Increase In Aerosol Optical Thickness Research Articles

Assessing the assimilation of Himawari-8 observations on aerosol forecasts and radiative effects during pollution transport from South Asia to the Tibetan Plateau

Abstract. Emissions from South Asia (SA) represent a critical source of aerosols on the Tibetan Plateau (TP), and aerosols can significantly reduce the surface solar energy. To enhance the precision of aerosol forecasting and its radiative effects in SA and the TP, we employed a four-dimensional local ensemble transform Kalman filter (4D-LETKF) aerosol data assimilation (DA) system. This system was utilized to assimilate Himawari-8 aerosol optical thickness (AOT) into the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to depict one SA air pollution outbreak event in spring 2018. Sensitivity tests for the assimilation system were conducted first to tune temporal localization lengths. Comparisons with independent Moderate Resolution Imaging Spectroradiometer (MODIS) and AErosol RObotic NETwork (AERONET) observations demonstrate that the AOT analysis and forecast fields have more reasonable diurnal variations by assimilating all the observations within a 12 h window, which are both better than assimilating the hourly observations in the current assimilation time slot. Assimilation of the entire window of observations with aerosol radiative effect activation significantly improves the prediction of downward solar radiation compared to the free-run experiment. The AOT assimilation with aerosol radiative effect activation led to a reduction in aerosol concentrations over SA, resulting in increased surface radiation, temperature, boundary layer height, and atmospheric instability. These changes facilitated air uplift, promoting aerosol transport from SA to the southeastern TP and leading to an increase in AOT in this region.

Development of the method of remote sensing of low-concentration gases of the atmosphere taking into account the humidity fluctuations of the air

The emergence of strong desert vortices covering large areas, including the areas of large cities with dense population, the need to quickly obtain sufficient information on the state of desert ecosystems in large areas makes the application of remote sensing methods an urgent issue. At present, the method of measuring the desert aerosol using scattered solar radiation has been proposed, and the method of measuring the optical thickness of the dust aerosol has been substantiated. A new method of optimization of remote sensing systems and information-measurement systems in general is proposed in the article. A method of remote determination of aerosol optical thickness increase, which involves measuring the scattered radiation of the sun, has been proposed. The main goal of the study is to develop a remote sensing method that allows determining the dynamics and cause of the increase in aerosol optical thickness. The scientific novelty of the work is a new method that is invariant to the variations of the aerosol optical thickness under the influence of hydration and dehydration processes. By using the proposed three-wave biparametric Sun photometer, the effect of aerosol hygroscopic growth on the measurement results is eliminated.The given critical assessment of the state of atmospheric pollution monitoring and the consideration of the main shortcomings of the aerosol pollution monitoring carried out in the country will allow further improvement of such monitoring in the future. The proposed remote sensing method for determining the dynamics of aerosol optical thickness growth allows to determine the origin of aerosol optical thickness growth by measuring scattered solar radiation at two different wavelengths and in two optical air masses.

Physical-Optical Properties of Marine Aerosols over the South China Sea: Shipboard Measurements and MERRA-2 Reanalysis

Aerosols play an important role in the Earth–atmosphere system. Their impacts on the weather and climate are highly dependent on spatiotemporal distributions as well as physical-optical properties. Physical-optical properties of the aerosols over the Asian continent have been widely investigated, but there are relatively few observations in maritime locations, especially the South China Sea (SCS). Here, with the combination of in situ ship-based observations from June and July 2019 as well as long-term MERRA-2 reanalysis datasets from January 2012 to December 2021, the physical and optical properties of marine aerosols in the SCS are explored. The impacts of meteorological factors, particularly frontal systems, on the aerosol properties are further analyzed based on detailed observations. The observed results show that aerosols are vertically concentrated below 3 km and the extinction coefficient reaches the maximum value of 0.055 km−1 near 480 m. Moreover, the particles are composed of an accumulation and a coarse particle mode, and they conform to the lognormal distribution. The synoptic-scale case study demonstrates that both the cold front and stationary front lead to an increase in aerosol optical thickness (AOD), which is due to the enhanced wind speed and the hygroscopic growth of fine particles, respectively. The long-term analysis indicates that AOD decreases from northwest to southeast with the increasing distance away from the continent, and it reflects higher values in spring and winter than in summer and autumn. Sulfate and sea salt dominate AOD in this region when compared with other components. The overall AOD shows a significant negative trend of −0.0027 year−1. This work will help us further understand the physical and optical properties of marine aerosols over the SCS and then contribute to quantifying the aerosol radiative forcing in the future.

