Non-absorbing Aerosols Research Articles

Modelled and measured effects of clouds on UV Aerosol Indices on a local, regional, and global scale

Abstract. The UV Aerosol Indices (UVAI) form one of very few available tools in satellite remote sensing that provide information on aerosol absorption. The UVAI are also quite insensitive to surface type and are determined in the presence of clouds – situations where most aerosol retrieval algorithms do not work. The UVAI are most sensitive to elevated layers of absorbing aerosols, such as mineral dust and smoke, but they can also be used to study non-absorbing aerosols, such as sulphate and secondary organic aerosols. Although UVAI are determined for cloud-contaminated pixels, clouds do affect the value of UVAI in several ways: (1) they shield the underlying scene (potentially containing aerosols) from view, (2) they enhance the apparent surface albedo of an elevated aerosol layer, and (3) clouds unpolluted by aerosols also yield non-zero UVAI, here referred to as "cloudUVAI". The main purpose of this paper is to demonstrate that clouds can cause significant UVAI and that this cloudUVAI can be well modelled using simple assumptions on cloud properties. To this aim, we modelled cloudUVAI by using measured cloud optical parameters – either with low spatial resolution from SCIAMACHY, or high resolution from MERIS – as input. The modelled cloudUVAI were compared with UVAI determined from SCIAMACHY reflectances on different spatial (local, regional and global) and temporal scales (single measurement, daily means and seasonal means). The general dependencies of UVAI on cloud parameters were quite well reproduced, but several issues remain unclear: compared to the modelled cloudUVAI, measured UVAI show a bias, in particular for large cloud fractions. Also, the spread in measured UVAI is larger than in modelled cloudUVAI. In addition to the original, Lambert Equivalent Reflector (LER)-based UVAI algorithm, we have also investigated the effects of clouds on UVAI determined using the so-called Modified LER (MLER) algorithm (currently applied to TOMS and OMI data). For medium-sized clouds the MLER algorithm performs better (UVAI are closer to 0), but like for LER UVAI, MLER UVAI can become as large as −1.2 for small clouds and deviate significantly from zero for cloud fractions near 1. The effects of clouds should therefore also be taken into account when MLER UVAI data are used. Because the effects of clouds and aerosols on UVAI are not independent, a simple subtraction of modelled cloudUVAI from measured UVAI does not yield a UVAI representative of a cloud-free scene when aerosols are present. We here propose a first, simple approach for the correction of cloud effects on UVAI. The method is shown to work reasonably well for small to medium-sized clouds located above aerosols.

On-the-fly cross flow laser guided separation of aerosol particles based on size, refractive index and density–theoretical analysis

Laser separation of particles is achieved using forces resulting from the momentum exchange between particles and photons constituting the laser radiation. Particles can experience different optical forces depending on their size and/or optical properties, such as refractive index. Thus, particles can move at different speeds in the presence of an optical force, leading to spatial separations. In this paper, we present a theoretical analysis on laser separation of non-absorbing aerosol particles moving at speeds (1-10 cm/sec) which are several orders of magnitude greater than typical particle speeds used in previous studies in liquid medium. The calculations are presented for particle deflection by a loosely focused Gaussian 1064 nm laser, which simultaneously holds and deflects particles entrained in flow perpendicular to their direction of travel. The gradient force holds the particles against the viscous drag for a short period of time. The scattering force simultaneously pushes the particles, perpendicular to the flow, during this period. Our calculations show particle deflections of over 2500 µm for 15 µm aerosol particles, and a separation of over 1500 µm between 5 µm and 10 µm particles when the laser is operated at 10 W. We show that a separation of about 421 µm can be achieved between two particles of the same size (10 µm) but having a refractive index difference of 0.1. Density based separations are also possible. Two 10 µm particles with a density difference of 600 kg/m3 can be separated by 193 µm. Examples are shown for separation distances between polystyrene, poly(methylmethacrylate), silica and water particles. These large laser guided deflections represent a novel achievement for optical separation in the gas phase.

