On the effect of the chemical composition of atmospheric aerosol particles on nucleation scavenging and the formation of a cloud interstitial aerosol

Nucleation scavenging and the formation of a cloud interstitial aerosol (CIA) were theoretically studied in terms of the chemical composition of atmospheric aerosol particles. For this study, we used our air-parcel cloud model, which includes the entrainment of air and detailed microphysics, for determining the growth and interaction of aerosol particles and drops. Maritime and remote continental aerosol particle spectrums were used whose size distributions were superpositions of three log-normal distributions, each of a prescribed chemical composition. Our results show (1) that the CIA exhibits a size distribution with a distinctive cut-off at a specific radius of the dry as well as of the wet particle size distribution. All particles above this limiting size become activated to cloud drops and, thus, are not present in the CIA spectrum. This limiting size was found to be independent of the chemical composition of the particles and only dependent on the prevailing supersaturation. Below this specific size, the CIA spectrum becomes depleted of dry aerosol particles in a manner which does depend on their chemical composition and on the supersaturation in the air. (2) The number of aerosol particles nucleated to cloud drops depends critically on the chemical composition of the particles and on the prevailing supersaturation.     

Microphysical, chemical and optical aerosol properties in the Baltic Sea region

The microphysical structure, chemical composition and prehistory of aerosol are related to the aerosol optical properties and radiative effect in the UV spectral range. The aim of this work is the statistical mapping of typical aerosol scenarios and adjustment of regional aerosol parameters. The investigation is based on the in situ measurements in Preila (55.55° N, 21.00° E), Lithuania, and the AERONET data from the Gustav Dalen Tower (58 N, 17 E), Sweden.Clustering of multiple characteristics enabled to distinguish three aerosol types for clear-sky periods: 1) clean maritime–continental aerosol; 2) moderately polluted maritime–continental aerosol; 3) polluted continental aerosol. Differences between these types are due to significant differences in aerosol number and volume concentration, effective radius of volume distribution, content of SO₄⁻ ions and Black Carbon, as well as different vertical profiles of atmospheric relative humidity. The UV extinction, aerosol optical depth (AOD) and the Ångstrom coefficient α increased with the increasing pollution. The value α = 1.96 was observed in the polluted continental aerosol that has passed over central and eastern Europe and southern Russia. Reduction of the clear-sky UV index against the aerosol-free atmosphere was of 4.5%, 27% and 41% for the aerosol types 1, 2 and 3, respectively.     

Impacts of aerosol chemical composition on microphysics and precipitation in deep convection

Aerosols affect precipitation by modifying cloud properties such as cloud droplet number concentration (CDNC). Aerosol effects on CDNC depend on aerosol properties such as number concentration, size spectrum, and chemical composition. This study focuses on the effects of aerosol chemical composition on CDNC and, thereby, precipitation in a mesoscale cloud ensemble (MCE) driven by deep convective clouds. The MCE was observed during the 1997 department of energy's Atmospheric Radiation Measurement (ARM) summer experiment. Double-moment microphysics with explicit nucleation parameterization, able to take into account those three properties of aerosols, is used to investigate the effects of aerosol chemical composition on CDNC and precipitation. The effects of aerosol chemical compositions are investigated for both soluble and insoluble substances in aerosol particles. The effects of soluble substances are examined by varying mass fractions of two representative soluble components of aerosols in the continental air mass: sulfate and organics. The increase in organics with decreasing sulfate lowers critical supersaturation (S_c) and leads to higher CDNC. Higher CDNC results in smaller autoconversion of cloud liquid to rain. This provides more abundant cloud liquid as a source of evaporative cooling, leading to more intense downdrafts, low-level convergence, and updrafts. The resultant stronger updrafts produce more condensation and thus precipitation, as compared to the case of 100% sulfate aerosols. The conventional assumption of sulfate aerosol as a surrogate for the whole aerosol mass can be inapplicable for the case with the strong sources of organics. The less precipitation is simulated when an insoluble substance replaces organics as compared to when it replaces sulfate. When the effects of organics on the surface tension of droplet and solution term in the Köhler curve are deactivated by the insoluble substance, S_c is raised more than when the effects of sulfate on the solution term are deactivated by the insoluble substance. This leads to lower CDNC and, thus, larger autoconversion of cloud liquid to rain, providing less abundant cloud liquid as a source of evaporative cooling. The resultant less evaporative cooling produces less intense downdrafts, weaker low-level convergence, updrafts, condensation and, thereby, less precipitation in the case where organics is replaced by the insoluble substance than in the case where sulfate is replaced by the insoluble substance. The variation of precipitation caused by the change in the mass fraction between the soluble and insoluble substances is larger than that caused by the change in the mass fraction between the soluble substances.     

