Aerosols (particulate matter) have a dramatic effect on radiative forcing of the climate, in some cases cooling and in other cases warming. The Fourth Assessment Report of the IPCC estimates that direct radiative forcing due to all aerosols is a cooling of -0.50 W m-2 with absorbing aerosol (black carbon) responsible for a warming of +0.22 W m-2, but the uncertainties associated with these numbers are very large. Better measurements of the optical properties of aerosols, especially absorption coefficient and asymmetry parameter, and their spatial and temporal distribution are required to reduce these uncertainties and improve the ability of models to predict climate change. Aero3X was designed to provide such measurements. It is a light weight (11 kg), compact (0.25 x 0.30 x 0.6 m), and fast (1 Hz sample rate) instrument intended for use on an Unmanned Aerial System (UAS) but suitable for flight on other aircraft and for surface measurements. Aero3X uses an off-axis cavity ring-down technique to measure extinction coefficient and a reciprocal nephelometry technique for measurement of total-, forward- and back-scatter coefficients at wavelengths of 405 nm and 675 nm. Its outstanding precision (0.1 Mm-1) and sensitivity (0.2 Mm- 1) allow the determination of absorption coefficient, single-scattering albedo, estimates of backscatter to extinction ratio and asymmetry parameter at both wavelengths, and Angstrom exponent. Together with its humidification system for measurement of the dependence of aerosol optical properties on relative humidity, these represent a complete set of the aerosol optical properties important to climate and air quality. Aero3X was designed to operate in pollution plumes where NO2 may cause interference with the measurement, therefore, a measurement of NO2 mixing ratio is also made.
