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Lindenberg Aerosol Characterization Experiment 1998 (LACE 98): Overview
Abstract and Figures
Back{scattering and absorption of solar radiation by aerosol particles are an important source of uncertainty in climate predictions. Integrated research o n t h e radiative properties of aerosol may reduce this uncertainty. The Lindenberg Aerosol Characterization Experiment 1 9 9 8 (L A CE 98) contributes to this aim. LACE 98 took place between July 13 and August 12, 1998, near Berlin, Germany. The Lindenberg Meteorological Observatory (52.2 o N, 14.1 o E) was chosen as the central eld site because of its long record with aerosol{optical{depth data. Measurements were performed from three aircraft, with one airborne and four ground{based lidars, and at a ground station. The meteorological situations in which i n tensive observations were carried out included clean and polluted air masses as characterized by l o w and high aerosol optical depths. This introductory paper gives an overview of the LACE 98 goals, instrumentation, meteorological and aerosol properties, and reports on the key ndings as a guide to the results presented in the more detailed papers that follow. A very remarkable nding should be mentioned beforehand becaue of its unique character: on August 9{10, 1998, a free{tropopsheric aerosol layer was observed that originated from forest res in western Canada.
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... Crucial for the in-cabin instrumentation is the sampling system (consisting of inlets and sampling lines) aboard the aircraft which transports the ambient aerosol particles to the instruments. One of the greatest challenges of airborne measurements is to design and operate the sampling system such that biases in the measurements of aerosol properties are avoided despite facing different ambient conditions (e.g., varying true airspeed of the aircraft, changing temperature, and pressure; Baumgardner and Huebert, 1993;Wendisch et al., 2004). In particular, coarsemode aerosol particles (particle diameter > 1 µm) 1 are affected by sampling effects due to their high inertia, which can result in an artificial depletion or enhancement (Hinds, 1999;Brockmann, 2011). ...
... Here, we carry out a comprehensive characterization of the sampling system of the Falcon, which was used as the re-search aircraft during the A-LIFE field campaign and has been used for many aircraft missions in the past decades (e.g., SALTRACE, Weinzierl et al., 2017;ACCESS, Moore et al., 2017). Fiebig (2001) investigated the Falcon aerosol inlet's cutoff diameter D p,50 (particle diameter at which the overall sampling efficiency equals 50 %) as a function of flight altitude during the Lindenberger Aerosol Characterization Experiment 1998 (LACE 98;Ansmann et al., 2002), but the analysis was restricted to six flight sequences within the planetary boundary layer. In this study, we use the entire data set of the A-LIFE mission for the characterization which covers the entire altitude range of the Falcon from the ground to about 12 km. ...
Atmospheric aerosol particles have a profound impact on Earth's climate by scattering and absorbing solar and terrestrial radiation and by impacting the properties of clouds. Research aircraft such as the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) Falcon are widely used to study aerosol particles in the troposphere and lower stratosphere. However, transporting a representative sample to the instrumentation inside the aircraft remains a challenge due to high airspeeds and changing ambient conditions. In particular, for high-quality coarse-mode aerosol measurements, knowledge about losses or enhancements in the aerosol sampling system is crucial. In this study, the sampling efficiency of the aerosol inlet aboard the Falcon research aircraft is characterized for the first time with state-of-the art in situ measurements including sizing instruments operated behind the Falcon aerosol inlet and mounted at the aircraft wing not affected by the aerosol inlet. Sampling efficiencies were derived for different true airspeed ranges by comparing the in-cabin and ”full”-size-range particle number size distributions during 174 flight sequences with a major contribution of mineral dust particles during the ”Absorbing aerosol layers in a changing climate: aging, lifetime and dynamics” project (A-LIFE). Additionally, experimentally derived Stokes numbers were used to calculate the cutoff diameter of the aerosol sampling system for different particle densities as a function of true airspeed. As expected, the results show that the velocity of the research aircraft has a major impact on the sampling of coarse-mode aerosol particles with in-cabin instruments. For true airspeeds up to about 190 m s−1, aerosol particles larger than about 1 µm are depleted in the sampling system of the Falcon during the A-LIFE project. In contrast, for true airspeeds higher than 190 m s−1, an enhancement of particles up to a diameter of 4 µm is observed. For even larger particles, the enhancement effect at the inlet is still present, but inertial and gravitational particle losses in the transport system get more and more pronounced, which leads to a decreasing overall sampling efficiency. In summary, aerosol particles are either depleted or enhanced in the Falcon aerosol inlet, whereas transport in sampling lines always leads to a loss of particles. Here, we have considered both effects and determined the cutoff diameter for the A-LIFE transport system (i.e., the sampling lines only), the cutoff diameter of the Falcon aerosol inlet (i.e., the effect of the inlet only), and the combined effect of the inlet and sampling lines.
