Aerosol Air Mass Research Articles

The influence of a south Asian dust storm on aerosol radiative forcing at a high-altitude station in central Himalayas

The impact of long-range transported dust aerosols, originating from the Thar Desert region, to a high-altitude station in the central Himalayas was studied with the help of micro-pulse lidar (MPL) observations. A drastic change in lidar back-scatter profile was observed on a dust day as compared with that on a pre-dust day. The back-scatter coefficient on a dust day revealed that the dust layer peaked at an altitude ∼1300 m above ground level (AGL) and extended up to ∼3000 m AGL, with maximum value ∼3 × 10–5 m–1 sr–1. Aerosol Index (AI) and air mass back-trajectory analysis substantiate the transport of dust aerosols from the far-off Thar Desert region to the experimental site. A significant effect of dust aerosols was also observed over the station on the spectral aerosol optical depths (AODs), measured using a Microtops-II Sunphotometer. It showed significantly different spectral behaviour of AOD on a dust day as compared with that on a pre-dust day. The Ångström exponent (α) showed a marked decrease from 0.42 to 0.04 from the pre-dust day to the dust day. The aerosol radiative forcing estimated using the Santa Barbara DISORT (discrete ordinate radiative transfer) atmospheric radiative transfer (SBDART) model, in conjunction with the optical properties of aerosol and cloud (OPAC) model, showed values of about –30, –45 and +15 W m–2, respectively, at top-of-atmosphere (TOA), surface and in the atmosphere on the dust day. The positive atmosphere forcing caused an estimated heating of the lower atmosphere by ∼0.4 K day–1.

Biomass burning aerosol detection over Buenos Aires City, August 2009

At the end of August 2009, a biomass burning aerosol intrusion event was detected at the Laser and Applications Research Center, CEILAP (CITEFA-CONICET) (34.5° S – 58.5° W) at Villa Martelli, in Buenos Aires, Argentina. This center has a sunphotometer from the AERONET-NASA global network, UV solar radiation sensors, a meteorological station and an aerosol lidar system. The aerosol origin was determined by means of back-trajectories and satellite images. This work studies the aerosol air mass optical characterization and their effect in UV solar radiation.

MISR Aerosol Product Attributes and Statistical Comparisons With MODIS

In this paper, Multi-angle Imaging SpectroRadiometer (MISR) aerosol product attributes are described, including geometry and algorithm performance flags. Actual retrieval coverage is mapped and explained in detail using representative global monthly data. Statistical comparisons are made with coincident aerosol optical depth (AOD) and Angstrom exponent (ANG) retrieval results from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. The relationship between these results and the ones previously obtained for MISR and MODIS individually, based on comparisons with coincident ground-truth observations, is established. For the data examined, MISR and MODIS each obtain successful aerosol retrievals about 15% of the time, and coincident MISR-MODIS aerosol retrievals are obtained for about 6%-7% of the total overlap region. Cloud avoidance, glint and oblique-Sun exclusions, and other algorithm physical limitations account for these results. For both MISR and MODIS, successful retrievals are obtained for over 75% of locations where attempts are made. Where coincident AOD retrievals are obtained over ocean, the MISR-MODIS correlation coefficient is about 0.9; over land, the correlation coefficient is about 0.7. Differences are traced to specific known algorithm issues or conditions. Over-ocean ANG comparisons yield a correlation of 0.67, showing consistency in distinguishing aerosol air masses dominated by coarse-mode versus fine-mode particles. Sampling considerations imply that care must be taken when assessing monthly global aerosol direct radiative forcing and AOD trends with these products, but they can be used directly for many other applications, such as regional AOD gradient and aerosol air mass type mapping and aerosol transport model validation. Users are urged to take seriously the published product data-quality statements.

