Aerosol light scattering measurements as a function of relative humidity

The hygroscopic nature of atmospheric fine aerosol was investigated at a rural site in the Great Smoky Mountains National Park during July and August 1995. Passing the sample aerosol through an inlet, which housed an array of Perma Pure diffusion dryers, controlled the sample aerosol's relative humidity (RH). After conditioning the aerosol sample in the inlet, the light scattering coefficient and the aerosol size distribution were simultaneously measured. During this study, the conditioned aerosol's humidity ranged between 5% < RH < 95%. Aerosol response curves were produced using the ratio bspw/bspd; where bspw is the scattering coefficient measured at some RH greater than 20% and bspd is the scattering coefficient of the "dry" aerosol. For this work, any sample RH values below 15% were considered dry. Results of this investigation showed that the light scattering ratio increased continuously and smoothly over the entire range of relative humidity. The magnitude of the ratio at a particular RH value, however, varied considerably in time, particularly for RH values greater than approximately 60%. Curves of the scattering coefficient ratios as a function of RH were generated for each day and compared to the average 12-hour chemical composition of the aerosol. This comparison showed that for any particular RH value the ratio was highest during time periods of high sulfate concentrations and lowest during time periods of high soil or high organic carbon concentrations.     

Light scattering characteristics of aerosols as a function of relative humidity: Part I--A comparison of measured scattering and aerosol concentrations using the theoretical models

The Southeastern Aerosol and Visibility Study (SEAVS) was undertaken to characterize the size-dependent composition, thermodynamic properties, and optical characteristics of the ambient atmospheric particles in the southeastern United States. The field portion of the study was carried out from July 15 to August 25, 1995. As part of the study a relative humidity controlled inlet was built to raise or lower the relative humidity to predetermined levels before the aerosol was passed into an integrating nephelometer or particle-sizing device. Five other integrating nephelometers were operated in various configurations, two of which were fitted with a 2.5 microns inlet. Fine particle (< 2.5 microns) samplers were operated to measure concentrations of sulfate, nitrate, and ammonium ions, organic and elemental carbon, and fine soil. Mass size distributions were measured with an eight-stage, single orifice cascade impactor. Four different strategies for estimating scattering were used. First, an externally mixed model with constant specific scattering coefficients, sulfate ion mass interpreted as ammonium bisulfate, and ammonium bisulfate growth as a function of relative humidity, is assumed. Second, an externally mixed aerosol model, assuming constant dry specific scattering but with sulfate ammoniation and associated composition-dependent hygroscopicity explicitly accounted for, is used. Third, an externally mixed aerosol model, but with sulfate ammoniation, associated growth as a function of relative humidity, and sulfate size distributions, is applied. Fourth, an internally mixed aerosol model with measured sulfur size distributions and estimated size distributions for other species is used with the growth characteristics of the mixture being estimated using the Zdanovskii-Stokes-Robinson (ZSR) assumptions. Only ionic species were considered to be hygroscopic. The second, third, and fourth approaches yield similar results with reconstructed scattering comparing quite favorably with measured scattering. Accounting for sulfate ammoniation and associated water uptake was the most important detail in achieving closure between measurements and modeled scattering. In general, differences between estimated scattering, assuming internally or externally mixed models, was small. These same models were used to estimate wet to dry scattering ratios. The R2 for an ordinary least-squares regression between measured and predicted ratios was high (0.71-0.92), and in most cases the scattering ratio was insensitive to modeling assumptions. However, during some sample periods differences between predicted scattering ratios for the different modeling assumptions were as high as 30%.     

