Thick Aerosol Layers Research Articles

Retrieval of Aerosol Single-Scattering Albedo from MODIS Data Using an Artificial Neural Network

Aerosol single-scattering albedo (SSA) is one of the largest sources of uncertainty in the evaluation of the aerosol radiative forcing effect. The SSA signal, coupled with aerosol optical depth (AOD) and surface reflectance in satellite images, is difficult to retrieve by the look-up table approach. In this study, we proposed an artificial neural network- (ANN) based approach that retrieves SSA over land based on MODIS (moderate resolution imaging spectroradiometer) visible (red band) reflectance variations among nearby pixels that have different surface reflectivities. Using the training dataset generated by the radiative transfer model, the ANN model was trained to establish the relationship among SSA, surface reflectance, and top of atmosphere (TOA) reflectance. Then, based on the trained ANN model, SSA can be retrieved using the surface and apparent reflectance of several heterogeneous pixels. According to sensitivity analysis, this method works well on nonuniform land surfaces with high AODs. The root mean square error (RMSE) of retrieved and measured SSA (from 28 sites of AErosol RObotic NETwork, AERONET) was 0.042, of which the results with an error less than 0.03 accounted for 51%. In addition, the SSA retrieval method was applied to several thick aerosol layer events over different areas (South Asia, South America, and North China Plain) and compared with the ozone monitoring instrument near-UV aerosol data product (OMAERUV). The comparison results of the images show that the retrieval method of visible wavelength proposed in this study has similar outcomes to those from the ultraviolet wavelengths in these regions. The retrieval algorithm we propose provides an effective way to produce an SSA product in visible wavelength and might help to better estimate the aerosol radiative and optical properties over high heterogeneous areas, which is important for the aerosol radiative impact estimate at a regional scale.

Assessing CALIOP-Derived Planetary Boundary Layer Height Using Ground-Based Lidar

Coincident profiles from the space-borne and ground-based lidar measurements provide a unique opportunity to estimate the planetary boundary layer height (PBLH). In this study, PBLHs derived from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) were assessed by comparing them with those obtained from the ground-based lidar at Seoul National University (SNU) in Korea for both day and night from 2006 to 2019, and sounding data. CALIOP-derived PBLHs using wavelet covariance transform (WCT) are generally higher than those derived from the SNU lidar for both day and night. The difference in PBLH tends to increase as the signal-to-noise ratio for CALIOP decreases. The difference also increases as aerosol optical depth increases, implying that the PBLH estimated from CALIOP could be higher than that determined from the SNU lidar because of the signal attenuation within the aerosol layer under optically thick aerosol layer conditions. The higher PBLH for CALIOP in this study is mainly attributed to multiple aerosol layers. After eliminating multilayer cases, the PBLHs estimated from both the lidars showed significantly improved agreement: a mean difference of 0.09 km (R = 0.81) for daytime and 0.25 km (R = 0.51) for nighttime. The results from this study suggest that PBL detection using CALIOP is reliable for daytime if multilayer cases are removed. For nighttime, PBLHs derived from the SNU lidar and CALIOP showed a relatively large difference in frequency distribution compared with sounding data. It suggests that further investigations are needed for nighttime PBLHs, such as investigations about discriminating the residual layer and the difference between lidar-derived PBLH based on the aerosol layer and thermally derived PBLH from radiosonde data for the stable boundary layer during the nighttime.

