High frequency of new particle formation events driven by summer monsoon in the central Tibetan Plateau, China.

New particle formation (NPF) is an important source of cloud condensation nuclei (CCN), which affects Earth's radiative balance and global climate. The mechanism and CCN contribution of NPF at the high-altitude mountains, especially in the Tibetan Plateau (TP), was unclear due to lack of measurements. In this study, intensive measurements were conducted at the Nam Co station (4730 m a.s.l.) in the central TP during both the pre-monsoon and summer monsoon seasons. The frequencies of NPF events exhibited evident seasonal differences with 15 % in the pre-monsoon season and 80 % in the monsoon season. The comprehensive analysis of the measured condensation sink (CS), gaseous precursors and meteorological conditions, supplemented by the model simulations of SO 2 and volatile organic compound (VOC), points to the organic involved nucleation as the dominant mechanism. Condensation sink and gaseous sulfuric acid could have no significant effect on the occurrence of NPF events. The frequent NPF events in the summer monsoon season may result from the higher frequency of southerly and southwesterly air masses, which brought the organic precursors to participate in the NPF process. It had increased the aerosol number concentrations and CCN at supersaturation of 1.2 % by more than 2 and 0.6 times compared with those in the pre-monsoon season, respectively. Considering that the smaller particles formed by NPF may further grow and reach CCN size during the following days due to the low-level coagulation sink, the amount of potential CCN in the monsoon season could be much larger than our local measurement results. Our results emphasized the importance of considering the seasonal effect of NPF when simulating the amounts of aerosols and CCN in the high-altitude atmosphere. Long-term investigations with a full set of instrumentation are required for deeper scientific understanding of NPF process and its role in the global budget in the TP. [ABSTRACT FROM AUTHOR]