Columnar Aerosol Size Distributions Research Articles

Aerosol size distribution retrieved from optical depth measurements in Tamanrasset and Blida

Since we are often faced with limited, insufficient or contaminated observations in remote sensing. We will employ a theoretical technique for the retrieval of the particle size distribution. In this study, the columnar aerosol size distributions have been inferred by numerically inverting particulate optical depth measurements obtained in Blida and Tamanrasset from AERONET database at wavelengths region between 0.340 - 1.640 . Our results are in good agreement with those found by other authors and the correlation between the aerosol optical thickness measurements and the reproduced AOTs by the inverted columnar aerosol size distribution is very good with a correlation coefficient more than 0.97.

Impact of aerosols on surface ozone during COVID-19 pandemic in southern India: A multi-instrumental approach from ground and satellite observations, and model simulations

The World Health Organization (WHO) declared the coronavirus disease of 2019 (COVID-19) as a pandemic due to its widespread global infection. This has resulted in lockdown under different phases in many nations, including India, around the globe. In the present study, we report the impact of aerosols on surface ozone in the context of pre-lockdown (01st - 24th March 2020 (PLD)), lockdown phase1 (25th March to 14th April 2020 (LDP1)), and lockdown phase 2 (15th April to 03rd May 2020 (LDP2)) on clear days at a semi-arid site, Anantapur in southern India using both in situ observations and model simulations. Collocated measurements of surface ozone (O3), aerosol optical depth (AOD), black carbon mass concentration (BC), total columnar ozone (TCO), solar radiation (SR), and ultraviolet radiation (UV-A) data were collected using an Ozone analyzer, MICROTOPS sunphotometer, Ozonometer, Aethalometer, and net radiometer during the study period. The diurnal variations of O3 and BC exhibited an opposite trend during three phases. The concentrations of ozone were ~10.7% higher during LDP1 (44.8 ± 5.2 ppbv) than the PLD (40.5 ± 6.0 ppbv), which mainly due to an unprecedented reduction in NOx emissions leading to a lower O3 titration by NO. The prominent increase in the surface zone during LDP1 is reasonably consistent with the observed photolysis frequencies (j (O1D)) through Tropospheric Ultraviolet and Visible (TUV) model. The results show that a pronounced spectral and temporal variability in the AOD during three lockdown phases is mainly due to distinct aerosol sources. The increase in AOD during LDP2 due to long-range transport can bring large amounts of mineral dust and smoke aerosols from the west Asian region and central India, and which is reasonably consistent with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) air mass back trajectories and Moderate Resolution Imaging Spectroradiometer (MODIS) fire counts analysis over the measurement location. Overall, a drastic reduction in BC concentration (~8.4%) and AOD (10.8%) were observed in the semi-arid area during LDP1 with correspondence to PLD. The columnar aerosol size distributions retrieved from the spectral AODs followed power-law plus unimodal during three phases. The absorption angstrom exponent (AAE) analysis reveals a predominant contribution to the BC from biomass burning activities during the lockdown period over the measurement location.

Dependence of columnar aerosol size distribution, optical properties, and chemical components on regional transport in Beijing

Seasonal dependence of the columnar aerosol optical and chemical properties on regional transport in Beijing over 10 years (from January 2005 to December 2014) were analyzed by using the ground-based remote sensing combined with backward trajectory analysis. Daily air mass backward trajectories terminated in Beijing were computed with HYSPLIT-4 model and were categorized into five clusters. The columnar mass concentrations of black carbon (BC), brown carbon (BrC), dust (DU), aerosol water content (AW), and ammonium sulfate like aerosol (AS) of each cluster were retrieved from the optical data obtained from the Aerosol Robotic NETwork (AERONET) with five-component model. It was found that the columnar aerosol properties in different seasons were changed, and they were related to the air mass origins. In spring, aerosol was dominated by coarse particles. Summer was characterized by higher single scattering albedo (SSA), lower real part of complex refractive index ( n ), and obvious hygroscopic growth due to humid air from the south. During autumn and winter, there was an observable increase in absorption aerosol optical thickness (AAOT) and the imaginary part of complex refraction ( k ), with high levels of retrieved BC and BrC. However, concentrations of BC showed less dependence on the clusters during the two seasons owing to the widely spread coal heating in north China.

