scholarly journals A global study of hygroscopicity-driven light-scattering enhancement in the context of other in situ aerosol optical properties

2021 ◽  
Vol 21 (17) ◽  
pp. 13031-13050
Author(s):  
Gloria Titos ◽  
María A. Burgos ◽  
Paul Zieger ◽  
Lucas Alados-Arboledas ◽  
Urs Baltensperger ◽  
...  
Keyword(s):  
Light Scattering ◽  
Radiative Forcing ◽  
Incident Light ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Clear Pattern ◽  
Opposite Pattern ◽  
Aerosol Radiative Forcing ◽  
Dry Conditions ◽  
The Impact

Abstract. The scattering and backscattering enhancement factors (f(RH) and fb(RH)) describe how aerosol particle light scattering and backscattering, respectively, change with relative humidity (RH). They are important parameters in estimating direct aerosol radiative forcing (DARF). In this study we use the dataset presented in Burgos et al. (2019) that compiles f(RH) and fb(RH) measurements at three wavelengths (i.e., 450, 550 and 700 nm) performed with tandem nephelometer systems at multiple sites around the world. We present an overview of f(RH) and fb(RH) based on both long-term and campaign observations from 23 sites representing a range of aerosol types. The scattering enhancement shows a strong variability from site to site, with no clear pattern with respect to the total scattering coefficient. In general, higher f(RH) is observed at Arctic and marine sites, while lower values are found at urban and desert sites, although a consistent pattern as a function of site type is not observed. The backscattering enhancement fb(RH) is consistently lower than f(RH) at all sites, with the difference between f(RH) and fb(RH) increasing for aerosol with higher f(RH). This is consistent with Mie theory, which predicts higher enhancement of the light scattering in the forward than in the backward direction as the particle takes up water. Our results show that the scattering enhancement is higher for PM1 than PM10 at most sites, which is also supported by theory due to the change in scattering efficiency with the size parameter that relates particle size and the wavelength of incident light. At marine-influenced sites this difference is enhanced when coarse particles (likely sea salt) predominate. For most sites, f(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of f(RH) is found to be low or even exhibit the opposite pattern. The impact of RH on aerosol properties used to calculate radiative forcing (e.g., single-scattering albedo, ω0, and backscattered fraction, b) is evaluated. The single-scattering albedo generally increases with RH, while b decreases. The net effect of aerosol hygroscopicity on radiative forcing efficiency (RFE) is an increase in the absolute forcing effect (negative sign) by a factor of up to 4 at RH = 90 % compared to dry conditions (RH < 40 %). Because of the scarcity of scattering enhancement measurements, an attempt was made to use other more commonly available aerosol parameters (i.e., ω0 and scattering Ångström exponent, αsp) to parameterize f(RH). The majority of sites (75 %) showed a consistent trend with ω0 (higher f(RH = 85 %) for higher ω0), while no clear pattern was observed between f(RH = 85 %) and αsp. This suggests that aerosol ω0 is more promising than αsp as a surrogate for the scattering enhancement factor, although neither parameter is ideal. Nonetheless, the qualitative relationship observed between ω0 and f(RH) could serve as a constraint on global model simulations.

scholarly journals A global study of hygroscopicity-driven light scattering enhancement in the context of other in-situ aerosol optical properties

2020 ◽  
Author(s):  
Gloria Titos ◽  
María A. Burgos ◽  
Paul Zieger ◽  
Lucas Alados-Arboledas ◽  
Urs Baltensperger ◽  
...  
Keyword(s):  
Light Scattering ◽  
Radiative Forcing ◽  
Incident Light ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Opposite Pattern ◽  
Aerosol Radiative Forcing ◽  
Dry Conditions ◽  
The Difference ◽  
The Impact

