Remote Sensing of Spectral Aerosol Properties: A Classroom Experience

From my graduate school experience I find that the best way to learn and understand science is by getting your hands dirty with the relavant data when it comes to understanding remote sensing. Like remote sensing courses at many other universities in the United States, the University of Alabama in Huntsville offers two courses in Satellite Remote Sensing, ATS670 and ATS770. These courses are tailored in a manner to that allows students to get hands on experience with state-of-the art remote sensing datasets such as the MODIS . In ATS670, students select a MODIS image of their interest and perform a supervised and unsupervisd classification of the image to identify different features in the image such as land, water, clouds, aerosols, vegetation etc. The beauty of doing all this is that the student doesn't get to use any classification software! They write their own routines to perform all the required tasks including trivial tasks such as calculating minimum, maximum, mean, standard deviation to more sophisticated tasks such as histogram equalization, contrast streching, gray flipping, edge detection, fire detection, cloud detection in images to name a few. The journey begins with learning the basic principles of remote sensing and understanding the fundamentals behind seperating features in a remotely sensed image based on spectral signatures. Once the basics unfold, students write their own programs to read the MODIS image, perform a true color three band overlay, pick samples, perform image classification using several techniques such as the parellelopiped method, migrating means method, minimum distance, maximum likelihood methods and the mahalonobis classifier. The ingredients of this course a perfect blend of theoritical and practical classroom learning. ATS770 is more advanced and students use several radiative transfer models and other remote sensing tools to perform retrievals such as for cloud and aerosol properties.

Having taken these courses I have a great appreciation of hands-on experience in learning remote sensing and this is what attracted my attention to a paper by Robert Levy that appeared in the BAMS, 2007 (reference below). This paper talks about the challenge instructors face in bridging the gap between current research and the classroom and how the University of Maryland and NASA Goddard Space Flight Center teamed up to "design a graduate class project intended to provide a hands-on introduction to the physical basis for the retrieval of aerosol properties from state-of-the-art Moderate Resolution Imaging Spectroradiometer (MODIS) observations". "This paper reviews the basic physics of the remote sensing of aerosols and describes selected findings and lessons learned by the students." Students use both hand calculations based on given look-up tables of aerosol properties and the operational MODIS aerosol retrieval algorithm to carry out the class project. Aerosol retrievals are done over selected AERONET sites (shown in figure below) that aid validation of retrieved products.

Students investigated the reflectance v/s wavelength relations over these sites and they find a surface dependence. Figure alongside shows retrievals obtained by hand calculations by using selected fine and coarse mode geometries of aerosols and the fitting error was estimated to find the best fit. Overlaid is the MODIS retrieval in black. Best fit spectral AOT retrievals were then compared with AERONET AOTs . Details on the codes used and the instructions to perform the exercises is given at :

www.atmos.umd.edu/~levy/MODIS_Aerosol_Project/

This paper illustrates how " Projects such as this provide an opportunity for students and young scientists to become familiar with (and less apprehensive of) datasets of this magnitude". This paper is a must read for all those interested in learning aerosol retrieval techniques.

References :

Levy, R.C., and R.T. Pinker, 2007: Remote Sensing of Spectral Aerosol Properties: A Classroom Experience, Bull. Amer. Meteor. Soc., 88, 25–30

http://www.nsstc.uah.edu/~sundar/ats670.html

http://www.nsstc.uah.edu/~sundar/ats770.html

Levoglucosan: a unique tracer of biomass burning aerosols

Atmospheric aerosols in general and biomass burning aerosols in particular have recently attracted extensive interest owing to their ability to affect the climate on local to global scales. These climatic effects include a direct radiative effect due to the aerosols’ ability to scatter and absorb incoming sunlight, an indirect effect due to the aerosols’ ability to serve as cloud condensation nuclei (CCN), increasing the cloud’s reflectivity and lifetime, a semidirect effect which leads to reduction in cloud cover, owing to aerosols’ ability to absorb sunlight, changes in precipitation patterns, and export of pollutants and water vapor to the stratosphere. Therefore, it is important to assess human contribution to aerosol emissions, and to assign a source to both anthropogenic and natural aerosols, for understanding the respective contribution of different aerosol types to climate change.