A physically‐based algorithm for estimating the relationship between aerosol mass and cloud droplet number

In this study, we present a relationship between total accumulation mode aerosol mass concentrations and cloud droplet number concentrations ( N _d). The fundamental aim with the present method is to arrive at a physically‐based conversion algorithm in which each step in the conversion is based on real physical processes that occur and can be observed in the atmosphere, and in which all of the fields involved can be observed or modeled. In the last conversion (the critical part in the algorithm), we use measurements of the size distributions of cloud droplet residual particles for different pollution conditions. This conversion assumes that the size of the residual particles can be described with a lognormal distribution function and uses the Hatch–Choate relationship to convert between residual volume and number. The relatively sparse data set with which we have developed the present algorithm results in a course classification of the aerosol mass field. Consequently, uncertainties need to be recognized when using the algorithm in its present form in model calculations. The algorithm has been used on data from 15 days and the agreement between calculated and observed N _d values is, with one exception, within a factor of 2 and for many of these cases also much better than a factor of 2. In addition to the results of the algorithm itself, we also present a least‐squares fit to the predicted N _d values. To improve the algorithm in the longer‐term requires more data of scavenging fractions, particle chemical composition and density, and residual particle size distributions as a function of aerosol mass loading and cloud type.     

Size-resolved aerosol chemical concentrations at rural and urban sites in Central California, USA

Aerosol size distributions were measured with Micro Orifice Uniform Deposit Impactor (MOUDI) cascade impactors at the rural Angiola and urban Fresno Supersites in California's San Joaquin Valley during the California Regional PM₁₀/PM_2.5 Air Quality Study (CRPAQS) winter campaign from December 15, 2000 to February 3, 2001. PM_2.5 filter samples were collected concurrently at both sites with Sequential Filter Samplers (SFS). MOUDI nitrate (NO₃⁻) concentrations reached 66 μg/m³ on January 6, 2001 during the 1000–1600 PST (GMT-8) period. Pair-wise comparisons between PM_2.5 MOUDI and SFS concentrations revealed high correlations at the Angiola site (r > 0.93) but more variability (r < 0.85) at the Fresno site for NO₃⁻, sulfate (SO₄⁼), and ammonium (NH₄⁺). Correlations were higher at Fresno (r > 0.87) than at Angiola (r < 0.7) for organic carbon (OC), elemental carbon (EC), and total carbon (TC). NO₃⁻ and SO₄⁼ size distributions in Fresno were multi-modal and wider than the uni-modal distributions observed at Angiola. Geometric mean diameters (GMD) were smaller for OC and EC than for NO₃⁻ and SO₄⁼ at both sites. OC and EC were more concentrated on the lowest MOUDI stage (0.056 µm) at Angiola than at Fresno. The NO₃⁻ GMD increased from 0.97 to 1.02 µm as the NO₃⁻ concentration at Angiola increased from 43 to 66 µg m^− 3 during a PM_2.5 episode from January 4–7, 2001. There was a direct relationship between GMD and NO₃⁻ and SO₄⁼ concentrations at Angiola but no such relationships for OC or EC. This demonstrates that secondary aerosol formation increases both concentration and particle size for the rural California environment.     

Microphysical and chemical characteristics of cloud droplet residuals and interstitial particles in continental stratocumulus clouds

During June and July 2003 the Sources and Origins of Atmospheric Cloud Droplets experiment (SOACED) was carried out on a mountain-top site in central Sweden. The main objective of the experiment was to characterise the microphysical and chemical properties of cloud droplet residuals and interstitial aerosol particles in continental clouds and to understand the processes controlling cloud properties at this location.Interstitial and residual aerosol size distributions, cloud liquid water content and species- and size-resolved aerosol mass concentrations are the main variables employed to address questions pertaining to the cloud droplet number concentration and scavenging efficiency during a stratocumulus cloud event observed on July 28, 2003. In this cloud event, about 56% of the aerosol mass was associated with organic species, whilst SO₄ accounted for 23% and NH₄ for 14%. NO₃ and Cl made up about 7% of the total mass.The partitioning of the aerosol particles between cloud droplets and interstitial air has been studied in terms of their microphysical properties. The scavenging efficiency, defined as the fraction of particles activated into cloud elements compared to the total amount of particles, was investigated as a function of size. The scavenging efficiency curves displayed different shapes during the cloud event, from an S-shaped curve, with low scavenging efficiency in the Aitken mode and larger scavenging efficiency in the accumulation mode, to more unusual shapes where Aitken-mode particles were either solely activated or activated in addition to accumulation-mode particles.This study suggests that alterations of the aerosol chemical composition occurred during the measurement period, changing the hygroscopic nature of the CCN and decreasing their activation diameter. It is also hypothesized that entrainment of drier air aloft may have introduced inhomogeneities in the supersaturation field and modified the S-shaped scavenging curves.     

