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1.
Radon has been measured continuously at Cape Grim, Tasmania with a detection limit of about 0.05 Bq m-3 (1.4 pCi m-3). The annual average level from a close continental source is 3.8 Bq m-3, whereas the level in maritime air streams is typically 0.5 Bq m-3. The contribution of the ocean is about 0.04 Bq m-3, which indicates that more than 90% of radon in maritime air is from a continent. Thus radon can provide much less equivocal evidence of the degree of continental influence on an air stream than can other atmospheric constituents, whose sources and loss mechanisms are more complex. A knowledge of the past history of an air stream is important in the interpretation of atmospheric measurements.There is a close correlation between low radon and low Aitken particle concentrations, which suggests a continental origin for these particles. On the basis of present data it is difficult to reconcile this with evidence that the particles are generated photochemically over the ocean. An extension of the radon program to other latitudes would help to resolve this discrepancy.  相似文献   

2.
《Atmospheric Research》2009,91(2-4):187-194
Heterogeneous nucleation of supersaturated n-nonane vapour on seed particles of different size and composition has been investigated using a fast expansion chamber. Monodisperse seed particle sizes were ranging from about 4 nm up to about 24 nm in diameter. By using different types of particle generators WOx, Ag and (NH4)2SO4 particles were generated. For direct comparison between different particle compositions overlapping sizes have been generated for WOx and Ag at about 7 nm particle diameter as well as for Ag and (NH4)2SO4 at about 15 nm. Nucleation temperature was kept constant at about 278 K. Experimental data were compared to Kelvin equation and Fletcher theory including the effect of line tension. It was found that heterogeneous nucleation of n-nonane seems to be independent of seed particle composition and starts well below the Kelvin curve. Good agreement was achieved with Fletcher theory including the effect of line tension.  相似文献   

3.
Electrical charges on aerosol particles and droplets modify the droplet–particle collision efficiencies involved in scavenging, and the droplet–droplet and particle–particle collision efficiencies involved in coalescence of droplets and particles, even in only weakly electrified clouds and aerosol layers. This work places electrically enhanced scavenging, and the electrical inhibition of scavenging in the context of the microphysics of weakly electrified clouds.Collision efficiencies are calculated by numerical integration to obtain particle trajectories, that are determined by the complex interplay of electrical, gravitational and phoretic forces together with inertia. These modify the trajectory of a particle as it is carried by flow around the falling droplet. Conversely, the flow around the particle also modifies the trajectory of the droplet. The flows are specified analytically, using a hybrid of the Proudman–Pearson stream function for that region close to the droplet or particle, where it is accurate, merging into the exact Oseen stream function for larger distances, where that becomes accurate. The effect of the flow around the particle on the motion of the droplet was simulated using Langmuir's superposition technique on the hybrid stream functions. The treatment of inertia in the present calculations allows an extension of the scope of our previous work by a factor of 10 larger in particle size (103 in mass). The coverage is extended to a wide range of atmospheric conditions and particle densities.The pressures and temperatures used in the models ranged from a representation of the lower troposphere at  1 km altitude (900 hPa, 10 °C) to that of the middle stratosphere at  30 km altitude (12 hPa, − 47 °C). The particles considered range from 0.1 μm to 10 μm radius; the droplet radii range from 4 μm to 50 μm; particle densities range from 300 kg m− 3 to 2500 kg m− 3; particle charges range from 2e to 100e with droplet charges of like sign of 100e; and relative humidities range from 10% to 100%.For the larger particles (radii greater than about 3 μm) interacting with the larger droplets (radii greater than about 15 μm) the effects of inertia increase with particle density and dominate at the larger densities. For particles with radii in the range 1–3 μm the ‘Greenfield Gap’ of very low collision efficiencies was found, and was determined to be due to the effects of the gravitational force causing a reduction of collisions of particles with the front of the droplet, and the effect of inertia overcoming the tendency for the weight to produce a collision in the slow velocity region in the rear. When the electrical or phoretic forces are sufficiently large the Greenfield Gap is closed.When the particles have radii < 3 μm inertial effects no longer dominate the collisions, although inertia modifies the weight effects for particles with radii down to about 0.5 μm. For charged aerosol particles with radii smaller than about 0.1 μm interacting with droplets or background aerosol particles smaller than a radius of about 15 μm, the long range electrical repulsive force is effective in opposing the phoretic forces and keeping the particle out of range of the short range attractive image force. Thus ‘electroscavenging’ gives way to ‘electroprotection’ against the scavenging or coagulation processes otherwise caused by Browninan diffusion or phoretic forces.In an atmosphere of temperature 10 °C and pressure 900 hPa the net phoretic force reduces to zero and becomes repulsive for particles with radii above about 2 μm (depending on particle conductivity). This enhances the development of the Greenfield Gap. However, the value of this radius (at which the net phoretic force is zero) increases strongly with decreasing temperature and pressure (increasing altitude) as expected from theory, and is about 5 μm in the middle troposphere and more than 10 μm in the stratosphere. Thus a net attractive phoretic force on particles extends into the 1–3 μm radius range in the upper troposphere; however, the weight and inertial effects can ensure the presence of the Greenfield Gap in that range for 2000 kg m− 3 particles up to the middle stratosphere.  相似文献   

