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.     

Intercomparison of photoacoustic and thermal-optical methods for the measurement of atmospheric elemental carbon

Two atmospheric elemental carbon measurement methods based on different analytical principles have been compared using data collected during the summer of 1987 at the Claremont, California site of the Southern California Air Quality Study (SCAQS). An optical absorption method, photoacoustic spectroscopy, measured the visible light absorption (γ = 514.5 nm) of atmospheric elemental carbon in its natural aerosol-state in real time. Elemental carbon concentrations were obtained by applying the appropriate value of the absorption cross-section for elemental carbon to the optical absorption data. The other method was a thermal-optical technique, which measures elemental and organic carbon concentrations on a filte--collected sample by combustion and corrects for the pyrolytic conversion of organic to elemental carbon by measuring the transmittance of laser light through the sample. Aerosol was collected on a filter mounted inside the carbon analyzer and analyzed in place. A 2-h collection and analysis cycle was used. The real-time photoacoustic data integrated over ∼100 min were compared with thermal-optical data. The two methods compare quite well. The linear least squares fit gave a correlation coefficient of R=0.905, and no significant difference was seen between the two data sets at the 95% confidence level.     

中国秸杆焚烧及民用燃煤棕色碳排放的初步研究

近年来的研究表明,在BC(黑碳)和OC(有机碳)之间,还存在着一种有弱吸光能力的OC,因大多显棕黄色而被称为BrC(brown carbon,棕色碳). 广泛存在的秸杆焚烧和冬季大量民用燃煤的使用,使国内BrC排放严重,但鲜见对其排放量的测算. 采用七波段黑碳仪(aethalometer)方法,对夏季小麦秸杆焚烧过程及冬季民用炉燃煤过程产生的烟气进行现场监测,根据BrC与BC的光谱关联性差别,分化出R_BrC/BC(总光学衰减中BrC和BC的相对贡献). 结果表明:麦秆焚烧和民用燃煤烟气的R_BrC/BC分别为1.754±0.278和0.183±0.142. 借助R_BrC/BC值,结合现有的BC排放清单(2000年),初步推算出中国民用燃煤和秸杆田间焚烧BrC的排放总量(以BC当量计,下同)为(270.6±101.6)Gg,接近同期BC排放量的一半;其中秸杆焚烧的BrC排放量为(175.4±27.8)Gg,约占二者总量的65%;民用燃煤的BrC排放量为(95.2±73.7)Gg,约占35%. 该研究结果可为更全面的BrC排放测算奠定基础,并为研究BrC的大气化学及辐射强迫提供依据.      

水稻对气态单质汞的吸收与挥发

湿地是汞的重要富集场所,其还原性底质会促进气态单质汞的产生.湿地植物已进化出一系列适应淹水环境的生态特征,其中发达的通气组织可以从大气向根部输送氧气满足根系的呼吸作用.以水稻这一典型的湿地植物为例,研究其根系及通气组织是否也能成为根际气态汞向大气运输的通道,并探究其过程和影响因素.通过设计密闭分室培养装置将水稻根部与地上部分隔开,对水稻根部进行汞蒸气暴露,并通过气体吸收剂吸收由叶片挥发出的气态汞.结果表明,水稻可以从根系吸收气态单质汞并向地上组织迁移,其中根系的汞含量与水稻根孔隙度呈显著负相关,并呈二次拟合(R=0.8309,P0.01);而水稻地上部汞迁移量与水稻根表面积和根体积均呈线性正相关(R=0.896,P0.01;R=0.871,P0.01).由根系吸收的汞最终可通过水稻叶片挥发进入大气,叶片汞的挥发量随着叶片面积的增加呈线性增加(R=0.897,P0.01),单位叶面积汞挥发量与水稻蒸腾作用强度呈线性显著正相关(R=0.73,P0.01).证明了水稻根系不仅能吸收气态单质汞,并且能将汞从根部转运到地上部,再通过叶片的气孔释放到空气中.为进一步揭示湿地生态系统中汞的迁移及调控机制提供了科学依据.     