Abstract. The scattering and backscattering enhancement factors (f(RH) and fb(RH)) describe how aerosol particle light scattering and backscattering, respectively, change with relative humidity (RH). They are important parameters in estimating direct aerosol radiative forcing (DARF). In this study we use a dataset presented in Burgos et al. (2019) that compiles f(RH) and fb(RH) measurements at three wavelengths (i.e. 450, 550 and 700 nm) performed with tandem nephelometer systems at multiple sites around the world. We present an overview of f(RH) and fb(RH) based on both long-term and campaign observations from 23 sites representing a range of aerosol types. The scattering enhancement shows a strong variability from site to site, with no clear pattern with respect to total scattering coefficient. In general, higher f(RH) is observed at Arctic and marine sites while lower values are found at urban and desert sites, although a consistent pattern as a function of site type is not observed. The backscattering enhancement fb(RH) is consistently lower tan f(RH) at all sites, with the difference between f(RH) and fb(RH) increasing for aerosol with higher f(RH). This is consistent with Mie theory which predicts higher enhancement of the light-scattering in the forward than in the backward direction as the particle takes up water. Our results show that the scattering enhancement is higher for PM1 than PM10 at most sites, which is also supported by theory due to the change in scattering efficiency with the size parameter that relates particle size and wavelength of incident light. At marine-influenced sites this difference is enhanced when coarse particles (likely sea salt) predominate. For most sites, f(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of f(RH) is found to be low or even exhibit the opposite pattern. The impact of RH on aerosol properties used to calculate radiative forcing (e.g., single scattering albedo, ω0, and backscattered fraction, b) is evaluated. The single scattering albedo generally increases with RH while b decreases. The net effect of aerosol hygroscopicity on radiative forcing efficiency (RFE) is an increase in the absolute forcing effect (negative sign) by a factor of up to 4 at RH = 90 % compared to dry conditions (RH 

scholarly journals Measured and predicted aerosol light scattering enhancement factors at the high alpine site Jungfraujoch

2010 ◽  
Vol 10 (5) ◽  
pp. 2319-2333 ◽  
Author(s):  
R. Fierz-Schmidhauser ◽  
P. Zieger ◽  
M. Gysel ◽  
L. Kammermann ◽  
P. F. DeCarlo ◽  
...  
Keyword(s):  
Light Scattering ◽  
Radiative Forcing ◽  
Single Scattering ◽  
Important Variable ◽  
Single Scattering Albedo ◽  
Climate Forcing ◽  
Ambient Relative Humidity ◽  
Enhancement Factors ◽  
Dry Conditions ◽  
Aerosol Type

Abstract. Ambient relative humidity (RH) determines the water content of atmospheric aerosol particles and thus has an important influence on the amount of visible light scattered by particles. The RH dependence of the particle light scattering coefficient (σsp) is therefore an important variable for climate forcing calculations. We used a humidification system for a nephelometer which allows for the measurement of σsp at a defined RH in the range of 20–95%. In this paper we present measurements of light scattering enhancement factors f(RH)=σsp(RH)/σsp(dry) from a 1-month campaign (May 2008) at the high alpine site Jungfraujoch (3580 m a.s.l.), Switzerland. Measurements at the Jungfraujoch are representative for the lower free troposphere above Central Europe. For this aerosol type hardly any information about the f(RH) is available so far. At this site, f(RH=85%) varied between 1.2 and 3.3. Measured f(RH) agreed well with f(RH) calculated with Mie theory using measurements of the size distribution, chemical composition and hygroscopic diameter growth factors as input. Good f(RH) predictions at RH<85% were also obtained with a simplified model, which uses the Ångström exponent of σsp(dry) as input. RH influences further intensive optical aerosol properties. The backscatter fraction decreased by about 30% from 0.128 to 0.089, and the single scattering albedo increased on average by 0.05 at 85% RH compared to dry conditions. These changes in σsp, backscatter fraction and single scattering albedo have a distinct impact on the radiative forcing of the Jungfraujoch aerosol.

