Nitrate in the atmospheric boundary layer of the tropical South Pacific: Implications regarding sources and transport

Weekly bulk aerosol samples collected at Funafuti, Tuvalu (8°30S, 179°12E), American Samoa (14°15S, 170°35W), and Rarotonga (21°15S, 159°45W), from 1983 through most of 1987 have been analyzed for nitrate and other constituents. The mean nitrate concentration is about 0.11 g m^–3 at each of these stations: 0.107±0.011 g m^–3 at Funafuti; 0.116±0.008 at American Samoa; and 0.117±0.010 at Rarotonga. Previous measurements of mineral aerosol and trace metal concentrations at American Samoa are among the lowest ever recorded for the near-surface troposphere and indicate that this region is minimally affected by transport of soil material and pollutants from the continents. Consequently, the nitrate concentration of 0.11 g m^–3 can be regarded as the natural level for the remote marine boundary layer of the tropical South Pacific Ocean. In contrast, over the tropical North Pacific which is significantly impacted by the transport of material from Asia and North America, the mean nitrate concentrations are about three times higher, 0.29 and 0.36 g m^–3 at Midway and Oahu, respectively. The major sources of the nitrate over the tropical South Pacific are still very uncertain. A very significant correlation between the nitrate concentrations at American Samoa and the concentrations of ²¹⁰Pb suggests that transport from continental sources might be important. This continental source could be lightning, which occurs most frequently over the tropical continents. A near-zero correlation with ⁷Be indicates that the stratosphere and upper troposphere are probably not the major sources. A significant biogenic source would be consistent with the higher mean nitrate concentrations, 0.16 to 0.17 g m^–3, found over the equatorial Pacific at Fanning Island (3°55N, 159°20W) and Nauru (0°32S, 166°57E). The lack of correlation between nitrate and nss sulfate at American Samoa does not necessarily preclude an important role for marine biogenic sources.     

气溶胶对云中水成物粒子荷电情况的影响

利用已有的二维雷暴云起电模式,加入气溶胶模块,建立一个完善的雷暴云起电模式.结合SEET个例,初步探讨了气溶胶浓度对雷暴云内各种水成物粒子荷电情况的影响.发现气溶胶的浓度与雷暴云内云滴、霰粒、冰雹以及雨滴等水成物粒子在空间所携带的最大电荷面密度值以及电荷量有很好的正相关性;同时气溶胶粒子浓度的增加使得雨滴在空间携带电荷量达到峰值的时间有一定提前.     

Characteristic features of atmospheric aerosols over India and their role in warm rain process

Atmospheric aerosols are a crucial link in the physical processes, involved in the formation and growth of precipitating clouds. Extensive aerosol measurements in surface air and in the lower troposphere were made at inland and coastal stations of different regions in India. At inland stations, the hygroscopic fraction of the total aerosol content is found to be a useful characteristic for distinguishing between the monsoon and summer airflow, as well as an indicator for a good or a badly developed monsoon. At coastal stations, however, this feature is not observed.Measurements as a function of height brought out that the aerosol varied widely in air over different seasons. During monsoon, the hygroscopic fraction was found highest at the cloud base level and was closely linked to the development of rain. Details of these investigations are presented.     

ISORROPIA: A New Thermodynamic Equilibrium Model for Multiphase Multicomponent Inorganic Aerosols

A computationally efficient and rigorous thermodynamic model that predicts the physical state and composition of inorganic atmospheric aerosol is presented. One of the main features of the model is the implementation of mutual deliquescence of multicomponent salt particles, which lowers the deliquescence point of the aerosol phase.The model is used to examine the behavior of four types of tropospheric aerosol (marine, urban, remote continental and non-urban continental), and the results are compared with the predictions of two other models currently in use. The results of all three models were generally in good agreement. Differences were found primarily in the mutual deliquescence humidity regions, where the new model predicted the existence of water, and the other two did not. Differences in the behavior (speciation and water absorbing properties) between the aerosol types are pointed out. The new model also needed considerably less CPU time, and always shows stability and robust convergence.     

Ground-Based In Situ Measurements of Near-Surface Aerosol Mass Concentration over Anantapur: Heterogeneity in Source Impacts