4.
A new instrument (LOPAP: LOng Path liquid Absorption Photometer) for the sensitive detection of nitric acid (HNO3) in the atmosphere is described. HNO3 is sampled in a temperature controlled stripping coil mounted in an external sampling module to minimize sampling artefacts in sampling lines. After conversion into a strongly absorbing dye, HNO3 is detected in long path absorption in special Teflon® AF 2400 tubes used as liquid core wave guides. For the correction of some interferences, due to for example HONO and particle nitrate, two channels are used in series. The interferences from several potential interfering compounds including particle nitrate were quantified in the laboratory and in a large outdoor simulation chamber. With the exception of the interference caused by N2O5, which is quantitatively measured by the instrument, all tested interferences can be corrected under atmospheric conditions. Thus, in the instrument only the sum of N(V) from HNO3 and N2O5 is determined, which is expected to be a common problem of wet chemical HNO3 instruments. The instrument has a detection limit of 5–30 pptv for a time response of 6–2 min, respectively and was validated against the FTIR technique in a large outdoor simulation chamber. In addition, the applicability of the instrument was demonstrated in a field campaign.  相似文献   

5.
Radon daughters are produced as free ions, but they become attached to aerosol particles at a rate depending on the particle concentration. In the lower marine boundary layer, most of those which do not become attached plate out on the ocean surface. In this paper a simple model is used to examine the influence of several parameters on radon/radon daughter disequilibria in maritime air. The model is compared with experimental data from Cape Grim.The radon daughter to radon ratio, f, decreases from 0.86 to 0.1 as the particle concentration falls from 1000 to 10 cm-3. Estimates of radon concentration at sea level, based on daughter measurements may therefore be in error by as much as a factor of 10 unless allowance is made for particle concentration. At 100 cm-3, the standard deviation of the distribution of measured f values is about 30%, indicating that the particle concentration is not the only factor influencing the loss of radon daughters. The implication is that radon daughters can be used to measure radon concentrations at sea level with an accuracy of about 30%, provided the particle concentration is known.The measurements show that there is a very low proportion of unattached daughers in the air between about 30 and 165 m above sea level. According to the model, this implies that mixing of air up to about 200 m is usually rapid enough to result in plate-out of radon daughters on the ocean on a time scale of less than 100 s.  相似文献   

6.
If a spot of tracer is released into a turbulent flow, the peak concentration at some subsequent time will initially be much greater than that implied by a solution for the ensemble average concentration at fixed points. For two-dimensional turbulence three areas may be defined: (1) an area Ad related to the ensemble average concentration field; (2) an area Ap defined in terms of the relative dispersion of particles seeded into the patch after a short initial diffusion time; and (3) the area At occupied by tracer. It is argued that Ad grows linearly with time, whereas Ap and At grow exponentially; Ap faster than At. Thus, the concentration field is significantly streaky, even within the particle domain, until At becomes comparable with Ad. The time taken for this to occur is estimated; after this time, fluctuations about the ensemble average concentration field should not be greater than those given by a simple mixing length argument. In three-dimensional turbulence the volume Vt of the tracer domain grows much more rapidly than the volume Vp of the particle domain if the merging of streaks is ignored. However, Vt cannot be greater than Vp so streaks must merge and Vp can be used to provide a rough estimate of peak concentration, or concentration variance.  相似文献   