Spatial distribution analysis of the OMI aerosol layer height: a pixel-by-pixel comparison to CALIOP observations

Abstract. A global picture of atmospheric aerosol vertical distribution with a high temporal resolution is of key importance not only for climate, cloud formation, and air quality research studies but also for correcting scattered radiation induced by aerosols in absorbing trace gas retrievals from passive satellite sensors. Aerosol layer height (ALH) was retrieved from the OMI 477 nm O2−O2 band and its spatial pattern evaluated over selected cloud-free scenes. Such retrievals benefit from a synergy with MODIS data to provide complementary information on aerosols and cloudy pixels. We used a neural network approach previously trained and developed. Comparison with CALIOP aerosol level 2 products over urban and industrial pollution in eastern China shows consistent spatial patterns with an uncertainty in the range of 462–648 m. In addition, we show the possibility to determine the height of thick aerosol layers released by intensive biomass burning events in South America and Russia from OMI visible measurements. A Saharan dust outbreak over sea is finally discussed. Complementary detailed analyses show that the assumed aerosol properties in the forward modelling are the key factors affecting the accuracy of the results, together with potential cloud residuals in the observation pixels. Furthermore, we demonstrate that the physical meaning of the retrieved ALH scalar corresponds to the weighted average of the vertical aerosol extinction profile. These encouraging findings strongly suggest the potential of the OMI ALH product, and in more general the use of the 477 nm O2−O2 band from present and future similar satellite sensors, for climate studies as well as for future aerosol correction in air quality trace gas retrievals.

Spatiotemporal evolution of a severe winter dust event in the western Mediterranean: Aerosol optical and physical properties

AbstractAn outstanding Saharan dust event affected the Iberian Peninsula during winter 2016 (20 to 25 February). The impact of this event on ambient PM10 surface levels and its spatial and temporal evolution was investigated by means of data from 250 air quality stations across mainland Spain and the Balearic Islands. The event had a significant impact on surface PM10 levels, and on 22 February, 90% of the air quality monitoring sites registered PM10 concentrations above the EU daily limit value of 50 μg/m3. The study of the attenuated backscattering vertical profiles associated with African dust evidenced a complex structure, with a thick aerosol layer that was at higher altitudes over the pre‐Pyrenees compared to the coastal area of Barcelona but closer to the surface than typically observed at both sites. Optical and physical properties of dust particles were investigated at the continental background Global Atmosphere Watch mountain observatory of Montsec (MSA) in the pre‐Pyrenees. Good agreement was found between in situ and passive remote sensing methodologies once the aloft dust layer reached the MSA station. Scattering Ångström exponent values decreased to values close to zero (even below zero for surface in situ measurements) indicating the predominance of coarse particles. On the contrary, absorption Ångström exponent values increased during the Saharan dust outbreak denoting an absorption enhancement at shorter wavelengths, characteristic of mineral dust particles. Furthermore, the performance of NMMB/BSC‐Dust and BSC‐DREAM8b models has been qualitatively evaluated for the dust spatial distribution across Spain and the vertical structure over MSA and Barcelona showing good agreement.

Utilization of O4 slant column density to derive aerosol layer height from a spaceborne UV-Visible hyperspectral sensor: Sensitivity and case study.

The sensitivities of oxygen-dimer (O4) slant column densities (SCDs) to changes in aerosol layer height are investigated using the simulated radiances by a radiative transfer model, the Linearlized pseudo-spherical vector discrete ordinate radiative transfer (VLIDORT), and the Differential Optical Absorption Spectroscopy (DOAS) technique. The sensitivities of the O4 index (O4I), which is defined as dividing O4 SCD by 1040 molecules2cm-5, to aerosol types and optical properties are also evaluated and compared. Among the O4 absorption bands at 340, 360, 380, and 477 nm, the O4 absorption band at 477 nm is found to be the most suitable to retrieve the aerosol effective height. However, the O4I at 477 nm is significantly influenced not only by the aerosol layer effective height but also by aerosol vertical profiles, optical properties including single scattering albedo (SSA), aerosol optical depth (AOD), particle size, and surface albedo. Overall, the error of the retrieved aerosol effective height is estimated to be 1276, 846, and 739 m for dust, non-absorbing, and absorbing aerosol, respectively, assuming knowledge on the aerosol vertical distribution shape. Using radiance data from the Ozone Monitoring Instrument (OMI), a new algorithm is developed to derive the aerosol effective height over East Asia after the determination of the aerosol type and AOD from the MODerate resolution Imaging Spectroradiometer (MODIS). About 80% of retrieved aerosol effective heights are within the error range of 1 km compared to those obtained from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) measurements on thick aerosol layer cases.