A relationship between liquid water content and chemical composition in clouds

In this paper we present two-year results from the Cloud Chemistry Measurements Programme, carried out at Mt. Brocken/Hart (Germany) to assess the influence of cloud physical parameters on the chemical composition of clouds. There are large variations in the chemical composition of cloud water. We found that the liquid water content of clouds predominantly determines the ionic content. The relationship is best approximated by a power function, nearly identical for all chemical species. We identified deviations of data points from the general relationship accompanying events with different air pollution situations. An important conclusion is that cloud chemistry monitoring in the sense of air pollution studies is meaningless without measurement of cloud physical parameters. These are liquid water content, cloud base altitudes, and to some extent, droplet size distribution.     

On the observability of chemical and physical aerosol properties by optical observations: Inverse modelling with variational data assimilation

Determining size-resolved chemical composition of aerosols is important for modelling the aerosols' direct and indirect climate impact, for source–receptor modelling, and for understanding adverse health effects of particulate pollutants. Obtaining this kind of information from optical remote sensing observations is an ill-posed inverse problem. It can be solved by variational data assimilation in conjunction with an aerosol transport model. One important question is how much information about the particles' physical and chemical properties is contained in the observations. We perform a numerical experiment to test the observability of size-dependent aerosol composition by remote sensing observations. An aerosol transport model is employed to produce a reference and a perturbed aerosol field. The perturbed field is taken as a proxy for a background estimate subject to uncertainties. The reference result represents the 'true' state of the system. Optical properties are computed from the reference results and are assimilated into the perturbed model. The assimilation results reveal that inverse modelling of optical observations significantly improves the background estimate. However, the optical observations alone do not contain sufficient information for producing a faithful retrieval of the size-resolved aerosol composition. The total mass mixing ratios, on the other hand, are retrieved with remarkable accuracy.     

A review of global stratospheric aerosol: Measurements, importance, life cycle, and local stratospheric aerosol

Stratospheric aerosol, noted after large volcanic eruptions since at least the late 1800s, were first measured in the late 1950s, with the modern continuous record beginning in the 1970s. Stratospheric aerosol, both volcanic and non-volcanic are sulfuric acid droplets with radii (concentrations) on the order of 0.1–0.5 µm (0.5–0.005 cm^− 3), increasing by factors of 2–4 (10–10³) after large volcanic eruptions. The source of the sulfur for the aerosol is either through direct injection from sulfur-rich volcanic eruptions, or from tropical injection of tropospheric air containing OCS, SO₂, and sulfate particles. The life cycle of non-volcanic stratospheric aerosol, consisting of photo-dissociation and oxidation of sulfur source gases, nucleation/condensation in the tropics, transport pole-ward and downward in the global planetary wave driven tropical pump, leads to a quasi steady state relative maximum in particle number concentration at around 20 km in the mid latitudes. Stratospheric aerosol have significant impacts on the Earth's radiation balance for several years following volcanic eruptions. Away from large eruptions, the direct radiation impact is small and well characterized; however, these particles also may play a role in the nucleation of near tropopause cirrus, and thus indirectly affect radiation. Stratospheric aerosol play a larger role in the chemical, particularly ozone, balance of the stratosphere. In the mid latitudes they interact with both nitrous oxides and chlorine reservoirs, thus indirectly affecting ozone. In the polar regions they provide condensation sites for polar stratospheric clouds which then provide the surfaces necessary to convert inactive to active chlorine leading to polar ozone loss. Until the mid 1990s the modern record has been dominated by three large sulfur-rich eruptions: Fuego (1974), El Chichón (1982) and Pinatubo (1991), thus definitive conclusions concerning the trend of non-volcanic stratospheric aerosol could only recently be made. Although anthropogenic emissions of SO₂ have changed somewhat over the past 30 years, the measurements during volcanically quiescent periods indicate no long term trend in non-volcanic stratospheric aerosol.     