... We applied the iterative procedure for the indirect determination of the single scattering albedo on a case extracted from the LACE98 experiment (Ansmann et al., 2002). We used global irradiance measurements performed with a single Brewer at Lindenberg and aerosol optical depth measurements at 399 nm performed with a sunphotometer 20 during 10 August 1998, a cloud free day. ...
... This change indicates the presence of different type of aerosols above the measuring site between the morning and the afternoon hours, when more absorbing aerosols are expected to be present. in Ansmann et al. (2002) the flow was from the north during most of that day with low aerosol optical depth values, while in the late afternoon the winds changed to east and southeast advecting air from the polluted western and southwestern parts of Europe. The existence of different aerosol layers in the afternoon hours relative to the morning hours observed with the MPI-UV lidar is shown in Fig. 2b is associated with the wind 5 change and provides further indication that the aerosol type has changed over the measuring site. ...
Routine lidar measurements of the vertical distribution of the aerosol extinction coefficient and the extinction-to-backscatter ratio have been performed at Thessaloniki, Greece using a Raman lidar system in the frame of the EARLINET project since 2000. Spectral and broadband UV-B irradiance measurements, as well as total ozone observations, were available whenever lidar measurements were obtained. From the available measurements several cases could be identified that allowed the study of the effect of different types of aerosol on the levels of the UV-B solar irradiance at the Earth's surface. The TUV radiative transfer model has been used to simulate the irradiance measurements, using total ozone and the lidar aerosol data as input. From the comparison of the model results with the measured spectra the effective single scattering albedo was determined using an iterative procedure, which has been verified against results from the 1998 Lindenberg Aerosol Characterization Experiment. It is shown that the same aerosol optical depth and same total ozone values can show differences up to 10% in the UV-B irradiance at the Earth's surface, which can be attributed to differences in the aerosol type. It is shown that the combined use of the estimated single scattering albedo and the measured extinction-to-backscatter ratio leads to a better characterization of the aerosol type probed.
... Two complex lidar systems named MARTHA (Multiwavelength Atmospheric Raman Lidar for Temperature, Humidity, and Aerosol) and BERTHA (Backscatter Extinction lidar-Ratio Temperature Humidity profiling Apparatus) have been developed using several different lidar-specific techniques (Raman, polarization, multiwavelength, high-spectral-resolution, etc.) throughout the years. The mobile container-based BERTHA has been deployed in several field campaigns since the end of the 1990s (e.g., LACE98: Ansmann et al., 2002b;ACE-2: Ansmann et al., 2002a;INDOEX: Ansmann et al., 2000;COPS: Herold et al., 2011;SAMUM-1 and SAMUM-2: Ansmann et al., 2011;SALTRACE: Haarig et al., 2017a), while the lab-based MARTHA (Mattis et al., 2002a) was used for EARLINET observations (Mattis et al., 2004(Mattis et al., , 2008 and testing new methodologies (e.g., Mattis et al., 2002a;Schmidt et al., 2013;Jimenez et al., 2020a). In parallel, novel data retrieval techniques have been developed and steadily improved, which have become state-ofthe-art for active aerosol profiling , e.g., inversion techniques (Müller et al., 1998), sepa-ration of aerosol components with polarization (POLIPHON; Mamouri and Ansmann, 2016), and automatic and unsupervised data retrievals (e.g., Baars et al., 2008Baars et al., , 2016D'Amico et al., 2015;Baars and Yin, 2020;Yin and Baars, 2021). ...