7.
黄山地区气溶胶吸湿增长特性数值模拟研究   总被引：4，自引：2，他引：2  
江琪  银燕  秦彦硕  陈魁  杨素英 《气象科学》2013,33(3):237-245
应用多种化学组分气溶胶的绝热气块分档模式,对2008年春季黄山地区气溶胶吸湿增长特性进行了模拟分析.结果表明:黄山地区气溶胶吸湿增长因子f的大小与粒子半径、相对湿度、粒子化学组分、上升速度及上升高度密切相关,且小粒子吸湿增长比大粒子显著.吸湿增长因子与相对湿度呈正相关,相对湿度越接近粒子的临界饱和比,吸湿增长因子变化越显著.可溶性有机气溶胶,通过增加溶液中溶质的百分比来影响临界饱和比,使吸湿增长因子增大.若不考虑不可溶粒子的成核作用,会高估粒子的吸湿性.随着上升速度增大,吸湿增长因子降低,降低程度与粒子初始高度的相对湿度有关.上升高度通过改变气块相对湿度的变化来影响气溶胶吸湿增长因子.  相似文献   

8.
气溶胶光学厚度谱特征判断粒子大小方法初探   总被引：4，自引：0，他引：4       下载免费PDF全文
当气溶胶谱满足Junge分布时,Angstrom指数 (α) 可以准确地描述粒子大小,但真实大气气溶胶很少完全满足这一条件,仅用α判断粒子大小会有较大出入。基于北京、香河、兴隆、太湖4个Aeronet观测站21世纪以来各站历时都超过1年的气溶胶光学厚度资料,获得5511组lnτ与lnλ的二次拟合参数a2,a1,尝试找到一种结合α,a2,a1判断粒子大小的方法。结果表明:当气溶胶为粗粒子时 (Vfine/Vtotal＜0.2),α均小于0.75,仅用α就可以较好地判断粒子大小,但当气溶胶以细粒子为主时 (Vfine/Vtotal＞0.7),该方法会有较大出入,此时a2,a1可以有效地辅助α判断粒子大小,α＞0.75,a2＜-0.5或a2＜-0.5,a1＜-1.0是较好的判据。此外,分析发现国外研究提出的用a2-a1判断粒子大小的方法效果并不理想,尤其在1＜a2-a1＜2的情况下,粒子的组成有多种可能。  相似文献   

9.
A favoured method of assimilating information from state-of-the-art climate models into integrated assessment models of climate impacts is to use the transient climate response (TCR) of the climate models as an input, sometimes accompanied by a pattern matching approach to provide spatial information. More recent approaches to the problem use TCR with another independent piece of climate model output: the land-sea surface warming ratio (φ). In this paper we show why the use of φ in addition to TCR has such utility. Multiple linear regressions of surface temperature change onto TCR and φ in 22 climate models from the CMIP3 multi-model database show that the inclusion of φ explains a much greater fraction of the inter-model variance than using TCR alone. The improvement is particularly pronounced in North America and Eurasia in the boreal summer season, and in the Amazon all year round. The use of φ as the second metric is beneficial for three reasons: firstly it is uncorrelated with TCR in state-of-the-art climate models and can therefore be considered as an independent metric; secondly, because of its projected time-invariance, the magnitude of φ is better constrained than TCR in the immediate future; thirdly, the use of two variables is much simpler than approaches such as pattern scaling from climate models. Finally we show how using the latest estimates of φ from climate models with a mean value of 1.6—as opposed to previously reported values of 1.4—can significantly increase the mean time-integrated discounted damage projections in a state-of-the-art integrated assessment model by about 15 %. When compared to damages calculated without the inclusion of the land-sea warming ratio, this figure rises to 65 %, equivalent to almost 200 trillion dollars over 200 years.  相似文献   

