The distribution of ammonium salts among a size and composition dispersed aerosol

The chemical and physical processes that govern the distribution of ammonium salt condensate over a size- and composition-dispersed aerosol particle population are considered. From an analysis of the concentration profiles of ammonia, nitric acid, and hydrochloric acid vapors surrounding an aerosol particle, the single particle fluxes of these species are derived. By evaluating the time scales for equilibration of the vapor-phase species with a population of aerosol particles, it is found that ammonium salts in the gas and aerosol phases are not always in equilibrium, especially under less polluted and cooler conditions. The principles that govern the distribution of ammonium salts on aerosol particles of different size and composition are identified, and it is found that thermodynamic equilibrium often does not uniquely determine the distribution of ammonium salt condensate. Thus it is concluded that both transport and thermodynamic properties of the aerosol population govern the distribution of ammonium salt condensate.     

南京北郊大气气溶胶的吸湿性观测研究

大气气溶胶的吸湿性不仅影响气溶胶的光学特性进而影响大气能见度,并且影响气溶胶的直接和间接气候效应.本文利用加湿串联差分迁移分析仪H-TDMA,于2012年5~7月在南京地区,对40~200nm大气气溶胶粒子在不同相对湿度下的吸湿增长因子进行了观测研究.统计结果表明:在90%相对湿度下,颗粒物的吸湿增长因子GF多为双峰分布,分为GF1.15的强吸湿组分.弱吸湿组分的吸湿增长因子(GFLH)在1.10~1.14之间;对应的强吸湿组分增长因子(GFMH )范围在1.47~1.58之间变化.相同粒径下的离散程度(σ)强吸湿组大于弱吸湿组,说明强吸湿组的粒子化学成分更复杂,异质性更强.相对湿度的变化对粒子吸湿增长的影响与粒子大小及化学组分有关,爱根核模态和积聚模态粒子在相同的相对湿度下潮解点不同,硝酸铵和硫酸铵是颗粒物中的主要吸湿成分.对不同天气条件下的气溶胶吸湿性分析,发现污染期间的吸湿增长因子(GF)和强吸湿组的数目比例(NFMH)都高于清洁期间,这与当时的气象条件以及粒子的内外部混合状态相关.观测还发现气溶胶粒子的吸湿性有明显的日变化特征,白天光照所引发的光化学反应以及混合层演变而造成粒子的吸湿性较强.机动车尾气排放的黑碳等不吸湿或弱吸湿颗粒物也会因为影响颗粒物的化学成分并进而对气溶胶吸湿性产生影响.     

Determination of aerosol strong acidity losses due to interactions of collected particles: Results from laboratory and field studies

Existing methods of measuring atmospheric aerosol strong acidity adequately prevent neutralization of fine-particle acidity by removing course alkaline particles and gaseous ammonia from air samples. However, these techniques do not consider particle interactions on the collection medium; therefore, they may still underestimate the actual aerosol acidity. Assessment of acid neutralization due to such interactions is made possible using annular denuder technology in conjuction with a newly designed filter pack. The amount of sulfate-related acidity neutralized by the collected ammonium nitrate (and possibly ammonium chloride and organic acid ammonium salts) is determined. Laboratory data suggest that large fractions of sulfate-related aerosol acidity are neutralized by ammonium nitrate particles during collection on filter media. Field data from the Harvard Acid Aerosol Health Effects Study also suggest that ammonium nitrate and possibly other ammonium salts, such as ammonium chloride, neutralize collected acid aerosols. For low-acid aerosol concentrations, the correction factor is significant; whereas, for high-acid concentrations, correction is negligible.     

中国3个AERONET站点气溶胶大小的识别及特征分析

?ngstr?m波长指数a是判断气溶胶粒子大小的重要参数,但当气溶胶尺度分布不满足Junge分布时,仅用a不能很好的反映气溶胶粒子的尺度信息.利用Mie散射理论和AERONET站点实测资料分析说明在0.750.6;当a2>0.3时,多以粗粒子为主,Vfine/Vtotal<0.4.利用Gobbi气溶胶图解法分析显示SACOL站AOD的大值主要是由于沙尘气溶胶的影响造成的;香河站沙尘和细粒子气溶胶都会产生大值AOD,并且图解法可以很好的区分这两种情况;太湖站受沙尘的影响较小,大值AOD绝大多数是由于细粒子气溶胶造成的.     