This paper presents a collection of lidar-derived aerosol intensive optical properties for several aerosol types, namely the particle linear depolarization ratio, the extinction-to-backscatter ratio (lidar ratio) and the Ångström exponent. The data collection, named DeLiAn, is based on globally distributed, long-term, ground-based, multiwavelength, Raman and polarization lidar measurements, conducted mainly with lidars that have been developed at the Leibniz Institute for Tropospheric Research. The intensive optical properties are presented at two wavelengths, 355 and 532 nm, for 13 aerosol categories. The categories cover the basic aerosol types (i.e., marine, pollution, continental European background, volcanic ash, smoke, mineral dust), as well as the most frequently observed mixtures they form. This extensive collection also incorporates more peculiar aerosol categories, including dried marine aerosol that, compared to marine aerosol, exhibits a significantly enhanced depolarization ratio (up to 15 %). Besides Saharan dust, additional mineral dust types related to their source region were identified due to their lower lidar ratios (Central Asian and Middle Eastern dust). In addition, extreme wildfire events (such as in north America and Australia) emitted smoke into the stratosphere showing significantly different optical properties, i.e., high depolarization values (up to 25 %), compared to tropospheric smoke. The data collection reflects and underlines the variety of aerosol mixtures in the atmosphere and can be used for the development of aerosol-typing schemes. The paper contains the most up-to-date and comprehensive overview of optical properties from aerosol lidar measurements and, therefore, provides a solid basis for future aerosol retrievals in the frame of both spaceborne and ground-based lidars. Furthermore, DeLiAn can assist the efforts for the harmonization of satellite records of aerosol properties performed at different wavelengths.
... SALTRACE (Weinzierl et al., 2017) or ACCESS (Moore et al., 2017)). Fiebig (2001) investigated the Falcon aerosol inlet's cut-off diameter Dp,50 (particle diameter at which the overall sampling efficiency equals 50 %) as a function of flight altitude during the Lindenberger Aerosol Characterisation Experiment (LACE 98;Ansmann et al., 2002), but the analysis was restricted to six flight sequences within the planetary boundary layer. In this study, we use the entire data set of the A-LIFE mission for the characterization which covers the entire altitude range of the Falcon from the ground to about 90 12 km. ...
Atmospheric aerosol particles have a profound impact on Earth’s climate by scattering and absorbing solar and terrestrial radiation and by impacting the properties of clouds. Research aircraft such as the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) Falcon are widely used to study aerosol particles in the troposphere and lower stratosphere. However, transporting a representative sample to the instrumentation inside the aircraft remains a challenge due to high airspeeds and changing ambient conditions. In particular, for high-quality coarse mode aerosol measurements, knowledge about losses or enhancements in the aerosol sampling system is crucial. In this study, we present a detailed characterization of the Falcon aerosol sampling system. Aerosol number size distributions were measured during the A-LIFE field campaign simultaneously with in-cabin and out-cabin/wing-mounted instrumentation. Sampling efficiencies were derived for different true airspeed ranges by comparing the in-cabin and the out-cabin particle number size distributions during flight sequences with a major contribution of mineral dust particles in the coarse mode size range. Additionally, experimentally derived Stokes numbers were used to calculate the cut-off diameter of the A-LIFE aerosol sampling system for different particle densities as a function of true airspeed. The results show that the velocity of the research aircraft has a major impact on the sampling of coarse mode aerosol particles with in-cabin instruments. For true airspeeds up to about 190 m s-1, aerosol particles larger than about 1 µm are depleted in the sampling system of the Falcon during the A-LIFE project. In contrast, for true airspeeds higher than 190 m s-1, an enhancement of particles up to a diameter of 4 µm is observed. For even larger particles, the enhancement effect at the inlet is still present, but inertial and gravitational particle losses in the transport system get more and more pronounced which leads to a decreasing overall sampling efficiency. In summary, aerosol particles can either be depleted or enhanced at an aerosol inlet, whereas transport in sampling lines always leads to a loss of particles. Therefore, it is important to consider both, inlet and transport efficiency, when quantifying the sampling efficiency of an aerosol sampling system.