10.
The long-standing academic and public debate on economic growth, prosperity and environmental sustainability has recently gained new momentum. It lacks, however, a broad perspective on public opinion. Prior opinion surveys typically offered a simple dichotomous choice between growth and environmental protection. This study examines public beliefs and attitudes about a wider range of aspects of the growth debate. To this end, we conducted an online questionnaire survey including a country-wide, representative sample of 1008 Spanish citizens. Using factor analysis, we identify six distinct dimensions of public attitudes, referred to as: prosperity with growth; environmental limits to growth; general optimism; wrong priority; overrated GDP; and governmental control. We further analyze several specific questions associated with the growth debate, such as those concerning the desired GDP growth rate, the preferred growth-environment position, and beliefs about, as well as reasons for, a possible end or continuation of growth. We find that most respondents favor GDP growth rates of more than 3%. A majority views growth and environmental sustainability as compatible (green growth), while about one-third prefers either ignoring growth as a policy aim (agrowth), or stopping it altogether (degrowth). Only very few people want growth unconditionally (growth-at-all-costs). About one-third of the respondents believe that growth may be never-ending. We examine how support for or disagreement with different statements on growth are related to each other, as well as how they are influenced by socio-demographic, knowledge and ideology/values variables. Overall, our findings can inform public debates about the growth paradigm and its potential alternatives by providing a more nuanced understanding of public opinion. We make suggestions for future research, including modifying poll questions on growth and environment through offering a more diverse set of response options.  相似文献   

11.
Place attachment, the emotional bonds that people form to significant places, influences adaptation to climate change. Within this context, weather is garnering greater attention for its dynamic, mediating role, yet its political and cultural significance remains under-researched. Here we draw on Serres and McCarren’s (1992) idea of the Natural Contract and Vannini et al.’s (2012) contributions on weathering to integrate contractarianism with a deep account of people’s relationship with weather in place. We analyse attitudes and adaptation to climate change in communities of the geographically-remote and climate-vulnerable Outer Hebrides in Scotland, using video-elicitation to generate data on significant places. Our results show that changing, difficult and unpredictable weather binds people to place and influences how they think about themselves, their place and adaptation in dynamic ways. Through this connection, we demonstrate that people knowingly enter into what we term Weather Contracts and that accepting the volatility of the weather allows people to react positively to changes brought with climate. Finally, we show that the common ideology of a community living with weather generates wider discourses around independence and resisting modernisation that we term a weathered ideology. Thus, climate change is not always a destabilising force. For those who are accustomed to changing weather it can be a dimension of place around which people can organise. Uncertainty and anxiety about the future of the climate is caused more by a lack of control over adaptation processes than by a fear of unknown weather. This has implications for people living at the margins across the globe, where unpredictable weather is a part of local identities, but influence on adaptation policy making is low.  相似文献   

12.
Virtually all reviews dealing with aerosol-sized particle deposition onto forested ecosystems stress the significance of topographic variations, yet only a handful of studies considered the effects of these variations on the deposition velocity (V d ). Here, the interplay between the foliage collection mechanisms within a dense canopy for different particle sizes and the flow dynamics for a neutrally stratified boundary layer on a gentle and repeating cosine hill are considered. In particular, how topography alters the spatial structure of V d and its two constitutive components, particle fluxes and particle mean concentration within and immediately above the canopy, is examined in reference to a uniform flat-terrain case. A two-dimensional and particle-size resolving model based on first-order closure principles that explicitly accounts for (i) the flow dynamics, including the two advective terms, (ii) the spatial variation in turbulent viscosity, and (iii) the three foliage collection mechanisms that include Brownian diffusion, turbo-phoresis, and inertial impaction is developed and used. The model calculations suggest that, individually, the advective terms can be large just above the canopy and comparable to the canopy collection mechanisms in magnitude but tend to be opposite to each other in sign. Moreover, these two advective terms are not precisely out of phase with each other, and hence, do not readily cancel each other upon averaging across the hill wavelength. For the larger aerosol-sized particles, differences between flat-terrain and hill-averaged V d can be significant, especially in the layers just above the canopy. We also found that the hill-induced variations in turbulent shear stress, which are out-of-phase with the topography in the canopy sublayer, play a significant role in explaining variations in V d across the hill near the canopy top. Just after the hill summit, the model results suggest that V d fell to 30% of its flat terrain value for particle sizes in the range of 1–10 μm. This reduction appears consistent with maximum reductions reported in wind-tunnel experiments for similar sized particle deposition on ridges with no canopies.  相似文献   

