms of primary aerosol formation, and 15 are correlated with a variety of experimental measurements on aerosol PMs and VOCs. 16 Copyright statement. This work is distributed under the Creative Commons Attribution 3.0 License. 17

Here we use MD simulations to study the physisorption of gaseous organic compounds on BC nanoparticles, which is 40 correlated to the primary formation process of aerosol PM from VOCs. The binding energy and morphology of the molecular 41 clusters obtained from molecular simulations are analyzed as a way to gain insight into the role of BC in the primary growth 42 of organic aerosols. The molecular clusters formed on the NP are found to be energetically mores table than those formed in 43 the gas phase, which points to a catalytic role of black carbon in the primary formation of aerosols from VOCs. Furthermore,   Fig. 1 (a).  Fig. 1 (b). Initial velocities are sampled from a Maxwell-Boltzmann distribution. A thermostat is then used to let the NP 69 progressively reach thermal equilibrium at 300 K in about 0.5 ns, during which most of the molecules interact with the NP 70 surface; this interval was determined to be enough after testing with the case of toluene as a reference. The case without NP is 71 also simulated under the same conditions for a purpose of comparison, as illustrated in Fig. 1 (c). In this case, the thermostat is 72 applied directly to the molecules instead.

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A repeated heating-annealing process is used to compute the statistical energy of atomization of the system. Energy mini-74 mization is performed via an annealing process after reaching thermal equilibrium, in order to take a "frozen" picture of the 75 system, from which the energy is calculated. After optimization, the temperature is raised again and molecules are free to 76 move at 300 K. This heating-annealing process is repeated for 13 times (determined after a convergence test) in each case to 77 let the system hop among metastable states and compute an average. The full set of Lammps inputs required to replicate these 78 simulations is provided with the online version.

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A key coefficient influencing the clustering of molecules, the per-molecule binding energy ε is calculated as the difference 80 in the energy of atomization between the whole system and the sum of that of the NP and clustering adsorbates that are isolated 81 from each other, divided by N , the total number of organic molecules in the simulation cell:

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where ε np is zero for the case without NP.  The physisorption of organic compounds could be an important primary stage in the formation of aerosols.  In Fig. 2 (a), the four aromatic compounds (para-xylene, ethylbenzene, styrene and toluene) have clearly higher absolute 98 values of ε than the two aliphatic compounds. This is not only due to different numbers of atoms in the molecule, but also 99 due to the difference in the NP surface coverage. Examining the morphology of the formed clusters, we find that most of the 100 aromatic molecules aggregate on the surface of NP more readily than the aliphatic ones, as shown in Fig. 3 as examples. For To quantify how the case shown in Fig. 3 is a general situation in the simulation results, we compute a molecular aggregation 111 factor for all simulations, which is defined as

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When there is no NP in the simulation box, the |ε| for aromatic compounds is also generally higher than for aliphatic ones as 135 shown in Fig. 2 (b). This difference is consistent with their different clustering behaviors, as evidenced in Fig. 4 (b). Propylene only a few small aggregates, whereas the clusters of ethylbenzene molecules are much larger at the same number density.

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An exception, styrene, has much lower values of |ε| [ Fig. 2 (b)] and α [ Fig. 4 (b)] in gas phase than other aromatic compounds 139 of similar molecular structures. Indeed, it is found to be much harder for styrene molecules to form aggregates in the gas phase 140 than for the three other aromatic species, as shown in Fig. 5 (c). Although the underlying reason for this behavior remains 141 unclear, we assume that this may come from the sp 2 -type hybridization of the benzene and the vinyl groups of styrene, so  single particle characterization using the soot particle aerosol mass spectrometer (SP-AMS), Atmos. Chem. Phys., 15, 1823, 2015.
coating formation and evaporation: chamber studies using black carbon seed aerosol and the single-particle soot photometer, Aerosol Sci.