Growth of atmospheric freshly nucleated particles: a semi-empirical molecular dynamics study
<p>When simulating new particle formation rates, collisions in the system are approximated as hard spheres without long-range interactions. This simplification may lead to an underestimation of the actual formation rate. In this study, we employ semi-empirical molecular dynamics (SEMD) at the...
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| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-05-01
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| Series: | Aerosol Research |
| Online Access: | https://ar.copernicus.org/articles/3/237/2025/ar-3-237-2025.pdf |
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| Summary: | <p>When simulating new particle formation rates, collisions in the system are approximated as hard spheres without long-range interactions. This simplification may lead to an underestimation of the actual formation rate. In this study, we employ semi-empirical molecular dynamics (SEMD) at the GFN1-xTB level of theory to probe the sticking process of the monomers sulfuric acid (SA), methanesulfonic acid (MSA), nitric acid (NA), formic acid (FA), ammonia (AM), methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA) onto freshly nucleated particles (FNPs). The FNPs considered are (SA)<span class="inline-formula"><sub>10</sub></span>(AM)<span class="inline-formula"><sub>10</sub></span>, (SA)<span class="inline-formula"><sub>10</sub></span>(MA)<span class="inline-formula"><sub>10</sub></span>, (SA)<span class="inline-formula"><sub>10</sub></span>(DMA)<span class="inline-formula"><sub>10</sub></span>, and (SA)<span class="inline-formula"><sub>10</sub></span>(TMA)<span class="inline-formula"><sub>10</sub></span>.</p>
<p>In general, we find that the hard-sphere kinetic approximation, which neglects long-range interactions, significantly underestimates the number of collisions leading to sticking. By calculating the sticking coefficient from SEMD simulations, we obtain enhancement factors of 2.3 and 1.5 for the SA <span class="inline-formula">+</span> (SA)<span class="inline-formula"><sub>10</sub></span>(AM)<span class="inline-formula"><sub>10</sub></span> and AM <span class="inline-formula">+</span> (SA)<span class="inline-formula"><sub>10</sub></span>(AM)<span class="inline-formula"><sub>10</sub></span> collisions, respectively. A comparison with OPLS (optimized potentials for liquid simulations) all-atom force field simulations shows similar enhancement factors of 2.4 and 1.6 for the SA <span class="inline-formula">+</span> (SA)<span class="inline-formula"><sub>10</sub></span>(AM)<span class="inline-formula"><sub>10</sub></span> and AM <span class="inline-formula">+</span> (SA)<span class="inline-formula"><sub>10</sub></span>(AM)<span class="inline-formula"><sub>10</sub></span> collisions, respectively.</p>
<p>Compared to the force field simulations, SEMD exhibits a more isotropic sticking behavior, with the probability remaining near unity for small offsets before rapidly dropping to 0 % beyond a certain offset. In contrast, the force field simulations show a more gradual decline in sticking probability due to certain orientations still leading to sticking. The largest discrepancy between the two methods occurs at lower collision velocities – below 200 m s<span class="inline-formula"><sup>−1</sup></span> for SA and below 400 m s<span class="inline-formula"><sup>−1</sup></span> for AM – where force field simulations, even for head-on collisions, predict low or zero sticking probability. This has previously been attributed to periodic repulsions between the rotating collision partners caused by fluctuations in their charge distributions. In contrast, SEMD simulations do not exhibit this behavior. Since these low velocities are not significantly populated in our simulations, both methods yield similar enhancement factors. However, for systems with larger effective masses, where such velocities are more prevalent, we would expect the two methods to diverge.</p> |
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| ISSN: | 2940-3391 |