scholarly journals Measured and predicted aerosol light scattering enhancement factors at the high alpine site Jungfraujoch

2009 ◽  
Vol 9 (5) ◽  
pp. 20063-20101 ◽  
Author(s):  
R. Fierz-Schmidhauser ◽  
P. Zieger ◽  
M. Gysel ◽  
L. Kammermann ◽  
P. F. DeCarlo ◽  
...  
Keyword(s):  
Light Scattering ◽  
Radiative Forcing ◽  
Mie Theory ◽  
Single Scattering ◽  
Important Variable ◽  
Single Scattering Albedo ◽  
Climate Forcing ◽  
Ambient Relative Humidity ◽  
Enhancement Factors ◽  
Dry Conditions

Abstract. Ambient relative humidity (RH) determines the water content of atmospheric aerosol particles and thus has an important influence on the amount of visible light scattered by particles. The RH dependence of the particle light scattering coefficient (σsp) is therefore an important variable for climate forcing calculations. We used a humidification system for a nephelometer which allows for the measurement of σsp at a defined RH in the range of 20–95%. In this paper we present measurements of light scattering enhancement factors f(RH)=σsp(RH)/σsp(dry) from a 1-month campaign (May 2008) at the high alpine site Jungfraujoch (3580 m a.s.l.), Switzerland. At this site, f(RH=85%) varied between 1.2 and 3.3. Measured f(RH) agreed well with f(RH) calculated with Mie theory using measurements of the size distribution, chemical composition and hygroscopic diameter growth factors as input. Good f(RH) predictions at RH<85% were also obtained with a simplified model, which uses the Ångström exponent of σsp(dry) as input. RH influences further intensive optical aerosol properties. The backscatter fraction decreased by about 30% from 0.128 to 0.089, and the single scattering albedo increased on average by 0.05 at 85% RH compared to dry conditions. These changes in σsp, backscatter fraction and single scattering albedo have a distinct impact on the radiative forcing of the Jungfraujoch aerosol.

scholarly journals Direct Aerosol Radiative Forcing Uncertainty Based on a Radiative Perturbation Analysis

2010 ◽  
Vol 23 (19) ◽  
pp. 5288-5293 ◽  
Author(s):  
Norman G. Loeb ◽  
Wenying Su
Keyword(s):  
Radiative Forcing ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Aerosol Radiative Forcing ◽  
Clear Sky ◽  
Radiative Perturbation ◽  
Sky Conditions ◽  
Global Mean ◽  
Aerosol Properties ◽  
Aerosol Radiative

Abstract To provide a lower bound for the uncertainty in measurement-based clear- and all-sky direct aerosol radiative forcing (DARF), a radiative perturbation analysis is performed for the ideal case in which the perturbations in global mean aerosol properties are given by published values of systematic uncertainty in Aerosol Robotic Network (AERONET) aerosol measurements. DARF calculations for base-state climatological cloud and aerosol properties over ocean and land are performed, and then repeated after perturbing individual aerosol optical properties (aerosol optical depth, single-scattering albedo, asymmetry parameter, scale height, and anthropogenic fraction) from their base values, keeping all other parameters fixed. The total DARF uncertainty from all aerosol parameters combined is 0.5–1.0 W m−2, a factor of 2–4 greater than the value cited in the Intergovernmental Panel on Climate Change’s (IPCC’s) Fourth Assessment Report. Most of the total DARF uncertainty in this analysis is associated with single-scattering albedo uncertainty. Owing to the greater sensitivity to single-scattering albedo in cloudy columns, DARF uncertainty in all-sky conditions is greater than in clear-sky conditions, even though the global mean clear-sky DARF is more than twice as large as the all-sky DARF.