... Lindenberg Aerosol Characterization Experiment, Germany (1998;Ansmann et al., 2002) 2 European integrated project on Aerosol Cloud Climate and Air Quality Interactions, Long-range experiment (2008;Kulmala et al., 2009) ...
... Therefore, we begin with a historical overview of published (pioneering) attempts to simultaneously measure the INPC and ICNC and to perform icenucleation-related aerosol-cloud closure studies. The concept of aerosol-related closure experiments (Russel et al., 1979;Bates et al., 1998;Russell and Heintzenberg, 2000;Ansmann et al., 2002) was originally introduced to investigate the complex relationships between physical, chemical, optical, and radiative properties of atmospheric aerosol particles and thus to study the direct effect on climate solely based on aerosol observations (Quinn et al., 1996). In closure experiments, the measured value of a dependent variable is compared with the modeled or predicted value that is calculated from measured values of independent variables by using an appropriate model. ...
For the first time, a closure study of the relationship between the ice-nucleating particle concentration (INP; INPC) and ice crystal number concentration (ICNC) in altocumulus and cirrus layers, solely based on ground-based active remote sensing, is presented. Such aerosol–cloud closure experiments are required (a) to better understand aerosol–cloud interaction in the case of mixed-phase clouds, (b) to explore to what extent heterogeneous ice nucleation can contribute to cirrus formation, which is usually controlled by homogeneous freezing, and (c) to check the usefulness of available INPC parameterization schemes, applied to lidar profiles of aerosol optical and microphysical properties up to the tropopause level. The INPC–ICNC closure studies were conducted in Cyprus (Limassol and Nicosia) during a 6-week field campaign in March–April 2015 and during the 17-month CyCARE (Cyprus Clouds Aerosol and Rain Experiment) campaign. The focus was on altocumulus and cirrus layers which developed in pronounced Saharan dust layers at heights from 5 to 11 km. As a highlight, a long-lasting cirrus event was studied which was linked to the development of a very strong dust-infused baroclinic storm (DIBS) over Algeria. The DIBS was associated with strong convective cloud development and lifted large amounts of Saharan dust into the upper troposphere, where the dust influenced the evolution of an unusually large anvil cirrus shield and the subsequent transformation into an cirrus uncinus cloud system extending from the eastern Mediterranean to central Asia, and thus over more than 3500 km. Cloud top temperatures of the three discussed closure study cases ranged from −20 to −57 ∘C. The INPC was estimated from polarization/Raman lidar observations in combination with published INPC parameterization schemes, whereas the ICNC was retrieved from combined Doppler lidar, aerosol lidar, and cloud radar observations of the terminal velocity of falling ice crystals, radar reflectivity, and lidar backscatter in combination with the modeling of backscattering at the 532 and 8.5 mm wavelengths. A good-to-acceptable agreement between INPC (observed before and after the occurrence of the cloud layer under investigation) and ICNC values was found in the discussed three proof-of-concept closure experiments. In these case studies, INPC and ICNC values matched within an order of magnitude (i.e., within the uncertainty ranges of the INPC and ICNC estimates), and they ranged from 0.1 to 10 L−1 in the altocumulus layers and 1 to 50 L−1 in the cirrus layers observed between 8 and 11 km height. The successful closure experiments corroborate the important role of heterogeneous ice nucleation in atmospheric ice formation processes when mineral dust is present. The observed long-lasting cirrus event could be fully explained by the presence of dust, i.e., without the need for homogeneous ice nucleation processes.