Desert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging

Desert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging

Desert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging

Coincident observations made over the Moroccan desert during the Sahara mineral dust experiment (SAMUM) 2006 field campaign are used both to validate aerosol amount and type retrieved from multi-angle imaging spectroradiometer (MISR) observations, and to place the suborbital aerosol measurements into the satellite’s larger regional context. On three moderately dusty days during which coincident observations were made, MISR mid-visible aerosol optical thickness (AOT) agrees with field measurements point-by-point to within 0.05–0.1. This is about as well as can be expected given spatial sampling differences; the space-based observations capture AOT trends and variability over an extended region. The field data also validate MISR’s ability to distinguish and to map aerosol air masses, from the combination of retrieved constraints on particle size, shape and single-scattering albedo. For the three study days, the satellite observations (1) highlight regional gradients in the mix of dust and background spherical particles, (2) identify a dust plume most likely part of a density flow and (3) show an aerosol air mass containing a higher proportion of small, spherical particles than the surroundings, that appears to be aerosol pollution transported from several thousand kilometres away.

Aerosol Optical Depth, Ozone and Water Vapor Measurements over Gadanki, A Tropical Station in Peninsular India

This paper reports the results of a study related to the optical and physical characteristics of columnar aerosols and variation in total column ozone (TCO) and precipitable water content (PWC) over Gadanki (13.45°N, 79.18°E), a tropical station in peninsular India, for the first time, using MICROTOPS-II (Microprocessor-based Total Ozone Portable Spectrometer), comprising of both sun photometer and ozonometer. Results show wavelength dependence of AOD, having mean value of ~0.4 (±0.09) at 500 nm optical channel. Daily mean aerosol size spectra shows, most of the time, power-law distribution. However, its diurnal variations show significant changes in aerosol size spectra modulated by a combination of both power-law and bi-modal distributions. To characterize AOD, the Angstrom parameters (i.e., α and β) were used. The day-to-day variations in TCO were found to be in fair agreement with that derived from TOMS satellite data for all the experimental days, having mean observed value of ~253 (±8) DU over the station. Interestingly, an inverse relationship between TCO and AOD or PWC was observed over the station, on some times of the day, which could be attributed to the mixing of significant fraction of ozone with aerosol and water vapor-rich air mass. However, a significant positive correlation was observed between AOD and PWC.

Application of satellite and ground-based data to investigate the UV radiative effects of Australian aerosols

An understanding of the effect of aerosols on biologically- and photochemically-active UV radiation reaching the Earth's surface is important for many ongoing climate, biophysical, and air pollution studies. In particular, estimates of the UV characteristics of the most common Australian aerosols will be valuable inputs to UV Index forecasts, air quality studies, and assessments of the impact of regional environmental changes. By analyzing climatological distributions of Australian aerosols we have identified sites where co-located ground-based UV-B and ozone measurements were available during episodes of relatively high aerosol activity. Since at least June 2003, surface UV global irradiance spectra (285–450 nm) have been measured routinely at Darwin and Alice Springs in Australia by the Australian Bureau of Meteorology (BoM). Using co-located sunphotometer measurements at Darwin and Alice Springs, we identified several episodes of relatively high aerosol activity. Aerosol air mass types were analyzed from sunphotometer-derived angstrom parameter, MODIS fire maps and MISR aerosol property retrievals. To assess aerosol effects we compared the measured UV irradiances for aerosol-loaded and clear-sky conditions with each other and with irradiances simulated using the libRadtran radiative transfer model for aerosol-free conditions. We found that for otherwise similar atmospheric conditions, smoke aerosols over Darwin reduced the surface UV irradiance by as much as 40–50% at 290–300 nm and 20–25% at 320–400 nm near active fires (aerosol optical depth, AOD, at 500 nm ∼ 0.6). Downwind of fires, the smoke aerosols over Darwin reduced the surface irradiance by 15–25% at 290–300 nm and ∼ 10% at 320–350 nm (AOD at 500 nm ∼ 0.2). The effect of smoke increased with decrease of wavelength and is strongest in the UV-B. The aerosol attenuation factors calculated for the selected cases suggest smoke over Darwin has an effect on surface 340–380 nm irradiances that is comparable to that produced by smoke over Sub-Saharan Africa. Dust activity was very low at Alice Springs during 2004, therefore we were not able to identify strong dust events to fully assess the UV effect of dust. For the cases studied, smoke aerosols seem to produce a stronger reduction in surface UV irradiances than dust aerosols.

