scholarly journals Scattering and absorption by aerosols during EUCAARI-LONGREX: can airborne measurements and models agree?

2011 ◽  
Vol 11 (6) ◽  
pp. 18487-18525 ◽  
Author(s):  
E. J. Highwood ◽  
M. J. Northway ◽  
G. R. McMeeking ◽  
W. T. Morgan ◽  
D. Liu ◽  
...  
Keyword(s):  
Optical Properties ◽  
Black Carbon ◽  
Single Scattering ◽  
Temporal Variations ◽  
Mie Scattering ◽  
Single Scattering Albedo ◽  
Chemical Components ◽  
Atmospheric Measurements ◽  
Data Set ◽  
Airborne Measurements

Abstract. Scattering and absorption by aerosol in anthropogenically perturbed air masses over Europe has been measured using instrumentation flown on the UK's BAe-146-301 large Atmospheric Research Aircraft (ARA) operated by the Facility for Airborne Atmospheric Measurements (FAAM) on 14 flights during the EUCAARI-LONGREX campaign in May 2008. The geographical and temporal variations of the derived shortwave optical properties of aerosol are presented. Values of single scattering albedo of dry aerosol at 550 nm varied considerably over the data set from 0.86 to near unity. Dry aerosol optical depths ranged from 0.03 to 0.24. An optical properties closure study comparing calculations from composition data and Mie scattering code with the measured properties is presented. Very good agreement (to within 30 %) can be achieved for scattering, but the modelling of absorption is shown to be sensitive to the refractive indices chosen for organic aerosols, and to a lesser extent black carbon. Agreement with the measured absorption can only be achieved if organic carbon is assumed to be only weakly absorbing. Hygroscopic growth curves derived from the wet nephelometer indicate moderate water uptake by the aerosol with a campaign mean f(RH) value (change in scattering) of 1.3 at 80 % relative humidity. This value is consistent with the major chemical components of the aerosol measured by the aerosol mass spectrometer (AMS), which are primarily mixed organics and nitrate and some sulphate. As expected the effect of humidity is to raise the single scattering albedo, and to increase the aerosol optical depth. This study represents an important new body of data regarding European aerosol amounts, composition and optical properties and additionally demonstrates the importance of airborne measurements of black carbon mass and aerosol hygroscopicity.

scholarly journals Aerosol scattering and absorption during the EUCAARI-LONGREX flights of the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146: can measurements and models agree?

2012 ◽  
Vol 12 (15) ◽  
pp. 7251-7267 ◽  
Author(s):  
E. J. Highwood ◽  
M. J. Northway ◽  
G. R. McMeeking ◽  
W. T. Morgan ◽  
D. Liu ◽  
...  
Keyword(s):  
Optical Properties ◽  
Organic Carbon ◽  
Black Carbon ◽  
Growth Curves ◽  
Temporal Variations ◽  
Mie Scattering ◽  
Chemical Components ◽  
Refractive Indices ◽  
Hygroscopic Growth ◽  
Atmospheric Measurements

Abstract. Scattering and absorption by aerosol in anthropogenically perturbed air masses over Europe has been measured using instrumentation flown on the UK's BAe-146-301 large Atmospheric Research Aircraft (ARA) operated by the Facility for Airborne Atmospheric Measurements (FAAM) on 14 flights during the EUCAARI-LONGREX campaign in May 2008. The geographical and temporal variations of the derived shortwave optical properties of aerosol are presented. Values of single scattering albedo of dry aerosol at 550 nm varied considerably from 0.86 to near unity, with a campaign average of 0.93 ± 0.03. Dry aerosol optical depths ranged from 0.030 ± 0.009 to 0.24 ± 0.07. An optical properties closure study comparing calculations from composition data and Mie scattering code with the measured properties is presented. Agreement to within measurement uncertainties of 30% can be achieved for both scattering and absorption, but the latter is shown to be sensitive to the refractive indices chosen for organic aerosols, and to a lesser extent black carbon, as well as being highly dependent on the accuracy of the absorption measurements. Agreement with the measured absorption can be achieved either if organic carbon is assumed to be weakly absorbing, or if the organic aerosol is purely scattering and the absorption measurement is an overestimate due to the presence of large amounts of organic carbon. Refractive indices could not be inferred conclusively due to this uncertainty, despite the enhancement in methodology compared to previous studies that derived from the use of the black carbon measurements. Hygroscopic growth curves derived from the wet nephelometer indicate moderate water uptake by the aerosol with a campaign mean f(RH) value (ratio in scattering) of 1.5 (range from 1.23 to 1.63) at 80% relative humidity. This value is qualitatively consistent with the major chemical components of the aerosol measured by the aerosol mass spectrometer, which are primarily mixed organics and nitrate and some sulphate.

