The Four-Wavelength Photoacoustic Aerosol Absorption Spectrometer PAAS-4λ

Abstract. In this paper the Photoacoustic Aerosol Absorption Spectrometer PAAS-4λ is introduced. PAAS-4λ was specifically developed for long-term monitoring tasks in (unattended) air quality stations. It uses four wavelengths coupled to a single acoustic resonator in a compact and robust set-up. The instrument has been thoroughly characterized and carefully calibrated in the laboratory using NO₂/air mixtures and Nigrosin aerosol. It has an ultimate 1σ detection limit below 0.1Mm^-1 at a measurement precision and accuracy of 3 % and 10 %, respectively. In order to demonstrate the PAAS-4λ suitability for long-term monitoring tasks, the instrument is currently validated at the air quality monitoring station Pallas in Finland, about 140 km north of the Arctic circle. Eleven months of PAAS-4λ data from this deployment are presented and discussed in terms of instrument performance. Intercomparisons with the filter-based photometers COSMOS, MAAP, and AE33 demonstrate the capabilities and value of PAAS-4λ, also for the validation of the widely used filter-based instruments.