Aerosol optical properties during the 2004 New England Air Quality Study–Intercontinental Transport and Chemical Transformation: Gulf of Maine surface measurements—Regional and case studies

We report results from the 2004 New England Air Quality Study–Intercontinental Transport and Chemical Transformation (NEAQS‐ITCT) based on measurements of aerosol optical properties in the marine boundary layer of the Gulf of Maine. The in situ data collected on board the NOAA research vessel Ronald H. Brown includes extensive properties of aerosol scattering coefficient, absorption coefficient, and optical depth at multiple wavelengths in the visible. From these, intensive properties were derived, including the Ångström exponent of scattering, single scattering albedo, and submicrometer fraction of scattering. The optical particle properties were sorted by time and location on the basis of aerosol source regions and air mass back trajectories. The results indicate a large degree of temporal and spatial variability in the observed parameters: up to 80% in the scattering and absorption coefficients, 55% in the Ångström exponent, 16% in the single scattering albedo, and 25% in the submicrometer fraction of scattering. The variability is mainly due to the multitude of aerosol sources and transport pathways in the study area. During NEAQS 2004 the mean scattering coefficients were about 20 to 40% lower than those measured during NEAQS 2002 but showed a similar variability. The mean absorption coefficient for the aerosol during NEAQS 2002 was about 80% larger than that measured during NEAQS 2004. The modal parameters of the volume‐size distributions and refractive index and density of the particles were consistent with the aerosol models used in the MODIS land and ocean aerosol retrieval algorithms.

Columnar characteristics of aerosols by spectroradiometer measurements in the maritime area of the Cadiz Gulf (Spain)

AbstractAtmospheric aerosol characteristics represented by the spectral aerosol optical depth AOD) and the α Ångström turbidity parameter were determined in the coastal area of the Gulf of Cádiz, (southwest of Spain). The columnar aerosol properties presented here correspond to the 1996–1999 period, and were obtained by solar direct irradiance measurements carried out by a Licor1800 spectroradiometer. The performance of this type of medium‐spectral resolution radiometric system is analysed over the measured period. The detailed spectral information of these irradiance measurements enabled the use of selected non‐absorption gases spectral windows to determine the columnar spectral AOD that was modelled by Ångström formula to obtain the α coefficient. Temporal evolutions of instantaneous values together with a general statistical analysis represented by seasonal values, frequency distributions and some representative correlations for the AOD and the derived α Ångström coefficient gave us the first insight of aerosol characteristics in this coastal area. Special attention was paid to the analysis of these aerosol properties at the nominal wavelengths of 440 nm, 670 nm, 870 nm and 1020 nm for the near‐future comparisons with the Cimel sun‐photometer data. However, taking the most representative aerosol wavelength of 500 nm, the variability of the AOD ranges from 0.005 to 0.53, with a mean of 0.12 (s.d = 0.07) and that of the α parameter is given by a mean value of 0.93 (s.d. = 0.58) falling inside the range of marine aerosols. A quantitative discrimination of aerosol types was conducted on the basis of the spectral aerosol properties and air mass back trajectory analysis, which resulted in a mixed type because of the specificity of this area, given by very frequent desert dust episodes, continental and polluted local influences. This study represents the first extended data characterization about columnar properties of aerosols in Spain which has been continued by Cimel‐AERONET data. Copyright © 2005 Royal Meteorological Society.