Sensitivity of Particle Extinction and Backscattering Calculation from Mie-Raman Lidar Measurements to the Choice of Ångström Exponent

Vibrational Raman scattering from nitrogen is commonly used in Mie-Raman lidars for evaluation of particle backscattering (β) and extinction (α) coefficients. However, vibrational scattering is characterized by significant frequency shift of the Raman component, so for the calculation of α and β the assumption about the extinction Angstrom exponent is needed. Simulation results presented in this study demonstrate that ambiguity in the choice of this exponent can be the significant source of uncertainty in the calculation of backscattering coefficients when optically thick aerosol layers are considered. Examples of lidar measurements and optical data calculated for different values of Angstrom exponent are given.

Aerosol influence on energy balance of the middle atmosphere of Jupiter.

Aerosols are ubiquitous in planetary atmospheres in the Solar System. However, radiative forcing on Jupiter has traditionally been attributed to solar heating and infrared cooling of gaseous constituents only, while the significance of aerosol radiative effects has been a long-standing controversy. Here we show, based on observations from the NASA spacecraft Voyager and Cassini, that gases alone cannot maintain the global energy balance in the middle atmosphere of Jupiter. Instead, a thick aerosol layer consisting of fluffy, fractal aggregate particles produced by photochemistry and auroral chemistry dominates the stratospheric radiative heating at middle and high latitudes, exceeding the local gas heating rate by a factor of 5–10. On a global average, aerosol heating is comparable to the gas contribution and aerosol cooling is more important than previously thought. We argue that fractal aggregate particles may also have a significant role in controlling the atmospheric radiative energy balance on other planets, as on Jupiter.

Evaluation of the operational Aerosol Layer Height retrieval algorithm for Sentinel-5 Precursor: application to O<sub>2</sub> A band observations from GOME-2A