Examining the impact of overlying aerosols on the retrieval of cloud optical properties from passive remote sensing

Haywood et al. (2004) show that an aerosol layer above a cloud can cause a bias in the retrieved cloud optical thickness and effective radius. Monitoring for this potential bias is difficult because space‐based passive remote sensing cannot unambiguously detect or characterize aerosol above cloud. We show that cloud retrievals from aircraft measurements above cloud and below an overlying aerosol layer are a means to test this bias. The data were collected during the Intercontinental Chemical Transport Experiment (INTEX‐A) study based out of Portsmouth, New Hampshire, United States, above extensive, marine stratus cloud banks affected by industrial outflow. Solar Spectral Flux Radiometer (SSFR) irradiance measurements taken along a lower level flight leg above cloud and below aerosol were unaffected by the overlying aerosol. Along upper level flight legs, the irradiance reflected from cloud top was transmitted through an aerosol layer. We compare SSFR cloud retrievals from below‐aerosol legs to satellite retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) in order to detect an aerosol‐induced bias. In regions of small variation in cloud properties, we find that SSFR and MODIS‐retrieved cloud optical thickness compares within the uncertainty range for each instrument while SSFR effective radius tend to be smaller than MODIS values (by 1–2 μm) and at the low end of MODIS uncertainty estimates. In regions of large variation in cloud properties, differences in SSFR and MODIS‐retrieved cloud optical thickness and effective radius can reach values of 10 and 10 μm, respectively. We include aerosols in forward modeling to test the sensitivity of SSFR cloud retrievals to overlying aerosol layers. We find an overlying absorbing aerosol layer biases SSFR cloud retrievals to smaller effective radii and optical thickness while nonabsorbing aerosols had no impact.

Measurements of aerosol optical properties in central Tokyo during summertime using cavity ring-down spectroscopy: Comparison with conventional techniques

A highly sensitive cavity ring-down spectrometer (CRDS) was used to monitor the aerosol extinction coefficient at 532 nm. The performance of the spectrometer was evaluated using measurements of nearly monodisperse polystyrene particles with diameters between 150 and 500 nm. By comparing the observed results with those determined using Mie theory, the accuracy of the CRDS instrument was determined to be >97%, while the upper limit for the precision of the instrument was estimated to be 0.6–3.5% (typically 2%), depending on the particle number concentration, which was in the range of 30–2300 particles cm −3. Simultaneous measurements of the extinction ( b ext), scattering ( b sca) and absorption ( b abs) coefficients of ambient aerosols were performed in central Tokyo from 14 August to 2 September 2007 using the CRDS instrument, two nephelometers and a particle/soot absorption photometer (PSAP), respectively. The value of b ext measured using the CRDS instrument was compared with the sum of the b sca and b abs values measured with a nephelometer and a PSAP, respectively. Good agreement between the b ext and b sca + b abs values was obtained except for data on days when high ozone mixing ratios (>130 ppbv) were observed. During the high-O 3 days, the values for b sca + b abs were ∼7% larger than the value for b ext, possibly because the value for b abs measured by the PSAP was overestimated due to interference from coexisting non-absorbing aerosols such as secondary organic aerosols.

Algorithm for retrieval of aerosol optical properties over the ocean from the Geostationary Ocean Color Imager

An aerosol retrieval algorithm for the first Geostationary Ocean Color Imager (GOCI) to be launched in March 2010 onboard the Communication, Ocean, and Meteorological Satellite (COMS) is presented. The algorithm retrieves aerosol optical depth (AOD), fine-mode fraction (FMF), and aerosol type in 500 m × 500 m resolution. All the products are retrieved over clear water which is defined by surface reflectance ratio between 640 nm and 860 nm (SRR) less or equal to 2.5, while only AOD is retrieved over turbid water (SRR > 2.5) due to high surface reflectance. To develop optimized algorithm for the target area of GOCI, optical properties of aerosol are analyzed from extensive observation of AERONET sunphotometers to generate lookup table. Surface reflectance of turbid water is determined from 30-day composite of Rayleigh- and gas corrected reflectance. By applying the present algorithm to MODIS top-of-the atmosphere reflectance, three different aerosol cases dominated by anthropogenic aerosol contains black carbon (BC), dust, and non-absorbing aerosol are analyzed to test the algorithm. The algorithm retrieves AOD, and size information together with aerosol type which are consistent with results inferred by RGB image in a qualitative way. The comparison of the retrieved AOD with those of MODIS collection 5 and AERONET sunphotometer observations shows reliable results. Especially, the application of turbid water algorithm significantly increases the accuracy in retrieving AOD at Anmyon station. The sensitivity study between MODIS and GOCI instruments in terms of relative sensitivity and scattering angle shows promising applicability of the present algorithm to future GOCI measurements.