scholarly journals Direct SW aerosol radiative forcing over Portugal

2008 ◽  
Vol 8 (3) ◽  
pp. 8585-8628 ◽  
Author(s):  
D. Santos ◽  
M. J. Costa ◽  
A. M. Silva
Keyword(s):  
Forest Fires ◽  
Spectral Reflectance ◽  
Radiative Forcing ◽  
Longwave Radiation ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Aerosol Radiative Forcing ◽  
Desert Dust ◽  
Average Value ◽  
Aerosol Radiative

Abstract. The estimation of radiative forcing due to desert dust and forest fires aerosols is a very important issue since these particles are very efficient at scattering and absorbing both short and longwave radiation. In this work, the evaluation of the aerosol radiative forcing at the top of the atmosphere over the south of Portugal is made, particularly in the regions of Évora and of Cabo da Roca. The radiative transfer calculations combine ground-based and satellite measurements, to estimate the top of the atmosphere direct SW aerosol radiative forcing. The method developed to retrieve the surface spectral reflectance is also presented, based on ground-based measurements of the aerosol optical properties combined with the satellite-measured radiances. The aerosol direct radiative effect is shown to be very sensitive to the underlying surface, since different surface spectral reflectance values may originate different forcing values. The results obtained also illustrate the importance of considering the actual aerosol properties, in this case measured by ground-based instrumentation, particularly the aerosol single scattering albedo, because different aerosol single scattering albedo values can flip the sign of the direct SW aerosol radiative forcing. The instantaneous direct SW aerosol radiative forcing values obtained at the top of the atmosphere are, in the majority of the cases, negative, indicating a tendency for cooling the Earth. For Desert Dust aerosols, over Évora land region, the average forcing efficiency is estimated to be −25 W/m2/AOT0.55 whereas for Cabo da Roca area, the average forcing efficiency is −46 W/m2/AOT0.55. In the presence of Forest Fire aerosols, over Cabo da Roca region, the average value of forcing efficiency is −28 W/m2/AOT0.55 and over Évora region an average value of −33 W/m2/AOT0.55 is found.

scholarly journals Multi-satellite retrieval of single scattering albedo using the OMI–MODIS algorithm

2019 ◽  
Vol 19 (5) ◽  
pp. 3307-3324 ◽  
Author(s):  
Kruthika Eswaran ◽  
Sreedharan Krishnakumari Satheesh ◽  
Jayaraman Srinivasan
Keyword(s):  
Bay Of Bengal ◽  
Radiative Forcing ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Retrieval Algorithm ◽  
Ozone Monitoring Instrument ◽  
Aerosol Composition ◽  
Aerosol Radiative Forcing ◽  
Moderate Resolution Imaging Spectroradiometer ◽  
Aerosol Type

Abstract. Single scattering albedo (SSA) represents a unique identification of aerosol type and can be a determinant factor in the estimation of aerosol radiative forcing. However, SSA retrievals are highly uncertain due to cloud contamination and aerosol composition. The recent improvement in the SSA retrieval algorithm has combined the superior cloud-masking technique of the Moderate Resolution Imaging Spectroradiometer (MODIS) and the higher sensitivity of the Ozone Monitoring Instrument (OMI) to aerosol absorption. The combined OMI–MODIS algorithm has only been validated over a small spatial and temporal scale. The present study validates the algorithm over global oceans for the period from 2008 to 2012. The geographical heterogeneity in the aerosol type and concentration over the Atlantic Ocean, the Arabian Sea and the Bay of Bengal was useful to delineate the effect of aerosol type on the retrieval algorithm. We also noted that OMI overestimated SSA when absorbing aerosols were present closer to the surface. We attribute this overestimation to data discontinuity in the aerosol height climatology derived from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. OMI uses predefined aerosol heights over regions where CALIPSO climatology is not present, leading to the overestimation of SSA. The importance of aerosol height was also studied using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The results from the joint retrievals were validated using cruise-based measurements. It was seen that OMI–MODIS SSA retrievals performed better than the OMI only retrieval over the Bay of Bengal during winter, when the aerosols are present closer to the surface. Discrepancy between satellite retrievals and cruise measurements was seen when elevated aerosols were present which might not have been detected by the cruise instruments.