Effects of inorganic seed aerosols on the growth and chemical composition of secondary organic aerosol formed from OH-initiated oxidation of toluene

(NH₄)₂SO₄, CaCl₂, Na₂SiO₃ and NaNO₃ were selected as surrogates of inorganic seed aerosols of ambient atmosphere of Chinese urban areas, respectively, to study their effects on the formation of secondary organic aerosol (SOA) in the toluene/CH₃ONO/NO_x photooxidation system. The SMPS and aerosol laser time-of-flight mass spectrometer (ALTOFMS) was used to measure the aerodynamic size and chemical composition of individual SOA particles in real-time. Experimental results indicate that either the growth or products of SOA is affected by the presence of inorganic seed aerosol. Inorganic seed aerosols would promote growth rates of SOA formation at the start of the reaction and inhibits its formation rate with prolonging the reaction time. In the case of about 100 μg m^?3 seed aerosol load, the addition of Na₂SiO₃ induced a same growth rate of SOA formation as NaNO₃. The influence of four individual seed aerosols on the generation of SOA decreased in the order of CaCl₂ > (NH₄)₂SO₄ > NaNO₃, Na₂SiO₃. The presence of Na₂SiO₃ or NaNO₃ has no obvious effect on the growth rates of SOA formation, but it does increase the yield of organic acid and nitrogen-containing organic compounds, respectively. Besides the significantly effect on the growth rate of SOA formation, the presence of CaCl₂ or (NH₄)₂SO₄ can lead to the formation of high-molecular weight species which is found to be positively correlated with the hygroscopic behavior of seed aerosols. The CaCl₂ shows the strongest hygroscopic behavior among the four individual seed aerosols, and the most significant promotion effect on the formation of the high-molecular weight species. It is proposed that the SOA generation enhancement and high-molecular weight products are achieved by particle-phase heterogeneous reactions induced and catalyzed by the acidity of CaCl₂ and (NH₄)₂SO₄ seed aerosols.     

再论都市霾与雾的区别

都市霾的出现有重要的空气质量指示意义,而雾或轻雾与特定的天气系统相联系。由于经济规模的迅速扩大和城市化进程的加快,都市霾现象或灰霾天气日趋严重,霾与雾的区分成为一个非常现实,又迫切需要解决的问题。东南沿海各省用相对湿度区分的标准普遍偏低,将大量霾记成了轻雾或雾。实际上近地层大气中每时每刻总是有霾存在的,而雾滴的存在是少见或罕见的;霾滴要想通过吸湿增长成为雾滴,必须有足够的过饱和度,能够越过过饱和驼峰才行,这在自然界并不容易。在非饱和条件下,不但非水溶性的霾不能转化成雾滴,即便是水溶性的霾粒子一般也不可能吸湿转化为雾滴。实测资料表明,出现雾时,极端最小相对湿度是91％,在相对湿度低于90％的情况下,没有观测到雾。降温是达到饱和形成雾滴的最主要、最重要的物理过程,在自然界中的霾滴通过吸湿过程增长成雾滴几乎不可能。历史上我国各级气象部门从来不存在以相对湿度70％界定轻雾与霾的补充规定。区分霾和雾,应该根据影响天气系统的变化,结合宏观特征的各种判据来确定。建议将相对湿度的阈值定为90％,作为区分轻雾与霾的辅助判据是合理的。     

A modelling study of aerosol processing by stratocumulus clouds and its impact on general circulation model parameterisations of cloud and aerosol

A model of the aqueous phase processing of an aerosol population undergoing multiple cycling through a stratocumulus (Sc) cloud layer is presented. Results indicate that a significant modification of the aerosol properties is achieved following the first cycle through cloud. In a polluted atmosphere, further modification in subsequent cycles is seen to be hydrogen peroxide limited unless there is a flux of ammonia entering the system through cloud base (CB). The modification of the aerosol population is seen to have little effect on the microphysics (specifically the cloud droplet concentration and effective radius) of the processing cloud. However, it enables processed aerosols to subsequently act as efficient cloud condensation nuclei (CCN) in less vigorous clouds (as a result of reducing the critical supersaturation required to activate them). The effects of variations in the internal mixture of soluble components of aerosols on the microphysics of clouds forming on them are also investigated using the cloud model. A (K2) parameterisation of the effects of variations in internally mixed nitrate loadings on the cloud droplet number concentration is presented. The effects of applying this K2 correction to the droplet number (derived from a parameterisation based on sulphate) for the presence of nitrate in aerosol have been investigated using the HadAM3 version of the Hadley Centre General Circulation Model (GCM). The effect on global annual mean simulations of the indirect forcing and effective radius is small, but more pronounced regionally. Suggestions (based on model results and observations) for parameterising the size distribution and in-cloud growth of aerosols for use in GCMs are presented.     