Study of successive contrasting monsoons (2001–2002) in terms of aerosol variability over a tropical station Pune, India

Abstract. The present study suggests that aerosols play a major role in cloud formation and affect significantly the precipitation over a regional scale. The study reveals that there is a high variability of aerosol index during a bad monsoon year 2002, indicating an extension of cycle to more than 100 days from a normal 50 day cycle of absorbing and non-absorbing aerosols over a tropical urban station Pune. Pre-monsoon of 2002 shows a high loading of coarse-mode aerosols (absorbing dust aerosols) which indicate vertical and horizontal temperature variations in turn affecting the seasonal rainfall at a regional scale. Cloud formation highly depends on aerosol concentration, but the activation process is not monotonic. The surface meteorological features help to initiate the cloud process. The surface temperatures were high during the pre-monsoon of 2002 leading to increase of aerosol optical depth as compared to 2001. The effect of surface wind speed, though, complicated to understand, results in low values in 2002 with high aerosol optical depth and vice-versa in 2001.

Comparison of aerosol optical depths from the Ozone Monitoring Instrument (OMI) on Aura with results from airborne sunphotometry, other space and ground measurements during MILAGRO/INTEX-B

Abstract. Airborne sunphotometer measurements are used to evaluate retrievals of extinction aerosol optical depth (AOD) from spatially coincident and temporally near-coincident measurements by the Ozone Monitoring Instrument (OMI) aboard the Aura satellite during the March 2006 Megacity Initiative-Local And Global Research Observations/Phase B of the Intercontinental Chemical Transport Experiment (MILAGRO/INTEX-B). The 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS) flew on nine missions over the Gulf of Mexico and four in or near the Mexico City area. Retrievals of AOD from near-coincident AATS and OMI measurements are compared for three flights over the Gulf of Mexico for flight segments when the aircraft flew at altitudes 60–70 m above sea level, and for one flight over the Mexico City area where the aircraft was restricted to altitudes ~320–800 m above ground level over the rural area and ~550–750 m over the city. OMI-measured top of atmosphere (TOA) reflectances are routinely inverted to yield aerosol products such as AOD and aerosol absorption optical depth (AAOD) using two different retrieval algorithms: a near-UV (OMAERUV) and a multiwavelength (OMAERO) technique. This study uses the archived Collection 3 data products from both algorithms. In particular, AATS and OMI AOD comparisons are presented for AATS data acquired in 20 OMAERUV retrieval pixels (15 over water) and 19 OMAERO pixels (also 15 over water). At least four pixels for one of the over-water coincidences and all pixels for the over-land case were cloud-free. Coincident AOD retrievals from 17 pixels of the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard Aqua are available for two of the over-water flights and are shown to agree with AATS AODs to within root mean square (RMS) differences of 0.00–0.06, depending on wavelength. Near-coincident ground-based AOD measurements from ground-based sun/sky radiometers operated as part of the Aerosol Robotic Network (AERONET) at three sites in and near Mexico City are also shown and are generally consistent with the AATS AODs (which exclude any AOD below the aircraft) both in magnitude and spectral dependence. The OMAERUV algorithm retrieves AODs corresponding to a non-absorbing aerosol model for all three over-water comparisons whereas the OMAERO algorithm retrieves best-fit AODs corresponding to an absorbing biomass-burning aerosol model for two of the three over-water cases. For the four cloud-free pixels in one over-water coincidence (10 March), the OMAERUV retrievals underestimate the AATS AODs by ~0.20, which exceeds the expected retrieval uncertainty, but retrieved AODs agree with AATS values within uncertainties for the other two over-water events. When OMAERO retrieves AODs corresponding to a biomass-burning aerosol over water, the values significantly overestimate the AATS AODs (by up to 0.55). For the Mexico City coincidence, comparisons are presented for a non-urban region ~50–70 km northeast of the city and for a site near the center of the city. OMAERUV retrievals are consistent with AERONET AOD magnitudes for the non-urban site, but are nearly double the AATS and AERONET AODs (with differences of up to 0.29) in the center of the city. Corresponding OMAERO retrievals exceed the AATS and/or AERONET AODs by factors of 3 to 10.

