As part of the INDOEX Intensive Field Program during 1999, the University of Miami and NOAA CMDL made measurements of aerosol l

As part of the INDOEX Intensive Field Program during 1999, the University of Miami and NOAA CMDL made measurements of aerosol light scattering and absorption during February and March at the Kaashidhoo Climate Observatory in the Maldives. Both groups used the Radiance Research Particle/Soot Absorption Photometer to measure absorption. The CMDL used TSI Model 3563 Integrating Nephelometers while the University of Miami group used Radiance Research M903 Integrating Nephelometers. Both groups made measurements of scattering of aerosols <10 µm and <1 µm dried aerodynamic diameters. The CMDL’s measurements of aerosol absorption were also made at <10 µm and <1 µm dried aerodynamic diameters, while the Miami group’s measurement of absorption was performed without an intake impactor. A 10 m tall, lamiar flow sample air intake with an omni-directional inlet provided sample air dried to <50 % relative humidity to all instruments.

Here we compare the results of these measurement programs. In order to facilitate these comparisons:

all data were adjusted to 0°C and 1013 hPa,
all scattering values were either measured or adjusted to 550 nm and 55 % relative humidity,
angular non-idealities in nephelometer measurements were corrected according to Anderson and Ogren (AS&T 29:57, 1998), and
absorption measurements were corrected according to Bond et al. (AS&T 30:582, 1999).

Absorption versus time is presented in "bap Time Series.jpg". Absorption of aerosols <10 µm aerodynamic diameter (CMDL) versus absorption of all aerosols (UM) is presented in "<10 µm bap UM vs CMDL.jpg". Similarly, absorption of aerosols <1 µm aerodynamic diameter (CMDL) versus absorption of all aerosols (UM) is presented in "<1 µm bap UM vs CMDL.jpg". The error bars illustrate the magnitude of the uncertainty in the accuracy of these measurements. The linear regression of the <10 µm absorption (CMDL) versus total absorption (UM) shows the measurements to be highly correlated (R² = 0.94) with the CMDL measurement averaging nearly 20 % higher than the UM measurement. The linear regression of the <1 µm absorption (CMDL) versus total absorption (UM) shows the measurements to also be highly correlated (R² = 0.90) with the CMDL measurement averaging only about 2 % higher than the UM measurement. We expected the <10 µm absorption measurement by CMDL to be closest to the UM measurement of total absorption. However, that was not the case. Nevertheless, given the uncertainties in the measurements, the differences between the CMDL and UM measurements of absorption appear not to be significant.

Scattering versus time is presented in "<10 µm bsp Time Series.jpg" and "<1 µm bsp Time Series.jpg". Scattering of aerosols <10 µm aerodynamic diameter from both CMDL and UM instruments is presented in "<10 µm bsp UM vs CMDL.jpg". The measurements are highly correlated (R² = 0.97). The CMDL measurements average approximately 10 % lower than those by UM. The error bars, which represent one standard deviation of uncertainty in accuracy, indicate that the two measures of scattering are not significantly different.

Scattering of aerosols <1 µm aerodynamic diameter from both CMDL and UM instruments is presented in "<1 µm bsp UM vs CMDL.jpg". The measurements are correlated (R² = 0.86). The CMDL measurements average approximately 10 % higher than those by UM. The error bars represent one standard deviation of uncertainty in accuracy. There appear to be two populations of data points for scattering values above 50 Mm^-1. One group of points lies very near the 1:1 line. In the other group of points, the CMDL measurements appear to be systematically higher than the UM data. At present, we have no explanation for this ~20% discrepancy.