scholarly journals Seasonal cycle and source analyses of aerosol optical properties in a semi-urban environment at Puijo station in Eastern Finland

2012 ◽  
Vol 12 (12) ◽  
pp. 5647-5659 ◽  
Author(s):  
A. Leskinen ◽  
A. Arola ◽  
M. Komppula ◽  
H. Portin ◽  
P. Tiitta ◽  
...  
Keyword(s):  
Optical Properties ◽  
Paper Mill ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Scattering Coefficient ◽  
Traffic Density ◽  
Aerosol Optical Properties ◽  
Absorption Coefficients ◽  
Data Set ◽  
Pollutant Sources

Abstract. We introduce a four-year (in 2006–2010) continuous data set of aerosol optical properties at Puijo in Kuopio, Finland. We study the annual and diurnal variation of the aerosol scattering and absorption coefficients, hemispheric backscattering fraction, scattering Ångström exponent, and single scattering albedo, whose median values over this period were 7.2 Mm−1 (at 550 nm), 1.0 Mm−1 (at 637 nm), 0.15, 1.93 (between 450 and 550 nm), and 0.85, respectively. The scattering coefficient peaked in the spring and autumn, being 2–4 times those in the summer and winter. An exception was the summer of 2010, when the scattering coefficient was elevated to ~300 Mm−1 by plumes from forest fires in Russia. The absorption coefficient peaked in the winter when soot-containing particles derived from biomass burning were present. The higher relative absorption coefficients resulted in lower single scattering albedo in winter. The optical properties varied also with wind direction and time of the day, indicating the effect of the local pollutant sources and the age of the particles. Peak values in the single scattering albedo were observed when the wind blew from a paper mill and from the sector without local pollutant sources. These observations were linked, respectively, to the sulphate-rich aerosol from the paper mill and the oxygenated organics in the aged aerosol, which both are known to increase the scattering characteristics of aerosols. Decreases in the single scattering albedo in the morning and afternoon, distinct in the summertime, were linked to the increased traffic density at these hours. The scattering and absorption coefficients of residential and long-range transported aerosol (two separate cloud events) were found to be decreased by clouds. The effect was stronger for the scattering than absorption, indicating preferential activation of the more hygroscopic aerosol with higher scattering characteristics.

scholarly journals Vertical profiles of light absorption and scattering associated with black-carbon particle fractions in the springtime Arctic above 79° N

2019 ◽  
Author(s):  
W. Richard Leaitch ◽  
John K. Kodros ◽  
Megan D. Willis ◽  
Sarah Hanna ◽  
Hannes Schulz ◽  
...  
Keyword(s):  
Optical Properties ◽  
Absorption Coefficient ◽  
Black Carbon ◽  
Light Absorption ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
The Arctic ◽  
Vertical Profiles ◽  
Near Surface ◽  
Arctic Atmosphere

