Effects of physical and chemical nonlinearities on evolution of optical properties of biomass burning aerosol

2020 ◽  
Author(s):  
Nikolai Golovushkin ◽  
Igor Konovalov ◽  
Matthias Beekmann
Keyword(s):  
Optical Properties ◽  
Biomass Burning ◽  
Mass Concentration ◽  
Climate Models ◽  
Nonlinear Behavior ◽  
Basic Research ◽  
Chem Phys ◽  
Basis Set ◽  
Radiation Budget ◽  
Physical And Chemical

<p>Aerosol from open biomass burning (BB) is known to strongly impact the Earth radiation budget. Therefore, a good knowledge of its optical properties and their evolution is an important prerequisite for accurate assessments of contributions of various factors to climate change by means of chemistry-transport and climate models. As a major component of typical BB aerosol is organic matter, the atmospheric evolution of BB aerosol can be strongly affected by the physical and chemical processes governing the gas-particle partitioning of organic compounds. Recently, it has been shown [1] that these processes can give rise to strongly nonlinear behavior of mass concentration of organic fraction of BB aerosol during its atmospheric lifetime. It has been also argued that chemical and physical nonlinearities can explain part of the observed diversity of the effects of BB aerosol atmospheric aging. The present study has extended the previous analysis of the nonlinear behavior of BB aerosol, focusing on the evolution of BB aerosol optical properties, such as, specifically, mass absorption and scattering efficiencies (MAE and MSE) in the near-UV and optical wavelength ranges. The evolution of aerosol in BB plumes was simulated with the MDMOA [1] microphysical box model that involves a schematic parameterization of the dilution process and represents the oxidation and gas-particle partitioning processes within the volatility basis set (VBS) framework. The Mie-theory-based simulations of the optical properties of aging BB aerosol were performed with the OPTSIM module [2] coupled with MDMOA. The simulations show that both MAE and MSE can exhibit strong and diverse changes during BB aerosol evolution mostly due to significant changes in the aerosol particle size distribution. Furthermore, similar to the mass concentration, both MAE and MSC of the aged BB aerosol depend in a nonlinear manner on the initial BB aerosol concentration and the initial size of a smoke plume and are sensitive to the choice of a concrete VBS scheme. The results of this study may have important implications for modeling of radiative effects of BB aerosol with chemistry-transport and climate models and for interpretation of remote observations of BB aerosol.</p><p>The study was supported by the Russian Foundation for Basic Research (grant No. 18-05-00911).</p><p>References</p><ol><li>Konovalov, I. B., Beekmann, M., Golovushkin, N. A., and Andreae, M. O.: Nonlinear behavior of organic aerosol in biomass burning plumes: a microphysical model analysis, Atmos. Chem. Phys., 19, 12091–12119, https://doi.org/10.5194/acp-19-12091-2019, 2019.</li> <li>Stromatas, S., Turquety, S., Menut, L., Chepfer, H., Péré, J. C., Cesana, G., and Bessagnet, B.: Lidar signal simulation for the evaluation of aerosols in chemistry transport models, Geosci. Model Dev., 5, 1543–1564, https://doi.org/10.5194/gmd-5-1543-2012, 2012.</li> </ol>

New insights into physical and chemical atmospheric transformations of biomass burning aerosol from wildfires in Siberia

2020 ◽  
Author(s):  
Igor Konovalov ◽  
Nikolai Golovushkin ◽  
Matthias Beekmann ◽  
Valerii Kozlov
Keyword(s):  
Biomass Burning ◽  
Climate Models ◽  
Organic Aerosol ◽  
Chem Phys ◽  
Satellite Observations ◽  
Northern Eurasia ◽  
Brown Carbon ◽  
Radiative Balance ◽  
Biomass Burning Aerosol ◽  
Physical And Chemical