scholarly journals A Method to Estimate Aerosol Radiative Forcing from Spectral Optical Depths

2006 ◽  
Vol 63 (3) ◽  
pp. 1082-1092 ◽  
Author(s):  
S. K. Satheesh ◽  
J. Srinivasan
Keyword(s):  
Chemical Composition ◽  
Optical Depth ◽  
Radiative Forcing ◽  
Field Experiments ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Radiative Properties ◽  
Aerosol Radiative Forcing ◽  
Sky Radiance ◽  
Aerosol Radiative

Abstract Radiative forcing of aerosols is much more difficult to estimate than that of well-mixed gases due to the large spatial variability of aerosols and the lack of an adequate database on their radiative properties. Estimation of aerosol radiative forcing generally requires knowledge of its chemical composition, which is sparse. Ground-based sky radiance measurements [e.g., aerosol robotic network (AERONET)] can provide key parameters such as the single-scattering albedo, but in shipborne experiments over the ocean it is difficult to make sky radiance measurements and hence these experiments cannot provide parameters such as the single-scattering albedo. However, aerosol spectral optical depth data (cruise based as well as satellite retrieved) are available quite extensively over the ocean. Spectral optical depth measurements have been available since the 1970s, and spectral turbidity measurements (carried out at meteorological departments all over the world) have been available for several decades, while long-term continuous chemical composition information is not available. A new method to differentiate between scattering and absorbing aerosols is proposed here. This can be used to derive simple aerosol models that are optically equivalent and can simulate the observed aerosol optical properties and radiative fluxes, from spectral optical depth measurements. Thus, aerosol single-scattering albedo and, hence, aerosol radiative forcing can be estimated. Note that the proposed method is to estimate clear-sky aerosol radiative forcing (over regions where chemical composition data or sky radiance data are not available) and not to infer its exact chemical composition. Using several independent datasets from field experiments, it is demonstrated that the proposed method can be used to estimate aerosol radiative forcing (from spectral optical depths) with an accuracy of ±2 W m−2.

scholarly journals Sensitization Towards Aerosol Optical Properties And Radiative Forcing, Real Case In Morocco

2021 ◽  
Vol 319 ◽  
pp. 02027
Author(s):  
Somia Ssouaby ◽  
Hafida Naim ◽  
Abdelouahid Tahiri ◽  
Salmane Bourekkadi
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Radiative Forcing ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Regional Warming ◽  
Kuwait University ◽  
Aerosol Radiative Forcing ◽  
Desert Areas ◽  
Aerosol Radiative

Mineral dust is one of the most important aerosol components in the Earth’s atmosphere. Desert aerosol constitute the main types of tropospheric aerosols whose optical property uncertainties are still quite important. In this study, we analyse the variability of aerosol optical depth (AOD), Angstrôm Exponent (α), Single Scattering Albedo (ω0) and aerosol radiative forcing (ARF) of desert aerosol recent measurements, for six AERONET sites covering the belt desert areas: Ouarzazate (Morocco), Tamanrasset (Algeria), El Farafra (Egypt), Mezaira (Unites Arab Emirates), Kuwait University (Kuwait), Dalanzadgad (Mongolia). The annual cycle of the aerosol optical depth dialy averages shows variable values due to the changeable weather and the Sahara source. The highests were recorded at the Sahara site (2.2 at Tamanrasset) and (2.9 at Kuwait-University). The spectral single scattering albedo SSA annual averages varies in the interval (0,8-0.95) indicating dominant scattering. Desert aerosol radiative forcing shows always a negative ARF with a maximums registred in July, -90 W/m2 at surface (Mezaira) and -26 W/m2 at the top of the atmosphere (Kuwait) that imply a general trend towards regional warming of the total column atmosphere with a maximum near +55 W/m2 observed in July at UAE.

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