Abstract. An algorithm setup for the operational Aerosol Layer Height product for TROPOMI on the Sentinel-5 Precursor mission is described and discussed, applied to GOME-2A data, and evaluated with lidar measurements. The algorithm makes a spectral fit of reflectance at the O2 A band in the near-infrared and the fit window runs from 758 to 770 nm. The aerosol profile is parameterised by a scattering layer with constant aerosol volume extinction coefficient and aerosol single scattering albedo and with a fixed pressure thickness. The algorithm's target parameter is the height of this layer. In this paper, we apply the algorithm to observations from GOME-2A in a number of systematic and extensive case studies, and we compare retrieved aerosol layer heights with lidar measurements. Aerosol scenes cover various aerosol types, both elevated and boundary layer aerosols, and land and sea surfaces. The aerosol optical thicknesses for these scenes are relatively moderate. Retrieval experiments with GOME-2A spectra are used to investigate various sensitivities, in which particular attention is given to the role of the surface albedo. From retrieval simulations with the single-layer model, we learn that the surface albedo should be a fit parameter when retrieving aerosol layer height from the O2 A band. Current uncertainties in surface albedo climatologies cause biases and non-convergences when the surface albedo is fixed in the retrieval. Biases disappear and convergence improves when the surface albedo is fitted, while precision of retrieved aerosol layer pressure is still largely within requirement levels. Moreover, we show that fitting the surface albedo helps to ameliorate biases in retrieved aerosol layer height when the assumed aerosol model is inaccurate. Subsequent retrievals with GOME-2A spectra confirm that convergence is better when the surface albedo is retrieved simultaneously with aerosol parameters. However, retrieved aerosol layer pressures are systematically low (i.e., layer high in the atmosphere) to the extent that retrieved values no longer realistically represent actual extinction profiles. When the surface albedo is fixed in retrievals with GOME-2A spectra, convergence deteriorates as expected, but retrieved aerosol layer pressures become much higher (i.e., layer lower in atmosphere). The comparison with lidar measurements indicates that retrieved aerosol layer heights are indeed representative of the underlying profile in that case. Finally, subsequent retrieval simulations with two-layer aerosol profiles show that a model error in the assumed profile (two layers in the simulation but only one in the retrieval) is partly absorbed by the surface albedo when this parameter is fitted. This is expected in view of the correlations between errors in fit parameters and the effect is relatively small for elevated layers (less than 100 hPa). If one of the scattering layers is near the surface (boundary layer aerosols), the effect becomes surprisingly large, in such a way that the retrieved height of the single layer is above the two-layer profile. Furthermore, we find that the retrieval solution, once retrieval converges, hardly depends on the starting values for the fit. Sensitivity experiments with GOME-2A spectra also show that aerosol layer height is indeed relatively robust against inaccuracies in the assumed aerosol model, even when the surface albedo is not fitted. We show spectral fit residuals, which can be used for further investigations. Fit residuals may be partly explained by spectroscopic uncertainties, which is suggested by an experiment showing the improvement of convergence when the absorption cross section is scaled in agreement with Butz et al. (2013) and Crisp et al. (2012), and a temperature offset to the a priori ECMWF temperature profile is fitted. Retrieved temperature offsets are always negative and quite large (ranging between −4 and −8 K), which is not expected if temperature offsets absorb remaining inaccuracies in meteorological data. Other sensitivity experiments investigate fitting of stray light and fluorescence emissions. We find negative radiance offsets and negative fluorescence emissions, also for non-vegetated areas, but from the results it is not clear whether fitting these parameters improves the retrieval. Based on the present results, the operational baseline for the Aerosol Layer Height product currently will not fit the surface albedo. The product will be particularly suited for elevated, optically thick aerosol layers. In addition to its scientific value in climate research, anticipated applications of the product for TROPOMI are providing aerosol height information for aviation safety and improving interpretation of the Absorbing Aerosol Index.

Use of rotational Raman measurements in multiwavelength aerosol lidar for evaluation of particle backscattering and extinction

Abstract. Vibrational Raman scattering from nitrogen is commonly used in aerosol lidars for evaluation of particle backscattering (β) and extinction (α) coefficients. However, at mid-visible wavelengths, particularly in the daytime, previous measurements have possessed low signal-to-noise ratio. Also, vibrational scattering is characterized by a significant frequency shift of the Raman component, so for the calculation of α and β information about the extinction Ångström exponent is needed. Simulation results presented in this study demonstrate that ambiguity in the choice of Ångström exponent can be the a significant source of uncertainty in the calculation of backscattering coefficients when optically thick aerosol layers are considered. Both of these issues are addressed by the use of pure-rotational Raman (RR) scattering, which is characterized by a higher cross section compared to nitrogen vibrational scattering, and by a much smaller frequency shift, which essentially removes the sensitivity to changes in the Ångström exponent. We describe a practical implementation of rotational Raman measurements in an existing Mie–Raman lidar to obtain aerosol extinction and backscattering at 532 nm. A 2.3 nm width interference filter was used to select a spectral range characterized by low temperature sensitivity within the anti-Stokes branch of the RR spectrum. Simulations demonstrate that the temperature dependence of the scattering cross section does not exceed 1.5 % in the 230–300 K range, making correction for this dependence quite easy. With this upgrade, the NASA GSFC multiwavelength Raman lidar has demonstrated useful α532 measurements and was used for regular observations. Examples of lidar measurements and inversion of optical data to the particle microphysics are given.

