Photothermal interferometric aerosol absorption spectrometry

Aerosol light absorption still remains a difficult quantity to measure at the precision, accuracy and temporal resolution necessary to quantitatively bound the contribution of this direct effect on aerosol radiative forcing. These continuing difficulties are due, in part, because aerosol extinction is dominated by light scattering. In response to these and other issues, the aerosol community has been developing a new generation of instrumentation that can measure aerosol absorption without the need to deposit aerosols on a filter. Here we introduce work on the application of photothermal interferometry (PTI) towards this measurement problem. The advantages of this approach are: its complete insensitivity to aerosol scattering (true for any photothermal technique) and high sensitivity resulting from use of an interferometric technique. Using NO2 as a calibration standard, the accuracy of the PTI technique was measured to be 5 % (95 % confidence interval). Measurement at a 10-second time constant yields a precision of 0.2 Mm(-1) (95% confidence interval) and a lower limit of detection of 0.4 Mm(-1) for a sample pathlength of 5 cm. Using laboratory-generated nigrosin aerosols an intercomparison between the PTI and a 3-lambda Particle Soot Absorption Photometer (PSAP) gives a slope of 0.96 +/- 0.02. Acquisition of absorption coefficients for ambient aerosols reveals very good agreement between the two instruments except for periods of high relative humidity (>70%) whereupon the PSAP reports a larger absorption coefficient.