Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA.
Department of Mechanical Engineering, University of Minnesota, 111 Church Street SE, Minneapolis, Minnesota 55455, USA.
Faraday Discuss. 2013;165:25-43. doi: 10.1039/c3fd00039g.
The chemical composition of 20 nm diameter particles was measured with the Nano Aerosol Mass Spectrometer (NAMS) in a rural/coastal environment during days when new particle formation (NPF) occurred and days when NPF did not occur. NAMS provides a quantitative measure of nanoparticle elemental composition with high time resolution. These measurements show that nanoparticle chemical composition is dynamic on both types of days and that changes in nanoparticle chemical composition do not necessarily correlate with changes in aerosol mass or number concentration. On NPF days, NAMS can distinguish between elements associated with particle formation and early mass growth from those associated with later mass growth. In the early stage of NPF, the particle phase sulphur mole fraction (S) increases simultaneously with the increase in gas phase sulphuric acid. This composition change occurs before the mode diameter has grown into the NAMS-measured size range and is quantitatively described by sulphuric acid condensation. The nitrogen mole fraction (N) also increases during this time period. The N/S mole ratio is approximately 2, indicating that particulate sulphate is fully neutralized. As the mode diameter passes into and through the NAMS-measured size range, N increases at a faster rate than S (N/S mole ratio increases above 2), indicating that a separate, nitrogen-based growth process exists, possibly involving aminium salts, inorganic nitrate and/or organonitrates. Carbonaceous matter is the most abundant component (-50% by mass) of the growing nanoparticles, but it is the inorganic species that are preferentially enhanced during NPF relative to other times of day. Concurrent measurements of cloud condensation nucleation activity during NPF events suggest that these newly formed particles are hygroscopic. Nanoparticle composition on non-NPF days also shifts toward a more inorganic composition during the daytime, but the chemical species are different from NPF days and the particles are less hygroscopic. Incorporation of S into growing nanoparticles is adequately explained by existing models, but currently no models exist to satisfactorily explain incorporation of nitrogen-containing species or carbonaceous matter.
利用纳米气溶胶质谱仪（NAMS）在发生新粒子形成（NPF）和未发生 NPF 的农村/沿海环境中测量了 20nm 直径颗粒的化学成分。NAMS 以高时间分辨率提供纳米颗粒元素组成的定量测量。这些测量结果表明，纳米颗粒化学成分在这两种类型的日子里都是动态的，而且纳米颗粒化学成分的变化不一定与气溶胶质量或数浓度的变化相关。在 NPF 日，NAMS 可以区分与颗粒形成和早期质量增长相关的元素与与后期质量增长相关的元素。在 NPF 的早期阶段，颗粒相硫摩尔分数（S）与气相硫酸同时增加。这种成分变化发生在模态直径增长到 NAMS 测量的尺寸范围之前，并且可以通过硫酸冷凝定量描述。在此期间，氮摩尔分数（N）也增加。N/S 摩尔比约为 2，表明硫酸盐颗粒完全中和。随着模态直径进入并穿过 NAMS 测量的尺寸范围，N 的增加速度快于 S（N/S 摩尔比增加到 2 以上），表明存在单独的、基于氮的生长过程，可能涉及氨盐、无机硝酸盐和/或有机硝酸盐。碳质物质是增长中的纳米颗粒中最丰富的成分（质量的-50%），但在 NPF 期间相对于一天中的其他时间，优先增强的是无机物质。在 NPF 事件期间同时测量云凝结核活性表明，这些新形成的颗粒具有吸湿性。非 NPF 日的纳米颗粒组成在白天也向更无机的组成转变，但化学物质与 NPF 日不同，并且颗粒的吸湿性较小。现有模型可以充分解释 S 掺入生长中的纳米颗粒，但目前没有模型可以满意地解释含氮物质或碳质物质的掺入。