Impact of Atmospheric Optical Properties on Net Ecosystem Productivity of Peatland in Poland

Peatlands play an important role in the global carbon cycle due to the high carbon storage in the substrate. Ecosystem production depends, for example, on the solar energy amount that reaches the vegetation, however the diffuse component of this flux can substantially increase ecosystem net productivity. This phenomenon is observed in different ecosystems, but the study of the atmosphere optical properties on peatland production is lacking. In this paper, the presented methodology allowed us to disentangle the diffuse radiation impact on the net ecosystem production (NEP) of Rzecin peatland, Poland. It allowed us to assess the impact of the atmospheric scattering process determined by the aerosol presence in the air mass. An application of atmospheric radiation transfer (ART) and ecosystem production (EP) models showed that the increase of aerosol optical thickness from 0.09 to 0.17 caused NEP to rise by 3.4–5.7%. An increase of the diffusion index (DI) by 0.1 resulted in an NEP increase of 6.1–42.3%, while a DI decrease of 0.1 determined an NEP reduction of −49.0 to −10.5%. These results show that low peatland vegetation responds to changes in light scattering. This phenomenon should be taken into account when calculating the global CO2 uptake estimation of such ecosystems.

Variations of atmospheric aerosol parameters in periods of seismic activity in Tien–Shan

Combination of numerical modeling (MERRA) and ground based (AERONET) monitoring of atmospheric aerosol parameters were used to discover a possible connection to seismic activities within the territory of Tien-Shan. The obtained results demonstrated consistent behavior between increase of aerosol optical thickness (AOT) and powerful earthquakes, epicenters of which were located within 200 km from the “Issyk-Kul” station (AERONET). Thereat, the dominant role was played by fine-dispersed particles, which virtually determined the nature of AOT evolution. We also observed well-defined changes of the ratio between mass fraction PM1.0 (particles less than 1.0 μm aerodynamic diameter) and optical thickness of aerosol scattering several days before an earthquake of M > 5.0 magnitude.

Simulation analysis of aerosol effect on shortwave infrared remote sensing detection of atmospheric CO2

The research of carbon dioxide (CO2) sources and sinks within the carbon cycle is significant for enhancing our understanding of global climate change. Space based measurement of CO2 concentration in lower atmosphere by reflected sunlight in near infrared (NIR) band has become a hot research topic at present. The global characteristic of atmospheric CO2 retrieval from NIR is studied using the expected measurement performance of Tansat (Tan Satellite) mission. With the development of CO2 retrieval algorithms, the light-path modification due to multiple scattering by aerosol is identified as a major source of error when retrieving CO2 from high resolution near-infrared spectrum. The present study focuses on atmospheric CO2 retrieval sensitivity to aerosol properties such as aerosol types, aerosol modes, and profiles aiming at the demands for retrieval accuracy of CO2 no larger than 0.3%-0.5% on a regional scale. Here, we carry out the aerosol scattering effects analysis on retrieving atmospheric CO2 near 1610 nm using the simulated nadir observation for Tansat based on CALIPSO aerosol profile products and SCIATRAN radiative transfer model. The results show that light path modification due to aerosol scattering is closely related to their types, modes and vertical distributions. For aerosol types, on the one hand, urban aerosol has the most significant influence on the measured radiance, followed by maritime aerosols, and has a much smaller influence for rural aerosol, which will lead to overestimated CO2 concentration for the typical surface albedo. On the other hand, the measured radiance will decrease with the increase of aerosol optical thickness (AOT) for urban and rural aerosols, but exactly the opposite to maritime aerosols. For aerosol modes and vertical distributions, aerosols in accumulation mode, the continental aerosols with multilayer aerosol vertical distribution and maritime aerosols with AOT less than 0.3 will bring about less than 5% of negative radiance changes, and will cause positive changes with the increase of AOT. However, aerosols in coarse mode will always cause negative changes of radiance regardless of aerosol vertical distribution, and thus resulting in an overestimation of CO2. In addition, the higher the aerosol layer distributed, the smaller the negative radiance change is. If aerosol profiles can be successfully retrieved as a state vector, then it can be expected that satellite measurement can lead to tremendous improvement in CO2 retrieval precision. This study provides important information about estimations of the influence of aerosol property on CO2 retrieval algorithm. All these results can contribute to improving the accuracy of CO2 retrieval.