Light scattering characteristics of aerosols at ambient and as a function of relative humidity: Part II--A comparison of measured scattering and aerosol concentrations using statistical models

The eastern United States national parks experience some of the worst visibility conditions in the nation. To study these conditions, the Southeastern Aerosol and Visibility Study (SEAVS) was undertaken to characterize the size-dependent composition, thermodynamic properties, and optical characteristics of the ambient atmospheric particles. It is a cooperative three-year study that is sponsored by the National Park Service and the Electric Power Research Institute and its member utilities. The field portion of the study was carried out from July 15 to August 25, 1995. The study design, instrumental configuration, and estimation of aerosol types from particle measurements is presented in a companion paper. In the companion paper, we compare measurements of scattering at ambient conditions and as functions of relative humidity to theoretical predictions of scattering. In this paper, we make similar comparisons, but using statistical techniques. Statistically derived specific scattering associated with sulfates suggest that a reasonable estimate of sulfate scattering can be arrived at by assuming nominal dry specific scattering and treating the aerosols as an external mixture with ammoniation of sulfate accounted for and by the use of Tang's growth curves to predict water absorption. However, the regressions suggest that the sulfate scattering may be underestimated by about 10%. Regression coefficients on organics, to within the statistical uncertainty of the model, suggest that a reasonable estimate of organic scattering is about 4.0 m2/g. A new analysis technique is presented, which does not rely on comparing measured to model estimates of scattering to evoke an understanding of ambient aerosol growth properties, but rather relies on measurements of scattering as a function of relative humidity to develop actual estimates of f(RH) curves. The estimates of the study average f(RH) curve for sulfates compares favorably with the theoretical f(RH) curve for ammonium bisulfate, which is in turn consistent with the study average sulfate ammoniation corresponding to a molar ratio of NH4/SO4 of approximately one. The f(RH) curve for organics is not significantly different from one, suggesting that organics are weakly to nonhygroscopic.     

Aerosol light scattering measurements as a function of relative humidity: a comparison between measurements made at three different sites

The water uptake by fine aerosol particles in the atmosphere has been investigated at three rural National Parks in the United States (Great Smoky Mountains, Grand Canyon and Big Bend National Parks). The relative humidity (RH) of sample aerosols was varied from less than 20% to greater than 90% using Perma Pure drying tubes as the scattering coefficient of the aerosol was measured with a Radiance Research M903 nephelometer. Data from these studies show that growth curves at all the three sites are similar in shape but the magnitude of growth can vary considerably from day to day. The growth curves from Great Smoky Mountains show smooth continuous growth over the entire range of RH, while the growth curves from the Grand Canyon and Big Bend show smooth and continuous growth on some days and deliquescence on other days. Comparing 12-h filter samples of chemical composition data with the aerosol growth curves, we find that higher fractions of soluble inorganic compounds (sulfate and nitrate) produce growth curves of greater magnitude than do higher concentrations of either organic carbon or soil material.     

Influence of relative humidity on aerosol deposition by sedimentation

The deposition of particulate matter at the ground due to settlement of particles in the atmosphere increases greatly with increasing relative humidity (r.h.) when the r.h. becomes larger than 90 per cent. In fog, i.e. at r.h. of 99.9 percent, the flux of dry particulate material to the ground due to settling is about 15–25 times larger than at zero and moderate r.h. Thus, episodes of fog are very effective in removing particulate matter from the atmosphere. Secondly, mean deposition velocities of metals at Frankfurt, F.R. Germany, have been compared with the appropriate mean settling velocities. The results show the mean turbulent flux of material from the ground to the atmosphere to be smaller, but of the same order as the settling flux.     

Dependence of aerosol sulfur particle size on relative humidity

Ambient aerosol particle size fractions have been sampled by cascade impactors in laboratory experiments before and after humidification by air flow over a water surface. From PIXE analyses of each fraction mass median aerodynamic diameters, MMAD, of sulfur were computed. The increase in MMAD, found in 12 experiments covering the relative humidity range 10–97%, corresponded more closely to (NH₄)₂SO₄ than to H₂SO₄ as the principal sulfur species present in particles < 2 μm aerodynamic diameter.     