Abstract. Despite the potential importance of black carbon (BC) to radiative forcing of the Arctic atmosphere, vertically-resolved measurements of the particle light scattering coefficient (Bsp) and light absorption coefficient (Bap) in the springtime Arctic atmosphere are infrequent, especially measurements at latitudes at or above 80oN. Here, relationships among vertically-distributed aerosol optical properties Bap, Bsp, and single scattering albedo or SSA), particle microphysics and particle chemistry are examined for a region of the Canadian archipelago between 79.9oN and 83.4oN from near the surface to 500 hPa. Airborne data collected during April, 2015, are combined with ground-based observations from the observatory at Alert, Nunavut and simulations from the GEOS-Chem-TOMAS model (Kodros et al., 2018) to increase our knowledge of the effects of BC on light absorption in the Arctic troposphere. The results are constrained for Bsp less than 15 Mm-1, which represent 98% of the observed Bsp, because the single scattering albedo (SSA) has a tendency to be lower at lower Bsp, resulting in a larger relative contribution to Arctic warming. At 18.4 m2 g-1, the average BC mass absorption coefficient (MAC) from the combined airborne and Alert observations is substantially higher than the two averaged modelled MAC values (9.5 m2 g-1 and 7.0 m2 g-1) for two different internal mixing assumptions, the latter of which is based on previous observations. The higher observed MAC value may be explained by an underestimation of BC and possible differences in BC microphysics and morphologies between the observations and model. We present Bap and SSA based on the assumption that Bap is overestimated in the observations in addition to the assumption that the higher MAC is explained. Median values of the measured Bap, rBC and organic component of particles all increase by a factor of 1.8±0.1 going from near-surface to 750 hPa, and values higher than the surface persist to 600 hPa. Modelled BC, organics, and Bap agree with the near-surface measurements, but do not reproduce the higher values observed between 900 hPa and 600 hPa. The differences between modelled and observed optical properties follow the same trend as the differences between the modelled and observed concentrations of the carbonaceous components (black and organic). Some discrepancies in the model may be due to the use of a relatively low imaginary refractive index of BC as well as by the ejection of biomass burning particles only into the boundary layer at sources. For the assumption of the higher observed MAC value, the SSA range between 0.88 and 0.94, which is significantly lower than other recent estimates for the Arctic, in part reflecting the constraint of Bsp <15 Mm-1. The large uncertainties in measuring optical properties and BC as well as the large differences between measured and modelled values, here and in the literature, argue for improved measurements of BC and light absorption by BC as well as more vertical profiles of aerosol chemistry, microphysics, and other optical properties in the Arctic.

scholarly journals Closure on the single scattering albedo in the WRF-Chem framework using data from the MILAGRO campaign

2009 ◽  
Vol 9 (1) ◽  
pp. 5009-5054
Author(s):  
J. C. Barnard ◽  
J. D. Fast ◽  
G. Paredes-Miranda ◽  
W. P. Arnott
Keyword(s):  
Optical Properties ◽  
Black Carbon ◽  
Chemical Properties ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Aerosol Optical Properties ◽  
Mean Values ◽  
Coarse Mode ◽  
Fine Mode ◽  
Using Data

Abstract. Data from the MILAGRO field campaign, which took place in the Mexico City Metropolitan Area (MCMA) during March 2006, is used to perform a closure experiment between aerosol chemical properties and aerosol optical properties. Measured aerosol chemical properties, obtained from the MILAGRO T1 site, are fed to two different "chemical to optical properties" modules. One module uses a sectional approach and is identical to that used in the WRF-Chem model, while the other is based on a modal approach. This modal code is employed as an independent check on the WRF-Chem module. Both modules compute aerosol optical properties and, in particular, the single-scattering albedo, ϖ0, as a function of time. The single-scattering albedos are compared to independent measurements obtained from a photoacoustic spectrometer (PAS). Because chemical measurements of the aerosol coarse mode were not available, and the inlet of the PAS could not ingest aerosols larger than about 2 to 3 μm, we focus here on the fine-mode ϖ0. At 870 nm, the wavelength of the PAS measurements, the agreement between the computed (modal and WRF-Chem) and observed fine-mode ϖ0, averaged over the course of the campaign, is reasonably good. The observed ϖ0 value is 0.77, while for both modules, the calculated value was 0.75 resulting in a difference of 0.02 between observations and both computational approaches. This difference is less than the uncertainty of the observed ϖ0 values (6%, or 0.05), and therefore "closure" is achieved, at least for mean values. After adjusting some properties of black carbon absorption and mass concentration within plausible uncertainty limits, the two modules simulate well the diurnal variation of ϖ0, and the absorption coefficient, Babs, but are less successful in calculating the variation of the scattering coefficient, Bscat. This difficulty is probably caused by the presence of larger particles during the day when windblown dust is ubiquitous; this dust likely increases the proportion of large particles introduced into the PAS. The dust also contributes to a very large aerosol mass loading in the coarse mode, and neglect of the coarse mode may cause significant errors, estimated to be as large as 0.07, in the calculation and measurement of ambient ϖ0. Finally, the observed ϖ0 is compared to the ϖ0 computed by the full WRF-Chem model, which includes prognostic aerosol chemistry. Unlike the results discussed above, a comparison between observed and simulated ϖ0 values reveals major differences. This large discrepancy is probably due, in part, to poor characterization of emissions near the T1 site, particularly black carbon emissions.