<p>Wildfires in Siberia are a major source of aerosol in Northern Eurasia. Biomass burning (BB) aerosol can significantly impact the Earth’s radiative balance through absorption and scattering of solar radiation, interactions with clouds and changes of surface albedo due to deposition of black and brown carbon on ice and snow. There is growing evidence that atmospheric aging of BB aerosol can be associated with profound but diverse chemical and physical transformations which, in most cases, are not adequately represented in chemistry-transport and climate models that are widely used in assessments of radiative and climate effects of atmospheric pollutants.</p><p>An idea of this study is to identify changes in the optical properties of aging BB aerosol using absorption and extinction aerosol optical depths (AAOD and AOD) retrieved from the OMI and MODIS satellite observations and to elucidate key processes behind these changes using the Mie-theory-based calculations along with simulations with chemistry-transport and microphysical box models involving representation of the evolution of organic particulate matter within the VBS framework. The study focuses on a major outflow of BB plumes from Siberia into the European part of Russia in July 2016. The analysis of the satellite data is complemented by the original results of biomass burning aerosol aging experiments in a large aerosol chamber. </p><p>The results indicate that the BB aerosol evolution during the first 10-20 hours features strong secondary organic aerosol (SOA) formation resulting in a substantial increase in the particle single scattering albedo. Further evolution is affected by the loss of organic matter, probably due to evaporation and oxidation. The results also indicate that although brown carbon contained in the primary aerosol is rapidly lost (consistently with available independent observations) due to evaporation and photochemical destruction of chromospheres, it is partly replaced by weakly absorbing low-volatile SOA.</p><p>In general, this study reveals that aging BB aerosol from wildfires in Siberia undergoes major physical and chemical transformations that have to be taken into account in assessments of the impact of Siberian fires on the radiative balance in Northern Eurasia and the Arctic. It also proposes a practical way to address these complex transformations in chemistry-transport and climate models.</p><p>The study was supported by the Russian Science Foundation (grant agreement No. 19-77-20109).</p><p>References</p><ol><li>Konovalov, I.B., Beekmann, M., Berezin, E.V., Formenti, P., and Andreae, M.O.: Probing into the aging dynamics of biomass burning aerosol by using satellite measurements of aerosol optical depth and carbon monoxide, Atmos. Chem. Phys., 17, 4513–4537, 2017.</li> <li>Konovalov, I.B., Lvova, D.A., Beekmann, M., Jethva, H., Mikhailov, E.F., Paris, J.-D., Belan, B.D., Kozlov, V.S., Ciais, P., and Andreae, M.O.: Estimation of black carbon emissions from Siberian fires using satellite observations of absorption and extinction optical depths, Atmos. Chem. Phys., 18, 14889–14924, 2018.</li> <li>Konovalov, I.B., Beekmann, M., Golovushkin, N.A., and Andreae, M.O.: Nonlinear behavior of organic aerosol in biomass burning plumes: a microphysical model analysis, Atmos. Chem. Phys., 19, 12091–12119, 2019.</li> </ol>

scholarly journals Chemical apportionment of southern African aerosol mass and optical depth

2009 ◽  
Vol 9 (19) ◽  
pp. 7643-7655 ◽  
Author(s):  
B. I. Magi
Keyword(s):  
Optical Properties ◽  
Biomass Burning ◽  
Cross Sections ◽  
Southern Africa ◽  
Climate Models ◽  
Critical Role ◽  
Carbonaceous Aerosol ◽  
Aerosol Optical Properties ◽  
Aerosol Absorption ◽  
Aerosol Mass

Abstract. This study characterizes the aerosol over extratropical and tropical southern Africa during the biomass burning season by presenting an aerosol mass apportionment and aerosol optical properties. Carbonaceous aerosol species account for 54% and 83% of the extratropical and tropical aerosol mass, respectively, which is consistent with the fact that the major source of particulate matter in southern Africa is biomass burning. This mass apportionment implies that carbonaceous species in the form of organic carbon (OC) and black carbon (BC) play a critical role in the aerosol optical properties. By combining the in situ measurements of aerosol mass concentrations with concurrent measurements of aerosol optical properties at a wavelength of 550 nm, it is shown that 80–90% of the aerosol scattering is due to carbonaceous aerosol, and the derived mass scattering cross sections (MSC) for OC and BC are 3.9±0.6 m2/g and 1.6±0.2 m2/g, respectively. Derived values of mass absorption cross sections (MAC) for OC and BC are 0.7±0.6 m2/g and 8.2±1.1 m2/g, respectively. The values of MAC imply that ~26% of the aerosol absorption in southern Africa is due to OC, with the remainder due to BC. The results in this study provide important constraints for aerosol properties in a region dominated by biomass burning and should be integrated into climate models to improve aerosol simulations.

scholarly journals On the importance of cumulus penetration on the microphysical and optical properties of stratocumulus clouds