Impact of reflecting land surface on radiation environment over Hornsund, Spitsbergen – a model study for cloudless skies

Abstract This paper addresses the influence of land topography and cover on 3D radiative effects under cloudless skies in the Hornsund area, Spitsbergen, Svalbard. The authors used Monte Carlo simulations of solar radiation transfer over a heterogeneous surface to study the impact of a non-uniform surface on: (1) the spatial distribution of irradiance transmittance at the fjord surface under cloudless skies; (2) the spectral shortwave aerosol radiative forcing at the fjord surface; (3) normalized nadir radiance at the Top Of the Atmosphere (TOA) over the fjord. The modelled transmittances and radiances over the fjord are compared to the transmittances and radiances over the open ocean under the same conditions. The dependence of the 3D radiative effects on aerosol optical thickness, aerosol type, surface albedo distribution, solar azimuth and zenith angle and spectral channel is discussed. The analysis was done for channels 3 (459-479 nm) and 2 (841-876 nm) of the MODIS radiometer. In the simulations a flat water surface was assumed. The study shows that snow-covered land surrounding the fjord strongly modifies the radiation environment over the fjord surface. The enhancement of the mean irradiance transmittance over the fjord with respect to the open ocean is up to 0.06 for channel 3. The enhancement exceeds 0.11 in the vicinity of sunlit cliffs. The influence of the snow-covered land on the TOA radiance over the fjord in channel 3 is comparable to the impact of an increase in aerosol optical thickness of over 100%, and in lateral fjords of up to several hundred percent. The increase in TOA radiance is wavelength dependent. These effects may affect retrievals of aerosol optical thickness.

Aerosol direct shortwave radiative forcing effect based on SBDART model in the Pearl River Delta, Guangdong (China)

Aerosols play an important role in the energy budget of the earth-atmosphere system. In this paper, we studied aerosol shortwave direct radiative forcing (DRF) effects in Pearl River Delta based on SBDART and a 'two-layer-single-wavelength' model. Simulation results indicated that the underlying surface type and solar zenith angle have significant impacts on aerosol radiative forcing. The comparison between aerosol radiative forcing effects on urban asphalt surface and vegetation shows cooling and warming effects of aerosol shortwave radiative forcing on urban asphalt are much more apparent than that on vegetation, implying aerosols over asphalt-predominated cities will impact the local climate. Then we estimated variations of average DRF and net radiation flux with solar zenith angle in the Pearl River Delta. DRF indicates warming at solar zenith angles of 0°, 20°, 40° and 60°, but cooling at 80°. Net radiation flux increases with a decrease in aerosol optical thickness (AOT) at low elevation, but with an increase in AOT above 5 km.

Aerosol's optical and physical characteristics and direct radiative forcing during a shamal dust storm, a case study

Abstract. Dust aerosols are analyzed for their optical and physical properties during an episode of a dust storm that blew over Kuwait on 26 March 2003 when the military Operation Iraqi Freedom was in full swing. The intensity of the dust storm was such that it left a thick suspension of dust throughout the following day, 27 March. The synoptic sequence leading to the dust storm and the associated wind fields are discussed. Ground-based measurements of aerosol optical thickness reached 3.617 and 4.17 on 26 and 27 March respectively while the Ångstrom coefficient, α870/440, dropped to −0.0234 and −0.0318. Particulate matter concentration of 10 μm diameter or less, PM10, peaked at 4800 μg m−3 during dust storm hours of 26 March. Moderate Resolution Imaging Spectroradiometer (MODIS) retrieved aerosol optical depth (AOD) by Deep Blue algorithm and Total Ozone Mapping Spectrometer (TOMS) aerosol index (AI) exhibited high values. Latitude–longitude maps of AOD and AI were used to deduce source regions of dust transport over Kuwait. The vertical profile of the dust layer was simulated using the SKIRON atmospheric model. Instantaneous net direct radiative forcing is calculated at top of atmosphere (TOA) and surface level. The thick dust layer of 26 March resulted in cooling the TOA by −60 Wm−2 and surface level by −175 Wm−2 for a surface albedo of 0.35. Slightly higher values were obtained for 27 March due to the increase in aerosol optical thickness. Radiative heating/cooling rates in the shortwave and longwave bands were also examined. Shortwave heating rate reached a maximum value of 2 K day−1 between 3 and 5 km, dropped to 1.5 K day−1 at 6 km and diminished at 8 km. Longwave radiation initially heated the lower atmosphere by a maximum value of 0.2 K day−1 at surface level, declined sharply at increasing altitude and diminished at 4 km. Above 4 km longwave radiation started to cool the atmosphere slightly reaching a maximum rate of −0.1 K day−1 at 6 km.