Nadir airborne lidar observations of deep aerosol layers

Abstract. The observation of deep and optically thick aerosol layers by a nadir-pointing lidar poses a challenge in terms of the signal inversion into a geophysically meaningful quantity such as extinction coefficient. A far-end reference molecular layer will usually be unavailable if the observed layer is near the surface, and using a near-end reference results in an unstable mathematical solution. In this paper, it is demonstrated that a far-end reference, taken within the aerosol layer, yields a better solution, and that the influence of the reference reduces strongly when coming inward, so that 1–2 km above reference the solution can be trusted. A method is developed to set the reference using the assumption of a well-mixed layer near the surface, and its effect is tested on data collected during recent aircraft-based campaigns. The method is also tested on simulated profiles in order to verify its limits and accuracy. The assumption of a well-mixed layer can be relaxed if one is able to set the reference well within a layer rather than at its boundaries.

Satellite observations of desert dust‐induced Himalayan snow darkening

AbstractThe optically thick aerosol layer along the southern edge of the Himalaya has been subject of several recent investigations relating to its radiative impacts on the South Asian summer monsoon and regional climate forcing. Prior to the onset of summer monsoon, mineral dust from southwest Asian deserts is transported over the Himalayan foothills on an annual basis. Episodic dust plumes are also advected over the Himalaya, visible as dust‐laden snow surface in satellite imagery, particularly in western Himalaya. We examined spectral surface reflectance retrieved from spaceborne MODIS observations that show characteristic reduction in the visible wavelengths (0.47 µm) over western Himalaya, associated with dust‐induced solar absorption. Case studies as well as seasonal variations of reflectance indicate a significant gradient across the visible (0.47 µm) to near‐infrared (0.86 µm) spectrum (VIS‐NIR), during premonsoon period. Enhanced absorption at shorter visible wavelengths and the resulting VIS‐NIR gradient is consistent with model calculations of snow reflectance with dust impurity. While the role of black carbon in snow cannot be ruled out, our satellite‐based analysis suggests the observed spectral reflectance gradient dominated by dust‐induced solar absorption during premonsoon season. From an observational viewpoint, this study underscores the importance of mineral dust deposition toward darkening of the western Himalayan snow cover, with potential implications to accelerated seasonal snowmelt and regional snow albedo feedbacks.

Heavy air pollution suppresses summer thunderstorms in central China

Time series of rainfall, thunderstorms, temperatures, winds and aerosols of 50 years have been analyzed at the Xian valley (1951–2005, rain rates data are only available for the period of 1961–2000 for Xian) and the nearby Mount Hua (1951–2005) in central China, for assessing the impact of the increasing air pollution on convective precipitation. Adding aerosols to pristine air initially increases convective rainfall. However, aerosol amounts in the Xian valley (represented by large AOT and significant decreasing trend in visibility in the study area) have been shown to be sufficiently high so that added aerosols suppress convection and precipitation, by both radiative and microphysical effects, even at the starting of the analysis period in the 1950s.It was found that the aerosol's negative radiative forcing stabilized the lowest troposphere. The stabilization resulted in less vertical exchanges of air, which caused reduction in the lowland (Xian) surface winds and increase in the highland (Mount Hua) wind speeds. The decreased instability caused a decrease in the frequency of the thunderstorm normalized by rainfall amount in the lowland due to the thick aerosol layer above, but not at the highland, above which the aerosol layer was much thinner. The indicated decreasing trend of highland precipitation was associated with a similar size decreasing trend in thunderstorm frequency. This decrease was contributed by light and moderate (<25mmday−1) rainy days. These patterns of rainfall changes at the highland are consistent with the microphysical suppressive effects of aerosols.Despite the dramatic relative decrease in the already originally scarce thunderstorm activity in the Xian valley, the rainfall amount there appears to have little diurnal cycle, and shows little trend with the increasing aerosol amounts. Because only small fraction of the rainfall in Xian is generated by local instability, as indicated by the flat diurnal cycle, it appears to be a condition which is unsuitable for quantifying the impact of heavy aerosols on rainfall amounts. However, the dramatic relative decrease of the scarce thunderstorms in Xian suggests that aerosol's radiative effect can be substantial. Such study should be extended to other areas where local surface heating dominates rainfall amount.

