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首页> 外文期刊>Atmospheric chemistry and physics >The size-dependent charge fraction of sub-3-nm particles as a key diagnostic of competitive nucleation mechanisms under atmospheric conditions
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The size-dependent charge fraction of sub-3-nm particles as a key diagnostic of competitive nucleation mechanisms under atmospheric conditions

机译：亚3 nm颗粒的尺寸依赖性电荷分数是大气条件下竞争性成核机制的关键诊断

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A clear physical understanding of atmospheric particlenucleation mechanisms is critical in assessing the influences of aerosols onclimate and climate variability. Currently, several mechanisms have beenproposed and are being employed to interpret field observations ofnucleation events. Roughly speaking, the two most likely candidates areneutral cluster nucleation (NCN) and ion-mediated nucleation (IMN). Detailednucleation event data has been obtained in boreal forests. In one set ofanalyses of these measurements, NCN was suggested as the dominant formationmode, while in another, it was IMN. Information on the electrical chargedistribution carried by the nucleating clusters is one key for identifyingthe relative contributions of neutral and ion-mediated processes undervarious conditions. Fortunately, ground-breaking measurements of the chargedstates or fractions of ambient nanometer-sized particles soon afterundergoing nucleation are now available to help resolve the main pathways.In the present study, the size-dependent "apparent" formation rates andfractions of charged and neutral particles in a boreal forest setting aresimulated with a detailed kinetic model. We show that the predicted"apparent" formation rates of charged and neutral particles at 2 nm foreight representative case study days agree well with the correspondingvalues based on observations. In the simulations, the "apparent"contribution of ion-based nucleation increases by up to ~one order ofmagnitude as the size of "sampled" particles is decreased from 2 nm to~1.5 nm. These results suggest that most of the neutral particlessampled in the field at sizes around 2 nm are in reality initially formed onionic cores that are neutralized before the particles grow to this size.Thus, although the apparent rate of formation of neutral 2-nm particlesmight seem to be dominated by a neutral clustering process, in fact thoseparticles may be largely the result of an ion-induced nucleation mechanism.This point is clarified when the formation rates of smaller particles (e.g.,~1.5 nm) are explicitly analyzed (noting that measurements at thesesmaller sizes are not yet available), indicating that IMN dominates NCNprocesses under typical circumstances in the boreal forest casesinvestigated.
机译：在评估气溶胶对气候和气候变异性的影响时，对大气颗粒成核机制的清晰物理理解至关重要。目前，已经提出了几种机制，并且已经将其用于解释成核事件的现场观察。粗略地说，两个最可能的候选者是中性簇成核（NCN）和离子介导的成核（IMN）。已在北方森林中获得了详细的成核事件数据。在这些测量的一组分析中，建议使用NCN作为主要的形成模式，而在另一组中，建议使用IMN。由成核簇携带的电荷分布信息是确定在各种条件下中性和离子介导过程的相对贡献的关键之一。幸运的是，经过成核后不久，即可获得对带电态或周围纳米级粒子分数的突破性测量，以帮助解决主要途径。在本研究中，与尺寸有关的“表观”形成速率以及带电粒子和中性粒子的分数用详细的动力学模型模拟了在北方森林环境中的运动。我们表明，在代表性的案例研究天数为2 nm时，带电粒子和中性粒子的预测“表观”形成速率与基于观测值的相应值非常吻合。在模拟中，随着“采样”粒子的大小从2 nm减小到〜1.5 nm，基于离子的成核的“表观”贡献增加了大约一个数量级。这些结果表明，在现场以2 nm左右的尺寸采样的大多数中性粒子实际上都是最初形成的离子核，在粒子长至该尺寸之前已被中和。因此，尽管看起来中性2 nm粒子的表观形成速度似乎实际上，那些粒子可能主要是由离子诱导的成核机制所致。当明确分析较小粒子（例如，〜1.5 nm）的形成速率时，这一点便得到了澄清（注意到测量在较小的规模下尚无数据），这表明在调查的北方森林案例中，IMN在典型情况下主导了NCN过程。

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