Dual-aureole and sun spectrometer system for airborne measurements of aerosol optical properties

We have designed an airborne spectrometer system for the simultaneous measurement of the direct sun irradiance and the aureole radiance in two different solid angles. The high-resolution spectral radiation measurements are used to derive vertical profiles of aerosol optical properties. Combined measurements in two solid angles provide better information about the aerosol type without additional and elaborate measuring geometries. It is even possible to discriminate between absorbing and nonabsorbing aerosol types. Furthermore, they allow to apply additional calibration methods and simplify the detection of contaminated data (e.g., by thin cirrus clouds). For the characterization of the detected aerosol type a new index is introduced that is the slope of the aerosol phase function in the forward scattering region. The instrumentation is a flexible modular setup, which has already been successfully applied in airborne and ground-based field campaigns. We describe the setup as well as the calibration of the instrument. In addition, example vertical profiles of aerosol optical properties--including the aureole measurements--are shown and discussed.

Global aerosol optical properties and application to Moderate Resolution Imaging Spectroradiometer aerosol retrieval over land

As more information about global aerosol properties has become available from remotely sensed retrievals and in situ measurements, it is prudent to evaluate this new information, both on its own and in the context of satellite retrieval algorithms. Using the climatology of almucantur retrievals from global Aerosol Robotic Network (AERONET) Sun photometer sites, we perform cluster analysis to determine aerosol type as a function of location and season. We find that three spherical‐derived types (describing fine‐sized dominated aerosol) and one spheroid‐derived types (describing coarse‐sized dominated aerosol, presumably dust) generally describe the range of AERONET observed global aerosol properties. The fine‐dominated types are separated mainly by their single scattering albedo (ω0), ranging from nonabsorbing aerosol (ω0 ∼ 0.95) in developed urban/industrial regions, to moderately absorbing aerosol (ω0 ∼ 0.90) in forest fire burning and developing industrial regions, to absorbing aerosol (ω0 ∼ 0.85) in regions of savanna/grassland burning. We identify the dominant aerosol type at each site, and extrapolate to create seasonal 1° × 1° maps of expected aerosol types. Each aerosol type is bilognormal, with dynamic (function of optical depth) size parameters (radius, standard deviation, volume distribution) and complex refractive index. Not only are these parameters interesting in their own right, they can also be applied to aerosol retrieval algorithms, such as to aerosol retrieval over land from Moderate Resolution Imaging Spectroradiometer. Independent direct‐Sun AERONET observations of spectral aerosol optical depth (τ) are consistent the spectral dependence of the models, indicating that our derived aerosol models are relevant.

Case study of modeled aerosol optical properties during the SAFARI 2000 campaign

We present modeled aerosol optical properties (single scattering albedo, asymmetry parameter, and lidar ratio) in two layers with different aerosol loadings and particle sizes, observed during the Southern African Regional Science Initiative 2,000 (SAFARI 2,000) campaign. The optical properties were calculated from aerosol size distributions retrieved from aerosol layer optical thickness spectra, measured using the NASA Ames airborne tracking 14-channel sunphotometer (AATS-14) and the refractive index based on the available information on aerosol chemical composition. The study focuses on sensitivity of modeled optical properties in the 0.3-1.5 microm wavelength range to assumptions regarding the mixing scenario. We considered two models for the mixture of absorbing and nonabsorbing aerosol components commonly used to model optical properties of biomass burning aerosol: a layered sphere with absorbing core and nonabsorbing shell and the Maxwell-Garnett effective medium model. In addition, comparisons of modeled optical properties with the measurements are discussed. We also estimated the radiative effect of the difference in aerosol absorption implied by the large difference between the single scattering albedo values (approximately 0.1 at midvisible wavelengths) obtained from different measurement methods for the case with a high amount of biomass burning particles. For that purpose, the volume fraction of black carbon was varied to obtain a range of single scattering albedo values (0.81-0.91 at lambda=0.50 microm). The difference in absorption resulted in a significant difference in the instantaneous radiative forcing at the surface and the top of the atmosphere (TOA) and can result in a change of the sign of the aerosol forcing at TOA from negative to positive.