An attempt to interpret the humidity dependencies of the aerosol extinction and scattering coefficients

This paper shows that there exists a possibility of interpreting the variation of the aerosol scattering coefficient with relative humidity in terms of mean composition of the aerosol particles of all sizes (not the chemical compositions of specific size classes). Furthermore, the discussion of a specific example has demonstrated the most important requirements for such an analysis. These are:

• 1.(1) knowledge of the approximate shape of the size distribution,
• 2.(2) the approximate complex refractive index of the particles, and
• 3.(3) that the measurements are taken in one specific air mass.

     

Diurnal and seasonal patterns in light scattering,extinction, and relative humidity

Since 1988, several federal and state governmental agencies in the US have coordinated efforts to operate the interagency monitoring of protected visual environments (IMPROVE) network at sites in remote areas. Most IMPROVE sites are equipped with either a transmissometer to measure light extinction (B_ext) or a nephelometer to measure particle scattering (B_sp). Optical, temperature, and relative humidity (RH) measurements are made hourly at these sites. The diurnal and seasonal patterns in these data are examined and discussed here. At many IMPROVE sites the diurnal patterns in RH and therefore B_ext or B_sp are as expected based on average temperature. On average, RH is higher at night and during the winter than during warmer times of the day and year. Also as expected, based on RH alone, at many sites hourly mean B_ext or B_sp values are either in phase with RH or weakly dependent on time of day. Usually, the diurnal differences are not as large as the seasonal differences. Another group of IMPROVE sites have mean RH patterns similar to those described above but have a different diurnal pattern in measured scattering or extinction. At these sites, the highest mean B_sp or B_ext occurs during mid-day rather than at night. At several of these sites, especially those on ridge tops, it is hypothesized that this is because the diurnal shifts in mixing height only allow the surface layer of the atmosphere to reach the monitor during mid-day. Several other sites have unique diurnal or seasonal patterns in average B_sp or B_ext that can usually be linked to emissions in nearby source regions or local meteorology and terrain.     

相对湿度对ZSM-5分子筛低温等离子体再生特性的影响

低温等离子体再生法是一种低温、快速和高效的吸附材料再生新方法。运用管式单介质阻挡放电反应器产生的低温等离子体对吸附了苯的ZSM-5分子筛进行再生处理,分析了湿度对低温等离子体放电状态的影响,研究了湿度对材料吸附性能及低温等离子体再生效果的影响,考察了特定湿度条件下低温等离子体再生吸附材料的可重复性。实验结果表明,当相对湿度小于60%时,吸附材料的脱附效率和再生效率随湿度增加而升高;当相对湿度由60%升高至80%时,吸附材料的脱附效率和再生效率开始下降;但通过对比不同湿度条件再生后吸附材料的饱和吸附容量可以发现,湿度越低,再生后吸附材料的饱和吸附容量越大;在相对湿度30%的条件下,重复再生效率稳定,且重复再生后吸附材料仍有较好的吸附效果。     

Sampling at controlled relative humidity with a cascade impactor

The design and function of a device that regulates the relative humidity of an ambient aerosol sample is described. We use this RH controller upstream of MOUDI impactors to permit sampling at relative humidities in the 70–80% range. Humidity control is achieved by allowing the aerosol to approach equilibrium with a saturated salt solution. Benefits to sampling with impactors in this relative humidity range include greatly reduced bounce of fine, hygroscopic particles, minimal flow-induced sizing errors, and minimization of uncertainties in measured size distributions due to diurnal variations in relative humidity during sampling. Data from field measurements in a humid environment (Look Rock, TN) and arid environments (Las Vegas, NV and Meadview, AZ) are discussed.     

Product study of oleic acid ozonolysis as function of humidity

The heterogeneous reaction of ozone with oleic acid (OA) aerosol particles was studied as function of humidity and reaction time in an aerosol flow reactor using an off-line gas chromatography mass spectrometry (GC–MS) technique. We report quantitative yields of the major C9 ozonolysis products in both gas and condensed phases and the effect of relative humidity on the product distribution. The measurements were carried out with OA aerosol particles at room temperature. The results indicate that the product yields are increasing with increasing relative humidity during the reaction. Nonanal (NN) was detected as the major gas-phase product (55.6 ± 2.3%), with 94.5 ± 2.4% of the NN yield in the gas, and 5.5 ± 2.7% in the particulate phase, whereas nonanoic, oxononanoic and azelaic acids were detected exclusively in the particulate phase. Using UV-spectrometry, we observed that peroxides make up the largest fraction of products, about half of the product aerosol mass, and their concentration decreased with increasing humidity.     