Variability of aerosol properties during the 2007–2010 spring seasons over central Europe

Aerosol optical properties have been studied for spring seasons when increased values of PM10 are registered. Measurements of aerosol optical properties were taken by collocated lidar and sun-photometers at Belsk, Poland, and Minsk, Belarus. A significant increase of registered aerosol optical thickness (AOT) was found during episodes with elevated PM10 concentrations. An increase of AOT at 1020 nm amounted to 50% in the case of Minsk and 18% in the case of Belsk, while an increase of AOT at 400 nm was 66% and 33%, respectively. We noted an increase of Angstrom exponent by 6% at both stations and no significant increase of single scattering albedo. The LIDAR measurements together with NAAPS model results and backtrajectory analysis suggest that both the biomass burning products and the Saharan dust are responsible for increased PM10 concentrations and large AOT values during spring time. The smoke aerosol is transported over Central Europe mainly in the boundary layer, increasing both PM10 concentration and AOT. The dust aerosol transported in the free troposphere slightly affects the AOT values only. Statistically significant correlation between PM10 concentration and AOT was found during reporting period.

Remote sensing of aerosols over snow using infrared AATSR observations

Abstract. Infrared (IR) retrievals of aerosol optical thickness (AOT) are challenging because of the low reflectance of aerosol layer at longer wavelengths. In this paper we present a closer analysis of this problem, performed with radiative transfer (RT) simulations for coarse and accumulation mode of four main aerosol components. It shows the strong angular dependence of aerosol IR reflectance at low solar elevations resulting from the significant asymmetry of aerosol phase function at these wavelengths. This results in detectable values of aerosol IR reflectance at certain non-nadir observation angles providing the advantage of multiangle remote sensing instruments for a retrieval of AOT at longer wavelengths. Such retrievals can be of importance e.g. in case of a very strong effect of the surface on the top of atmosphere (TOA) reflectance in the visible spectral range. In the current work, a new method to retrieve AOT of the coarse and accumulation mode particles over snow has been developed using the measurements of Advanced Along Track Scanning Radiometer (AATSR) on board the ENVISAT satellite. The algorithm uses AATSR channel at 3.7 μm and utilizes its dual-viewing observation technique, implying the forward view with an observation zenith angle of around 55 degrees and the nadir view. It includes cloud/snow discrimination, extraction of the atmospheric reflectance out of measured brightness temperature (BT) at 3.7 μm, and interpolation of look-up tables (LUTs) for a given aerosol reflectance. The algorithm uses LUTs, separately simulated with RT forward calculations. The resulting AOT at 500 nm is estimated from the value at 3.7 μm using a fixed Angström parameter. The presented method has been validated against ground-based Aerosol Robotic Network (AERONET) data for 4 high Arctic stations and shows good agreement. A case study has been performed at W-Greenland on 5 July 2008. The day before was characterized by a noticeable dust event. The retrieved AOT maps of the region show a clear increase of AOT in the Kangerlussuaq area. The area of increased AOT was detected on 5 July on the ice sheet east of Kangelussuaq, opposite to the observed north easterly wind at ground level. This position can be explained by a small scale atmospheric circulation transporting the mobilized mineral dust upslope, after its intrusion into the upper branch of the circulation. The performed study of atmospheric reflectance at 3.7 μm also shows possibilities for the detection and retrievals of cloud properties over snow surfaces.