Retrieval of aerosol microphysical properties by means of sun and star photometry at Granada, Spain

Knowledge of aerosol microphysical properties is essential to understand the role of atmospheric aerosols in radiative forcing. Passive remote-sensing techniques allow the retrieval of columnar aerosol size distribution from spectral measurements of aerosol optical depth by applying numerical methods. These methods are widely applied to sun photometers. For night-time retrievals of aerosol optical depth, the GFAT (Atmospheric Physics Group of the University of Granada), operates a star photometer with channels at 380, 436, 500, 670, 880, and 1020 nm in the city of Granada (southeastern Spain, 36.83° N, 3.58° W, 680 m a.s.l.). The GFAT also operates a Cimel CE-318 sun photometer with filters at the same wavelengths. To our knowledge, the variations between daytime and night-time columnar aerosol microphysical properties have not been studied until now. In this work, the applicability of an inversion code based only on direct irradiance measurements is presented to retrieve aerosol size distributions, for both daytime and night-time. From March 2007 to February 2009, statistical analyses and time evolutions of the effective radius and the properties of the fine mode such as mean radius, width, and particle volume concentration are analysed. Daytime effective radius values are large in summer and small in winter, whereas no clear seasonal pattern emerges at night-time. During daytime, the fine mode radius does not show any remarkable pattern, whereas at night-time, this parameter shows a clear seasonal pattern with large values in summer and small values in winter. On the other hand, fine mode widths do not show any pattern, neither daytime nor night-time, although large values of this parameter are found during daytime in all seasons. The fine-particle volume concentration presents no seasonal pattern during the daytime, whereas at night-time it shows larger values in summer than in other seasons.

Influence of anomalous dry conditions on aerosols over India: Transport, distribution and properties

A synergy of satellite and ground‐based radiometric observations, along with chemical transport modeling, was used for the assessment of the influence of drought monsoon conditions of 2002 and prolonged dry pre‐monsoon period of 2003 on aerosol properties over south Asia, with emphasis over northern India. Reanalysis data are also examined for studying the dry anomalous period from the climatological mean, that show prevalence of westerlies under anticyclonic circulation and subsidence favoring the accumulation of aerosols. TRMM observations over south Asia indicate significant rainfall deficit over northwestern India in July 2002 and May–June 2003. Subsequently, the anomalous and prolonged dry conditions favored heavy aerosol buildup as indicated by strong positive anomalies (20–80%) of MODIS aerosol optical depth (AOD) as well as significant increase in TOMS aerosol index (AI) during July 2002 and May–June 2003 compared to the long‐term monthly means. The largest increase in aerosol loading is observed over northern India, encompassing the Indo‐Gangetic Plains (IGP) that is in the downwind region of dust outflow from the Thar Desert and long‐range transport from Arabia and Middle East. Ground‐based sunphotometer observations at Delhi and Kanpur also show enhanced presence of desert‐dust aerosols during July 2002 and May–June 2003, characterized by large AOD and significantly low Angstrom Exponent. In addition, modifications in columnar aerosol size distribution toward larger coarse‐mode fraction and higher single scattering albedo at longer wavelengths were observed, thus supporting the observation of enhanced dust influx. SPRINTARS model simulations also show the enhanced dust loading over northern India during the studied months, which is in general agreement with the satellite and ground‐based observations.

Analysis of the columnar radiative properties retrieved during African desert dust events over Granada (2005–2010) using principal plane sky radiances and spheroids retrieval procedure

The southern Iberian Peninsula is an important area for studying the columnar radiative properties of African desert dust air masses reaching Europe. The Aerosol Optical Depth (AOD) and Angström coefficient, α (440–1020nm), have been retrieved during the dust events reached at surface from 2005 to 2010 at Granada (37.18°N, 3.58°W, 680m a.m.s.l), using extinction measurements by means of a CIMEL CE 318-1 sun-photometer. In addition, sky radiance measurements performed in principal plane in conjunction with solar irradiance measurements were used to retrieve columnar aerosol size distributions, single scattering albedo and asymmetry parameter. During these desert dust intrusions, high values of AOD at 440nm (0.27±0.17) and low values of α (0.4±0.2) were found. These values indicate both high aerosol load and predominance of coarse particles during these events. The aerosol volume size distributions were bimodal, with the fine and coarse radius mode centered at 0.20μm and 2.41μm, respectively. The mean coarse to fine volume concentration ratio value was 11±6, showing a predominance of coarse particles in good agreement with α analysis. During these events, columnar aerosol single scattering albedo (ω0) increased with wavelength in accordance with previous works. However, the obtained values of ω0 in all wavelengths were lower than those reported by other authors during desert dust intrusions. The mixing of desert dust with absorbing particles from anthropogenic origin could explain the low ω0 values measured in the study area.

