National Oceanic and Atmospheric Administration (NOAA),
Earth System Research Laboratory (ESRL)
The aerosol Nephelometer and Particle Soot Absorption Photometer (PSAP) measure the aerosol scattering and absorption coefficients in units of meter-1. The Thermo Systems Incorporated (TSI) unit is a continuous-expansion Condensation Nucleus Counter (CNC). Particle number concentration is determined when particles are exposed to a high super saturation of butanol vapor. This causes the particles to grow to a size where they can be detected optically and counted. The instruments in use have lower particle-size detection limits of 10-20 nm diameter. The Three-Wavelength Nephelometer measures at 450, 550, and 700 nm and has a 2 minute 1/e response time
Aerosol absorption is measured with a Particle Soot Absorption Photometer ( PSAP). This instrument continuously measures the amount of light transmitted through a quartz filter, while particles are being deposited on the filter. The rate of decrease of transmission, divided by the sample flow rate, is directly proportional to the light absorption coefficient of the particles. Newer instruments have been calibrated in terms of the difference of light extinction and scattering in a long-path extinction cell, for laboratory test aerosols. Instruments at the baseline stations ("Aethalometers") have been calibrated by the manufacturer in terms of the equivalent amount of black carbon, from which the light absorption coefficient is calculated assuming a mass absorption efficiency of the calibration aerosols of 10m2g-1.
Integrating Nephelometers are used to determine the light scattering coefficient of the aerosol. These instruments operate by illuminating a fixed sample volume from the side, and observing the amount of light that is scattered by particles and gas molecules in the direction of a photo-multiplier tube. The instruments integrate over scattering angles of 7-170°. A particle filter is inserted periodically into the sample stream to measure the light scattered by gas molecules, which is subtracted from the total scattered signal to determine the contribution from the particles alone. The instruments are calibrated by filling the sample volume with carbon dioxide (CO2) gas, which has a known scattering coefficient.
Through scattering and absorption of solar radiation aerosols can cool or warm the Earth’s atmosphere and alter the climate. We are looking to see how the aerosol optical properties change with time, season and long range transport of pollutants.
Daily trends are observed between daytime up-slope airflow and night time down-slope flow. Up-slope air has higher aerosol concentrations from both pollution and vegetation, whereas down-slope air is relatively clean and representative of the lower free troposphere. Higher aerosol concentrations are observed in the spring when long range transport of aerosol from Asia is observed.
The effect of aerosols on solar radiation and climate change has a high uncertainty. Changes in aerosol concentration, size and composition directly alter the heating of the earth’s surface and atmosphere by scattering and absorption of solar radiation as well as indirectly by altering cloud properties. By measuring the scattering and absorption properties we intend to better quantify aerosol optical properties with changes in season and air mass source region. These optical parameters can then be used in global and regional climate models to predict aerosol related climate change.
The Mauna Loa site is one of the few places in the world with continuous measurements of free troposphere aerosol properties. The high altitude site is ideal for observation of long range transport of aerosol because aerosol have much longer lifetimes in the free troposphere than in the lower boundary layer.