氯元素对烟气中汞的形态和分布的影响

采用化学热力平衡分析方法研究了在煤燃烧在气化过程中产生的瀵气里痕量元素汞的形态及分布，在一个大气压下，400K－2000K温度范围里，研究了汞－煤系统和汞－煤－氯系统中汞在还原性气氛和氧化性气氛的烟气中的化学形态和分布，着重探讨了煤中的氯元素对汞在烟气中的形态和分布的影响，化学热力平衡分析结果表明，在煤燃烧和气化的最高温度区域里，单质汞是示是主要形式，少量的氯元素可以大大地增强汞元素的蒸发;在气化的还原性气氛烟气中，汞的主要形式是单质汞，在氧化性气氛的燃煤烟气中随着在烟气中温度的降低，单质汞将发生化学反应而生成氯化汞；烟气中氯元素的含量越大，氯化汞作为稳定相的温度范围越宽。     

Measurement of atmospheric elemental carbon: Real-time data for Los Angeles during summer 1987

Two fundamentally different techniques for measuring atmospheric elemental carbon (EC) aerosol were compared to validate the methods. One technique, photoacoustic spectroscopy, was used to measure the optical absorption (λ = 514.5 nm) of in situ atmospheric aerosol in real time. This optical absorption can be converted to EC concentration using the appropriate value of the absorption cross-section for C, so that a comparison could be made with the second technique, thermal-optical analysis of filter-collected samples, which measures the collected EC by combustion. Solvent extraction of the filter samples prior to the thermal analysis procedure was required to minimize errors due to pyrolysis of organic carbon. Excellent 1:1 correlation of atmospheric EC concentrations resulted for measurements by the photoacoustic method vs the thermal method over coincident sampling times. The linear regression gave y = 1.006 (±0.056) x+0.27 (±0.56) with r = 0.945 (n = 41), where y is the photoacoustic EC concentration and x is the thermal elemental carbon concentration, both in μg m⁻³. This data set was collected in Los Angeles as part of the Southern California Air Quality Study (SCAQS) during the summer 1987, and supplements the results of an earlier, more limited data set taken in Dearborn, MI. The diurnal variability of EC aerosol in Los Angeles during SCAQS, as determined by photoacoustic spectroscopy, is discussed.     

Influence of fuel moisture, charge size, feeding rate and air ventilation conditions on the emissions of PM, OC, EC, parent PAHs, and their derivatives from residential wood combustion

Controlled combustion experiments were conducted to investigate the influence of fuel charge size, moisture, air ventilation and feeding rate on the emission factors (EFs) of carbonaceous particulate matter, parent polycyclic aromatic hydrocarbons (pPAHs) and their derivatives from residential wood combustion in a typical brick cooking stove. Measured EFs were found to be independent of fuel charge size, but increased with increasing fuel moisture. Pollution emissions from the normal burning under an adequate air supply condition were the lowest for most pollutants, while more pollutants were emitted when an oxygen deficient atmosphere was formed in the stove chamber during fast burning. The impacts of these factors on the size distribution of emitted particles was also studied. Modified combustion efficiency and the four investigated factors explained 68%, 72%, and 64% of total variations in EFs of PM, organic carbon, and oxygenated PAHs, respectively, but only 36%, 38% and 42% of the total variations in EFs of elemental carbon, pPAHs and nitro-PAHs, respectively.     

Significant emission reductions of carbonaceous aerosols from residential coal burning by a novel stove

Carbonaceous aerosols (CA) are crucial components in the atmospheric PM_2.5 and derived from diverse sources. One of the major sources for CA is from the incomplete combustion of bituminous coal that has been prevailingly used by household stoves in rural areas for heating during winter. To efficiently eliminate the CA emission, a new household stove (NHS) was developed based on a novel combustion technology and CA emissions from the NHS and a traditional household stove (THS) were comparably investigated under the actual stove operation conditions in a farmer's house. Compared with the THS, the emission factors of organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) from the NHS were reduced by 96%±1%, 98%±1%, and 91%±1% under the flaming process and 95%±1%, 96%±2%, and 83%±4% under the smoldering process, respectively. Additionally, the mass absorption efficiency of WSOC from the NHS reduced by 3 folds and the radiative forcing by WSOC relative to EC shrank remarkably by a factor of 3-8. Based on the reduction of emissions and light absorption of WSOC, the promotion of the NHS offers a possible solution to achieve the clean combustion of residential solid fuel.     