Levoglucosan (1,6-anhydro-β-D-glucopyranose) is a unique tracer for biomass burning sources in atmospheric aerosol particles. It is a product of cellulose combustion, which has been recognized as a biomass burning tracer. When cellulose is heated to over 300°C, it undergoes various pyrolytic processes, yielding a highly combustible tar, a major constituent of which is levoglucosan, a dehydrated glucose containing a ketal functional group. Some of the levoglucosan is consumed in various reactions during combustion but it is nonetheless emitted in large quantities in the resulting smoke aerosol. Therefore, it can be utilize as a specific tracer for the presence of emissions from a biomass burning source in atmospheric particulate matter. Unlike other indicators used for the same purpose, levoglucosan is source-specific to burning of any fuel containing cellulose. Combustion of other materials (e.g., fossil fuels) or biodegradation and hydrolysis of cellulose do not produce levoglucosan. Levoglucosan is relatively stable in the atmosphere, showing no decay over 10 days in acidic conditions, similar to those of atmospheric liquid droplets. Levoglucosan is also used in other fields of chemistry and engineering, such as pyrolysis and fire-retardants research, biofuel research, biology, organic synthesis and as a biomass burning tracer in sediment analysis for the paleorecord.
For more information, please see the following paper and references therein.

Schkolnik G. and Rudich Y. (2006), Detection and quantification of levoglucosan in atmospheric aerosols: A review, Analytical and Bioanalytical Chemistry, 385, 26-33.

Atmospheric Aerosol and Ultraviolet Radiation

Ultraviolet (UV) radiation plays very important role in bio-geo-chemical cycle. Their harmful effects include skin cancer, cataract, immune suppression, reduction in crop yield, etc. Beneficial effects are synthesis of vitamin D in human body, treatment of psoriasis, etc (Lucas et al., 2006). My interest in UV radiation is how it interacts with atmospheric aerosols.

Aerosols are one of the many factors which determines amount of surface reaching UV radiation. While scattering type of aerosols may reduce surface reaching UV radiation, they increase the actinic flux which in turn increases the photolysis rate for smog formation (Dickerson et al., 1997). Relation of aerosol and UV radiation is not one-way; while aerosols affect surface reaching UV radiation, they are affected by surface reaching UV radiation. This is particularly true for naturally produced sulfate aerosol. Recently scientific community has shown a lot of interest to study UV induced sulfate aerosol production to better understand effect solar variability on climate change. Joyce Penner presented a talk on connection between Solar variability, Dimethyl sulfide (DMS) production, and climate change in Yoram Kaufman Symposium on Aerosols, Clouds and Climate (30-31 May 2007). The symposium was organized in honor of Yoram Kaufman at Goddard Space Flight Center, NASA, Maryland, USA. Presentations are available for download at this link.

Penner presented the details on solar variability and DMS production and showed how the matter is complicated due to cloud feedback. The connection works as following; increase in ultraviolet radiation decreases the marine biota, which in turn reduces production of DMS . Reduction in DMS reduces aerosol amount, which ultimately leads to cloud modification. The poorly understood connections between aerosol and cloud as well as cloud and marine biota makes it difficult to interpret solar variability connection of climate change. Two references cited repeatedly in her talk were Larsen (2005) and Vallina and Simo (2007).

References:-

Lucas, R., T. McMichael, W. Smith. and B. Armstrong (2006), Global burden of disease from solar ultraviolet radiation, Environmental burden of diseases series no. 13, ed. A. Pruss-Ustun, H. Zeeb, C. Mathers and M. Repacholi, World Health Organization Public Health and the Environment, Geneva, 2006.

R. R. Dickerson, S. Kondragunta, G. Stenchikov, K. L. Civerolo, B. G. Doddridge, and B. N. Holben, The Impact of Aerosols on Solar Ultraviolet Radiation and Photochemical Smog , Science 31 October 1997 278: 827-830 [DOI: 10.1126/science.278.5339.827]

Larsen, S. H. (2005), Solar variability, dimethyl sulphide, clouds, and climate, Global Biogeochem. Cycles, 19, GB1014, doi:10.1029/2004GB002333.

Vallina, S. M. and R. Simo (2007), Strong Relationship Between DMS and the Solar Radiation Dose over the Global Surface Ocean, Science, Vol. 315, No. 5811. pp. 506-508

Carbon Aerosols in Climate Models

"All models have assumed that particles are spherical and have chosen single values for the refractive index. If underlying model assumptions are inappropriate, then scattering, absorption and radiative forcing estimates will be incorrect."

This is one the statement made by Tami Bond and Robert Bergstrom in the review paper titled ‘Light Absorption by Carbonaceous Particles: An Investigative Review’ published in Aerosol Science and Technology, 2006.