scholarly journals Vertical profiles of light absorption and scattering associated with black carbon particle fractions in the springtime Arctic above 79° N

2020 ◽  
Vol 20 (17) ◽  
pp. 10545-10563 ◽  
Author(s):  
W. Richard Leaitch ◽  
John K. Kodros ◽  
Megan D. Willis ◽  
Sarah Hanna ◽  
Hannes Schulz ◽  
...  
Keyword(s):  
Optical Properties ◽  
Absorption Coefficient ◽  
Black Carbon ◽  
Light Absorption ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
The Arctic ◽  
Vertical Profiles ◽  
Near Surface ◽  
Arctic Atmosphere

Abstract. Despite the potential importance of black carbon (BC) for radiative forcing of the Arctic atmosphere, vertically resolved measurements of the particle light scattering coefficient (σsp) and light absorption coefficient (σap) in the springtime Arctic atmosphere are infrequent, especially measurements at latitudes at or above 80∘ N. Here, relationships among vertically distributed aerosol optical properties (σap, σsp and single scattering albedo or SSA), particle microphysics and particle chemistry are examined for a region of the Canadian archipelago between 79.9 and 83.4∘ N from near the surface to 500 hPa. Airborne data collected during April 2015 are combined with ground-based observations from the observatory at Alert, Nunavut and simulations from the Goddard Earth Observing System (GEOS) model, GEOS-Chem, coupled with the TwO-Moment Aerosol Sectional (TOMAS) model (collectively GEOS-Chem–TOMAS; Kodros et al., 2018) to further our knowledge of the effects of BC on light absorption in the Arctic troposphere. The results are constrained for σsp less than 15 Mm−1, which represent 98 % of the observed σsp, because the single scattering albedo (SSA) has a tendency to be lower at lower σsp, resulting in a larger relative contribution to Arctic warming. At 18.4 m2 g−1, the average BC mass absorption coefficient (MAC) from the combined airborne and Alert observations is substantially higher than the two averaged modelled MAC values (13.6 and 9.1 m2 g−1) for two different internal mixing assumptions, the latter of which is based on previous observations. The higher observed MAC value may be explained by an underestimation of BC, the presence of small amounts of dust and/or possible differences in BC microphysics and morphologies between the observations and model. In comparing the observations and simulations, we present σap and SSA, as measured, and σap∕2 and the corresponding SSA to encompass the lower modelled MAC that is more consistent with accepted MAC values. Median values of the measured σap, rBC and the organic component of particles all increase by a factor of 1.8±0.1, going from near-surface to 750 hPa, and values higher than the surface persist to 600 hPa. Modelled BC, organics and σap agree with the near-surface measurements but do not reproduce the higher values observed between 900 and 600 hPa. The differences between modelled and observed optical properties follow the same trend as the differences between the modelled and observed concentrations of the carbonaceous components (black and organic). Model-observation discrepancies may be mostly due to the modelled ejection of biomass burning particles only into the boundary layer at the sources. For the assumption of the observed MAC value, the SSA range between 0.88 and 0.94, which is significantly lower than other recent estimates for the Arctic, in part reflecting the constraint of σsp<15 Mm−1. The large uncertainties in measuring optical properties and BC, and the large differences between measured and modelled values here and in the literature, argue for improved measurements of BC and light absorption by BC and more vertical profiles of aerosol chemistry, microphysics and other optical properties in the Arctic.

scholarly journals Aircraft and ground measurements of dust aerosols over the west African coast in summer 2015 during ICE-D and AER-D