2005 ◽  
Vol 5 (3) ◽  
pp. 755-765 ◽  
Author(s):  
S. Ghosh ◽  
S. Osborne ◽  
M. H. Smith
Keyword(s):  
Optical Properties ◽  
Climate Models ◽  
Ionic Species ◽  
Radiation Budget ◽  
Cumulus Clouds ◽  
Microphysical Properties ◽  
Spatial Coverage ◽  
Stratocumulus Clouds ◽  
Modelling Studies ◽  
The Impact

Abstract. Owing to their extensive spatial coverage, stratocumulus clouds play a crucial role in the radiation budget of the earth. Climate models need an accurate characterisation of stratocumulus in order to provide an accurate forecast. However, remote sensing as well as in-situ observations reveal that on several occasions, cumulus clouds present below the stratocumulus, often have a significant impact on the main stratocumulus microphysical properties. This was observed during the ACE-2 (Aerosol Characterisation Experiment-2) campaign designed to study the impact of polluted continental air on stratocumulus formation. In this paper we used a detailed micro-physical chemical parcel model to quantify the extent of this cumulus-stratocumuls coupling. In addition, we made extensive use of microphysical observations from the C-130 aircraft that was operated during ACE-2. For the ACE-2 case studies considered in this paper, our analysis revealed that the chemical, microphysical and optical characteristics of the main stratocumulus cloud deck had significant contributions from cumulus clouds that often penetrated the stratocumulus deck. The amount of fine mode ionic species, the average droplet number concentrations, the effective radii and the optical depths during the flight A562 (when cumulus clouds interacted with the main stratocumulus) were estimated and model runs that included this effect yielded microphysical and optical properties which compared more favourably with the observations than the runs which did not. This study highlights the importance of including these cumulus effects in stratocumulus related modelling studies.

scholarly journals Evaluation of Cirrus Parameterizations for Radiative Flux Computations in Climate Models Using TOVS–ScaRaB Satellite Observations

2007 ◽  
Vol 20 (17) ◽  
pp. 4459-4475 ◽  
Author(s):  
C. J. Stubenrauch ◽  
F. Eddounia ◽  
J. M. Edwards ◽  
A. Macke
Keyword(s):  
Optical Properties ◽  
Radiative Transfer ◽  
Climate Models ◽  
Radiative Flux ◽  
Satellite Observations ◽  
Radiation Budget ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Ice Crystal ◽  
Infrared Observation

Abstract Combined simultaneous satellite observations are used to evaluate the performance of parameterizations of the microphysical and optical properties of cirrus clouds used for radiative flux computations in climate models. Atmospheric and cirrus properties retrieved from Television and Infrared Observation Satellite (TIROS-N) Operational Vertical Sounder (TOVS) observations are given as input to the radiative transfer model developed for the Met Office climate model to simulate radiative fluxes at the top of the atmosphere (TOA). Simulated cirrus shortwave (SW) albedos are then compared to those retrieved from collocated Scanner for Radiation Budget (ScaRaB) observations. For the retrieval, special care has been given to angular direction models. Three parameterizations of cirrus ice crystal optical properties are represented in the Met Office radiative transfer model. These parameterizations are based on different physical approximations and different hypotheses on crystal habit. One parameterization assumes pristine ice crystals and two ice crystal aggregates. By relating the cirrus ice water path (IWP) retrieved from the effective infrared emissivity to the cirrus SW albedo, differences between the parameterizations are amplified. This study shows that pristine crystals seem to be plausible only for cirrus with IWP less than 30 g m−2. For larger IWP, ice crystal aggregates lead to cirrus SW albedos in better agreement with the observations. The data also indicate that climate models should allow the cirrus effective ice crystal diameter (De) to increase with IWP, especially in the range up to 30 g m−2. For cirrus with IWP less than 20 g m−2, this would lead to SW albedos that are about 0.02 higher than the ones of a constant De of 55 μm.

scholarly journals Comparison of south Atlantic aerosol direct radiative effect overclouds from SCIAMACHY, POLDER and OMI/MODIS

2019 ◽  
Author(s):  
Martin de Graaf ◽  
Ruben Schulte ◽  
Fanny Peers ◽  
Fabien Waquet ◽  
L. Gijsbert Tilstra ◽  
...  
Keyword(s):  
Biomass Burning ◽  
Climate Models ◽  
Global Climate ◽  
Global Climate Models ◽  
Radiation Budget ◽  
Radiative Effect ◽  
The South ◽  
East Atlantic ◽  
The Difference ◽  
Aerosol Effects