Use of light-extinction method and inverse modeling to study aerosols in the Paso del Norte Airshed

A light-extinction technique for monitoring aerosols was used in conjunction with a robust inverse reconstruction model to retrieve aerosol size distribution. Results for the columnar aerosol size distribution for the Paso del Norte Airshed for two different pollution scenarios and at two different seasons were presented. These are well correlated with experimental surface aerosol data for this region. It was observed that, for the clean day scenario, the summer case showed a greater columnar concentration of smaller size aerosols than the winter case. Results for the columnar aerosol size distribution in the polluted day scenario demonstrated that the concentration of aerosols were greater in the summer than in the winter. In addition, HYSPLIT backward trajectories were used to identify pollution sources.

Detection of Sahara dust intrusions during mixed advection patterns over south-east Italy: A case study

Measurements from July 4 to July 8, 2005 by a high resolution visible radiometer, a Raman lidar, a ground particulate matter sampler, and ground meteorological sensors have been combined in synergy to infer the intrusion over south-east Italy, of air masses from north-west Sahara, the Atlantic Ocean, and the continental Europe. It is shown that backscatter coefficient, depolarization-ratio, and lidar ratio vertical profiles represent the best tools to detect the intrusion of long range transported air masses and to monitor their effects on the vertical distribution of aerosol optical and microphysical properties. High resolution radiometers are instead important tools to monitor changes on columnar aerosol properties and size distributions. Backscatter coefficient, depolarization-ratio, and lidar ratio vertical profiles have revealed that aerosol optical and microphysical properties significantly changed with time and space during African dust outbreaks: the intrusion of dust particles that at first occurred above 2 km of altitude extending up to 6 km, affected the all aerosol load down to ground within few hours. Aerosol size distributions showed during dust events a clear bimodality with an accumulation mode maximum at 0.24 µm and a coarse mode maximum at 0.94 μm. Conversely, we have found that during the advection of air masses from the Atlantic and continental Europe, aerosol particles were mainly located below 2 km, their optical and microphysical properties were affected by smaller changes in time and space, and were characterized by depolarization ratios rather close to those due to a pure molecular atmosphere. In this case bimodal size distributions with an accumulation mode showing two sub-modes at 0.16 μm and 0.24 μm, respectively and a coarse mode centred at 0.94 μm have also been observed.

An Investigation of Aerosol Size Distribution Properties at Dibrugarh: North-Eastern India

Columnaraerosol size distributions, retrieved from spectral AOD (aerosol optical depth) esti mates over a northeastern location of India (Dibrugarh) are, ingeneral, bimodal with the occurrence of primary (broad) mode at 0.04 - 0.17 m and the secondary mode at 0.88 - 1.29 m. The physical parameters of size distributions representing the microphysical properties of aerosols show distinct sea sonal variations with the highest value of the effec tiveradii (~ 0.55 m) during pre-mon soon (March to May) season which, along with the highest value of AOD (~ 0.46 ± 0.09) during the same season, is attributed to the maxi mum abundance of coarsepa ticles. Examining the re sults in the light of the HYSPLIT back tra jectory analy sis and the peruliar to pogr phy of northeast India allowing advection only from the Indo-Gangetic plains or Bay-of-Ben gal, it appears that the strong presence of the coarse mode aerosols are associated with either mineral dust or marine aerosol components or both.

In situ ship cruise measurements of mass concentration and size distribution of aerosols over Bay of Bengal and their radiative impacts

