National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506, Japan.
Sci Total Environ. 2012 Oct 15;437:339-47. doi: 10.1016/j.scitotenv.2012.07.085. Epub 2012 Sep 4.
We compared the effect of ambient temperature observed in two different seasons on the size distribution and particle number concentration (PNC) as a function of distance (up to ~250 m) from a major traffic road (25% of the vehicles are heavy-duty diesel vehicles). The modal particle diameter was found between 10 and 30 nm at the roadside in the winter. However, there was no peak for this size range in the summer, even at the roadside. Ambient temperature affects both the atmospheric dilution ratio (DR) and the evaporation rate of particles, thus it affects the decay rate of PNC. We corrected the DR effect in order to focus on the effect of particle evaporation on PNC decay. The decay rate of PNC with DR was found to depend on the season and particle diameter. During the winter, the decay rate for smaller particles (<30 nm) was much higher (i.e., the concentration decreased significantly against DR), whereas it was low during the summer. In contrast, for particles >30 nm in diameter, the decay rate was nearly the same during both seasons. This distinction between particles less than or greater than 30 nm in diameter reflects differences in particle volatility properties. Mass-transfer theory was used to estimate evaporation rates of C20-C36 n-alkane particles, which are the major n-alkanes in diesel exhaust particles. The C20-C28 n-alkanes of 30-nm particles completely evaporate at 31.2 °C (summer), and their lifetime is shorter than the transport time of air masses in our region of interest. Absence of the peak at 10-30 nm and the low decay rate of PNC <30 nm in diameter in the summer were likely due to the evaporation of compounds of similar volatilities comparable to the C20-C36 n-alkanes from particles near the exhaust pipes of vehicles, and complete evaporation of semivolatile materials before they reached the roadside. These results suggest that the lifetime of particles <30 nm in diameter depends on the ambient temperature, which differs between seasons. This leads us to conclude that these particles show distinctly different spatial distributions depending on the season.
我们比较了两个不同季节中观测到的环境温度对距离主干道（25%的车辆为重型柴油车）约 250 米处的粒径分布和粒子数浓度（PNC）的影响。在冬季，路边的模态粒径在 10 到 30nm 之间。然而，即使在路边，这个尺寸范围内也没有出现峰值。环境温度会影响大气稀释比（DR）和粒子的蒸发速率，从而影响 PNC 的衰减速率。我们校正了 DR 效应，以便重点研究粒子蒸发对 PNC 衰减的影响。结果发现，PNC 的衰减速率与 DR 随季节和粒径而变化。在冬季，较小粒径（<30nm）的粒子衰减速率要高得多（即浓度随 DR 显著降低），而在夏季则较低。相比之下，对于直径大于 30nm 的粒子，两个季节的衰减速率几乎相同。小于或大于 30nm 粒径的粒子之间的这种区别反映了粒子挥发性的差异。使用质量传递理论来估计 C20-C36 正构烷烃粒子的蒸发速率，这些粒子是柴油尾气粒子中的主要正构烷烃。直径为 30nm 的粒子中的 C20-C28 正构烷烃在 31.2°C（夏季）时完全蒸发，其寿命短于我们研究区域空气团的传输时间。夏季 10-30nm 处没有出现峰值，且 PNC<30nm 的粒径衰减率较低，这可能是由于来自车辆排气管附近的粒子中具有相似挥发性的化合物的蒸发，以及半挥发性物质在到达路边之前完全蒸发。这些结果表明，直径小于 30nm 的粒子的寿命取决于环境温度，而温度在季节之间有所不同。这使我们得出结论，这些粒子的空间分布取决于季节，表现出明显的不同。