Variations of meridional aerosol distribution and solar dimming

Meridional distribution of aerosol optical thickness (AOT) over the ocean was analyzed by using the 8‐year MISR and MODIS‐Terra data sets from March 2000 to February 2008, as well as the 5‐year MODIS‐Aqua data set from July 2002 to June 2007. The three satellite sensors show that there was a pronounced meridional aerosol asymmetry. It was found that there were strong seasonal variations in the meridional aerosol asymmetry: it was most pronounced in the April–July months. There was no noticeable asymmetry during the season from September to December. The Northern Hemisphere, where the main sources of natural and anthropogenic aerosols are located, contributed to the formation of noticeable aerosol asymmetry. During the season of pronounced hemispheric aerosol asymmetry, an increase in AOT was observed over the Northern Hemisphere, while a decrease in AOT was observed over the Southern Hemisphere. At midlatitudes in the Northern Hemisphere (30–60°N), the main contribution to seasonal variations of AOT over the ocean was made by Pacific Ocean aerosols. At low latitudes in the Northern Hemisphere (0–30°N), aerosols over the Atlantic Ocean contributed to seasonal variations of AOT more significantly than aerosols over the Pacific Ocean. During the 8‐year period under consideration, the brightening phenomenon, detected over the land, was not observed over the ocean at midlatitudes 30–60°N in cloudless conditions.

Sensitivity of aerosol optical thickness and aerosol direct radiative effect to relative humidity

Abstract. We present a sensitivity study of the effects of spatial and temporal resolution of atmospheric relative humidity (RH) on calculated aerosol optical thickness (AOT) and the aerosol direct radiative effects (DRE) in a global model. We carry out different modeling experiments using the same aerosol fields simulated in the Global Modeling Initiative (GMI) model at a resolution of 2° latitude by 2.5° longitude, using time-averaged fields archived every three hours by the Goddard Earth Observation System Version 4 (GEOS-4), but we change the horizontal and temporal resolution of the relative humidity fields. We find that, on a global average, the AOT calculated using RH at a 1°×1.25° horizontal resolution is 11% higher than that using RH at a 2°×2.5° resolution, and the corresponding DRE at the top of the atmosphere is 8–9% and 15% more negative (i.e., more cooling) for total aerosols and anthropogenic aerosol alone, respectively, in the finer spatial resolution case. The difference is largest over surface escarpment regions (e.g. >200% over the Andes Mountains) where RH varies substantially with surface terrain. The largest zonal mean AOT difference occurs at 50–60° N (16–21%), where AOT is also relatively larger. A similar impact is also found when the time resolution of RH is increased. This increase of AOT and aerosol cooling with the increase of model resolution is due to the highly non-linear relationship between RH and the aerosol mass extinction efficiency (MEE) at high RH (>80%). Our study is a specific example of the uncertainty in model results highlighted by multi-model comparisons such as AeroCom, and points out one of the many inter-model differences that can contribute to the overall spread among models.

Aerosol extinction in a remote continental region of the Iberian Peninsula during summer

Summer in Évora (38°34′N, 7°54′W), Portugal, is described in terms of aerosol properties of extinction of the solar radiation. We create a data set composed of (1) cloud‐screened half‐day averaged values of aerosol optical thickness (AOT) measured at 7 wavelengths by both a CIMEL Sun/sky‐photometer and a YES shadowband radiometer and (2) half day averaged values of aerosol scattering coefficient (ASC) measured at the surface level at two wavelengths by a TSI nephelometer. Spectral dependence of both AOT and ASC gives the column and the surface Ångström exponents, αC and αS, respectively. Measurements are acquired in both 2002 and 2003 summers. Back trajectories are computed. A statistical study of the data set provides thresholds in AOT and αC for a classification of the days. The classification is applied with success to the case study of the 2003 summer heat wave episode and is generalized to the whole data set. In 23% of the cases, the turbidity in Évora is very low, with AOT441 < 0.12 and AOT873 < 0.04. The air mass origin is the North Atlantic Ocean at 700 and 970 hPa. In 31% of the cases, the turbidity is high. Increase of AOT is due to forest fire emissions, originating in the Iberian Peninsula, with 0.30 < AOT441 < 1.10 and αC > 1.2, and to desert dust plumes transported from North Africa within 72 to 120 hours at 700 hPa, with 0.10 < AOT873 < 1.10 and 0.1 < αC < 1.0. The vertical profile is highly variable, and several cases of aerosol mixing in the column are identified. The duration of the aerosol episode during the 2003 summer heat wave is 16 days, which is exceptionally long.