Relationships between surface and column aerosol radiative properties and air mass transport at a rural New England site

Chemical, physical, and radiative properties of surface and vertical column aerosols were measured at a rural site in southern New Hampshire from July 2000 to September 2001. The primary objective was to determine how intensive and extensive aerosol properties vary in air masses originating in different upwind regions. The data set also allows for an investigation of some of the relationships between surface and column aerosol properties at the site, and provides an estimate of direct radiative forcing by aerosols during the study period. Extensive properties (e.g., optical depth and chemical concentration) were at maximum values during times of south‐southwest (S‐SW) transport, while minimum values were seen during north‐northeast (N‐NE) transport. Certain intensive properties such as fine particle mass scattering efficiency did not vary significantly between times of transport from different source regions. Mean optical depth (wavelength = 500 nm) was 0.24 during S‐SW transport, compared to 0.10 during N‐NE transport. The study period average scattering efficiency for (NH4)2SO4 was 6.54 ± 0.26 m2 g−1 (± standard error) and 3.36 ± 0.49 m2 g−1 for organic carbon, while the absorption efficiency of elemental carbon was 12.85 ± 0.80 m2 g−1. Top of the atmosphere aerosol direct radiative forcing was −0.35 ± 0.83 Wm−2 (±1 standard deviation) in winter 2000–2001 and −9.06 ± 3.77 Wm−2 in summer 2001, differences that can be primarily attributed to seasonal changes in surface reflectance (high in winter, low in summer) and the relatively low values of single scatter albedo observed in winter. The annual average direct radiative forcing was −5.14 ± 4.32 Wm−2. We generally observed a moderate correlation between surface and column aerosol light extinction, suggesting that vertical column aerosol radiative properties measured by surface‐based radiometers should be supplemented by boundary layer measurements of aerosol chemical, physical, and radiative properties to help understand the mechanisms contributing to global aerosol variability.

Sensitivity of multiangle imaging to natural mixtures of aerosols over ocean

Multiangle remote sensing data can discriminate among aerosol air mass types, as represented by climatologically probable, external mixtures of component particles. Retrievals are performed over a comparison space of four‐component mixtures, selected from six commonly observed components having assumed, fixed microphysical properties but with mixing ratios free to vary from 0% to 100%. We refer to this approach, which assumes climatologically probable component particles and derives aerosol mixtures from the observations, as a “climatological retrieval.” On the basis of simulated Multiangle Imaging Spectroradiometer (MISR) observations over dark water, the retrieval can distinguish mixtures containing large, spherical particles (sea salt), nonspherical particles (accumulation and coarse mode dust), and small, dark particles (black carbon) to within 20% or better of each component's true mixing ratio. This is sufficient to distinguish maritime from continental aerosol air masses. The retrievals, which use all nine MISR angles and the two wavelengths least affected by ocean surface reflectance (672 and 867 nm), are not good at distinguishing medium, spherical, nonabsorbing (sulfate) from medium, spherical, absorbing (carbonaceous) particles. However, the sum is retrieved to within 20% of the true mixing ratio or better. This is significantly more detail about the properties of particle mixtures than has previously been retrieved from satellite data, and in all cases, the derived total column aerosol optical depth remains well constrained, to at least 0.05 or 20%, whichever is larger. We expect the MISR data, with its frequent global coverage, to complement in situ and field data, which can provide greater detail about aerosol size and composition locally. This combined effort should advance our knowledge of aerosol behavior globally and our ability to model the impact of aerosols on the climatically important solar radiation budget.

Ozone and aerosol distributions and air mass characteristics over the South Pacific during the burning season

In situ and laser remote measurements of gases and aerosols were made with airborne instrumentation to establish a baseline chemical signature of the atmosphere above the South Pacific Ocean during the NASA Global Tropospheric Experiment (GTE)/Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A) conducted in August‐October 1996. This paper discusses general characteristics of the air masses encountered during this experiment using an airborne lidar system for measurements of the large‐scale variations in ozone (O3) and aerosol distributions across the troposphere, calculated potential vorticity (PV) from the European Centre for Medium‐Range Weather Forecasting (ECMWF), and in situ measurements for comprehensive air mass composition. Between 8°S and 52°S, biomass burning plumes containing elevated levels of O3, over 100 ppbv, were frequently encountered by the aircraft at altitudes ranging from 2 to 9 km. Air with elevated O3 was also observed remotely up to the tropopause, and these air masses were observed to have no enhanced aerosol loading. Frequently, these air masses had some enhanced PV associated with them, but not enough to explain the observed O3 levels. A relationship between PV and O3 was developed from cases of clearly defined O3 from stratospheric origin, and this relationship was used to estimate the stratospheric contribution to the air masses containing elevated O3 in the troposphere. The frequency of observation of the different air mass types and their average chemical composition is discussed in this paper.