... Besides radiative closure experiments, mass and optical closure studies are most important subtasks in the development of appropriate, well-evaluated and well-tested dust parameterizations to be implemented in atmospheric transport models. After a series of successful closure studies in marine conditions with little pollution (ACE 1, Bates et al., 1998) and in heavily polluted areas of North America (TARFOX, Russell et al., 1999), southern Europe (ACE 2, Raes et al., 2000), central Europe (LACE 98, Ansmann et al., 2002 ), southern Asia (IN- DOEX, Ramanathan et al., 2001), and eastern Asia (ACE–Asia, Huebert et al., 2003), the dust field studies mentioned in the next paragraph were the first specifically devoted to desert dust (non-spherical particles). Sokolik et al. (2001) recommended to arrange intensive field investigations on desert dust as column closure experiments. ...
... For that purpose, SAMUM-1 followed the closure approach which Ogren (1995) developed, andQuinn et al. (1996) extended. Ansmann et al. (2002), Russell and Heintzenberg (2000) and others had tested this approach in aerosol field experiments but never in an aerosol source region dominated by mineral dust. ...
For the first time, a closure study of the relationship between ice-nucleating particle concentration (INPC) and ice crystal number concentration (ICNC) in altocumulus and cirrus layers, solely based on ground-based active remote sensing, is presented. Such aerosol-cloud closure experiments are required (a) to better understand aerosol-cloud interaction in the case of mixed-phase clouds, (b) to explore to what extend heterogeneous ice nucleation can contribute to cirrus formation which is usually controlled by homogeneous freezing, and (c) to check the usefulness of available INPC parameterization schemes, applied to lidar profiles of aerosol optical and microphysical properties up to tropopause level. The INPC-vs-ICNC closure studies were conducted in Cyprus (Limassol and Nicosia) during a six-week field campaign in March–April 2015 and during the 17-month CyCARE (Cyprus Clouds Aerosol and Rain Experiment) campaign. Focus is on altocumulus and cirrus layers which developed in pronounced Saharan dust layers at heights from 5–11 km. Cloud top temperatures ranged from −20 °C to −57 °C. INPC was estimated from polarization/Raman lidar observations in combination with published INPC parameterization schemes for immersion and deposition nucleation. ICNC was estimated from combined Doppler lidar, aerosol lidar, and cloud radar observations of the terminal velocity of falling ice crystals, radar reflectivity and lidar backscatter in combination with modeling of backscattering at 532 nm and 8.5 mm wavelength. Good to acceptable agreement between INPC (observed before and after the occurrence of the cloud layer under investigation) and ICNC values was found in three proof-of-concept closure experiments. In these case studies, INPC and ICNC values matched within an order of magnitude (i.e., within the uncertainty ranges of the INPC and ICNC estimates), and ranged from 0.1–10 per liter in the altocumulus layers and 1–50 per liter in the cirrus layers observed between 8–11 km height.
The results of studies of the aerosol properties in different regions of America and western Europe, obtained during accomplishment of programmes of the field observation experiments discussed above, have demonstrated an exclusive diversity of physical properties and chemical composition of aerosol requiring further system-atization of observational data in order to substantiate more adequate models of aerosol than existing ones. Only on the basis of the use of such models it is possible to obtain more reliable estimates of possible aerosol climatic impact.
After a short survey I of the principles underlying the determination of the turbidity coefficient ?, earlier introduced by the author as a measure of the atmospheric turbidity, a condensed summary II is given of earlier and also of more recent determinations of ?. The variation of the turbidity with the time of the year, the airmass and with latitude is discussed. Finally a simple method of determining the wave length dependence of the extinction by atmospheric aerosol is outlined. The method is founded upon measurements of integral radiation values with aid of pyrheliometers and glass filters, III. Accuracy and probable error are considered, IV. DOI: 10.1111/j.2153-3490.1961.tb00078.x
In the past, different branches of the atmospheric sciences separately conducted research concerning the role of the atmospheric aerosol in cloud physics, atmospheric chemistry and radiative transfer. Findings of the last few years have emphasised the importance of the atmospheric aerosol for both climate change and the cycles of atmospheric trace substances. In this peer- reviewed editorial, arguments are given for research needed in order to include the atmospheric aerosol as a prognostic variable in global models of climate and trace substances. A program for the necessary integrated, interdisciplinary aerosol research is formulated, combining in-situ physical and chemical aerosol characterisation, remote sensing and modelling. It is necessary to emphasise the need for integration of the key activities, which individually, by themselves, cannot yield an adequate basis for a full characterisation of the climate effect of aerosols or of changes in atmospheric composition.
