Monday, May 28, 2007

14C, a proxy of ‘biomass burning’ versus ‘fossil fuel combustion’ contribution to carbonaceous aerosols

Carbonaceous aerosols (organic and elemental carbon) in the atmosphere are produced by biomass burning and fossil fuel combustion but their relative contribution is not properly known. Radiocarbon (14C) is present in living and recently living material at an approximate concentration of one 14C atom per 10^12 ordinary carbon atoms (12C + 13C). This equilibrium amount is a result of the gain of 14C from its steady production by cosmic rays spallation reaction with nitrogen in the atmosphere (some fraction of which is taken up by the biosphere through photosynthesis in the form of 14CO2) versus the loss of 14C from its radioactive decay (5730 y half-life). However, 14C is absent from fossil fuels because of the ancient age of fossil carbon (due to radioactive decay to unmeasurably small amounts). This dichotomy is the basis of inferring the fraction of fossil carbon in an ambient aerosol sample by comparing its 14C content to that of living material, a methodology that has become increasingly popular in recent years. For detail, please see Charles et al (2006) and references therein.

Charles W. L., John, V., James, N. B., William S. C., William A. L., and Ann P. M., 2006, Absence of 14C in PM2.5 Emissions from Gasohol Combustion in Small Engines, Aerosol Science and Technology, 40:657–663.

Monday, May 21, 2007

Cloud Properties Using Zenith Radiance Measurement

Last Thursday Warren Wiscombe was in my department (AOSC, Univ of Maryland) to give a talk. Many of us have greatly benefited from radiative transfer code DISORT. He is a co-author to that code. He has written few other excellent algorithms and code for atmospheric remote sensing. His code for the Mie scattering can be downloaded here.

His talk was about deriving cloud properties using ground based zenith radiance measurements. Focus of the talk was cloud optical depth. Cloud optical depth is very important parameter for climate modeling as can be understood from the fact that at any time more than 60% of the earth’s sky is covered with cloud. Cloud optical depth (COD) can range anywhere between 1 and 80+. In spite of such high importance, not only it is poorly understood parameter but also a challenging task to measure it. Turner et al. (2007) highlight this problem by showing five different techniques resulting in five different values for same cloud (see figure).
Figure from Turner et al. (2007)
Warren described narrow field of view zenith radiometer (NFOV) in detail. Unlike aerosol where ground based measurements are concerned mostly with transmitted photons, zenith radiance measurements in presence of cloud have to deal with both transmitted and backscattered photons. When a cloud is thin, increasing cloud optical depth increases number of scattered photons, but after reaching a peak value further increase in cloud optical depth results in attenuation of photons. Hence we are in a situation where we have same radiance value for two different cloud optical depths.

One can overcome this problem using spectral measurements, if geography of a place supports it. Green vegetation reflects predominantly near-infrared-radiation (NIR). If measurements are made at a place, which is surrounded by green vegetation, zenith radiance flux will have spectral signature depending on scattered photons are coming from the Sun (thin cloud) or reflected from the Earth (thick cloud). Wiscombe and his group are developing a new NFOV, which makes measurement over 240 wavelengths bands. That is just in one second!

I highly recommend reading Turner et al (2007) paper to get insight into challenges associated with cloud remote sensing.

Turner et al. (2007), “Thin Liquid Water Clouds: Their Importance and Our Challenge”, Bull. American Meteo. Soc, vol 88(2), 177-190, DOI:10.1175/BAMS-88-2-177


In a reply to my e-mail. Warren has pointed out following about NFOV.

I wrote " One can overcome this problem ... if geography of a place supports it"

It is not geography but only whether or not there is enough green vegetation around, for at least a horizontal distance equal to the vertical distance to cloud base.

I wrote "Wiscombe and his group are developing a new NFOV, which makes measurement over 240 wavelengths bands."

Peter Pilewski of U Colorado developed this spectrometer under contract to ARM. However, Wiscombe and his group urged its development for years and are its biggest users.

Sunday, May 13, 2007

AERONET Web Services: New look and Tools

AERONET (AERosol RObotic NETwork) is a well known, most used aerosol observation system, which provides most reliable measurements of different aerosol optical properties from their ground stations around the world. Some time ago, AERONET team has updated their web services. The new look of website is really cool and site contains almost everything you want to know about AERONET and their data. It contains all of the major publications, which used AERONET data.

The most fascinating and very useful tool they have is ‘data synergy tool’. It is really amazing for quick research and great visualization of various aerosol properties from AERONET, satellites, models, LIDARs, ozone, back trajectory, and even weather charts over any particular AERONET station.

Here is link to the synergy tool

and link to the main AERONET page

So, take some time and admire the AERONET facilities available for atmospheric aerosol research community.

Personally, I really thank the AERONET team and congrats them for their great work and services.

Monday, May 7, 2007

Long-Term Satellite Record Reveals Likely Recent Aerosol Trend

In the recent decade aerosols have gained much attention from a climate perspective. Researchers have been using both observations and modeling studies to address the question on radiative effects of aerosols and their role in global climate change. Ground based observations have provided unmatched understanding of aerosol optical and microphysical properties. Sophisticated remote sensing instruments such as MODIS, MISR, OMI, TOMS onboard various satellite platforms provide routine measurement of aerosol loading over the entire globe.

While much attention was being paid on characterizing aerosols and understanding their radiative effects not much speculation was done over their concentration trends over past decade or so until recently. An interesting paper appeared in Science on March 16, 2007 by Michael Mishchenko et al on “Long-Term Satellite Record Reveals Likely Recent Aerosol Trend “. They analyzed the Global Aerosol Climatology Project (GACP) data set to show a decrease in global tropospheric aerosol optical thickness by 0.03 during the period from 1991 to 2005.

Global Diming : Wikipedia

Comments on this paper also appeared in the same issue raising question on the dataset used, for example, the AVHRR observations that goes into GACP data set is questionable in the first place because AVHRR with limited channels was not really designed for aerosol retrieval. Cloud contamination was also in question. AVHRR retrieval was also the first question that came to both me and one of my colleagues Pawan Gupta. We discussed this issue and came up with the thought that MODIS and MISR being instruments meant to retrieve aerosol concentration (aerosol optical thickness => AOT) information should throw some light into this because they have been flying onboard Terra satellites since 1999. That gives a time series of 6 years (2000 – 2006). Mr Gupta generated the AOT trend plots shown below. The first figure overlays MODIS (red) and MISR (yellow) monthly mean AOT values over GACP AOT trend (blue). MODIS-MISR decreasing trends agree with each other both in trend and in magnitude though they do not agree in magnitude with GACP. But the key result is that all the three dataset do show a decreasing trend (see zero AOT line for reference).

A closer look annual mean AOT trends (figure below) separately over Land (from MISR) and Ocean (from MODIS) and global mean renders the same result – net decreasing trend in annual mean AOT.

These results along with study by Mishchenko et al., (2007) raise interesting questions on both global warming and global diming and their offset. How does this fit into what we observe around us? Is the decrease in aerosol concentration contributing to increase in global warming? What are some loop holes in understanding this debate and assimilating what data shows?

Acknowledgement : A special thanks to Mr Pawan Gupta for sharing some of his results (the two figures above) for this article.


Michael I. Mishchenko, Igor V. Geogdzhayev, William B. Rossow, Brian Cairns, Barbara E. Carlson, Andrew A. Lacis, Li Liu, and Larry D. Travis (16 March 2007)
Science 315 (5818), 1543. [DOI: 10.1126/science.1136709]