利用SPAMS分析北京市硫酸盐、硝酸盐和铵盐季节变化特征及潜在源区分布

采用单颗粒气溶胶质谱仪(SPAMS)于2013年12月至2014年11月在中国环境科学研究院内对大气颗粒物进行全年在线观测,采用特征离子法提取了观测结果中硫酸盐、硝酸盐和铵盐(SNA)的小时均值数据,分析了SNA混合特征和粒径分布随季节的变化.采用Hysplit模拟气团48 h的后向轨迹,并结合浓度权重轨迹方法(CWT),计算得到了影响北京市各季节SNA的潜在源区分布.结果表明,春、夏季颗粒物中硫酸盐、硝酸盐和铵盐混合比例较秋、冬季更加稳定.硫酸盐和硝酸盐的粒径分布特征十分相似,表明硫酸盐和硝酸盐成分在颗粒物中大多同时存在.颗粒物中SNA粒径分布的季节变化特征为:秋季SNA最大粒径段夏季春季冬季.SNA潜在源区分布有较为相似的空间分布特征,对北京市SNA高贡献的潜在源区主要分布在北京本地以及南部区域,以天津、廊坊、衡水、保定、石家庄等地为主.     

Second-generation inorganic aerosol model

Accurate prediction of the size distribution of the inorganic components of atmospheric aerosols must account for both therthermodynamic properties of the aerosol particles and transport between the gas and aerosol phases. For volatile inorganic species the transport rate is governed by the particle surface partial pressures which, in turn, is determined by the phase state and composition of the aerosol. We develop a model of the temporal composition of atmospheric aerosol particles based on their transport and thermodynamic properties. Included in the model is an improved theory of the temperature and composition dependence of deliquescence. Components of the model are tested against measurements of activity coefficients in single- and multicomponent aqueous solutions and general agreement is found. Aerosol water predictions are significantly higher under conditions of low relative humidity due to the improved theory of deliquescence.     

浙江金华秋季干气溶胶中主要化学组分的消光贡献解析

造成雾霾事件的主要原因是高浓度的大气细颗粒物污染.为了深入研究大气细颗粒物的消光来源,本研究采用高时间分辨率气溶胶观测仪器获得了浙江金华秋季PM1主要化学组分浓度及干气溶胶吸收系数和散射系数演变情况.结合有机气溶胶正矩阵因子解析模型(PMF)和多元线性回归方法,建立了拟合优度很高(R2=0.977)的细颗粒物中主要化学组分与干气溶胶消光系数间的定量关系模型.结果表明,观测期间消光贡献最大的是硫酸铵,贡献率为35.1%;其次是硝酸铵,贡献率为26.7%;二次有机气溶胶(SOA)、生物质燃烧有机气溶胶(BBOA)、黑碳(BC)及氯化铵的消光贡献率分别为14.3%、11.2%、8.7%、4.0%.在一些特定污染时段,BBOA具有最大的消光贡献,是导致此时大气能见度大幅度衰减的首要因子.     

Kinetic and thermodynamic behaviour of volatile ammonium compounds in industrial and marine atmospheres

Measurements of volatile ammonium aerosol compounds and respective acidic and basic gaseous precursors were taken at an industrial site emitting HCl, HNO₃ and NH₃ and at a marine suburban location.Because of fresh industrial emissions the gaseous acid and base concentration products show values orders of magnitude higher than thermodynamic equilibrium previsions, at the industrial site. To our knowledge these are the first published field measurements that strongly confirm that gas-particle equilibrium conditions are not attained instantaneously in the atmosphere and that kinetic constraints play an important role on gas-particle conversion for ammonium chloride and ammonium nitrate compounds. The results permit to conclude that, for temperature conditions lower than 15°C, transformation rates are not high, several minutes being necessary, at least, for the equilibrium between gaseous precursors and ammonium particulate products to be reached.At the marine suburban location a diurnal variation was observed for gaseous and particulate compounds in agreement with known emission and transformation mechanisms. The behaviour of nitric and hydrochloric acids in the marine atmosphere can be better explained if kinetic limitations are considered for the reaction of HNO₃ with NaCl aerosol particles and for the transference of acids and ammonia to the particulate phase.     

