中文摘要：

广东大气超级监测站新粒子生成事件中，在新粒子快速增长后观测到明显的颗粒物缩小过程（即新粒子增长一缩小过程）。结合3～1000nm颗粒物数谱分布、颗粒物化学组成和重要气态污染物的变化，具体分析这类新粒子生成事件出现颗粒物缩小过程的特性和成因。结果表明，秋季新粒子生成事件发生频率和新粒子增长一缩小过程出现频率均较高。新粒子增长速率为3．0～12．0nm／h，颗粒物缩小速率为2．2～10．9nm／h。新粒子增长过程中，颗粒有机物对PM2，5浓度贡献最大；颗粒物缩小过程中，SO2^4-对PM2．5浓度贡献最大，且SO2^4-、NH才和元素碳对PM2．5浓度的贡献有所提高；NO-3和颗粒有机物对PM2．5浓度的贡献下降，且颗粒有机物中二次有机颗粒物对PMzs浓度贡献的下降幅度明显大于颗粒有机物对PM2.5浓度贡献的下降幅度。经分析，固态NH4NO3分解和低挥发性有机物挥发是颗粒物缩小的重要直接原因，扩散条件改善和大气氧化性减弱可能是推动新粒子增长转为颗粒物缩小过程的重要条件。

英文摘要：

During some new particle formation （NPF） events at the atmospheric supersite in the Pearl River Delta （PRD） Region,particle shrinkage was observed following the fast new particle growth. Based on the variations of 3-1 000 nm particle number size distributions, chemical compositions of PM2.5, and important gaseous pollutants, the properties and causes of particle shrinkage were analyzed in this research. The results showed that.. 33.3% of the NPF events occurred with particle growth-shrinkage process, and such fraction was the highest in autumn. The growth rates of the newly formed particles were 3.0-12.0 nm/h,and the determined shrinkage rates spanned the range from 2.2 to 10.9 nm/h. Particulate organic matter took up the largest fraction of PM2.5 mass concentrations during the new particle growth process; and SO2^4- was the biggest contributor during the particle shrinkage process. The separate contributions of SO2^4- ,NH4＋ and elemental carbon to PMz.s increased,and those of NO-3 and particulate organic mat- ters decreased during particle shrinkage. And the decline of the secondary organic matters was significantly larger than that of particulate matters. It was deduced that dissociation of particulate NH4 NO3 and evaporation of low volatile or- ganic compounds were important direct causes for the particle shrinkage process. Improved dispersion conditions and diminished atmospheric oxidizability were favorable conditions to convert the new particle growth to particle shrinkage in the PRD region.