Aerosol optical depths for 35 wavelengths in the visible (VIS) and near infrared (NIR) are reported for measurements at the Lindenberg Meteorological Observatory from 1986 to 1995. During this period, episodic events in the stratosphere (e.g., the Mt. Pinatubo eruption) and troposphere (e.g., Saharan dust plumes) contributed significantly to the magnitude and the spectral dependence of the measurements. Other, more continuous processes, like the decrease of anthropogenic aerosols due to the decrease of European and regional SO 2 emissions, have resulted in a downward trend in the aerosol optical depth. The observations also demonstrate that the aerosol optical depth spectra are strongly modulated by air mass type and wind direction The seasonal dependence of the aerosol optical depth is characterized by minima in the summer and winter and maxima in the spring and autumn. In contrast the groundlayer related aerosol optical depth shows an annual cycle with a maximum in winter and a minimum in summer.
In recent years a monitoring system for water vapor and aerosol optical depths has been developed at the Observatory Lindenberg (52.14°N;14.12°E). The system runs 24 hours a day and measures in the visible and the near infrared region. During the day the observations are carried out by a fully automatized sunphotometer (approx. 60 channels, used water vapor channel at 951.7 nm) and during the night by an operator using a semi-automatic starphotometer (8 channels: 7 aerosol channels and 1 water vapor channel at 944.1 nm). This paper describes the observation and evalution methods and presents the results of a 21-day combined monitoring of the atmospheric water vapor and aerosol content in April/May 1996. The measuring results for water vapor of the infrared channel are compared to data derived from microwave radiometer and radiosonde measurements. The paper discusses the pros and cons of the different measuring methods. Finally, an example is given for the combined use of different measuring systems. For the parameter water vapor the combined results characterize the atmospheric column on a short-time monitoring scale of 0.5 to 1 hour. The paper points out the importance of obtaining almost continuous information about the variable parameters water vapor and aerosol. This information helps to solve many problems in meteorology, eg. concerning the cloud/aerosol/radiation complex or the validation of a small scale atmospheric simulation model.
Closure studies offer one approach for reducing the uncertainties associated with estimating the radiative forcing by tropospheric aerosols. A closure experiment involves an overdetermined set of observations such that the measured value of an important system property can be compared to a value calculated with an appropriate model based on independent measurements. This paper discusses how the concept of closure can be used to identify and reduce the uncertainties involved in calculating the direct shortwave radiative forcing by anthropogenic aerosols. It includes a review of the successes and shortcomings of previously published closure experiments as well as proposals for future closure experiments.
The optical properties of atmospheric aerosol particles were measured
close to ground level using different methods at Lindenberg/Falkenberg
(Germany) during the Lindenberg Aerosol Characterization Experiment
(LACE 98), 13 July 1998 to 14 August 1998 [, 2002]. The experimental
setup consisted of (a) an aerosol photometer, which measured a complete
set of aerosol optical properties, such as extinction, scattering, and
absorption coefficients, single scattering albedo, apparent complex
refractive index, asymmetry parameter of the phase function of
scattering, and apparent volume soot content, (b) an integrating plate,
which measured the absorption coefficient, and (c) a nephelometer and
(d) a particle soot absorption photometer (PSAP) for measurements of
scattering and absorption coefficients. All these measurements were
performed under dry conditions. A telephotometer and a horizontal lidar
were used to determine the aerosol extinction coefficient under ambient
conditions. This paper presents a closure study of the different
measurement methods. An empirical correction function for the PSAP is
introduced and compared with a semiempirical correction function
previously introduced by [1999]. A detailed model of humidity effects on
the extinction coefficient is presented using "coated sphere"
calculations based on measured input values of the size distribution,
the chemical composition, the growth factor at 90% relative humidity,
the water-soluble fraction of particulate materials, the temperature,
and the relative humidity. Model calculations allowed an intercomparison
of measured values of particles in the dry state and at ambient relative
humidity and showed good agreement.