Nature of ammonium containing particles in an urban site of Germany

Investigations of chemical properties of atmospheric aerosol particles were performed. Application of the spot techniques to individual ammonium containing particles in the urban site of Karlsruhe revealed the following results: (NH₄)₂SO₄ particles dominate the sub-μm size range, only a few particles of (NH₄)₃H(SO₄)₂ or NH₄HSO₄ were detected. Mixed sulfates and nitrates of ammonium and some particles of calcium/ammonium salts were found in the μm-size range. Reaction spots containing particles in the characteristic form of alkali nitrate were found only during a smog day in January.     

南京冬季大气气溶胶粒子谱分布及其对能见度的影响

2009年11～12月采用宽范围气溶胶粒径谱仪（WPS）、自动气象站和能见度仪等高分辨率仪器对南京北郊气溶胶粒子的谱分布特征与气象因子的关系及其对大气能见度的影响进行研究.结果表明,数浓度谱呈双峰分布,主峰集中在0.04～0.1μm;质量浓度谱的2个主峰位于0.5～0.7μm和2.7μm左右;表面积谱的2个主峰分别位于...     

Characteristics of secondary inorganic aerosol and sulfate species in size-fractionated aerosol particles in Shanghai

Sulfate, nitrate and ammonium(SNA) are the dominant species in secondary inorganic aerosol, and are considered an important factor in regional haze formation. Size-fractionated aerosol particles for a whole year were collected to study the size distribution of SNA as well as their chemical species in Shanghai. SNA mainly accumulated in fine particles and the highest average ratio of SNA to particulate matter(PM) was observed to be 47% in the fine size fraction(0.49–0.95 μm). Higher sulfur oxidation ratio and nitrogen oxidation ratio values were observed in PM of fine size less than 0.95 μm. Ion balance calculations indicated that more secondary sulfate and nitrate would be generated in PM of fine size(0.49–0.95 μm). Sulfur K-edge X-ray absorption near-edge structure(XANES) spectra of typical samples were analyzed. Results revealed that sulfur mainly existed as sulfate with a proportion(atomic basis) more than 73% in all size of PM and even higher at 90% in fine particles. Sulfate mainly existed as(NH4)2SO4 and gypsum in PM of Shanghai. Compared to non-haze days, a dramatic increase of(NH4)2SO4 content was found in fine particles on haze days only, which suggested the promoting impact of(NH4)2SO4 on haze formation. According to the result of air mass backward trajectory analysis, more(NH4)2SO4 would be generated during the periods of air mass stagnation. Based on XANES, analysis of sulfate species in size-fractionated aerosol particles can be an effective way to evaluate the impact of sulfate aerosols on regional haze formation.     

Second organic aerosol formation from the ozonolysis of α-pinene in the presence of dry submicron ammonium sulfate aerosol

An indoor chamber facility is described for investigation of atmospheric aerosol chemistry. Two sets of α-pinene ozonolysis experiments were conducted in the presence of dry ammonium sulfate seed particle: ozone limited experiments and α-pinene limited experiments. The concentration of gas phase and particle phase species was monitored continuously by on-line instruments and recorded automatically by data sampling system. The evolution of size distribution was measured by a scanning mobility particle sizer ...     