Optical properties of absorbing and non-absorbing aerosols retrieved by cavity ring down (CRD) spectroscopy

Abstract. Application of cavity ring down (CRD) spectrometry for measuring the optical properties of pure and mixed laboratory-generated aerosols is presented. The extinction coefficient (αext), extinction cross section (σext) and extinction efficiency (Qext) were measured for polystyrene spheres (PSS), ammonium sulphate ((NH4)2(SO4), sodium chloride (NaCl), glutaric acid (GA), and Rhodamine-590 aerosols. The refractive indices of the different aerosols were retrieved by comparing the measured extinction efficiency of each aerosol type to the extinction predicted by Mie theory. Aerosols composed of sodium chloride and glutaric acid in different mixing ratios were used as model for mixed aerosols of two non-absorbing materials, and their extinction and complex refractive index were derived. Aerosols composed of Rhodamine-590 and ammonium sulphate in different mixing ratios were used as model for mixing of absorbing and non-absorbing species, and their optical properties were derived. The refractive indices of the mixed aerosols were also calculated by various optical mixing rules. We found that for non-absorbing mixtures, the linear rule, Maxwell-Garnett rule, and extended effective medium approximation (EEMA), give comparable results, with the linear mixing rule giving a slightly better fit than the others. Overall, calculations for the mixed aerosols are not as good as for single component aerosols. For absorbing mixtures, the differences between the refractive indices calculated using the mixing rules and those retrieved by CRD are generally higher.

Laboratory and Ambient Particle Density Determinations using Light Scattering in Conjunction with Aerosol Mass Spectrometry

A light scattering module has been integrated into the current AMS instrument. This module provides the simultaneous measurement of vacuum aerodynamic diameter (d va) and scattered light intensity (RLS) for all particles sampled by the AMS above ∼180 nm geometric diameter. Particle counting statistics and correlated chemical ion signal intensities are obtained for every particle that scatters light. A single calibration curve converts RLS to an optical diameter (d o). Using the relationship between d va and d o the LS-AMS provides a real-time, per particle measurement of the density of the sampled aerosol particles. The current article is focused on LS-AMS measurements of spherical, non-absorbing aerosol particles. The laboratory characterization of LS-AMS shows that a single calibration curve yields the material density of spherical particles with real refractive indices (n) over a range from 1.41 < n < 1.60 with an accuracy of about ±10%. The density resolution of the current LS-AMS system is also shown...

Real-time detection of ambient aerosols using photothermal interferometry: Folded Jamin interferometer

Work in our laboratory has been directed at the development of a new class of instrumentation that can directly measure ambient aerosol absorption through photothermal interferometry. The hallmark of this approach is its ability to directly measure aerosol absorption without interference from aerosol scattering since the signal originates from the thermal dissipation of the spectrally absorbed energy. While the principle of the photothermal technique for the detection of aerosols was demonstrated in the mid-1980s, this interferometric technique remains a laboratory technique largely due to sensitivity to mechanical vibrations and other environmental factors that result in unwanted signal interference and commensurate reduction in detection sensitivity. In order to realize its application outside the laboratory, a folded Jamin interferometer design has replaced both the traditional Mach-Zehnder and unfolded Jamin configurations. The folded Jamin affords many advantages, which include high degree of common mode noise rejection, insensitivity to rotation and translation of optical components, inherent double pass configuration, and compact size. In this article, we report on the performance of this optical configuration and present representative data for both absorbing and nonabsorbing aerosols.