Interpretation of FRESCO cloud retrievals in case of absorbing aerosol events

Abstract. Cloud and aerosol information is needed in trace gas retrievals from satellite measurements. The Fast REtrieval Scheme for Clouds from the Oxygen A band (FRESCO) cloud algorithm employs reflectance spectra of the O2 A band around 760 nm to derive cloud pressure and effective cloud fraction. In general, clouds contribute more to the O2 A band reflectance than aerosols. Therefore, the FRESCO algorithm does not correct for aerosol effects in the retrievals and attributes the retrieved cloud information entirely to the presence of clouds, and not to aerosols. For events with high aerosol loading, aerosols may have a dominant effect, especially for almost cloud free scenes. We have analysed FRESCO cloud data and Absorbing Aerosol Index (AAI) data from the Global Ozone Monitoring Experiment (GOME-2) instrument on the Metop-A satellite for events with typical absorbing aerosol types, such as volcanic ash, desert dust and smoke. We find that the FRESCO effective cloud fractions are correlated with the AAI data for these absorbing aerosol events and that the FRESCO cloud pressure contains information on aerosol layer pressure. For cloud free scenes, the derived FRESCO cloud pressure is close to the aerosol layer pressure, especially for optically thick aerosol layers. For cloudy scenes, if the strongly absorbing aerosols are located above the clouds, then the retrieved FRESCO cloud pressure may represent the height of the aerosol layer rather than the height of the clouds. Combining FRESCO and AAI data, an estimate for the aerosol layer pressure can be given.

The radiative effect of an aged, internally mixed Arctic aerosol originating from lower-latitude biomass burning

Arctic-haze layers and their radiative effects have been investigated previously in numerous studies as they are known to have an impact on the regional climate. In this study, we report on an event of an elevated aerosol layer, notably consisting of high-absorbing soot particles, observed in the European Arctic free troposphere the 2007 April 14 during the ASTAR 2007 campaign. The ca. 0.5 km vertically thick aerosol layer located at an altitude of around 3 km had a particle-size distribution mode around 250 nm diameter. In this study, we quantify the radiative effect aerosol layers have on the Arctic atmosphere by using in situ observations. Measurements of particles size segregated temperature stability using thermal denuders, indicate that the aerosol in the optically active size range was chemically internally mixed. In the plume, maximum observed absorption and scattering coefficients were 3 × 10−6 and 20 × 10−6 m−1, respectively. Observed microphysical and optical properties were used to constrain calculations of heating rates of an internally mixed aerosol assuming two different surface albedos that represent snow/ice covered and open ocean. The average profile resulted in a heating rate in the layer of 0.2 K d−1 for the high-albedo case and 0.15 K d−1 for the low albedo case. This calculated dependence on albedo based on actual observations corroborates previous numerical simulations. The heating within the plume resulted in a measurable signal shown as an enhancement in the temperature of a few tenths of a degree. Although the origin of the aerosol plume could not unambiguously be determined, the microphysical properties of the aerosol had strong similarities with previously reported biomass burning plumes. With a changing climate, short-lived pollutants such as biomass plumes may become more frequent in the Arctic and have important radiative effects at regional scale.

Meteorological conditions and polar stratospheric clouds over Yakutsk in winter 2004/05

Regular observations of polar stratospheric clouds were started at a lidar stratospheric station in the city of Yakutsk. According to lidar measurements in the winter of 2004/05, thick aerosol layers in the lower stratosphere appeared over this region quite frequently. They were episodically observed as pearl clouds. In November 2004, polar stratospheric clouds were observed at the stratospheric temperatures that were much higher than those at which particles of the polar stratospheric clouds could condense. Analysis of air-mass trajectories that simultaneously passed over Yakutsk at different altitudes on the days of polar stratospheric cloud observations showed that clouds could be formed over the Norwegian Sea at altitudes of about 18–21 km, where the stratosphere was the coldest, and then were transported to Yakutsk by wind during 4–5 days.