[1] A novel mobile mass spectrometric acquisition and evaluation system for on-line analysis of single airborne particles and characterization of particle populations (aerosols), laser mass analyzer for particles in the airborne state (LAMPAS 2), was employed during the Lindenberg Aerosol Characterization Experiment 1998 in July and August 1998 (LACE 98). The major aim of this experiment was to perform a size-resolved determination of the chemical composition of atmospheric particles within populations in order to evaluate the climate forcing created by anthropogenic aerosol particles. During the field campaign, the LAMPAS 2 instrument continuously analyzed size and composition of individual particles from five size ranges with a time course resolution of 1 hour, recording several ten thousand single-particle spectra. By using a semiautomated data processing unit, the amount of data could be significantly reduced from several hundreds of particle mass spectra to only a few particle classes in every size range. Statistical evaluation of the mass spectral data measured during LACE 98 resulted in the determination of 10 basic classes of particles, yielding detailed information on the aerosol composition. Chemical interpretation of these particle classes and correlation to meteorological data and physicochemical particle properties (optical parameters) were performed.
We investigate the interannual variability of the seasonally recurring aerosol haze of the United States East Coast. The data consist of observations collected by five Sun/sky radiometer stations. We compare the data collected in 1996 in conjunction with the TARFOX experiment to similar data collected at the same locations in 1993. The regional mean optical thickness remains essentially constant in the two years. The Mount Pinatubo stratospheric mode decreases spectral dependence in 1993, especially for optical thickness below 0.20. The volume size distributions inverted from the sky measurements show an increase in accumulation mode particle size with increasing aerosol optical thickness in both years. At moderate to high optical thickness (tau670>0.20) the particles in the fine particle mode, 0.1-0.6 mum radius range, from both years have the same lognormal parameters (rm=0.21, sigma=0.45-0.50). The volume of these particles increases linearly with tau. These size particles dominate the optical properties of the aerosol in the visible spectrum and cause remarkable interannual consistency in the aerosol optical characteristics. The 1993 and 1996 phase functions agree to within 7-10% for optical thicknesses greater than 0.2.
The southern hemisphere marine Aerosol Characterization Experiment (ACE 1) was the first of a series of experiments that will quantify the chemical and physical processes controlling the evolution and properties of the atmospheric aerosol relevant to radiative forcing and climate. The goals of this series of process studies are to reduce the overall uncertainty in the calculation of climate forcing by aerosols and to understand the multiphase atmospheric chemical system sufficiently to be able to provide a prognostic analysis of future radiative forcing and climate response. ACE 1, which was conducted from November 15 to December 14, 1995, over the southwest Pacific Ocean, south of Australia, quantified the chemical, physical, radiative, and cloud nucleating properties and furthered our understanding of the processes controlling the aerosol properties in this minimally polluted marine atmosphere. The experiment involved the efforts of scientists from 45 research institutes in 11 countries. Atmospheric aerosol particles affect the Earth's ra- diative balance both directly by scattering and absorb- ing solar radiation and indirectly by modifying the op- tical properties and lifetime of clouds. The natural aerosol derived from biogenic sulfur and organic species, sea salt, and mineral aerosol has been substantially per- turbed by anthropogenic aerosols, particularly sulfates from SO2 emissions and organic condensates and soot from biomass and fossil fuel combustion (Intergovern- mental Panel on Climate Change (IPCC), 1996). The global mean radiative forcing due to the direct effect of anthropogenic sulfate aerosol particles alone is cal- culated to be of comparable magnitude (approximately -0.2 to-0.8 W m -2) but opposite in sign to the fore-