Regional-scale relationships between aerosol and summer monsoon circulation, and precipitation over northeast Asia

We investigated the regional-scale relationships between columnar aerosol loads and summer monsoon circulation, and also the precipitation over northeast Asia using aerosol optical depth (AOD) data obtained from the 8-year MODIS, AERONET Sun/sky radiometer, and precipitation data acquired under the Global Precipitation Climatology Project (GPCP). These high-quality data revealed the regional-scale link between AOD and summer monsoon circulation, precipitation in July over northeast Asian countries, and their distinct spatial and annual variabilities. Compared to the mean AOD for the entire period of 2001–2008, the increase of almost 40–50% in the AOD value in July 2005 and July 2007 was found over the downwind regions of China (Yellow Sea, Korean peninsula, and East Sea), with negative precipitation anomalies. This can be attributable to the strong westerly confluent flows, between cyclone flows by continental thermal low centered over the northern China and anticyclonic flows by the western North Pacific High, which transport anthropogenic pollution aerosols emitted from east China to aforementioned downwind high AOD regions along the rim of the Pacific marine airmass. In July 2002, however, the easterly flows transported anthropogenic aerosols from east China to the southwestern part of China in July 2002. As a result, the AOD off the coast of China was dramatically reduced in spite of decreasing rainfall. From the calculation of the cross-correlation coefficient between MODIS-derived AOD anomalies and GPCP precipitation anomalies in July over the period 2001–2008, we found negative correlations over the areas encompassed by 105–115°E and 30–35°N and by 120–140°E and 35–40°N (Yellow Sea, Korean peninsula, and East Sea). This suggests that aerosol loads over these regions are easily influenced by the Asian monsoon flow system and associated precipitation.     

浅析自动与人工雨量观测数据的差异

自动与人工2种雨量观测由于技术体制不同所获得的雨量数据存在差异。从理论和实践2个方面对2种观测方式的数据误差存在原因和2种方式体制差异进行比较分析,从中找出日常工作中正确的操作和维护方法,从而减少误差,保证2种观测方式所取得的观测数据具有代表性、准确性、比较性,为天气预报、气象信息、气候分析等提供可靠的依据。     

大气气溶胶散射吸湿增长特性研究进展

大气气溶胶的散射吸湿增长特性反映了相对湿度对气溶胶散射能力的影响。研究气溶胶散射吸湿增长对于评估气溶胶辐射强迫、了解气溶胶对大气能见度的影响具有重要意义。文中对气溶胶散射吸湿增长特性的研究意义、监测方法、中外研究进展、参数化拟合和模型模拟等方面进行了系统回顾和总结,并对未来发展方向做出展望。中国在该领域开展的研究数量偏少,广泛开展气溶胶散射吸湿增长特性研究十分必要。     

A new method of quantifying aerosol concentrations in atmosphere

Africa is one of the sources of biomass burning emissions. It is estimated that about 6 million tons of fuel per day is consumed in the southern hemisphere. Biomass burning has an important contribution on aerosol particle concentrations in the atmosphere. Efforts have been made to conduct research in Gaborone to monitor the concentration of atmospheric aerosol particles. These studies were mainly confined to measurement of concentration of aerosol particles and establishing a relation with determinants such as carbon dioxide concentration, biomass burning, and precipitation among others. However, very little seems to have been done in relating the empirical data to levels of aerosol concentrations through a mathematical model. In this paper an objective criterion of classifying levels of aerosol concentrations in terms of their severity is provided. A mathematical model for severity levels is built. Furthermore, two indices, namely, an index of dispersion when applied to the observed annual data indicated that intensity of atmospheric aerosol are on increase in the city of Gaborone, Botswana, and an index of drift which establishes that aerosol severity states showed larger drift during the year 2006–2007 than in the year 2007–2008.     

A closure study of sub-micrometer aerosol particle hygroscopic behaviour

The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11–1.15, 1.38–1.69 and 2.08–2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74–87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17–0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.