Abstract. The Direct Radiative Effect (DRE) of aerosols above clouds has been found to be significant over the south-east Atlantic Ocean during the African biomass burning season due to elevated smoke layers absorbing radiation above the cloud deck. So far, global climate models have been unsuccessful in reproducing the high DRE values measured by various satellite instruments. Meanwhile, the radiative effects by aerosols have been identified as the largest source of uncertainty in global climate models. In this paper, three independent satellite datasets of DRE during the biomass burning season in 2006 are compared to constrain the south-east Atlantic radiation budget. The DRE of aerosols above clouds is derived from the spectrometer SCIAMACHY, the polarimeter POLDER, and from collocated measurements by the spectrometer OMI and imager MODIS. All three confirm the high DRE values during the biomass season, underlining the relevance of local aerosol effects. Differences between the instruments can be attributed mainly to sampling issues. When these are accounted for, the remaining differences can be completely explained by the higher cloud optical thickness derived from POLDER compared to the other instruments. Additionally, a neglect of AOT at SWIR wavelengths in the method used for SCIAMACHY and OMI/MODIS accounts for 26 % of the difference between POLDER and OMI/MODIS DRE.

scholarly journals Chemical apportionment of southern African aerosol mass and optical depth

2009 ◽  
Vol 9 (3) ◽  
pp. 13439-13474
Author(s):  
B. I. Magi
Keyword(s):  
Optical Properties ◽  
Biomass Burning ◽  
Cross Sections ◽  
Southern Africa ◽  
Climate Models ◽  
Critical Role ◽  
Carbonaceous Aerosol ◽  
Aerosol Optical Properties ◽  
Carbonaceous Species ◽  
Aerosol Mass

Abstract. We investigate the aerosol mass apportionment and derive aerosol optical properties that characterize the aerosol over extratropical and tropical southern Africa during the biomass burning season. We find that 54% and 83% of the extratropical and tropical aerosol mass, respectively, is composed of carbonaceous species, consistent with the fact that the major source of particulate matter in southern Africa is biomass burning. This mass apportionment implies that carbonaceous species in the form of organic carbon (OC) and black carbon (BC) play a critical role in the aerosol optical properties. By combining the in situ measurements of aerosol mass concentrations with concurrent measurements of aerosol optical properties at a wavelength of 550 nm, we find that 80–90% of the aerosol scattering is due to carbonaceous aerosol, where our derived mass scattering cross sections (MSC) for OC and BC are 3.9±0.6 m2/g and 1.6±0.2 m2/g, respectively. Our derived values of mass absorption cross sections (MAC) for OC and BC are 0.7±0.6 m2/g and 8.2±1.1 m2/g, respectively. The values of MAC imply that 26–27% of the aerosol absorption in southern Africa is due to OC, with the remainder due to BC. Our results provide important constraints for aerosol properties in a region dominated by biomass burning and should be integrated into climate models to improve aerosol simulations.

scholarly journals Nonlinear behavior of organic aerosol in biomass burning plumes: a microphysical model analysis

2019 ◽  
Author(s):  
Igor B. Konovalov ◽  
Matthias Beekmann ◽  
Nikolai A. Golovushkin ◽  
Meinrat O. Andreae
Keyword(s):  
Biomass Burning ◽  
Linear Part ◽  
Nonlinear Behavior ◽  
Organic Aerosol ◽  
Basis Set ◽  
Aging Effects ◽  
Transport Models ◽  
Smoke Plume ◽  
Smoke Plumes ◽  
The Impact