13.
Many endangered species persist as a series of isolated populations, with some populations more genetically diverse than others. If climate change disproportionately threatens the most diverse populations, the species’ ability to adapt (and hence its long-term viability) may be affected more severely than would be apparent by its numerical reduction. In the present study, we combine genetic data with modelling of species distributions under climate change to document this situation in an endangered lizard (Eulamprus leuraensis) from montane southeastern Australia. The species is known from only about 40 isolated swamps. Genetic diversity of lizard populations is greater in some sites than others, presumably reflecting consistently high habitat suitability over evolutionary time. Species distribution modelling suggests that the most genetically diverse populations are the ones most at risk from climate change, so that global warming will erode the species’ genetic variability faster than it curtails the species’ geographic distribution.  相似文献   

14.
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15.
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16.
This paper describes the effects of convective adjustment time scale (τ) on the simulation of tropical climate. The NCAR-Community Atmosphere Model version 3 (CAM3) has been used for this study. In the default configuration of the model, the prescribed value of τ, a characteristic time scale with which convective available potential energy (CAPE) is removed at an exponential rate by convection, is assumed to be 1?h. However, some recent observational findings suggest that, it is larger by around one order of magnitude. In order to investigate the dependence of tropical climate simulation to this time scale, we conducted two simulations, one with a time scale of 1?h (CTRL) and another with 8?h (EXPT), and examined the differences in simulated climate. For this, we analyzed both the mean as well as transient features, viz., seasonal mean quantities, equatorial waves, and meridional migration of convective disturbances. The spatial distributions of seasonal mean precipitation are found to be better in EXPT. The spatial correlation coefficients of CTRL and EXPT with the observations are 0.79 and 0.83, respectively, for northern hemisphere winter. Similarly, for northern hemisphere summer, the values are 0.67 and 0.79, respectively. In addition, there is also an improvement in the simulation of equatorial waves, specifically, the Kelvin waves, Madden–Julian oscillation, and n?=?1 equatorial Rossby waves become more realistic in EXPT. The characteristics of meridional migration of convective activity over tropics also become more reasonable in EXPT. Thus, it is found that there is a clear improvement in some of the key aspects of the simulated tropical climate with the revised convective adjustment time scale.  相似文献   

17.
Both large-eddy simulations (LES) and water-tunnel experiments, using simultaneous stereoscopic particle image velocimetry and laser-induced fluorescence, have been used to investigate pollutant dispersion mechanisms in regions where the surface changes from rural to urban roughness. The urban roughness was characterized by an array of rectangular obstacles in an in-line arrangement. The streamwise length scale of the roughness was kept constant, while the spanwise length scale was varied by varying the obstacle aspect ratio l / h between 1 and 8, where l is the spanwise dimension of the obstacles and h is the height of the obstacles. Additionally, the case of two-dimensional roughness (riblets) was considered in LES. A smooth-wall turbulent boundary layer of depth 10h was used as the approaching flow, and a line source of passive tracer was placed 2h upstream of the urban canopy. The experimental and numerical results show good agreement, while minor discrepancies are readily explained. It is found that for \(l/h=2\) the drag induced by the urban canopy is largest of all considered cases, and is caused by a large-scale secondary flow. In addition, due to the roughness transition the vertical advective pollutant flux is the main ventilation mechanism in the first three streets. Furthermore, by means of linear stochastic estimation the mean flow structure is identified that is responsible for street-canyon ventilation for the sixth street and onwards. Moreover, it is shown that the vertical length scale of this structure increases with increasing aspect ratio of the obstacles in the canopy, while the streamwise length scale does not show a similar trend.  相似文献   