北京大气PM2.5中铅的同位素测定和来源研究

用石英滤膜连续采集北京2003-02～2003-03的12个PM_2.5样品,进行无机多元素、OC、EC和铅同位素丰度比的分析测定.对样品进行聚类分析,结合气象信息讨论了分组结果,描述了2003年春北京市的2个大气铅污染过程.2个污染过程均表现出铅含量高,²⁰⁶Pb/²⁰⁷Pb丰度比低的特点.Al、Ca、K等地壳元素的含量与Pb的含量负相关,表明土壤尘对铅的贡献不大.比较Pb、As、Zn、Se和Al、Ca等地壳元素含量的相关性,2个铅污染过程是由有色冶金排放和燃煤排放共同主导的.而02-28～03-02的铅污染过程中,燃煤排放贡献增大,有色冶金排放贡献下降.结果表明,汽油无铅化以后,北京市春季大气颗粒物中的铅主要来源于有色冶金排放与燃煤排放.     

大气棕色碳的研究进展与方向

有机气溶胶是大气气溶胶的重要组成部分.近年来,部分有机气溶胶被证实在紫外-近可见光波段能进行有效光吸收.吸光有机气溶胶(即棕色碳)已成为当前国际大气环境领域的研究热点之一,其吸光贡献对辐射强迫、区域空气质量、全球气候变化的影响备受关注.中国城市群区域大气复合污染严重,2013年1月以来,大尺度区域能见度降低、灰霾现象频繁发生.研究表明,中国大气棕色碳的负荷高,其重要贡献源类如生物质与化石燃料燃烧等排放量大,棕色碳在中国大气细颗粒物总消光中的贡献及其辐射强迫亟需评估.然而,棕色碳的基础研究还较为薄弱,特别是对它的组成及来源的认识仍十分有限.本文旨在指出在中国开展大气棕色碳研究的紧迫性和必要性,并从棕色碳的来源、组成、测量方法、浓度分布、光学特性、辐射强迫贡献等角度介绍目前国际研究进展,提出现有问题与不足,以及对未来研究方向的建议,以促进对棕色碳的认识与了解,为中国在该领域的研究提供参考和依据.     

Chemical composition, mass closure and sources of atmospheric PM10 from industrial sites in Shenzhen, China

Concentrations of atmospheric PM10 and chemical components (including twenty-one elements, nine ions, organic carbon (OC) and elemental carbon (EC)) were measured at five sites in a heavily industrial region of Shenzhen, China in 2005. Results showed that PM10 concentrations exhibited the highest values at 264 μg/m3 at the site near a harbor with the influence of harbor activities. Sulfur exhibited the highest concentrations (from 2419 to 3995 ng/m3) of all the studied elements, which may be related to the influence of coal used as fuel in this area for industrial plants. This was verified by the high mass percentages of SO42-, which accounted for 34.3%-39.7% of the total ions. NO3-/SO42- ratios varied from 0.64-0.71, which implies coal combustion was predominant compared with vehicle emission. The anion/cation ratios range was close to 0.95, indicating anion deficiency in this region. The harbor site showed the highest OC and EC concentrations, with the influence of emission from vessels. Secondary organic carbon accounted for about 22.6%-38.7% of OC, with the highest percentage occurring at the site adjacent to a coal-fired power plant and wood plant. The mass closure model performed well in this heavily industrial region, with significant correlation obtained between chemically determined and gravimetrically measured PM10 mass. The main constituents of PM10 were found to be organic materials (30.9%-69.5%), followed by secondary inorganic aerosol (7.9%-25.0%), crustal materials (6.7%-13.8%), elemental carbon (3.5%-10.8%), sea salt (2.4%-6.2%) and trace elements (2.0%-4.9%) in this heavily industrialized region. Principal component analysis indicated that the main sources for particulate matter in this industrial region were crustal materials and coal/wood combustion, oil combustion, secondary aerosols, industrial processes and vehicle emission.     