Simultaneous measurements of surface level aerosol mass concentrations, their size distribution, and aerosol optical depth (AOD) were made during a ship cruise study conducted over the Bay of Bengal (BoB) between 19 and 28 February 2003, when the prevailing surface level wind flow is predominantly from the continent toward the ocean, using a ten‐stage QCM cascade impactor and Microtops Sun photometer. On all cruise days, air parcels at different altitude levels were coming either from west or from northwest directions, crossing a significant portion of the Indian subcontinent before finally reaching over BoB. Average value of surface level aerosol mass concentration is found to be around 50, 37, and 13 μg/m3 for coarse mode (>1 μm), accumulation mode (between 1 μm and 0.1 μm), and nucleation mode (<0.1 μm) particles, respectively. Size distribution of aerosols measured during the cruise showed the presence of four distinct modes, all of which could be fitted using lognormal distribution. Mode radii for the distributions lie in the range of 0.025–0.036 μm for mode 1, between 0.15 and 0.165 for mode 2, between 0.39 and 0.55 for mode 3, and between 2.2 and 3.5 for mode 4. Over the study region, daily mean AOD values at 380 nm were in the range of 0.34 to 0.75 while those at 1020 nm varied from 0.09 to 0.25. The mean value of Angstrom wavelength exponent α is found to be 1.19 ± 0.12. Regression analysis for the scatterplots between AOD values and surface mass concentrations showed good correlation between them over the entire cruise region. Aerosol optical depths, as well as extinction coefficients calculated from surface level aerosol number concentrations, show higher values over northern and coastal areas of BoB. An estimate of aerosol scale height has been made from the ratio of columnar AOD values and surface extinction coefficients. Columnar aerosol size distributions were derived using King's inversion technique, and the results are found to be less sensitive to the choice of aerosol refractive index values used. Independently, aerosol scale height is also estimated by taking the ratio of derived columnar number distribution and measured surface level aerosol number concentrations. AOD values observed over BoB are used in estimating the aerosol radiative forcing over this region. The Average value of the net direct radiative forcing for the region is found to be in the range of −30.6 to −26.6 W/m2 at the surface and −8.4 to −10.6 W/m2 at the TOA. Magnitude of LW forcing is found to be about 10% of its SW counterpart, at both surface and TOA. Results of a sensitivity test showed that the magnitude of LW aerosol forcing at the surface level decreases with the increase in column water vapor.

Short‐wave radiative effects of biomass burning aerosol during SAFARI2000

AbstractWe present data from two case‐studies during the Southern AFricAn Research Initiative (SAFARI2000) of the radiative effects of a biomass burning plume advecting over the ocean. In situ aircraft measurements were made of the aerosol vertical structure, the aerosol microphysics, and the aerosol optical properties (scattering and absorption coefficients). Remote sensing measurements of the upwelling broadband short‐wave (SW) radiative fluxes (0.3–3.0µm) were made above the plume. Together with modelling of the clear‐air flux using a flexible radiation scheme, we have been able to determine the local mean direct radiative effects of the aerosol column for the two cases. Aerosol optical depths were derived using a number of techniques: the upwelling fluxes, vertical integration of the in situ extinction coefficient, and using downwelling sky radiances. These optical depths are compared and discussed; discrepancies are revealed through instrument sampling problems and horizontal inhomogeneity of the aerosol. Spectrally resolved downwelling SW sky radiances as a function of scattering angle were measured and modelled. The modelling technique illustrates the importance of including unpolluted marine boundary‐layer aerosol as part of the aerosol column. A retrieval algorithm that uses the measured downwelling sky radiances as input has also been utilized. This algorithm derives a columnar aerosol size distribution for comparison against the measured biomass burning and marine boundary‐layer size distributions. The retrieved columnar size distribution is acceptable in representing the atmospheric aerosol in terms of describing the effect on scattered radiances. © Crown copyright, 2004. Royal Meteorological Society

On the correlation between the depth of the boundary layer and the columnar aerosol size distribution

Most inversion algorithms used to retrieve the aerosol characteristics from ground-based solar radiation measurements, assume a vertically homogeneous atmosphere and constant aerosol optical characteristics over the whole air column. This assumption leads to correct results only if the extinction of the solar radiation incident on the earth surface depends on the mean optical characteristics of the entire atmospheric column, rather than on the details of the vertical distribution of air particles. The validity of this simplifying assumption is an issue of a certain importance, which requires a more definite answer based on observational evidence. The aerosol vertical profile is significantly affected by the extent of the atmospheric boundary layer, within which the physical properties of the suspended particulate matter show sharp variations. Therefore, if the above assumption were not correct, one should observe a significant correlation between the retrieved optical characteristics and the thickness of the boundary layer measured on the same site over time. In this paper we limit ourselves to study the possible correlations between the aerosol volume size distributions and the mixing height of the atmospheric boundary layer, in order to ascertain whether there exists any dependence between the volume size distributions and the vertical distribution of the aerosol, in a site where there are strong reasons to believe that most of the existing aerosol is confined below the mixing height level. The investigation is based on a dataset of measurements taken at the Southern Great Plains base station of the ARM Program, located in Oklahoma, USA. Measurements of the columnar aerosol characteristics are taken by means of a Cimel sun-sky radiometer of the Aeronet network, while vertical profiles of pressure and temperature, used for the determination of the height of the boundary layer, are taken by vertical sounding systems. In order to find a possible relationship between the columnar aerosol size distribution and the depth of the boundary layer, a statistical analysis has been carried out on this data. The analysis has not shown any significant correlation between those two parameters. This negative result supports the validity, for the Southern Great Plains site, of the assumption that the radiation incident on the earth surface depends only on the mean optical characteristics of the entire atmospheric column, rather than on the details of the aerosol vertical distribution.

