The Characteristics Of Aerosol Number Size Distribution And New Particle Formation Under Different Sources Of Air Masses At Mt. Huang

To investigate the physical/chemical properties, and the formation and growth processes of aerosol particles on mountainous regions in Southeast China, the field campaigns to measure the concentration, size distribution, chemical composition of aerosol particles and related quantities were conducted from 2008 to 2012 at the Bright Summit and Yungu Hotel of Mt. Huang. Based on the backward trajectories obtained using the HYSPLIT model, the transport pathways of observed air masses from April to July 2008, May to June 2009, and September to October 2012 at the Bright Summit were classified into the following four groups: northwest air mass,East China air mass, southwest air mass and northeast air mass. And combined with the observation data during the study periods, this paper discussed the characteristics of the aerosol number size distribution and the new particle formation (NPF) events under various types of air masses, along with meteorological data. Then based on the simultaneously data observed from September to October 2012 at the Bright Summit and Yungu Hotel, the characteristics of NPF events and its vertical distribution at different altitudes were discussed, and the possible reasons for the differences of spatial distribution were also explained. The main conclusions are as follows:(1) At the Bright Summit, the average total particle number concentration was 3.30×103 cm-3 during the study periods, with a domination of Aitken mode particles.The general level of ultrafine particle number concentration here was similar to that measured at those foreign mountainous regions, indicating that the Mt. Huang was relatively less polluted. Influenced by the specific sources and transport pathways of air masses, the total particle number concentration was significantly different under various types of air masses. But the particle number concentrations of each mode were all highest in northwest air masses, followed by northeast air masses, East China air masses, and southwest air masses. In addition, the average diurnal variations of particle number concentration in each mode under four types of air masses were obvious, especially for nucleation mode. Excluding the accumulation mode particles, the average daytime particle number concentrations under four types of air masses were prominently higher than those measured at nighttime, suggesting there was a diurnal pattern of changes between planetary boundary layer (PBL) and free troposphere (FT) air at the Bright Summit.(2) The characteristics of particle number size distribution were significantly affected by the source of air mass. The variation of particle number size distribution in northwest air masses was very strong, presenting obvious characteristics of new particle formation and growth. However, the variations of particle number size distribution in East China air masses, southwest air masses and northeast air masses were all weak (the intensity decreased in turn). Influenced by the background conditions in different sampling periods, the particle number size distributions under the four types of air masses in different seasons were also different. The aerosol size spectrum under each type of air masses all had bimodal distributions in spring, but it was bimodal only in East China air masses in summer and northwest air masses in autumn. In addition, temperature, relative humidity, wind speed and precipitation had different effects on number concentration distribution under different type of air masses.(3) New particle formation events occurred at the Bright Summit was closely related to the solar radiation and the development of boundary layer. The averge formation rate J10 and J15 was 0.25 cm-3 s-1 and 0.49 cm-3 s-1, and the average growth rate was 3.66 nm h-1 during the study periods. The NPF events tended to occur on sunny days and the environment with low temperature, low relative humidity, and high concentration of SO2 and O3. SO2 and O3 might be main potentially precursor gases for our NPF events. The formation and growth process of new particles at the Bright Summit were also affected by the air source and season, the East China air masses were more beneficial to the formation and growth of new particles in autumn.The analysis of the simultaneous data observated at the Bright Summit and Yungu Hotel indicated that the nucleation and subsequently growth process could have a large vertical extent at Mt. Huang. Based on the vertical scales and possible influencing factors, the NPF events could be divided into two categories. Compared with the results obtained at the Yungu Hotel, the NPF events occurred at the Bright Summit had showed the following characteristics: higher frequency, later start time,longer duration, and lower formation rate/growth rate.