珠江三角洲表层水中多环芳烃的季节分布、来源和原位分配

分别于2011年4月(春季)和2011年9月(夏季)采集珠江广州河段及东江东莞河段表层水体样品,对该区域表层水体中优控多环芳烃(PAHs)的时空分布、固液分配及其来源进行了分析和讨论.结果表明,珠江广州河段及东江东莞河段表层水体中多环芳烃浓度春季高于夏季.藻类有机碳是该水环境有机碳的主要成分.溶解有机碳(DOC)、颗粒态有机碳(POC)以及叶绿素a(Chl a)含量是控制水体PAHs浓度的主要因素,说明水环境的富营养化程度可以通过增长的浮游生物量来影响多环芳烃的生物地球化学过程,继而影响其环境行为和归宿.多环芳烃在水/颗粒物间的有机碳归一化分配系数(log Koc)与辛醇/水分配系数(log Kow)间存在明显的线性关系,其斜率是夏季大于春季,可能与多环芳烃的非平衡吸附有关.多环芳烃同系物比值法和主成分分析(PCA)的结果表明,研究区域水体中PAHs主要来源于石化燃料、煤和生物质的混合燃烧,并且PAHs的来源未体现出明显的季节变化.通过本研究我们能够比较全面的了解该流域多环芳烃的时空分布状况,固液分配及其可能的来源,并且为珠江广州河段及东江东莞河段多环芳烃污染的控制和生态风险评价提供科学依据.     

Anthropogenic contributions to the carbonaceous content of aerosols over the Pacific Ocean

During a joint U.S./U.S.S.R. research cruise from 3 May to 27 July 1987 both total and fine (< 2 μm) particulate material was sampled. This cruise started in the Hawaiian Islands and then proceeded to the Kamchatka peninsula, south to Wellington, New Zealand, south of Australia into the Indian Ocean, to Singapore and then returned to the Hawaiian Islands by sailing just north of the Equator. Particulate samples, collected on quartz fiber and Teflon filters, were analysed for organic and elemental carbon by transmission thermo-optical carbon analysis and for trace elements by energy dispersive X-ray fluorescence.In the Northern Hemisphere the total particulate, organic carbon and elemental carbon concentration ranges were, respectively, 10–25, 0.5–2.5 and 0–0.3 μg m⁻³. In the Southern Hemisphere they were, respectively, 5–10, <0.6 and <0.02 μg m⁻³. In the Northern Hemisphere the fine particulate concentration range was 2–15 μg m⁻³ and the fine fraction varied from 20 to 80% of total aerosol loading. In the Southern Hemisphere the fine particulate loading was 1.2–1.7 μg m⁻³ and was usually less than 20% of the total particulate mass.Chemical mass balance (CMB) modeling was used to determine possible anthropogenic particulate contributions to the ocean aerosol. Readily available source profiles were used for CMB modeling. Sea-salt aerosol was represented by either the conventional EPA marine source profile or by the average of ambient ocean aerosols sampled in very clean mid-ocean regions. Usually 60–90% of ambient particulate mass was “explained” by the CMB model. Sources such as soil, catalytic auto emissions and wood-burning emissions were found to be possible contributors to the ocean aerosol, especially in the Northern Hemisphere. Anthropogenic contributions were estimated to contribute from 10 to 30% of oceanic aerosol mass. Emissions from a hugh forest fire that burned in northern China during the spring of 1987 were possibly detected, but the CMB model cannot distinguish this source from emissions from heating and cooking with wood.Since anthropogenic emissions are mainly combustion emissions which usually contain a large carbon component, carbon data is essential to CMB modeling. The relatively good CMB results obtained in this study suggest that it might be useful to develop source profiles for major emission sources in those countries which contribute most directly to oceanic aerosols. Perhaps characteristic national or regional source profiles could be developed.     

棕色碳气溶胶来源、性质、测量与排放估算

随着对碳气溶胶吸光性认识的提高,近年来吸光有机碳——BrC(brown carbon,棕色碳)的吸光问题成为继BC(black carbon,黑碳)之后国际大气环境领域的新热点. 基于已有的研究报道,将BrC大体分为焦油类物质、类腐殖质(HULIS)和其他吸光性有机气溶胶三大类,其来源包括一次排放和二次生成2种. 由于BrC缺少BC所具有的类石墨烯结构,致使颗粒间较为分散,加之含氧官能团比重较高,因而在水及有机溶剂中均有较强的可溶性. BrC的光学性质通常借助AAE(ngstrm吸收波长指数)、MAE(质量吸收效率)、RI(折射率)及SSA(单次散射反照率)来表示,其中由于BrC分子结构中缺少sp2杂化成分,形成了区别于BC的典型特征,即AAE>1(而对于BC,其AAE=1)). 虽然已有借助于光学法、热光法、化学法和质谱法进行BrC测定的报道,但目前没有公认的标准测定方法和参考物质,测定结果实际依赖于选定的测定方式. 在排放估算研究方面,BrC远落后于BC,致使有些排放估算方法多以相伴的BC排放量作为参照. 建议今后对BrC研究应主要面向气候影响、生成机理、测定方法、排放因子与控制策略等领域来展开.      