Studies on aerosol optical properties over urban and semi-urban environments of Hyderabad and Anantapur

Aerosols in the troposphere exert an important influence on global climate and the environment through scattering, transmission and absorption of radiation as well as acting as nuclei for cloud formation. Atmospheric aerosol particles influence the earth's radiation balance directly by scattering of infrared energy and indirectly by modifying the properties of clouds through microphysical processes. The present study addresses visibility, radiative forcing, size distribution and attenuation of aerosols over the period from January to May, 2001 for urban and semi-urban regions of Hyderabad and Anantapur. High aerosol loading has been observed over urban environment compared to semi-urban environment. Aerosol optical depth values increased from January to April and then decreased during May over both urban and semi-urban regions. Over urban region, visibility decreased from January to April and increased during May. Similar trend has been observed over semi-urban region with relatively higher values of visibility. Radiative forcing estimated using aerosol optical depth values increased from January to April and then decreased during the month of May over urban and semi-urban areas. High visibility and low radiative forcing has been noticed over semi-urban area due to less aerosol loading. Wavelength exponent and turbidity coefficient registered high values over urban environment compared to semi-urban environment. Attenuation coefficient showed high values over urban region compared to semi-urban region. It reveals that semi-urban environment receives high solar flux than urban environment. Using 10 channel quartz crystal microbalance, measurements of total mass concentration and mass size distribution of near surface aerosols has been made over semi-urban environment and compared with size distribution derived from inversion methods based on aerosol optical depth variation with wavelength. The sensitivity of constrained linear inversions for inferring columnar aerosol size distributions has been discussed in detail in the paper.

Retrieval of columnar aerosol size distributions and radiative-forcing evaluations from sun-photometric measurements taken during the CLEARCOLUMN (ACE 2) experiment

Spectral measurements of direct solar irradiance were taken within 13 narrow-band channels in the 401–3676 nm wavelength range, using the IR–RAD sun-radiometer at the Sagres station in southern Portugal during the CLEARCOLUMN (ACE 2) experiment, from June 16 to 25 July 1997. The measurements performed on 21 clear-sky days were examined in terms of the Bouguer–Lambert–Beer law, following realistic correction procedures for Rayleigh scattering and absorption by minor atmospheric gases, to determine more than 2100 spectral series of aerosol optical depth. All these spectral series were examined with the King inversion method to retrieve the columnar aerosol particle size distributions over the 0.07–10 μm radius range, for values of the real part of the particulate refractive index ranging between 1.43 and 1.50 and values of the imaginary part between 0.003 and 0.010. Using the well-known 6S computer code for all the columnar aerosol size-distribution curves determined on ten “golden days” and both refractive index parts varying with wavelength, evaluations of the change Δ F↑ caused by aerosol particles in the outgoing solar radiation flux were made at solar zenith angles θ ranging between 15 and 76° and for spectral albedo features of both clear water and green vegetation surfaces. The results show that the radiative forcing Δ F↑ assumes positive values (associated with cooling effects) for the clear water surface on all the measurement days and at all the solar zenith angles, and mostly negative values (warming effects) for the green vegetation surface in the range θ<60°. The present evaluations also give clear evidence of the close dependence of Δ F↑ not only on the surface albedo, solar zenith angle and aerosol optical depth but also on the mean single scattering albedo of the columnar aerosols.

Stratification and size distribution of aerosols retrieved from simultaneous measurements with lidar, a sunphotometer, and an aureolemeter

Vertical profiles of backscattering coefficients, optical thicknesses, and columnar size distributions of aerosols were obtained by simultaneous measurements with lidar, a sunphotometer, and an aureolemeter in Tsukuba, Japan, from November 1991 to December 1992. Several conspicuous characteristics were found in the relationship between aerosol size distribution and stratification. In summer an accumulation mode is dominant, and aerosols were heavily loaded in the planetary boundary layer. Turbid atmospheres with an abundance of large particles are observed in the middle troposphere in the spring. In autumn and winter the troposphere is clear so that columnar aerosol size distributions reflect stratospheric aerosols. During the observation period, volcanic aerosols that are due to the Mt. Pinatubo eruption were being loaded in the stratosphere. The mode radius in the volume size distribution of the stratospheric aerosol was observed to increase from 0.45 mum in November 1991 to 0.6 mum in October 1992, and decreased after October 1992. Total aerosol loading in the stratosphere was estimated to be maximum in the spring of 1992, minimum in the autumn of 1992, and increased again after the autumn of 1992.