I had chance to present this paper as class seminar in my ‘Atmospheric Aerosol’ class teaching by Dr. Kirk Fuller during this summer. Let me tell you this, “this is one of the best written paper I ever read on carbon aerosols”. It reads very well and has almost all the information you need to know about atmospheric carbon aerosols and their optical properties. Personally, I congratulate and thanks Tami and Robert for putting together this wonderful paper. Below, I will briefly touch some important points from the paper.

Motivation:

Tabulation of optical properties needs update and measured absorptive properties demonstrate variability that has not been represented in climate models.

Key Conclusion:

Optical properties of Light Absorbing Carbon (LAC) are necessary to model its effects on climate.

Mass absorption cross-section is 7.5±1.2 m2/g for at λ=0.55 µm uncoated LAC.

Highest refractive index for strongly absorbing LAC is 1.95-i*0.79 with narrow range and refractive index values used in current climate models are in error.

Spherical aggregates treatment using realistic refractive indices under predict measured absorption by about 30%

Recommendations:

Mass absorption cross-section 7.5±1.2 m2/g at λ=0.55 µm for fresh LAC, this may increase due to coating and decrease due to particle coagulation.

Single scattering albedo: 0.2-0.3 for fresh LAC.

Absorption cross-section may be assumed to depend inversely on wavelength throughout the visible spectrum.

Refractive index (m=1.74-i0.44) commonly used by climate models should be retired and new values between 1.75 – i0.63 and 1.95- i0.79 should be used.

Mie calculation should not be used for fresh LAC but coated sphere type Mie calculation can be used for aged LAC.

Reference:

Light Absorption by Carbonaceous Particles: An Investigative Review TC Bond, RW Bergstrom - Aerosol Science & Technology, 2006, ISSN: 0278-6826.

Accurate Monitoring of Terrestrial Aerosols and Total Solar Irradiance : Introducing the Glory Mission

Its time for aerosol scientists to gear up for the upcoming Glory Mission that has a fantastic passive sensor for monitoring aerosols from space with unmatched retrieval accuracies of 0.02 over ocean and 0.04 over land!

This article on the Glory Mission appeared in BAMS in the May 2007 edition, volume 88, number 5. It has a very detailed description of the science objectives, the instruments on Glory, measurement objectives, the data products from these instruments, associated uncertainties and validation plans.

Glory is basically a part of the A-Train constellation of satellites. It is scheduled to be launched in 2008 and will carry two independent instruments :

1) The Total Irradiance Monitor (TIM)

2) The Aerosol Polarimetry Sensor (APS)

The main purpose of the Glory mission is to help address the challenge of reducing uncertainty in adequately contraining climate sensitivity. Glory is intented to specifically meet the following four scientific objectives :

" • improve the quantification of the effect of solar variability on the Earth’s climate by continuing the uninterrupted 28-yr satellite measurement record of TSI;

• facilitate the quantification of the aerosol direct and indirect effects on climate by determining the global distribution of the optical thickness and microphysical properties of natural and anthropogenic aerosols and clouds with much-improved accuracy;

• provide better aerosol representations for use in various remote sensing retrievals, thereby allowing improvements in aerosol assessments by other operational satellite instruments; and

• provide an improved framework for the formulation of future comprehensive satellite missions for aerosol, cloud, and ocean color research. "

The APS will offer along track measurements for 3 years of mission life. It has the following unique measurement capabilities to ameliorate the ill-posed inverse problem and hence improve aerosol retrieval accuracies:

" • to measure not only the intensity, I, but also the other Stokes parameters describing the polarization state of the reflected radiation (i.e., Q, U, and V; Hansen and Travis 1974);

• to increase the number of spectral channels and the total spectral range covered;

• to increase the number and range of viewing directions from which a scene location is observed; and

• to improve the measurement accuracy, especially for polarization "

The aerosol and cloud products from APS will be delivered at ~6 km spatial resolution (nadir) and initial data will be made available within 6 months after launch ! Aerosol data products include columnar spectral aerosol optical thickness, aerosol effective radius, effective variance of the aerosol size distribution, aerosol spectral real refractive index and aerosol spectral single scattering albedo.

For more details please refer to the following :

Mishchenko, M.I., B. Cairns, G. Kopp, C.F. Schueler, B.A. Fafaul, J.E. Hansen, R.J. Hooker, T. Itchkawich, H.B. Maring, and L.D. Travis, 2007: Precise and accurate monitoring of terrestrial aerosols and total solar irradiance: Introducing the Glory mission. Bull. Amer. Meteorol. Soc., 88, 677-691, doi:10.1175/BAMS-88-5-677.

http://glory.gsfc.nasa.gov/index.html

http://glory.giss.nasa.gov/aps/

http://www-calipso.larc.nasa.gov/about/atrain.php