污泥与煤在CO2/O2及N2/O2气氛条件下的混燃特性分析

采集了广州具有代表性的市政污泥,利用热重法对单一污泥、煤及其混合样品在CO_2/O_2及N_2/O_2气氛条件下进行了TG(热重)实验,同时计算了污泥和煤的4个燃烧特性指数,获得了燃烧的动力学参数.实验结果表明,污泥燃烧有4个燃烧阶段,包括水分析出、挥发分析出与燃烧、固定碳的燃烧和无机盐类挥发分解,其中,挥发分析出与燃烧是污泥燃烧的主要阶段;煤掺烧污泥可以提高煤的着火性能.在CO_2/O_2及N_2/O_2燃烧气氛下,增加O_2浓度,污泥与煤混合样的热重曲线整体向低温区移动,微商热重曲线峰值增大,峰宽变小,燃烧性能增强.在CO_2/O_2气氛下,O_2浓度从30%增加到60%时,污泥的挥发分释放特性指数D、可燃性指数C、燃尽指数Cb、综合燃烧特性指数S分别增加了45%、12%、18%、6%.相同O_2浓度条件下,污泥与煤在CO_2/O_2气氛下的着火性能较N_2/O_2气氛滞后,最大峰值变小,高浓度的CO_2抑制其混合样的燃烧.采用Coats-Redfern方程计算得到3个燃烧阶段反应的动力学参数,其中,增加O_2浓度,污泥与煤混合样品的质量平均表观反应活化能Em减小,并且O_2浓度越高越有利于燃烧反应的进行.污泥与煤混合燃烧取n=2可以对挥发分燃烧与固定碳燃尽峰峰前与峰后的反应模型进行描述.     

Volatilization of heavy metals during incineration of municipal solid wastes

Incineration experiments with MSW, which had been impregnated with heavy metals, were presented to obtain information on the volatilization behavior of the elements cadmium (Cd), lead (Pb), and zinc (Zn) under different conditions. Experiments were carried out in a bubbling fluid bed system connected to a customized inductively coupled plasma optical emission spectroscopy(ICP-OES) for analyzing metals in the flue gas. The results indicated that the combustion temperature, the gas atmosphere, and the chlorine content in the flue gas could affect the volatilization behavior of heavy metals. In the fluidized bed combustion, a large surface area was provided by the bed sand particles, and they may act as absorbents for the gaseous ash-forming compound. Comparer with the metals Cd and Pb, the vaporization of Zn was low. The formation of stable compounds such as ZnO.Al2O3 could greatly decrease the metals volatilization. The presence of chlorine would enhance the volatilization of heavy metals by increasing the formation of metal chlorides. However, when the oxygen content was high, the chlorinating reaction was kinetically hindered, which heavy metals release would be delayed.     

黑碳研究进展

黑碳由生物质和化石燃料不完全燃烧所产生的具有高度热稳定性的含碳物质,广泛存在于土壤、沉积物、大气乃至极地和高山的冰雪中。工业革命以来,人类活动促使大量的黑碳进入到环境中,导致全球黑碳的背景值显著升高。土壤和沉积物中的黑碳有着长时间尺度(约10000yr)的降解周期,在全球碳循环过程中起着部分"汇"的作用。大气和冰雪中的黑碳通过"辐射强迫"作用吸收太阳辐射,促进全球变暖进而影响全球气候变化。目前主要有光学(optical)、热氧化(TO)、化学氧化(CTO)等方法用于黑碳的分离和测试。大气中的黑碳研究一般采用光学法,而土壤和沉积物中的黑碳研究则主要采用化学氧化法。本文全面总结了黑碳在环境中的研究进展,并指出黑碳对全球变暖的贡献是今后黑碳的重要研究方向。     

Comparison of measurements of aerosol optical absorption by filter collection and a transmissometric method