2018 ◽  
Vol 18 (5) ◽  
pp. 3817-3838 ◽  
Author(s):  
Dantong Liu ◽  
Jonathan W. Taylor ◽  
Jonathan Crosier ◽  
Nicholas Marsden ◽  
Keith N. Bower ◽  
...  
Keyword(s):  
Optical Properties ◽  
Black Carbon ◽  
Marine Boundary Layer ◽  
Single Scattering ◽  
Effective Diameter ◽  
Radiative Effects ◽  
Airborne Measurements ◽  
Cloud Properties ◽  
The Uk ◽  
Santiago Island

Abstract. During the summertime, dust from the Sahara can be efficiently transported westwards within the Saharan air layer (SAL). This can lead to high aerosol loadings being observed above a relatively clean marine boundary layer (MBL) in the tropical Atlantic Ocean. These dust layers can impart significant radiative effects through strong visible and IR light absorption and scattering, and can also have indirect impacts by altering cloud properties. The processing of the dust aerosol can result in changes in both direct and indirect radiative effects, leading to significant uncertainty in climate prediction in this region. During August 2015, measurements of aerosol and cloud properties were conducted off the coast of west Africa as part of the Ice in Cloud Experiment – Dust (ICE-D) and AERosol properties – Dust (AER-D) campaigns. Observations were obtained over a 4-week period using the UK Facility for Atmospheric Airborne Measurements (FAAM) BAe 146 aircraft based on Santiago Island, Cabo Verde. Ground-based observations were collected from Praia (14∘57′ N, 23∘29′ W; 100 m a.s.l.), also located on Santiago Island. The dust in the SAL was mostly sampled in situ at altitudes of 2–4 km, and the potential dust age was estimated by backward trajectory analysis. The particle mass concentration (at diameter d = 0.1–20 µm) decreased with transport time. Mean effective diameter (Deff) for supermicron SAL dust (d = 1–20 µm) was found to be 5–6 µm regardless of dust age, whereas submicron Deff (d = 0.1–1 µm) showed a decreasing trend with longer transport. For the first time, an airborne laser-induced incandescence instrument (the single particle soot photometer – SP2) was deployed to measure the hematite content of dust. For the Sahel-influenced dust in the SAL, the observed hematite mass fraction of dust (FHm) was found to be anti-correlated with the single scattering albedo (SSA, λ = 550 nm, for particles d < 2.5 µm); as potential dust age increased from 2 to 7 days, FHm increased from 2.5 to 4.5 %, SSA decreased from 0.97 to 0.93 and the derived imaginary part (k) of the refractive index at 550 nm increased from 0.0015 to 0.0035. However, the optical properties of Sahara-influenced plumes (not influenced by the Sahel) were independent of dust age and hematite content with SSA ∼ 0.95 and k ∼ 0.0028. This indicates that the absorbing component of dust may be source dependent, or that gravitational settling of larger particles may lead to a higher fraction of more absorbing clay–iron aggregates at smaller sizes. Mie calculation using the measured size distribution and size-resolved refractive indices of the absorbing components (black carbon and hematite) reproduces the measured SSA to within ±0.02 for SAL dust by assuming a goethite ∕ hematite mass ratio of 2. Overall, hematite and goethite constituted 40–80 % of the absorption for particles d < 2.5 µm, and black carbon (BC) contributed 10–37 %. This highlights the importance of size-dependent composition in determining the optical properties of dust and also the contribution from BC within dust plumes.

scholarly journals Seasonal cycle and source analyses of aerosol optical properties in a semi-urban environment at Puijo station in Eastern Finland

2012 ◽  
Vol 12 (2) ◽  
pp. 4719-4754
Author(s):  
A. Leskinen ◽  
A. Arola ◽  
M. Komppula ◽  
H. Portin ◽  
P. Tiitta ◽  
...  
Keyword(s):  
Optical Properties ◽  
Paper Mill ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Scattering Coefficient ◽  
Traffic Density ◽  
Aerosol Optical Properties ◽  
Absorption Coefficients ◽  
Data Set ◽  
Pollutant Sources