Intercomparing the Twomey method with a multimodal lognormal approach to retrieve the aerosol size distribution

This paper concentrates on intercomparing an objective algorithm to estimate the columnar aerosol size distribution from aerosol spectral extinction and a widespread robust nonparametric inversion approach. The nonparametric approach which has been considered is the Phillips‐Twomey second‐derivative regularization method. The parametric approach assumes that the size distribution is the linear combination of a set of lognormal functions (typically, from 10 to 30 distributions are considered) which have suitable mode radii and standard deviations. Intercomparisons have been performed on the basis of aerosol optical thicknesses derived by high‐resolution solar radiation absorption spectra in the spectral region 0.40 − 1.1 μm. Absorption spectra were recorded by a grating spectrometer with a resolution of 0.5 nm. Each inversion, either parametric or nonparametric, was obtained by using more than 450 data points at a time which, from a statistical point of view, allowed us to obtain high‐quality results. For the cases presented here, we found that the two approaches agree fairly well in indicating the most significant characteristics of the aerosol size distribution.

Sea-Breeze Front Effects on Boundary-layer Aerosols at a Tropical Coastal Station

The effects of sea breeze on optical depth, size distribution, and columnar loading of aerosols at the tropical coastal station of Trivandrum are studied. It has been observed that sea-breeze front activity results in a significant and short-lived enhancement in aerosol optical depth and columnar loading in contrast to the effects seen on normal sea-breeze days. Examination of the changes in columnar aerosol size distribution associated with sea-breeze activity revealed an enhancement of small-particle (size less than 0.28 µ m) concentration. The aerosol size distributions deduced from optical depth measurements generally show a pronounced bimodal structure associated with the frontal activity.

Simultaneous determination of aerosol size distribution and refractive index and surface albedo from radiance—part II: Application

This paper presents and analyzes experimental results in simultaneous determination of atmospheric columnar aerosol size distribution, refractive index and surface albedo by use of the radiance data in almucantar measured by a radiometer. 32 groups of data measured in Beijing during winter show that the imaginary part of refractive index for 0.6943µm wavelength ranges from 0.022 to 0.079 with a mean of 0.0527. The mean real part and surface albedo are 1.537 and 0.287, respectively. The imaginary part was found to be less in autumn than that in winter, especially after raining. For 0.399µm and 0.6943µm wavelengths, the mean surface albedos are 0.101 and 0.222, and the mean imaginary parts are 0.0241 and 0.0129, respectively.

Latitudinal variation of spectral optical thickness and columnar size distribution of the El Chichon stratospheric aerosol layer

Multiwavelength solar radiometer observations of directly transmitted solar radiation have been obtained at 10 discrete wavelengths between 0.440 and 2.233 μm during an airborne latitudinal survey in late April and early May 1983. These observations have been used to derive the spectral optical thickness of the El Chichon stratospheric aerosol layer from 68°N to 56°S. The stratospheric optical thicknesses 1 year after the eruptions of El Chichon were still greatly enhanced over background levels for the stratosphere. The maximum optical thickness of 0.14 (0.670‐μm wavelength) was found at 50°N, rather than at 20°N as was found during an airborne latitudinal survey in late October and early November 1982. Columnar aerosol size distributions have been inferred from the 1982 and 1983 data sets by numerically inverting the aerosol optical thickness measurements as a function of wavelength. In May 1983, latitudes north of 5°S had bimodal stratospheric aerosol size distributions, while latitudes south of 5°S had monomodal size distributions. During October 1982, when optical thickness measurements were obtained only from 0.440 to 0.871 μm, the size distributions showed a strong latitudinal variation but were everywhere monomodal. For both dates the northern hemisphere distribution indicated a significantly higher ratio of large particles to small particles than the southern hemisphere distribution. The addition of the near‐infrared wavelengths in the May 1983 experiment increased the accuracy of the size distribution determinations at the large‐particle end of the spectrum, as anticipated, but was also found to increase the accuracy at the small‐particle end.

Experimental study of remote sensing of atmospheric aerosol size distribution by combined solar extinction and forward scattering method

Combined observations of solar spectral extinction and forward angular scattering are presented. The atmospheric columnar aerosol size distributions in Beijing are retrieved from observed data and the characteristics discussed.