Light-absorbing aerosol particles are present in the atmosphere all over the world, and they play an important role in the energy transfer to the atmosphere. The main substance causing light absorption in the atmosphere is graphitic (black) carbon, the major constituent of soot. It is generated at any kind of combustion, and in source regions soot particles are mostly mixed externally with other particles. After some time the soot particles may become attached to other particles, thus becoming mixed internally.Many methods for the determination of the light absorption by particles exists. The integrating plate technique offers a fast and reproducible way to determine light absorption coefficients of aerosol particles deposited on filters. Comparisons of this method with other determinations of light absorption coefficients have given partially agreement and partially disagreement. Therefore a comparison with an absolute method has been performed in order to determine possible inaccuracies.An externally mixed aerosol was made by independently generating a light-absorbing aerosol and a non-light-absorbing aerosol and mixing them such that within the measuring period coagulation was negligible. The light-absorbing aerosol was generated by a spark discharge between carbon electrodes. The sizes of the particles were small, usually more than 90% of the particle mass was below 50 nm. The non-absorbing was generated by a constant output atomizer.For small carbon particles the light extinction is dominated by the light absorption, therefore a measurement of the light extinction coefficient in a long path photometer provides an absolute method to determine the absorption coefficient.A comparison with the absorption coefficient determined simultaneously with the integrating plate method permits the estimation of the error. For pure carbon the light absorption coefficient determined by the integrating plate method is approximately 20% higher than the “true” value. For mixed aerosols the deviation depends on the mixing rate, but the integrating plate method always overestimates the light absorption coefficient.     

Progressively narrow the gap of PM2.5 pollution characteristics at urban and suburban sites in a megacity of Sichuan Basin, China

Nowadays, the fine particle pollution is still severe in some megacities of China, especially in the Sichuan Basin, southwestern China. In order to understand the causes, sources, and impacts of fine particles, we collected PM_2.5 samples and analyzed their chemical composition in typical months from July 2018 to May 2019 at an urban and a suburban (background) site of Chengdu, a megacity in this region. The daily average concentrations of PM_2.5 ranged from 5.6-102.3 µg/m³ and 4.3-110.4 µg/m³ at each site. Secondary inorganics and organic matters were the major components in PM_2.5 at both sites. The proportion of nitrate in PM_2.5 has exceeded sulfate and become the primary inorganic component. SO₂ was easier to transform into sulfate in urban areas because of Mn-catalytic heterogeneous reactions. In contrast, NO₂ was easily converted in suburbs with high aerosol water content. Furthermore, organic carbon in urban was much greater than that in rural, other than elemental carbon. Element Cr and As were the key cancer risk drivers. The main sources of PM_2.5 in urban and suburban areas were all secondary aerosols (42.9%, 32.1%), combustion (16.0%, 25.2%) and vehicle emission (15.2%, 19.2%). From clean period to pollution period, the contributions from combustion and secondary aerosols increased markedly. In addition to tightening vehicle controls, urban areas need to restrict emissions from steel smelters, and suburbs need to minimize coal and biomass combustion in autumn and winter.     

Carbonaceous particles in the atmosphere and precipitation of the Nam Co region, central Tibet

A continuous air and precipitation sampling for carbonaceous particles was conducted in a field observatory beside Nam Co, Central Tibetan Plateau during July of 2006 through January of 2007. Organic carbon (OC) was the dominant composition of the carbonaceous particles both in the atmosphere (1660 ng/m³) and precipitation (476 ng/g) in this area, while the average elemental carbon (BC) concentrations in the atmosphere and precipitation were only 82 ng/m³ and 8 ng/g, respectively. Very high OC/BC ratio suggested local secondary organic carbon could be a dominant contribution to OC over the Nam Co region, while BC could be mainly originated from Southern Asia, as indicated by trajectory analysis and aerosol optical depth. Comparison between the BC concentrations measured in Lhasa, those at “Nepal Climate Observatory at Pyramid (NCO-P)” site on the southern slope of the Himalayas, and Nam Co suggested BC in the Nam Co region reflected a background with weak anthropogenic disturbances and the emissions from Lhasa might have little impact on the atmospheric environment here, while the pollutants from the Indo-Gangetic Basin of Southern Asia could be transported to the Nam Co region by both the summer monsoon and the westerly.