环境相对湿度对颗粒物过滤性能的影响

以吸湿性氯化钠 (NaCl) 颗粒和非吸湿性氧化铝 (Al₂O₃) 颗粒为实验对象,使用纤维材料过滤介质,引入纳米和微米两类颗粒物尺度,考察不同相对湿度 (RH) 环境下颗粒物过滤性能的变化规律及内在机理。结果表明：低湿度下 (RH20%和50%) 过滤吸湿性颗粒时,压降呈线性增长趋势,高湿度下 (≥RH80%) 会出现压降的急剧升高;非吸湿性颗粒受湿度变化的影响较小,但在饱和湿度下 (RH100%) 压降增长较为显著;滤料初始过滤效率对环境中相对湿度的变化不敏感,也不会受到颗粒吸湿性能差别的影响;在颗粒加载过程中,受滤料纤维表面颗粒物沉积的影响,过滤效率持续缓慢增长,但湿度超过颗粒潮解点后,对吸湿性颗粒的过滤效率出现一定程度下降,类似液体过滤机制;此外,环境湿度的变化对于纳米颗粒物过滤性能的影响远比微米颗粒更为显著。研究结果可为空气污染防治中颗粒物过滤技术的应用与改进提供参考。     

宝鸡市大气PM_(10)中水溶性物质的组成及特征研究

2008年冬、春季在宝鸡市4个不同功能区采集PM10样品,探讨了PM10中水溶性物质的化学组成、时空分布特征以及来源。结果表明,冬、春季PM10的平均质量浓度分别为(402±100)、(410±160)μg/m3,无明显季节差异,冬季以交通干道区的PM10浓度为最高,而春季则以商贸区的PM10浓度为最高;冬、春季PM10中水溶性有机碳(WSOC)浓度最高值均出现在商贸区,最低值则分别出现在背景点和交通干道区,水溶性无机碳(WSIC)浓度最高值分别出现在交通干道区和商贸区,最低值均出现在背景点;冬、春季PM10中所含大多数无机离子浓度不存在显著空间差异,但不同功能区PM10中无机离子所占质量分数差异较明显;冬、春季PM10中的水溶性物质质量浓度分别为207、151μg/m3,在PM10中所占质量分数分别为51%和40%,其中,冬、春季水溶性物质浓度最高的分别为居民区和商贸区;冬季PM10中WSOC浓度与SO24-、NO3-浓度有较好的相关性,说明冬季PM10中WSOC的主要组分为二次有机气溶胶,而春季PM10中WSOC浓度与SO42-、NO3-浓度的相关性相对较差,这是由于一次有机气溶胶对WSOC的贡献率较冬季显著增大;宝鸡市与北京市大气PM10浓度、PM10中的SO42-、NO3-、NH4+浓度最为接近;广州市大气PM10中的SO42-所占质量分数(14%)要高于北方城市(宝鸡市和北京市均为9%)。     

Effect of composition on aerosol light scattering efficiencies

Forecasts of the impact of emissions changes on visibility use light scattering efficiencies—the change in the amount of light scattered with a change in mass of an aerosol constituent. This paper demonstrates how light scattering efficiencies depend on interactions between the aerosol constituents. Calculations are presented for two model aerosols: NH₄HSO₄ droplets and a ‘typical’ urban aerosol. These calculations demonstrate that the traditional means of predicting visibility impairment for increases in atmospheric emissions is not generally appropriate. Different light scattering efficiencies are needed for distinct meteorological conditions and aerosol compositions thus for typical high and low visibility events.