Surface measurements of aerosol physical properties were made at Anantapur(14.62°N,77.65 °E,331 m a.s.l),a semiarid rural site in India,during August 2008-July 2009.Measurements included the segregated sizes of aerosolsas as well as total mass concentration and size distributions of aerosols measured at low relative humidity(RH<75%) using a Quartz Crystal Microbalance(QCM) in the 25-0.05 μm aerodynamic diameter range.The hourly average total surface aerosol mass concentration in a day varied from 15 to 70 μg m-3,with a mean value of 34.02±9.05μgm-3 for the entire study period.A clear diurnal pattern appeared in coarse,accumulation and nucleation-mode particle concentrations,with two local maxima occurring in early morning and late evening hours.The concentration of coarse-mode particles was high during the summer season,with a maximum concentration of 11.81±0.98μgm-3 in the month of April,whereas accumulationmode concentration was observed to be high in the winter period contributed >68% to the total aerosol mass concentration.Accumulation aerosol mass fraction,A f(=Ma/Mt) was highest during winter(mean value of Af～0.80) and lowest(Af～0.64) during the monsoon season.The regression analysis shows that both R eff and R m are dependent on coarse-mode aerosols.The relationship between the simultaneous measurements of daily mean aerosol optical depth at 500 nm(AOD500) and PM 2.5 mass concentration([PM2.5]) shows that surface-level aerosol mass concentration increases with the increase in columnar aerosol optical depth over the observation period.     

南岭大瑶山高速公路浓雾的宏微观结构与能见度研究

南岭山地地处南亚热带湿润型季风气候区,每年9月至次年5月有华南准静止锋活动时均会有浓雾发生,每月浓雾日可高达15-18 d,尤其是中国目前最长的京珠高速公路通过南岭主脉大瑶山的乐昌-乳源段,路面海拔高度从200 m增至800多米,山地的抬升使雾害更加严重.在南岭大瑶山高速公路开展的两次多学科综合野外观测,内容包括目测能见度、器测能见度、雾滴谱、雾含水量、系留探空、双参数低空探空、雾层湍流扩散、气溶胶粒子谱、气溶胶成分谱、雾水样品成分、雨水样品成分.对典型个例进行了天气学分析,雾的宏观结构特征分析,雾的微物理特征分析.认识到南岭山地浓雾发生频率高,雾害十分严重,是典型的平流雾和上坡雾,实质上是出现在相对较高海拔高度上的低云,与华南锋面活动尤其是华南准静止峰的活动密切相关,局地地形的作用也非常重要.其特点是浓雾持续时间长、能见度极其恶劣、团块结构明显、雾滴尺度大、浓度不高、含水量较大、雾层内的湍流扩散能力比晴空区强,与中国过去研究较多的辐射雾差别较大.发现雾含水量与能见度呈明显的反相关关系,含水量较大时能见度较小;南岭山地雾含水量等微结构特征量的起伏变化,除与雾体本身的结构不均匀有关外,一个重要的原因是平流因素的影响,南岭山地下垫面的不均匀性,雾体随环境风的平移过程中,不规则的爬坡、翻越山坡的运动是造成雾体结构不均匀、振荡起伏变化的另一个重要原因.该地气溶胶粒子谱是呈单调下降的幂函数谱,次微米粒子浓度甚高,南岭山地气溶胶中含有高浓度的硫酸盐粒子,是优质凝结核,有利于雾的形成.雾的存在可以清除大气中的微量成分,雾滴可以包含浓度很高的污染物成分.同时,较之云滴而言,雾滴也很容易被地表物体(如植被、建筑物等)的垂直表面所截获,构成另一类清除过程.在南岭这样的大面积森林地区,这类清除过程可能是很重要的.研究本地区雾的特征变化,对建立本地区雾的预警预报系统有很大的现实意义,并为开展消雾试验提供了基本资料.     

Mountain wave motions determined by the Esrange MST radar

A European campaign of ground-based radar, lidar and optical measurements was carried out during the winter of 1996/1997 (28 December–2 February) to study lee waves in the northern part of Scandinavia. The participants operated ozone lidars, backscatter lidars and MST radars at ALOMAR/Andoya and Esrange/Kiruna, and an ALIS imaging system in Kiruna. The Andoya site was generally windward of the Scandinavian mountains, the Kiruna site on the leeward side. The goal of the experiment was to examine the influence of lee waves on the formation of Polar Stratospheric Clouds (PSCs). This paper studies the radar data from MST-radar ESRAD located at Esrange [68.°N, 21.°E], i.e. in the lee of the mountains. We present three cases where strong lee waves were observed: in one case they propagated upwards to the lower stratosphere and in the other two cases they were trapped or absorbed in the troposphere. We examine the local waves and the direction and strength of the local wind using the radar, the synoptic meteorological situation using weather maps (European Meteorological Bulletin) and the synoptic stratospheric temperatures using ECMWF data. We observed that waves propagate up to the stratosphere during frontal passages. When anticyclonic ridges are present, the propagation to the stratosphere is very weak. This is due to trapping of the waves at or below the tropopause. We also show that the radar data alone can be used to characterise the different weather conditions for the three cases studied (through the variation of the height of the tropopause). The synoptic stratospheric temperatures in the three cases were similar, and were above the expected threshold for PSC formation. Lidar and visual observation of PSCs and nacreous clouds, respectively, showed that these were present only in the case when the lee waves propagated up to the lower stratosphere.     