A Three-Wavelength Optical Extinction Cell for Measuring Aerosol Light Extinction and Its Application to Determining Light Absorption Coefficient

An optical extinction cell (OEC) was used to measure aerosol particle extinction coefficient σEP at three wavelengths, 467, 530, and 660 nm. The details of the design and the results of its use in the Reno Aerosol Optics Study (RAOS) in June 2002 are presented. The OEC agreed well with the scattering coefficient σSP measured using an integrating nephelometer for nonabsorbing aerosols. Linear regression of 1 min-averaged σEP versus σSP yielded slopes 1.00 ± 0.02 and offsets < 4 Mm−1 in the range ∼50 to 900 Mm−1. The difference, (σEP − σSP) was taken as a standard measure of absorption coefficient, σAP. The noise calculated as the standard deviation of (σEP − σSP) from 1 min-averaged white aerosol data was about 10, 11, and 5 Mm− 1 for the blue, green and red wavelengths, respectively. The absorption coefficients calculated from σEP − σSP were compared with the absorption coefficients measured using a photoacoustic instrument. In general, the two methods agreed better than 10%. The data gives also a way for estimating indirectly the relative uncertainty of the nephelometer. The theoretical relative uncertainties of the absorption coefficient were closest to the measured ones when the relative uncertainty of the scattering coefficient was ∼ 4%.

Evaluation of Multiangle Absorption Photometry for Measuring Aerosol Light Absorption

A new multiangle absorption photometer for the measurement of aerosol light absorption was recently introduced that builds on the simultaneous measurement of radiation transmitted through and scattered back from a particle-loaded fiber filter at multiple detection angles. The absorption coefficient of the filter-deposited aerosol is calculated from the optical properties of the entire filter system, which are determined by a two-stream-approximation radiative transfer scheme. In the course of the Reno Aerosol Optics Study (RAOS), the response characteristics of multiangle absorption photometry (MAAP) for white aerosol, pure black carbon aerosol from different sources, external mixtures of black and white aerosol, and ambient aerosol was investigated. The MAAP response characteristics were compared to basic filter transmittance and filter reflectance measurements. MAAP showed close agreement with a reference absorption measurement by extinction minus scattering. The slopes of regression lines vary between 0.99 ± 0.01 and 1.07 ± 0.02 for pure black carbon particles and external mixtures with ammonium sulphate to 1.03 ± 0.05 for ambient aerosol. No effect of the filter aerosol loading or the single-scattering albedo ω0 of the sampled aerosol on the MAAP response characteristics was observed. In contrast, transmittance and reflectance methods showed a clear impact of ω0 and the filter loading on the response characteristics, which requires the application of a correction function for the reliable determination of the aerosol absorption coefficient. In the case of nonabsorbing aerosol, the MAAP approach reduced the magnitude of the apparently measured absorption coefficient by one order of magnitude compared to a basic transmittance measurement.

Characterizing the annual-mean climatic effect of anthropogenic CO2 and aerosol emissions in eight coupled atmosphere-ocean GCMs

