The Volatility and Effective Density of Ambient Ultrafine Particles in the Outskirt of an Urban City in Taiwan

Ambient ultrafine particles (UFPs; < 100 nm) play a profound role in human health and global climate. These particles need to grow fast and large enough to become optically and climatically relevant. Such growth process is dependent on both the particle size and composition, many of which are volatile materials. With that in mind, the aim of this study was to measure the particle size-dependent volatility and subsequently the volatility-dependent hygroscopicity of ambient UFPs nearby an urban city in Taiwan. We adopted the volatility-hygroscopicity tandem differential mobility analyzer (VH-TDMA) system, which occasionally coupled with an Aerosol Particle Mass (APM) analyzer, in a one-year field campaign. The volatility was determined under three temperatures (25°C, 125°C, 300°C), while the hygroscopicity under two humidity (15%, 90%). Preliminary results show that the measured UFPs were mostly externally mixed, with the extent increased with increasing particle size and heating temperature. The mixing state of UFPs was similar between spring/summer and between fall/winter, possibly related to the similarity of prevailing winds for each season. The volatile materials show notable particle size and seasonal dependency. In general, the 35-nm particles contained the highest fraction of non-volatile materials, the 55-nm particles contained the highest fraction of high-volatile materials, and the 100-nm particles contained the highest fraction of low-volatile materials. In addition, short-intensive campaigns show that the particle effectively density increased with increasing particle size and the progress of time. Overall, these results indicate that the volatile materials in ambient UFPs are both size- and time-dependent, as well as highly variable over time and season. It is also indicative of the dynamical nature of ambient UFPs with regard to sources and atmospheric processing. Our following effort will begin to examine how these volatile materials affect the particles’ hygroscopic growth. The outputs are expected to provide highly time-resolved physiochemical properties of UFPs that are crucial to an improved knowledge of their impacts on the formation and activation of cloud condensation nuclei.