Ozone and aerosol distributions and air mass characteristics over the South Atlantic Basin during the burning season

In situ and laser remote measurements of gases and aerosols were made with airborne instrumentation to investigate the sources and sinks of tropospheric gases and aerosols over the tropical South Atlantic during the NASA Global Tropospheric Experiment (GTE)/Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE A) field experiment conducted in September–October 1992. Gases from extensive fires in Brazil were transported by convective storms into the upper troposphere where tropospheric ozone (O3) was photochemically produced and advected eastward over the South Atlantic. In central Africa, the fires were widespread, and in the absence of deep convection, the fire plumes were advected at low altitudes (below ∼6 km) over the Atlantic. There was a positive correlation between O3 and aerosols found in the plumes that were not involved in convection. High O3 (>75 parts per billion by volume (ppbv)) was observed in the low‐altitude plumes, and also in the upper troposphere where O3 often exceeded 100 ppbv with low aerosol loading. The average tropospheric O3 distributions were determined for the following: Brazil and western South Atlantic, eastern and central South Atlantic, central and east coast of Africa, and the entire South Atlantic Basin. The tropopause heights and O3 columns across the troposphere were calculated for individual flights and for the average O3 distributions in the above regions. A maximum tropospheric O3 column of 56 Dobson units (DU) was found over the biomass burning region in Zambia and in the subsidence region over the central South Atlantic. The high O3 region over the South Atlantic from 4° to 18°S corresponded with the latitudinal extent of the fires in Africa. In situ and laser remote measurements were used to determine the frequency of observation and chemical composition of nine major air mass types. Biomass burning emissions contributed to most of the air masses observed over the South Atlantic Basin, and biomass burning was found to contribute up to half (28 DU) of the O3 column across this region.

Bias in a solar constant determination by the Langley method due to structured atmospheric aerosol: comment

A recent article by Schotland and Lea [Appl. Opt. 25, 2486 (1986)] discusses the errors in radiometer calibration by the Langley method and the detail required for very precise work. Their numerical methods on aerosol air mass can be simplified for use in the calibration of the worldwide sunphotometer network by using geometric considerations, assuming aerosol scale heights of ~1 Km and using the apparent solar zenith angle at the instrument as the independent variable in these formulas, so that refraction effects are incorporated. By using multiple-pass Langley analyses the errors due to air mass assignments can be reduced significantly.

The influence of air origin on the chemical composition and size distribution of tropospheric aerosols

Chemical and size distribution data and the corresponding air mass trajectories were obtained for aerosols collected at Death Valley, California and over the Pacific Ocean 250 km west of Santa Barbara, California. For air masses from the western Pacific, vertical profiles of Si, an element associated with continental sources, showed virtually no change in concentration with altitude at the offshore location. At this location the concentrations of Si and Cl in the lowest kilometer of an air mass which had recently passed over land indicated that modification had taken place. Differences in the profiles of Cl concentration over Death Valley and over the Pacific were consistent with the model of Toba for modification of sea salt profiles over land. At altitudes higher than 2.5 km, the concentrations of all of the elements measured were nearly constant. In the lowest kilometer at the ocean and Death Valley locations, size distributions of the aerosols were not greatly different in air masses of different origin. Modification of the chemical composition of aerosol at 300 m altitude in a maritime air mass was observed in measurements made at successive distances from the ocean to 180 km inland over the Orinoco Delta. In general, the relative concentration of particles in the size range 0.5 μm ≤ r ≤ 2 μm was larger at altitudes above 2.5 km than at lower altitudes. The experimental results and qualitative arguments based on the mechanics of aerosols indicate a significant relationship between aerosol properties and air mass only near the surface.