Macro-and Micro-Structures of Heavy Fogs and Visibility in the Dayaoshan Expressway

Belonging to the southern subtropical moist type of monsoon climate,the Nanling mountainous area experiences heavy fogs whenever quasi-stationary fronts appear there from September to May.There can be as many as 15-18 days of heavy fogs per month.Fogs have more serious consequences in the Lechang-Ruyuan section of the Beijing-Zhuhai Expressway(the longest expressway in China)that passes through the main part of the Nanling Mts.,where the road rises from 200m to more than 800m above sea level(ASL).For a major motorway in the mountainous areas of Nanling Mts.,two multidisciplinary integrated field observations were carried out,which measured visibility by the naked eyes,visibility by instrument,spectrum of fog- drops,liquid water content(LWC)of fog,tethered sounding,dual-parameter low-level sounding,turbulence diffusion within fog layers,aerosol spectra of size and composition,sampled fog water compositions,and sampled rainwater compositions.Typical cases were probed for their analyses of synoptics,micro- and macro-structures and microphysics.It is understood that heavy fogs take place with high frequency in the area and bring about serious consequences.Being typical advection and upslope fogs,they are in essence low-lying clouds appearing at high altitudes,which are closely related with the activity of South China frontal processes,especially the South China quasi-stationary fronts,and reflect on the role of local terrain as well.The heavy fogs are characteristic of long duration,extremely low visibility,well-organized lump- shaped structure,large-size fog-drops,moderate concentration,high LWC,and stronger turbulent diffusion within the fog layers than in fine sky.They differ much from radiation fogs,which are better documented in previous study in China.It is found that fog LWC is in significant anti-correlation with visibility so that large LWC is associated with small visual range.It is also noted that one of the reasons for the fluctuation of characteristic quantities of micro-structure such as the LWC of fog in the area is,in addition to the inhomogeneous structure of the fog itself,the effect of advection and inhomogeneous underlying surface; during the translation of fog with the ambient wind,irregular upslope and cross-over movement is another reason for the inhomogeneous structure and fluctuation of fog.The spectrum of the aerosol size displays itself as the power function of monotonous descent.The concentration of submicrometer particles is even higher.The high-concentration sulfate particles found in the aerosols of Nanling Mts.are actually good nuclei for condensation,which are favorable for the formation of fog.The presence of fog can help cleanse the trace compositions in the atmosphere so that fog droplets contain high levels of polluting elements.In the meantime,compared to cloud droplets,fog droplets are easier to be captured by the vertical surfaces of objects on the land surface,such as vegetation and buildings to constitute another kird of cleansing process. In vast stretches of forest like the Nanling Mts.,this kind of cleansing may be quite important.Studying the characteristic variation of fogs in the area realistically assists in setting up a forecast and warning system for local fogs and provides basic information for fog dispersal experiments.     

硫酸盐气溶胶对长江中下游夏季降水年代际转型的影响

为了研究人为硫酸盐气溶胶增长对1970年代末长江中下游夏季降水年代际转型的影响,使用全球气候模式（GFDL—AM2）,对硫酸盐直接气候效应进行了模拟。结果表明,硫酸盐气溶胶增长引起的降水年代际变化与观测到的降水转型有很好的时空一致性;观测中包括副热带高压西伸南移、中国东部近地面异常北风等夏季风年代际减弱信号以及对应的垂直温度、上升运动分布等均能很好地被模式再现。机制上,硫酸盐气溶胶通过引起负辐射强迫,造成中国中东部的大部分地区地面到对流层中层降温,海陆热力对比减小,使东亚夏季风减弱,雨带容易在长江中下游停留,从而导致该区域降水增多。于是,硫酸盐气溶胶增多对长江中下游降水年代际转型有重要贡献。     

全球和亚洲人为气溶胶影响东亚气候的数值模拟研究

利用海气耦合的全球气候模式CSIRO-Mk3.6分析比较了全球和亚洲人为气溶胶对东亚各季节气候的不同影响。结果表明,全球和亚洲外人为气溶胶使得东亚地区年平均地表温度分别下降0.9℃和0.55℃。亚洲区域气溶胶强迫决定了东亚近地面降温的时空分布特征,而亚洲区域外气溶胶进一步增强了我国北方夏季的近地面降温。各个季节对流层中上层的降温主要受区域外气溶胶的影响,并引起东亚高空急流强度和位置的变化,造成夏季和秋季明显的经向环流异常。同时,亚洲气溶胶影响各季节东亚低层环流场的响应,使得东亚陆地降水减少,而区域外气溶胶则主要影响冬季中高纬度和夏、秋季南海地区的低层风场。总体上,亚洲区域内、外人为气溶胶会增强我国冬、夏季风低层环流,并共同决定南海地区的降水变化。