The "apparent" transmission, or transmission ratio (Ellis & Pueschel, Science, 1971), is derived from broadband (0.3 to 2.8um) direct solar irradiance observations at the Mauna Loa Observatory (19.533 ° N, 155.578 ° W, elev. 3.4 km) in Hawaii. Data are for clear-sky mornings between solar elevations of 11.3 and 30 degrees. The plotted points are monthly averages and the plotted curve results from a 6 month lowess statistical smoother.
The current dataset available as of March 12, 2012 is Version C (available here). Version C is being published in the 2012 Bulletin of the American Meteorological Society’s State of the Climate 2011 report due out in mid 2012. This version replaces all previously distributed Mauna Loa Transmission records and includes a refinement to the time-to-integer-airmass interpolation scheme between 1993 and 2012, a minor revision of the clear-sky selection criteria used between 1993 and 2012, and the removal of spurious contributions from a backup instrument between 2000 and 2002. The changes to Versions C do not significantly alter previously published conclusions concerning this dataset by NOAA/ESRL authors, and strengthen the case for a general decline in transmission (or transmittance) between 2000 and 2009.
Solomon, S., J. S. Daniel, R. R. Neely III, J. P. Vernier, E. G. Dutton, and L. W. Thomason, 2011: The Persistently Variable “Background” Stratospheric Aerosol Layer and Global Climate Change. Science, Published online 21 July, Science Express, 2011,DOI:10.1126/science.1206027]
Ellis, H. T. and Pueschel, R. F. 1971. Solar radiation: absence of air pollution trends at Mauna Loa, Science, 172, 845-846.
Dutton, E. G., J. J. DeLuisi, and A. P. Austring, 1985. Interpretation of Mauna Loa Atmospheric Transmission Relative to Aerosols, Using Photometric Precipitable Water Amounts. J. Atmos. Chem., 3, 53-68.
Dutton, E.G., 1992. A Coherence Between the QBO and the Amplitude of the Mauna Loa Atmospheric Transmission Annual Cycle. International J. Climatology 12, 383-396.
Dutton, E.G., P. Reddy, S. Ryan, and J.J. DeLuisi, 1994. Features and effects of aerosol optical depth observed at Mauna Loa, Hawaii: 1982-1992. J. Geophys. Res. 99, 8295- 8306.
Dutton, E.G. and B.A. Bodhaine, 2001: Solar irradiance anomalies caused by clear-sky transmission variations above Mauna Loa 1957-1999. J. Clim., 14, 3255-3262.