Influence of the Pinatubo eruption on the aerosol optical depth in the Arctic in the summer of 1993

In the summer of 1993, measurements of the spatial distribution of atmospheric optical aerosol thickness were conducted in the region of the Laptev Sea, the Kara Sea and the Taimyr peninsula, using an aircraft visible and UV band spectrophotometer. The Arctic atmosphere's aerosol optical depth was measured using I1-18 ‘Cyclone’ aircraft-meteolab as a platform at an altitude ranges of 100–8500 m. It was observed that the troposphere aerosol was concentrated in the altitude range 100–4000 m. The light extinction of the troposphere column was approximately 0.05 for a wavelength of 400 nm. A comparison with the results of measurements made in the same region in March-April 1990 shows a decrease of more than 3–5 times in troposphere aerosol optical depth in the Arctic during summer months. It was also found that there was a relatively clean area above 4000 m in this season in the Arctic troposphere. In addition, an increase in aerosol optical thickness in the stratosphere is observed. The value of aerosol optical depth measured from the level 8200 m was 0.1 for a wavelength of 500 nm. The spectral dependence of the stratospheric optical thickness has a complicated form with at least two local maxima. Estimates of the size and concentration of stratospheric aerosol particles are in agreement with in-situ measurements after the Mount Pinatubo eruption.

Effect of dry‐season biomass burning on Amazon basin aerosol concentrations and optical properties, 1992–1994

Aerosol concentrations and properties have been derived from a network of ground‐based Sun‐sky radiometer measurements in Brazil's Amazon basin region since 1992. The measurements characterize the background aerosol environment and aerosol emissions from biomass burning at eight selected sites. The duration and frequency of the measurements provide the foundation of an aerosol climatology based on direct sun measurements of aerosol optical thickness and retrievals of size distribution from solar aureole measurements. The aerosol optical thickness measurements clearly illustrate that for sites located within regions of biomass burning the duration of smoke above background levels often exceeds 2 months and frequently at levels an order of magnitude above background. The aerosol optical thickness range during preburning conditions was 0.11 to 0.27 at 440 nm. Under these conditions, stratospheric aerosols from Pinatubo constituted a significant part of the signal in 1993 but were about 50% less in 1994. During the burning season, smoke elevated the aerosol optical thickness above 1.0 for seasonally averaged values measured at 440 nm at sites located in active source regions in Mato Grosso, Rondonia, and Tocantins states. The measurement sites are located in the cerrado and forest conversion areas. Analysis of the size distribution of the particles indicated that the increase in aerosol optical thickness was associated with an increase of an accumulation and coarse particle modes. The asymmetry factor “g”, computed from the phase function, showed considerable spectral dependence between the preburning and burning seasonal phases. The 1020‐nm channel was reduced from 0.66 to ∼0.53, while at 440 nm little seasonal phase variation was noted. Conditions of burning were sufficiently strong that the atmospheric conditions associated with the climatological definition of a dry season was subdivided into (1) preburning, (2) transition to burning, (3) burning, and (4) transition to wet season phases for most sites. Averages and frequency distributions were used to characterize each seasonal phase by site. Changes in total column water vapor amount, also retrieved from direct sun measurements, did not have an apparent effect on the optical properties of the aerosols.

On the relation between haze layer and air mass aerosol at an urban location — case studies

Aerosol extinction spectra of the lowest part of the urban boundary layer at Leipzig measured by means of sunphotometers in the wavelength range between 0.36 μm and 1.05 μm are presented. The temporal behaviour of these spectra is compared with that of relevant atmospheric turbidity parameters. Special attention is paid to the analysis of atmospheric aerosol optical thickness spectra. It is shown that, concerning aerosol properties, the ageing of an air mass caused by urban activities is identical with an increase of aerosol optical thickness, especially in the short-wavelength-range. This fact could be attributed to the convective rise of, primarily, small aerosol particles ( r < 0.3 μm) being emitted by anthropogenic sources near the ground. This process also seems to be responsible for a vertical aerosol stratification within the haze layer. It was found that in the case of turbulent mixing the optical state of the around layer is mainly determined by accumulation and coarse mode particles, whereas the upper layers contain a large amount of fine particles.