18.
《Atmospheric Research》2009,91(2-4):125-131
Particle detection by condensation particle counters (CPCs) is ultimately limited by the onset of homogeneous nucleation. At vapour supersaturations around the homogeneous nucleation limit the diameter of critical clusters is typically about 2 nm. It is widely assumed that only particles larger than critical clusters can be activated by vapour condensation and the general detection limit of CPCs is therefore currently accepted to be around 2 nm particle diameter. Using an expansion type CPC with n-propanol as working fluid we investigated the transition from heterogeneous to homogeneous nucleation, clearly showing that particles are activated much before the onset of homogeneous nucleation, even at particle diameters as small as 1.4 nm. For particle diameters below 2 nm we have usually found condensation particle counting to be influenced by the simultaneous presence of ions as generated in a bipolar diffusion charger. In this paper we illustrate how the presence of ions influences particle number concentration measurement and how ions can be removed in order to obtain accurate seed particle number concentrations for particle diameters down to 1 nm.  相似文献   

19.
Particle detection by condensation particle counters (CPCs) is ultimately limited by the onset of homogeneous nucleation. At vapour supersaturations around the homogeneous nucleation limit the diameter of critical clusters is typically about 2 nm. It is widely assumed that only particles larger than critical clusters can be activated by vapour condensation and the general detection limit of CPCs is therefore currently accepted to be around 2 nm particle diameter. Using an expansion type CPC with n-propanol as working fluid we investigated the transition from heterogeneous to homogeneous nucleation, clearly showing that particles are activated much before the onset of homogeneous nucleation, even at particle diameters as small as 1.4 nm. For particle diameters below 2 nm we have usually found condensation particle counting to be influenced by the simultaneous presence of ions as generated in a bipolar diffusion charger. In this paper we illustrate how the presence of ions influences particle number concentration measurement and how ions can be removed in order to obtain accurate seed particle number concentrations for particle diameters down to 1 nm.  相似文献   

20.
南京雾-霾天气个例湍流运动特征的对比研究   总被引：4，自引：0，他引：4       下载免费PDF全文
利用超声风速仪、能见度仪等探测资料,采用Fortran、Matlab、Origin等软件处理数据并绘图分析,对比了南京郊区2013年12月4日一次霾过程、2013年12月7—9日一次雾过程和2013年12月3日一个晴天的湍流运动特征。结果表明,霾天和晴天平均水平风速、平均动能、湍流动能、湍流强度、摩擦速度、动量通量和热量通量都有明显的日变化,而这些参量在雾天没有明显的日变化,不同天气湍流参量大小有差异;霾、雾、晴天近中性层结下,u、v、w三个方向风速归一化标准差近似为常数,霾天分别为3.15、2.72、1.17,雾天分别为3.11、2.45、1.25,晴天分别为3.40、3.45、1.50;不稳定条件下霾、雾、晴天风速归一化标准差和湍流动能归一化标准差符合1/3幂次律,稳定条件下霾、雾、晴天无因次湍流动能均满足1/3幂次律;不稳定条件下霾、雾、晴天温度和湿度归一化标准差满足-1/3幂次律,稳定条件下霾、雾、晴天温度归一化标准差符合-2/3幂次律;雾天归一化u、v、w谱与Kansas谱吻合度比霾天和晴天高,霾天u、v、w谱峰值频率约为0.01 Hz,雾天u谱峰值频率约为0.004 Hz,v、w谱峰值频率约为0.1 Hz,晴天u、v、w谱峰值频率约为0.01 Hz。得出结论:雾天的边界层结构与霾天和晴天有所不同;不稳定条件下霾、雾、晴天风速、湍流动能、温度、湿度归一化标准差均符合莫宁-奥布霍夫相似理论;雾天归一化u、v、w谱比霾天和晴天更加符合莫宁-奥布霍夫局地各向同性理论,且霾天和晴天以机械湍流为主,雾天既有机械湍流也有热力湍流。  相似文献   

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