Abstract. We introduce a four-year (2006–2010) continuous data set of aerosol optical properties at Puijo in Kuopio, Finland. We study the annual and diurnal variation of the aerosol scattering and absorption coefficients, hemispheric backscattering fraction, scattering Ångström exponent, and single scattering albedo, whose averages over this period were 11.1 Mm−1 (at 550 nm), 1.5 Mm−1 (at 670 nm), 0.13, 1.9, and 0.83, respectively. The scattering coefficient peaked in the spring and autumn, being 2–4 times those in the summer and winter. An exception was the summer of 2010, when the the scattering coefficient was elevated to ~300 Mm−1 by the plumes from forest fires in Russia. The absorption coefficient peaked in the winter with values of 2–3 times those in the summer. The single scattering albedo was lowest in the winter when more biomass burning derived, soot-containing aerosols were present. The optical properties varied also with wind direction and time of the day, indicating the effect of the local pollutant sources and the age of the particles. Peak values in the single scattering albedo were observed when the wind blew from a paper mill and from the sector without local pollutant sources. These observations were linked to the sulphate-rich aerosol from the paper mill and the oxygenated organics in the aged aerosol, which both are known to increase the scattering characteristics of aerosols. Changes in the single scattering albedo in the morning and afternoon in the summertime were linked to the increased traffic density at these hours. The scattering and absorption coefficients were found to be decreased by clouds. The effect was stronger for the scattering than absorption, indicating preferential activation of the more hygroscopic aerosol with higher scattering characteristics. What happens to the aerosol optical properties during a cloud event when the air masses come from different directions with different local sources, is under a more detailed inspection. Also, more aerosol mass spectrometry data will be analyzed in order to strengthen our knowledge about the role of the chemical composition of the aerosol particles in their activation into cloud droplets.

scholarly journals Airborne observations of mineral dust over Western Africa in the summer monsoon season: spatial and vertical variability of physico-chemical and optical properties

2011 ◽  
Vol 11 (1) ◽  
pp. 2549-2609 ◽  
Author(s):  
P. Formenti ◽  
J. L. Rajot ◽  
K. Desboeufs ◽  
F. Saïd ◽  
N. Grand ◽  
...  
Keyword(s):  
Optical Properties ◽  
Vertical Distribution ◽  
Size Distribution ◽  
Mineral Dust ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Airborne Measurements ◽  
Number Size Distribution ◽  
Western Africa ◽  
June July

Abstract. We performed airborne measurements of aerosol particle concentration, composition, size distribution and optical properties over Western Africa in the corridor 2–17° N and 3–5° E. Data were collected on board the French ATR-42 research aircraft in June–July 2006 as part of the African Monsoon Multidisciplinary Analysis (AMMA) intensive field phases in June–July 2006 using the AVIRAD airborne aerosol sampling system. The aerosol vertical distribution was documented on an almost daily basis. In particular, the vertical distribution of mineral dust emitted locally by Mesoscale Convective Systems (MSC) was distinguished from that of mineral dust that was transported from the Saharan by the African Easterly Jet (AEJ). Mineral dust emitted in the Sahel by convection-driven erosion was mostly confined in the boundary layer. One episode of injection of Sahelian mineral dust in the AEJ was observed. The single scattering albedo of Sahelian dust is lower than that of Saharan dust, owing to different mineralogy and size distribution. Nonetheless this difference is within 0.02 (single scattering albedo unit) and does not significantly alter the radiative perturbation at the surface or at the top of the atmosphere. Our investigation provides with further experimental evidence of the limited dynamical evolution of the number size distribution during two days of transport. Whereas the chemical composition is clearly depending on the origin of mineral dust, no indications are of a possible dependence on the number size distribution.

scholarly journals Modelling black carbon absorption of solar radiation: combining external and internal mixing assumptions

2019 ◽  
Vol 19 (1) ◽  
pp. 181-204 ◽  
Author(s):  
Gabriele Curci ◽  
Ummugulsum Alyuz ◽  
Rocio Barò ◽  
Roberto Bianconi ◽  
Johannes Bieser ◽  
...  
Keyword(s):  
Optical Properties ◽  
Black Carbon ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Absorption Enhancement ◽  
Core Shell ◽  
Mixing State ◽  
Aerosol Optical Properties ◽  
Internal Mixing ◽  
Carbon Absorption