This work examines the spatial patterns of the transient response of mean annual temperature and precipitation to CO2 (or CO2 plus aerosol or aerosol proxy) radiative forcing in eight coupled AOGCMs, generally for the period 1900–2099. Response patterns are characterized using empirical orthogonal functions (EOFs) and the quasi-EOFs of Harvey and Wigley (the first qEOF field, discussed here, is given by the correlation between local year-by-year temperature changes and the global mean temperature change). The first temperature EOF accounts for 80–95% of the space-time variation of the CO2 run in all of the models, and is almost identical to qEOF1 of the temperature response or to the temperature change pattern averaged over the last 30 years of the simulations. EOF1 accounts for 80–95% of the space-time variation in the CO2+aerosol runs in six of the eight models. The CO2 response patterns of different models are highly correlated with one another (R 2 generally >0.5), and are also highly correlated with the CO2+aerosol response patterns (R 2≥ 0.85 in all except one model). The difference between CO2 and CO2+aerosol runs can be represented by EOF1 of the year-by-year differences, by qEOF1 of the year-by-year differences, or by the difference in temperature averaged over the last 30 years of each run. In models where these representations are highly correlated with each other, they are also highly correlated with CO2 EOF1. In other cases, aerosol EOF1 is modestly to highly correlated with control EOF1 (i.e.: the year-by-year differences between CO2 and CO2+aerosol runs are dominated by internal variability), while aerosol qEOF1 and the 30-year difference are highly correlated with each other. For all models, the decadal mean temperature change can be closely replicated by scaling the CO2 EOF1 pattern based on the global mean temperature changes (RMSE for the last decade is <6% of the RMS temperature change for CO2 runs, <8% for CO2+aerosol runs). The first EOF of the precipitation response to increasing CO2 accounts for only 10–30% of the space-time variation, and is generally highly correlated (R 2 up to 0.85) with control EOF1. In all of the models, there is an increase in precipitation in the ITCZ and a decrease in bands at or near 30°S and 30°N. In many models there is an El Nino-like response, including a substantial decrease in precipitation over the Amazon. Global-mean precipitation increases in all models due to CO2 forcing, but aerosols appear to have a disproportionally large effect in suppressing the increase compared to their effect in suppressing the warming. There is evidence in some models that the non-absorbing aerosols considered here reduce summer monsoon rainfall compared to the changes that would be expected based on the globally averaged effect of aerosols on precipitation. When regional precipitation changes over time are predicted by scaling a fixed precipitation-change pattern with the global mean temperature change, the global mean RMSE in the predicted change in decadal-mean precipitation is 25–35% of the global RMS precipitation changes by the end of the simulation.

Iron oxides and light absorption by pure desert dust: An experimental study

Theoretical computations based on Mie theory indicate that absorption of light by desert dust aerosols should be highly sensitive to their content in iron oxides (hematite, goethite, etc.). A selective extraction method that has been developed recently now allows quantification of these minerals in aerosol samples less than 500 μg in mass. Thus it is now possible to assess experimentally the part played by iron oxide minerals on dust absorption properties. In this paper we present an adaptation of Bond et al.'s [1999] method for measuring mineral dust mass absorption efficiencies and deriving single scattering albedo values at two wavelengths (325 and 660 nm). It consists in measuring simultaneously the aerosol mass concentration with a TEOM microbalance, its scattering properties in the visible spectrum with a 3 wavelength nephelometer, and attenuation at the 2 aforementioned λ with a dual‐wavelength aethalometer. At first the method is applied to nonabsorbing (iron oxide‐free) aerosols in order to check the magnitude of the “apparent absorption” due to scattering artifacts. In good agreement with Bond et al.'s results for visible wavelengths, it is found that 2% of scattering is misinterpreted as absorption. This proportion is also found to be practically insensitive to the aerosol size distribution. After these preliminary measurements the method is applied to aerosols generated by shaking three natural soil samples collected in one of the main Chinese dust source (Gobi desert), in northern Sahara (Tunisia), and in the Sahel (Niger). For these aerosols, mass absorption efficiencies are found to range between 10−2 and 2 10−2 m2 per gram of aerosol at 660 nm and to increase linearly with the iron oxide content at the rate of 0.56 m2 per gram of iron oxide. Owing to the larger absorbing potential of iron oxides at short wavelengths, mass absorption efficiencies at 325 nm are about 6 times larger than at 660 nm. At this last wavelength the single scattering albedo (SSA) is found to decrease from 0.97 for Chinese and Tunisian aerosols to 0.95 for the Niger one that also happens to have the largest iron oxide content (6.5% in mass). At 325 nm the SSA is much lower for the three aerosols (∼0.80) than at 660 nm. These values are similar to recent results obtained close to major mineral dust sources by inversion of Sun photometer or satellite data. Finally, simple computations performed for conditions that prevail at regional scale in the vicinity of important dust sources show that, even when mineral dust is mixed with strongly absorbent particles such as black carbon (BC), the effect of iron oxides on light absorption is in the same order of magnitude as the one of BC.