Second organic aerosol formation from the ozonolysis of -pinene in the presence of dry submicron ammonium sulfate aerosol

An indoor chamber facility is described for investigation of atmospheric aerosol chemistry. Two sets of α-pinene ozonolysisexperiments were conducted in the presence of dry ammonium sulfate seed particle: ozone limited experiments and α-pinene limitedexperiments. The concentration of gas phase and particle phase species was monitored continuously by on-line instruments andrecorded automatically by data sampling system. The evolution of size distribution was measured by a scanning mobility particlesizer (SMPS), and α-pinene consumed was measured using GC-FID. Secondary organic aerosol (SOA) produced for seed-free systemis 100% organic in content, resulting from a sufficient supersaturation of low volatility organics to produce homogeneous nucleationfollowed by condensation to the aerosol. Secondary organic aerosol produced in seeded system is a mixture of organic and inorganicconstituents, initially forms via condensation onto the inorganic particles, and subsequent growth occurs via absorption into the organicsurface coating the inorganic core. Although the formation process and the size distribution for seed-free system and seeded system isdifferent, the ultimate mass of SOA formed is equal, and SOA yield for the two system located in the same regression line when usingone-product model, suggesting that the presence of dry ammonium sulfate seed has no measurable effect on the total aerosol yield, and the dry seed particle acts solely as a site upon which organic deposition occurs.     

水平表面上多机理作用下气溶胶沉积特性数值分析

采用离散随机模型模拟了水平表面上气溶胶粒子沉积的微观过程.基于随机游走方法,求解了微粒在布朗扩散和外场迁移两种输运机制耦合作用下的运动轨迹,分析了沉积物形结构特征与微粒输运机制间的作用关系,并讨论了不同沉积机制和粒径分布下的表面沉积水平.结果表明,低Pe数下沉积物形态呈现疏松的粉尘絮团结构,而在高Pe数下沉积物则表现出较为紧密的堆积结构.不同沉积机制下粒子沉积形态结构的差异取决于微粒运动轨迹的随机强度;水平表面最大沉积微粒数存在一上限值,且与微粒输运机制和粒径分布均有关.     

Atmospheric light absorption—A review

The atmosphere interacts both with incoming as well as outgoing light. Two main processes take place: light scattering and light absorption. Whereas light scattering redistributes any ligh energy in the atmosphere, light absorption converts the light energy to internal energy of the absorbing molecules and eventually transfers it to the surrounding gas as heat.Atmospheric gases absorb light in distinct spectral regions usually at more or less broad bands. Best known is the broad absorption of ozone in the far u.v., being essential for the existence of the biological macromolecules on Earth. Gases known as greenhouse gases, e.g. CO₂, CH₄, N₂O and water vapor absorb a wide range of infrared radiation, and thus are responsible for the greenhouse effects. Since the lifetime of these gases (except water vapor) is considerable, their distribution around the globe is fairly homogeneous.The atmospheric aerosol gives the major contribution to the atmospheric light absorption in the visible and near u.v. and near i.r. Graphitic (black) carbon, the main constituent of soot, is almost exclusively responsible for the light absorption of the particles. The light absorption by aerosols is continuous and covers the whole visible spectral range. It only depends slightly on wavelength.The optical properties of elemental carbon are determined by the size of the particles and their complex refractive index. A variety of refractive indices can be found in the literature for elemental carbon, most likely caused by different production and thus composition of the particles. Soot particles are very efficient in attenuating light; for the average size the particles have more than twice the mass extinction coefficient compared to transparent particles such as ammonium sulfate. The light absorption coefficient of a mixture of elemental carbon and transparent materials is higher for internal than for external mixtures. When incorporated into transparent particles, the absorption properties of elemental carbon can be multiplied and the single scattering albedo will decrease in comparison to an external mixture of the same materials.There are different methods to measure the light absorption coefficient of suspended particles. They can be separated in three groups, depending on the effect or methodology they use, but no standard procedure has been adopted so far.Soot is produced by all combusttion processes. Since most fires on Earth are due to humans, then indirectly humans are the major source of light-absorbing aerosol particles. On a global scale black carbon amounts to 1.1–2.5% of the anthropogenic particles and to 0.2–1% of the total emitted particles. The emission factors for elemental carbon are highest for small sources such as diesel motors or fireplaces.The light-absorbing aerosol consists of fine particles, with most particles having diameters less than a few tenths of a micrometer. Particles in the size range of soot particles have an average lifetime of 7 days in the atmosphere, therefore they can be transported over large distances and are also found in remote regions. Since light-absorbing particles have a variety of sources and sinks and they are involved in precipitation cycles, their distribution on the globe is inhomogeneous. Light-absorption coefficients of the atmospheric aerosol reported in the literature differ by more than four orders of magnitudes at different locations, but nevertheless black carbon particles have been found even at very remote areas, such as the South Pole.Although light-absorbing particles are a minority component in the atmospheric aerosol, their effects cannot be neglected: since the mass extinction coefficient of soot is higher by a factor of two to three compared to transparent particles, light-absorbing substances in the atmosphere can cause at some locations up to half of the visibility reduction; light-absorbing substances in the atmosphere can be responsible for the brown appearance of urban hazes and the discoloration of the sky.The light absorption of the atmosphere in the visible (which mainly is due to particulate matter) has to be taken into account when considering radiative properties and climatic consequences. A small temperature increase due to absorption in the visible is to be expected. The value is around a few tenths of a Kelvin, but no general statement on its magnitude is possible, since a large spatial and temporal variation exists and other factors like surface albedo, the optical depth of the aerosol, its incorporation in clouds and humidity growth of the aerosol have to be considered.     

