| To investigate the vertical variation of cloud condensation nuclei (CCN) and water soluble inorganic ions (WSI) in aerosols and the influence of aerosol chemical composition to CCN at a regional background mountainous site. The concentration of CCN, WSI and hygroscopicity in aerosols were collected at different altitudes simultaneously at Huangshan in 2011 and 2012. This study examines the characteristics of CCN and WSI at different altitudes at Huangshan. The impact of aerosol chemical composition to hygroscopicity and activity is discussed in this paper. The overall results are shown below:The CCN concentrations of Huangshan are lower than which in some pollution cities. CCN size distribution is shown of clean continental type. Huangshan is hardly impacted on local pollution and the atmospheric environment is relatively clean. The air mass impacted on aerosols in different altitudes is the same due to the principal component analysis and strong relativity of CCN concentrations among three altitudes. Pollution air masses aged during the long-range process make the soluble components accounting for large amount in aerosol particles and increase the hygroscopicity in aerosols. The concentrations of aerosol particles and secondary ions at the foot of mountain are higher than that at at the summit of mountain, which investigate that anthropological activities are intensive at at the foot of mountain. In addition, polluted air masses can be transported from the ground to the mountain top by upslope winds along the mountain slopes and by the uplifting PBL in the daytime. Aerosols can lose through dry or wet deposition processes and the dilution effects during transportation processes. The pollutants from lower levels can be removed during transport to the summit which may also lead to the decrease the concentrations of aerosol particles, CCN, and ions. In general, due to the complicated terrain, not only long-range transport but also vertical transport in high mountain impact on the physical and chemical properties of aerosols. Specific analysis on CCN, WSI and hygroscopicity are following.The number concentrations of CCN were obtained by using cloud condensation nuclei counter (CCNC) at three altitudes of Huangshan in June,2011. The results show that the time series of CCN concentration at three altitudes are nearly consistent. CCN concentration decreases with altitude. The average CCN number concentration is 1105.62,1218.39, and 1777.78cm’3 at supersaturation of 0.8%, at the foot, mid height and the summit of the mountain, respectively. The occurrence frequency of high CCN concentration (more than 1000cm’3) at the foot of the mountain is higher than which at the mid height and summit of the mountain. The results suggest that the environment of foot of the mountain were influenced by surrounding areas, which was heavier than that of mid height and the summit of the mountain. A bimodal pattern at both the summit and the foot of the mountain is presented in the diurnal variation. Two peaks appear at forenoon and afternoon respectively, which is related to the variation of atmospheric boundary layer height and valley wind. The fitted CCN spectra on different weather conditions at the summit of the mountain are obtained by using the expression N=CSk, which properties are analyzed as well. The results show that the fitted spectra parameters C is 2798、384、765, respectively, on sunny day, rainy day and foggy day, and which is lower than that of some polluted cities. The size distribution of CCN is shown of clean continental type.Nine size fractions of particles (10.0-9.0,9.0-5.8,5.8-4.7,4.7-3.3,3.3-2.1, 2.1-1.1,1.1-0.65,0.65-0.43 and <0.43μm) were collected at two different altitudes simultaneously at Huangshan in southeast China, from 14 September to 26 October of 2012. The mass concentrations of PM1.1, PM2.1 and PMio are 17.07,21.28 and 39.25μg/m3 at the summit (SM,1840m), respectively, and are 24.79,29.02 and 42.39μg/m3 at a lower height site (LL,869m). The average mass concentrations of total WSI for PM1.1, PM2.1 and PM10 are 9.59,11.73 and 17.16μg/m3 at SM, and are 16.88,19.38 and 27.61μg/m3 at LL. The concentrations of particulates and WSI both decrease with altitude increasing from 869m to 1840m. SO42- and NH4+ exhibit peak values 0.43-0.65μm at SM, whereas maintained peak values 0.65-1.1μm at LL. NO3-are mostly concentrated in fine mode for SM but in coarse mode for LL. Further analyses show that at LL, the heterogeneous reaction on coarse particles containing more calcium and magnesium may explain the higher concentrations of NO3- in coarse mode and also the higher temperature may reduce the concentrations of NO3" in fine mode. Na+, Cl" and K+ exhibit bimodal size distributions; Ca2+ and Mg2+ show maximum values in coarse mode. Aerosol acidity analysis shows a higher acidity of aerosol particles at LL when compared with those at SM. The average concentration of [H+] is relatively low when compared with those observed at two other mountains in China. This corresponds with the relatively low concentrations of SO42- and HSO4-and lower water content at Huangshan.The impact of aerosol composition on CCN activity were analyzed in this study based on field experiments carried out at Huangshan by using Moudi-type particle sampler and aerosol size resolved activity system, in September and October,2012. The results show that the critical supersaturation (Sc) is influenced by aerosol size distribution and composition simultaneously. Sc decrease when solubility increasing. While the increased amount of organic species decrease the activity, CCN activity increased with the concentration of water soluble ions increasing, which is notable under low supersaturation (SS) condition. Based on the curve of size-resolved CCN activation, the average values of critical dry diameter (Dd) is 145.5,90.6,58.0, 40.9nm under SS of 0.1%,0.2%,0.4%,0.7%. The high level and positive correlation between SO42- and NH4+ make Dd at Huangshan closed with that of (NH4)SO4. The average values of hygroscopicity parameter κa calculated by size-resolved CCN activation curves is 0.43, increasing with the increasing particle size as well as the concentration of WSI. The average values of hygroscopicity parameter κp calculated by aerosol chemical composition is 0.37, higher than the average values in cities. These suggest that Huangshan is little polluted by local pollutant, so that the long-range transport processes led to the high aerosol hygroscopicity at Huangshan. A case study of κ and CCN closure analyses are not very good. It is possible related to lacking organic matter measurement used by filter-based methods and is related to the low time resolution. |
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