Simulation of secondary aerosols over North China in summer

The comparisons of observed and simulated NOx, CO, O3, NH3, HNO3, SO2 and PM2.5 indicate that CMAQ model can simulate variations of pollutants over North China well. Moreover, the model results show that high NH3 is in Hebei, Henan and Shandong provinces, with average concentration of (30-35)×10-9. The results of the sensitive experiment indicate that high concentration of NH3 has the efficiency of the production of secondary sulfate aerosol increase by more than 30%, especially at the juncture of Handan, Anyang and Changzhi that increased by 50%. In addition, NH3 also produces secondary ammonia and nitrate aerosol, and the sum of them is approximately equal to sulfate aerosol. The height of planetary boundary layer (PBL) in Beijing is higher in daytime, with average height of 1500 m at noon. This makes SO2, NH3 and HNO3 transported into upper PBL of 850 hPa. The high secondary sulfate, and ammonia and nitrate aerosol happen in the upper and lower PBL, respectively. Because PM2.5 lifetime is relatively long, it can be transported into the middle troposphere to form a thick aerosol layer, which is the arched roof of aerosol. The model result suggests that if the aerosol concentration in North China would be controlled, the reduction of NH3 emission is one of efficient ways besides the reduction of primary SO2, NOx and aerosol emission.

Spectral absorption of solar radiation by aerosols during ACE‐Asia

As part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia), the upward and downward spectral solar radiant fluxes were measured with the Spectral Solar Flux Radiometer (SSFR), and the aerosol optical depth was measured with the Ames Airborne Tracking Sunphotometer (AATS‐14) aboard the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. In this paper, we examine the data obtained for two cases: a moderately thick aerosol layer, 12 April, and a relatively thin aerosol case, 16 April 2001. On both days, the Twin Otter flew vertical profiles in the Korean Strait southeast of Gosan Island. For both days we determine the aerosol spectral absorption of the layer and estimate the spectral aerosol absorption optical depth and single‐scattering albedo. The results for 12 April show that the single‐scattering albedo increases with wavelength from 0.8 at 400 nm to 0.95 at 900 nm and remains essentially constant from 950 to 1700 nm. On 16 April the amount of aerosol absorption was very low; however, the aerosol single‐scattering albedo appears to decrease slightly with wavelength in the visible region. We interpret these results in light of the two absorbing aerosol species observed during the ACE‐Asia study: mineral dust and black carbon. The results for 12 April are indicative of a mineral dust‐black carbon mixture. The 16 April results are possibly caused by black carbon mixed with nonabsorbing pollution aerosols. For the 12 April case we attempt to estimate the relative contributions of the black carbon particles and the mineral dust particles. We compare our results with other estimates of the aerosol properties from a Sea‐Viewing Wide Field‐of View Sensor (SeaWiFS) satellite analysis and aerosol measurements made aboard the Twin Otter, aboard the National Oceanic and Atmospheric Administration Ronald H. Brown ship, and at ground sites in Gosan and Japan. The results indicate a relatively complicated aerosol mixture of both industrial pollution (including black carbon) and mineral dust. This underscores the need for careful measurements and analysis to separate out the absorption effects of mineral dust and black carbon in the east Asia region.

Synergetic measurements of aerosols over São Paulo, Brazil using LIDAR, sunphotometer and satellite data during the dry season

