dc.description.abstract | In addition to scattering and absorbing solar radiation, atmospheric aerosols change the formation of clouds and indirectly affect the climate. The microphysical and chemical composition of cloud condensation nuclei is one of the main reasons of climate change and precipitation. It is useful to know the characteristics of cloud condensation nuclei (CCN) under different spatial, temporal distributions and environmental conditions. The study is categorized into two parts. The first part is a three-year observation experiment in Fuguei Cape from 2014 to 2016. This measures hygroscopicity of the particles in the spring and winter seasons under the long-range transport (LRT) from Northeast Asia. The second part is conducted in Taichung from 2017 to 2018. This section explores the physicochemical and hygroscopic properties of local polluted particles in Taiwan during the fall and winter seasons.
In this study, CCN activities are quantified by activation rate (AR = CCN / CN). The experimental data showed that the wind direction of Fuguei Cape is almost northeast wind and southwest wind. Northeast wind particles are composed by LRT and non-LRT. Southwest wind particles are local pollution from Fuguei Cape. In spring, AR of the northeast wind particles under LRT is not significantly higher than that of the non-LRT period. The correlation with the geometric mean diameter (GMD) and AR is low, and the black carbon ratio is relatively low. In non-LRT period, black carbon ratio (BC/PM2.5) is more concentrative than winter, and the AR is slightly positively correlated with the GMD. In winter, AR of northeast wind particles under LRT with obvious pollution event is higher than non-LRT event. And black carbon ratio is negatively correlated with the particle size. The GMD probability distribution is wider than that spring, and it is prone to appear large particles, which may imply the source of pollution in spring may be simple and weaker aging process, while the path in winter is more complex and a higher degree of aging. In non-LRT period, the value of black carbon ratio is more dispersed than spring and AR is positively correlated with the GMD regardless of LRT or non-LRT. On the other hand, the properties of particles in southwest wind are similar in spring and winter seasons. Pollution sources should be the same which mainly originate from the ship emissions in nearby harbor.
The average AR in Taichung area was 0.13, and fall and winter seasons are divided by the time of first cold front arriving Taiwan in October. Experimental data shows that the AR in fall is higher than winter. Furthermore, the analysis of the two seasons and the wind direction shows that the AR and particle size are the highest in the southerly wind and the lowest in the northeast wind. However, black carbon ratio is highest in the northwest wind, and this is mainly affected by traffic emissions. The experimental hygroscopicity, κCCN, proposed by Petters and Kreidenweis (2007) under the scheme of Kӧhler theory was further derived using AR and collocated particle size distribution (PSD) measurements. Analyzing AR, particle size and κ showed that particles in central Taiwan have two different characteristics. One group showed a high positive correlation between GMD and AR, where the hygroscopic parameters are high. The other group showed a relatively weak correlation, where the hygroscopic parameters are low. In fall, higher hygroscopicity particles contain more volume fraction of sulfate, which is presumably related to temperature or PH value of particles; in the winter, higher hygroscopicity particles contain lower portion of black carbon, which is more likely to be affected by LRT. This phenomenon is also consistent with Fuguei Cape. | en_US |