Size distribution and chemical composition of secondary organic aerosol formed from Cl-initiated oxidation of toluene

Secondary organic aerosol (SOA) formed from Cl-initiated oxidation of toluene was investigated in a home-made smog chamber. The size distribution and chemical composition of SOA particles were measured using aerodynamic particle sizer spectrometer and the aerosol laser time-of-flight mass spectrometer (ALTOFMS), respectively. According to a large number of single aerosol diameter and mass spectra, the size distribution and chemical composition of SOA were obtained statistically. Experimental results showed that SOA particles created by Cl-initiated oxidation of toluene is predominantly in the form of fine particles, which have diameters less than 2.5 m (i.e., PM2.5), and glyoxal, benzaldehyde, benzyl alcohol, benzoquinone, benzoic acid, benzyl hydroperoxide and benzyl methyl nitrate are the major products components in the SOA. The possible reaction mechanisms leading to these products are also proposed.     

Kinetics of evaporation of ammonium chloride and ammonium nitrate aerosols

The evaporation rates of ammonium chloride and ammonium nitrate were measured by continuously and rapidly removing gaseous NH₃ and HNO₃ or HCl from aerosols in an annular denuder. The experiments gave the evaporation rates in terms of mass loss of chloride or nitrate which can be expressed conveniently as the rates of reduction of aerosol radius with time. Both dry aerosols (humidity 30–60% r.h.) and aqueous aerosols (humidity ca 97% r.h.) were studied. Dry aerosols evaporate at rates of −1.05 Å s⁻¹ for NH₄Cl and −0.45Å s⁻¹ for NH₄NO₃, while the evaporation rates of aqueous aerosols expressed as for equivalent dry particles are −4.52 Å s⁻¹ for NH₄Cl and −0.49 Å s⁻¹ for NH₄NO₃. The experimentally measured rates are independent of particle radius and remarkably low compared with those predicted from existing theories of aerosol evaporation, thus implying that there is an unknown kinetic constraint to the achievement of equilibrium at atmospheric temperature and pressures.     

Field measurements of dissociation of ammonium nitrate at a Beijing site

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Field measurements of dissociation of ammonium nitrate at a Beijing site

The atmospheric nitric acid, ammonia and ammonium nitrate aerosol was determined simultaneously in a Beijing site to teste quilibrium. Rather good agreement between measurements and theory was found at ambient temperature above 9oC at relative humidity below 70%. Below 9oC the kinetic constraints preventing rapid attainment equilibrium were observed. A procedure for calculation of dependence of NH4NO3 aerosol dissociation constants on temperature and relative humidity was given. The seasonal variation of concentration of NH3, total NH4+ and HNO3 was observed.The seasonal variation of concentration of HNO3 was caused by dissociation of aerosol of NH4NO3. The concentrations of NH3 were 1-2 order higher than those of HNO3. For formation of NH4NO3 aerosol the HNO3 was the control reagent. Any increase of HNO3 to formed from NOx would react with NH3 form NH4NO3 aerosol in Beijing area except for someday in summer time.