Relation between TOMS-AI and air masses in the Northeast Atlantic free troposphere

One of the most useful technique to evaluate the presence of atmospheric aerosol is based on the aerosol index (AI) calculated from the Total Ozone Mapping Spectrometer (TOMS) data. For this methodology, positive values of AI are related with UV-absorbing aerosols, mainly dust, smoke and volcanic aerosols, and negative values of AI are associated with non-absorbing aerosols. Using the aerosol index (AI) provided by TOMS and isentropic backwards trajectories, we have investigated the relation between the aerosol load of the air masses and the Al values. The back trajectories have been classified in eight categories attending to the contribution of the main aerosols sources, taking into account the geographical regions, the residence time in these sectors and the altitude of the air mass during its evolution towards the Izaña GAW station (IZO, 28.3 °N 16.5 °W, 2367m asl), Canary Islands. The categories are: African, Maritimes, Atlantic Free Troposphere, European, Maritimes-European, Maritimes-African, European-African, and European-African-Maritimes. Thus this station is characterized mainly by clean air masses with the origin in the middle troposphere and air masses from Africa with an annual frequency of 36% and 10% respectively of the total cases study. Each point of these back trajectories has been associated with the corresponding AI value, obtaining statistical information about the relation between AI values and expected aerosol type at the station. In this Atlantic region, the most important aerosol events are associated with Saharan dust outbreaks. For these episodes we have investigated the influence of the altitude of the air masses that reach IZO station, on the AI value. The main difference is the time that these air masses spend above 2km over the African continent. For air masses associated with Al positive values greater than 0.4 this time is around the 40%, whereas for air masses associated with AI values smaller than 0.4 is around 20%. 0 2003 COSPAR.

Chemical processes in a young biomass‐burning plume

The photochemistry in young biomass‐burning plumes depends on the emissions from the fire and their mixing with the background atmosphere as well as on the actinic flux. In the present study a three‐dimensional plume model is used to investigate the photochemical evolution of a biomass‐burning plume during the first tens of minutes after the fire emissions have been released into the atmosphere. The model results represent the evolution of the plume from the Quinault prescribed fire conducted during the Smoke, Cloud, and Radiation‐C (SCAR‐C) experiment. The modeled ozone concentrations of about 70 ppb are close to observations. The main nitrogen reservoir species downwind of the fire are HNO3and peroxyacetyl nitrate, accounting for about ∼60% and ∼30% of the total nitrogen reservoir species, respectively. Photolysis of formaldehyde, which is emitted from the fire, is the primary source of radicals in the plume. Omitting the emissions of oxygenated volatile organic compounds in the modeled fire plume leads to unrealistically low ozone concentrations in the simulations. A nonabsorbing aerosol as well as the lower emission of NOxin the simulations enhance the radical concentration, the photochemical ozone formation, and the oxidation efficiency, at least at the timescales considered here. Further investigations of the atmospheric processes in young biomass‐burning plumes will increase our understanding of the interaction of transport and chemical processes not only in biomass‐burning plumes but also in other convective systems.

Aerosol optical properties from columnar data during the second Aerosol Characterization Experiment on the south coast of Portugal

Measurements of aerosol optical properties over the atmospheric column (aerosol optical thickness, spectral angular sky radiance (sky brightness), and downwelling hemispheric flux) have been used to derive climate‐relevant aerosol parameters such as the phase function, the broadband single‐scattering albedo, and the refractive index. These parameters are needed to estimate the direct short‐wave radiative forcing by aerosols and to validate aerosol models in the satellite retrieval algorithms. Values of the broadband single‐scattering albedo obtained in this study range between ω0 = 0.98 (marine aerosols) and 0.90 (continental pollution aerosols). The columnar ambient broadband refractive index is found to be m = 1.39 ± 0.044 − i (&lt;0.003) for marine conditions and m = 1.48 ± 0.058 − i (0.01 ± 0.003) for polluted continental aerosols. Nonsphericity is shown to be important in the case of marine aerosols. Moreover, aerosol nonsphericity gives an additional contribution to the negative short‐wave radiative forcing of marine aerosols under clear‐sky conditions, which can be estimated as being 30 up to 50% of the radiative forcing estimated for spherical marine aerosols. In the case of continental polluted aerosols the optical properties can be represented by spherical particles, and no additional shape effect has to be considered. However, the aerosol absorption leads to an increase of about 40% of the radiative forcing estimated for nonabsorbing aerosol of the same size distribution.