Abstract. An accurate simulation of the absorption properties is key for assessing the radiative effects of aerosol on meteorology and climate. The representation of how chemical species are mixed inside the particles (the mixing state) is one of the major uncertainty factors in the assessment of these effects. Here we compare aerosol optical properties simulations over Europe and North America, coordinated in the framework of the third phase of the Air Quality Model Evaluation International Initiative (AQMEII), to 1 year of AERONET sunphotometer retrievals, in an attempt to identify a mixing state representation that better reproduces the observed single scattering albedo and its spectral variation. We use a single post-processing tool (FlexAOD) to derive aerosol optical properties from simulated aerosol speciation profiles, and focus on the absorption enhancement of black carbon when it is internally mixed with more scattering material, discarding from the analysis scenes dominated by dust. We found that the single scattering albedo at 440 nm (ω0,440) is on average overestimated (underestimated) by 3–5 % when external (core-shell internal) mixing of particles is assumed, a bias comparable in magnitude with the typical variability of the quantity. The (unphysical) homogeneous internal mixing assumption underestimates ω0,440 by ∼14 %. The combination of external and core-shell configurations (partial internal mixing), parameterized using a simplified function of air mass aging, reduces the ω0,440 bias to -1/-3 %. The black carbon absorption enhancement (Eabs) in core-shell with respect to the externally mixed state is in the range 1.8–2.5, which is above the currently most accepted upper limit of ∼1.5. The partial internal mixing reduces Eabs to values more consistent with this limit. However, the spectral dependence of the absorption is not well reproduced, and the absorption Ångström exponent AAE675440 is overestimated by 70–120 %. Further testing against more comprehensive campaign data, including a full characterization of the aerosol profile in terms of chemical speciation, mixing state, and related optical properties, would help in putting a better constraint on these calculations.

scholarly journals Modelling black carbon absorption of solar radiation: combining external and internal mixing assumptions

2018 ◽  
Author(s):  
Gabriele Curci ◽  
Ummugulsum Alyuz ◽  
Rocio Barò ◽  
Roberto Bianconi ◽  
Johannes Bieser ◽  
...  
Keyword(s):  
Optical Properties ◽  
Black Carbon ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Absorption Enhancement ◽  
Core Shell ◽  
Mixing State ◽  
Aerosol Optical Properties ◽  
Internal Mixing ◽  
Carbon Absorption

Abstract. An accurate simulation of the absorption properties is key for assessing the radiative effects of aerosol on meteorology and climate. The representation of how chemical species are mixed inside the particles (the mixing state) is one of the major uncertainty factors in the assessment of these effects. Here we compare aerosol optical properties simulations over Europe and North America, coordinated in the framework of the third phase of the Air Quality Model Evaluation International Initiative (AQMEII), to one year of AERONET sunphotometer retrievals, in an attempt to identify a mixing state representation that better reproduces the observed single scattering albedo and its spectral variation. We use a single post-processing tool (FlexAOD) to derive aerosol optical properties from simulated aerosol speciation profiles, and focus on the absorption enhancement of black carbon when it is internally mixed with more scattering material, discarding from the analysis scenes dominated by dust. We found that the single scattering albedo at 440 nm (ω0,440) is on average overestimated (underestimated) by 3–5 % when external (core-shell internal) mixing of particles is assumed, a bias comparable in magnitude with the typical variability of the quantity. The (unphysical) homogeneous internal mixing assumption underestimates ω0,440 by ~ 14 %. The combination of external and core-shell configurations (partial internal mixing), parameterized using a simplified function of air mass aging, reduces the ω0,440 bias to −1/−3 %. The black carbon absorption enhancement (Eabs) in core-shell with respect to the externally mixed state is in the range 1.8–2.5, which is above the currently most accepted upper limit of ~ 1.5. The partial internal mixing reduces Eabs to values more consistent with this limit. However, the spectral dependence of the absorption is not well reproduced, and the absorption Angostrom exponent AAE440675 is overestimated by 70–120 %. Further testing against more comprehensive campaign data, including a full characterization of the aerosol profile in terms of chemical speciation, mixing state, and related optical properties, would help in putting a better constraint on these calculations.

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