Abstract. A backscattering LIDAR system, the first of this kind in Brazil, has been set-up in a suburban area in the city of São Paulo (23º33' S, 46º44' W) to provide the vertical profile of the aerosol backscatter coefficient at 532 nm up to an altitude of 4-6 km above sea level (asl). The measurements have been carried out during the second half of the so-called Brazilian dry season, September and October 2001 and during the first half of the dry season in August and September 2002. The LIDAR data are presented and analysed in synergy with aerosol optical thickness (AOT) measurements obtained by a CIMEL sun-tracking photometer in the visible spectral region and with satellite measurements obtained by the MODIS sensor. This synergetic approach has been used, not only to validate the LIDAR data, but also to derive a typical value (45 sr) of the so-called extinction-to-backscatter ratio (LIDAR ratio) during the dry season. The satellite data analysis offers additional information on the spatial distribution of aerosols over Brazil including the determination of aerosol source regions over the country. The LIDAR data were also used to retrieve the Planetary Boundary Layer (PBL) height, aerosol layering and the structure of the lower troposphere over the city of São Paulo. These first LIDAR measurements over the city of São Paulo during the dry season showed a significant variability of the AOT in the lower troposphere (0.5-5 km) at 532 nm. It was also found that the aerosol load is maximized in the 1-3 km height region, although up to 3 km thick aerosol layers were also detected in the 2.5-5.5 km region in certain cases. Three-dimensional 96-hours air mass back-trajectory analysis was also performed in selected cases to determine the source regions of aerosols around São Paulo during the dry season.

Africa burning: A thematic analysis of the Southern African Regional Science Initiative (SAFARI 2000)

The Southern African Regional Science Initiative (SAFARI 2000) was a major surface, airborne, and spaceborne field campaign carried out in southern Africa in 2000 and 2001 that addressed a broad range of phenomena related to land‐atmosphere interactions and the biogeochemical functioning of the southern African system. This paper presents a thematic analysis and integration of the Journal of Geophysical Research SAFARI 2000 Special Issue, presenting key findings of an intensive field campaign over southern Africa in August and September of 2000. The integrating themes deal with surface emissions characterization; airborne characterizations of aerosols and trace gases; regional haze and trace gas characterization; and radiant measurements by surface, aircraft, and remote sensing platforms. Enhanced regional fuel loads associated with the moist La Niña phase of the El Niño‐Southern Oscillation (ENSO) cycle produced above average biomass burning emissions, which consequently dominated all other aerosol and trace gas emissions during the dry season. Southward transport of a broad plume of smoke originating in equatorial Africa and exiting off the east coast toward the Indian Ocean (the river of smoke) is attributed to unusual synoptic airflows associated the ENSO phase. New and revised biogenic and pyrogenic emission factors are reported, including a number of previously unreported oxygenated organic compounds and inorganic compounds from biomass combustion. Emission factors are scaled up to regional emission surfaces for biogenic species utilizing species specific and light‐dependent emission factors. Fire scar estimates reveal contradictory information on the timing of the peak and extent of the biomass‐burning season. Integrated tall stack coordinated measurements (between ground, airborne and remotely sensing platforms) of upwelling and downwelling radiation in massive thick aerosol layers covering much of southern Africa yield consistent estimates of large negative forcing for both surface and top of atmosphere radiative forcing. Radiation calculations are supported by novel information on chemical speciation and internal aerosol particle structure. The overall conclusion is that SAFARI 2000, as an integrating theme, has been able to give significant new insights into the regional scale biogeochemical cycling of southern Africa and contributed in important ways to the validation of remote sensing instruments on board the NASA Terra spacecraft.

Coordinated airborne, spaceborne, and ground‐based measurements of massive thick aerosol layers during the dry season in southern Africa

During the dry season airborne campaign of the Southern African Regional Science Initiative (SAFARI 2000), coordinated observations were made of massive thick aerosol layers. These layers were often dominated by aerosols from biomass burning. We report on airborne Sun photometer measurements of aerosol optical depth (λ = 0.354–1.557 μm), columnar water vapor, and vertical profiles of aerosol extinction and water vapor density that were obtained aboard the University of Washington's Convair‐580 research aircraft. We compare these with ground‐based AERONET Sun/sky radiometer results, with ground based lidar data (MPL‐Net), and with measurements from a downward pointing lidar aboard the high‐flying NASA ER‐2 aircraft. Finally, we show comparisons between aerosol optical depths from the Sun photometer and those retrieved over land and over water using four spaceborne sensors (TOMS, MODIS, MISR, and ATSR‐2).