Air Pollution, Aerosols and Clouds in the Climate System

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 increas...

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). Warren descri...

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 http://aeronet.gsfc.nasa.gov/cgi-bin/bamgomas_interactive and link to the main AERONET page http://aeronet.gsfc.nasa.gov/ So, take some time and admire the AERONET facilities available for atmospheric ae...

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 g...