Abstract. Organic aerosol (OA) is a major component of smoke plumes from open biomass burning (BB). Therefore, adequate representation of the atmospheric transformations of BB OA in chemistry-transport and climate models is an important prerequisite for accurate estimates of the impact of BB emissions on air quality and climate. However, field and laboratory studies of atmospheric transformations (aging) of BB OA have yielded a wide diversity of observed effects. This diversity is still not sufficiently understood and thus not addressed in models. As OA evolution is governed by complex nonlinear processes, it is likely that at least a part of the observed variability of the BB OA aging effects is due to the factors associated with the intrinsic nonlinearity of the OA system. In this study, we performed a numerical analysis in order to gain a deeper understanding of such factors. We employ a microphysical dynamic model that represents gas-particle partitioning and OA oxidation chemistry within the volatility basis set (VBS) framework and includes a schematic parameterization of BB OA dilution due to dispersion of an isolated smoke plume. Several VBS schemes of different complexity, which have been suggested in the literature to represent BB OA aging in regional and global chemistry-transport models, are applied to simulate BB OA evolution over a five-day period representative of a BB aerosol lifetime in the dry atmosphere. We consider the BB OA mass enhancement ratio (EnR), which is defined as the ratio of the mass concentration of BB OA to that of an inert tracer and allows us to eliminate the linear part of the dilution effects. We also analyze the behavior of the hygroscopicity parameter, κ, that was simulated in a part of our numerical experiments. As a result, five qualitatively different regimes of OA evolution are identified, which comprise (1) a monotonic saturating increase of EnR, (2) an increase of EnR followed by a decrease, (3) an initial rapid decrease of EnR followed by a gradual increase, (4) an EnR increase between two intermittent stages of its decrease, or (5) a gradual decrease of EnR. We find that the EnR for BB aerosol aged from a few hours to a few tens of hours typically increases for larger initial sizes of the smoke plume (and therefore, smaller dilution rates) or for lower initial OA concentrations (and thus more organic gases available to form secondary OA). However, these dependencies can be weakened or even reversed, depending on the BB OA age and on the ratio between the fragmentation and functionalization oxidation pathways. Nonlinear behavior of BB OA is also exhibited in the dependencies of κ on the parameters of the plume. Application of the different VBS schemes results in large quantitative and qualitative differences between the simulations, although our analysis suggests also that the main qualitative features of OA evolution simulated with a complex two-dimensional VBS scheme can also be reproduced with a much simpler scheme. Overall, this study indicates that the BB aerosol evolution may strongly depend on parameters of the individual BB smoke plumes (such as the initial organic aerosol concentration and plume size) that are typically not resolved in chemistry transport models.

scholarly journals Nonlinear behavior of organic aerosol in biomass burning plumes: a microphysical model analysis

2019 ◽  
Vol 19 (19) ◽  
pp. 12091-12119 ◽  
Author(s):  
Igor B. Konovalov ◽  
Matthias Beekmann ◽  
Nikolai A. Golovushkin ◽  
Meinrat O. Andreae
Keyword(s):  
Biomass Burning ◽  
Linear Part ◽  
Nonlinear Behavior ◽  
Organic Aerosol ◽  
Basis Set ◽  
Aging Effects ◽  
Transport Models ◽  
Smoke Plume ◽  
Smoke Plumes ◽  
The Impact

Abstract. Organic aerosol (OA) is a major component of smoke plumes from open biomass burning (BB). Therefore, adequate representation of the atmospheric transformations of BB OA in chemistry-transport and climate models is an important prerequisite for accurate estimates of the impact of BB emissions on air quality and climate. However, field and laboratory studies of atmospheric transformations (aging) of BB OA have yielded a wide diversity of observed effects. This diversity is still not sufficiently understood and thus not addressed in models. As OA evolution is governed by complex nonlinear processes, it is likely that at least a part of the observed variability in the BB OA aging effects is due to the factors associated with the intrinsic nonlinearity of the OA system. In this study, we performed a numerical analysis in order to gain a deeper understanding of these factors. We employ a microphysical dynamic model that represents gas–particle partitioning and OA oxidation chemistry within the volatility basis set (VBS) framework and includes a schematic parameterization of BB OA dilution due to dispersion of an isolated smoke plume. Several VBS schemes of different complexity, which have been suggested in the literature to represent BB OA aging in regional and global chemistry-transport models, are applied to simulate BB OA evolution over a 5 d period representative of the BB aerosol lifetime in the dry atmosphere. We consider the BB OA mass enhancement ratio (EnR), which is defined as the ratio of the mass concentration of BB OA to that of an inert tracer and allows us to eliminate the linear part of the dilution effects. We also analyze the behavior of the hygroscopicity parameter, κ, that was simulated in a part of our numerical experiments. As a result, five qualitatively different regimes of OA evolution are identified, which comprise (1) a monotonic saturating increase in EnR, (2) an increase in EnR followed by a decrease, (3) an initial rapid decrease in EnR followed by a gradual increase, (4) an EnR increase between two intermittent stages of its decrease, or (5) a gradual decrease in EnR. We find that the EnR for BB aerosol aged from a few hours to a few tens of hours typically increases for larger initial sizes of the smoke plume (and therefore smaller dilution rates) or for lower initial OA concentrations (and thus more organic gases available to form secondary OA – SOA). However, these dependencies can be weakened or even reversed, depending on the BB OA age and on the ratio between the fragmentation and functionalization oxidation pathways. Nonlinear behavior of BB OA is also exhibited in the dependencies of κ on the parameters of the plume. Application of the different VBS schemes results in large quantitative and qualitative differences between the simulations, although our analysis suggests also that the main qualitative features of OA evolution simulated with a complex two-dimensional VBS scheme can also be reproduced with a much simpler scheme. Overall, this study indicates that the BB aerosol evolution may strongly depend on parameters of the individual BB smoke plumes (such as the initial organic aerosol concentration and plume size) that are typically not resolved in chemistry-transport models.

scholarly journals Source-Based Size-Resolved Optical Properties of Carbonaceous Aerosols

2021 ◽  
Vol 11 (4) ◽  
pp. 1434
Author(s):  
Chang Hoon Jung ◽  
Sang Hee Han ◽  
Ji Yi Lee ◽  
Yong Pyo Kim
Keyword(s):  
Refractive Index ◽  
Optical Properties ◽  
Biomass Burning ◽  
Mass Concentration ◽  
Geometric Mean ◽  
Physicochemical Characteristics ◽  
Carbonaceous Aerosols ◽  
Multilinear Regression ◽  
Lognormal Size Distribution ◽  
Range Transport

In this study, the source-based optical properties of polydisperse carbonaceous aerosols were determined from PM2.5 concentrations measured at a Global Atmospheric Watch station in South Korea. The extinction and absorption coefficients of carbonaceous aerosols were calculated using the Mie theory and assuming a lognormal size distribution. Based on the mass concentration from the EPA’s Positive Matrix Factorization (PMF) receptor model, which considers five source identification and apportionment factors (biogenic source, local biomass burning, secondary organic aerosol, transported biomass burning, and mixed sources), the source-based size-resolved mass extinction and absorption efficiencies were estimated for each source using a multilinear regression model. The results show that the source-based optical properties depend on the aerosol size and physicochemical characteristics of the chemical compounds. The long-range transport of biomass burning (LBB) aerosol, which has a mass concentration of 20%, holds a 12.1–23.1% total extinction efficiency—depending on the size and refractive index—in the range of 0.1–0.5 μm in geometric mean diameter and humic-like substances (HULIS) imaginary refractive index of 0.006–0.3. Biogenic sources of aerosols with small diameters have higher mass absorption efficiencies (MAE) than other sources, depending on the size and refractive index.

scholarly journals On the importance of cumulus penetration on the microphysical and optical properties of stratocumulus clouds

2004 ◽  
Vol 4 (4) ◽  
pp. 4611-4640
Author(s):  
S. Ghosh ◽  
S. Osborne ◽  
M. H. Smith
Keyword(s):  
Optical Properties ◽  
Climate Models ◽  
Ionic Species ◽  
Radiation Budget ◽  
Cumulus Clouds ◽  
Microphysical Properties ◽  
Spatial Coverage ◽  
Stratocumulus Clouds ◽  
Modelling Studies ◽  
The Impact

Abstract. Owing to their extensive spatial coverage, stratocumulus clouds play a crucial role in the radiation budget of the earth. Climate models need an accurate characterisation of stratocumulus in order to provide an accurate forecast. However, remote sensing as well as in-situ observations reveal that on several occasions, cumulus clouds present below the stratocumulus, often have a significant impact on the main stratocumulus microphysical properties. This was observed during the ACE-2 (Aerosol Characterisation Experiment 2) campaign designed to study the impact of polluted continental air on stratocumulus formation. In this paper we used a detailed micro-physical chemical parcel model to quantify the extent of this cumulus-stratocumuls coupling. In addition, we made extensive use of microphysical observations from the C-130 aircraft that was operated during ACE-2. For the ACE-2 case studies considered in this paper, our analysis revealed that the chemical, microphysical and optical characteristics of the main stratocumulus cloud deck had significant contributions from cumulus clouds that often penetrated the stratocumulus deck. The amount of fine mode ionic species, the average droplet number concentrations, the effective radii and the optical depths during the flight A562 (when cumulus clouds interacted with the main stratocumulus) were estimated and model runs that included this effect yielded microphysical and optical properties which compared more favourably with the observations than the runs which did not. This study highlights the importance of including these cumulus effects in stratocumulus related modelling studies.

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