The four-wavelength Photoacoustic Aerosol Absorption Spectrometer (PAAS-4λ).

In this paper, the Photoacoustic Aerosol Absorption Spectrometer (PAAS-4 λ) is introduced. PAAS-4 λ 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 NO 2 /air mixtures and Nigrosin aerosol. It has an ultimate 1σ detection limit below 0.1 Mm -1 , at a measurement precision and accuracy of 3 % and 10 %, respectively. In order to demonstrate the PAAS-4 λ 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. A total of 11 months of PAAS-4 λ data from this deployment are presented and discussed in terms of instrument performance. Intercomparisons with the filter-based photometers of a continuous soot monitoring system (COSMOS), the Multi-Angle Absorption Photometer (MAAP), and Aethalometer (AE33) demonstrate the capabilities and value of PAAS-4 λ , as well as for the validation of the widely used filter-based instruments. [ABSTRACT FROM AUTHOR]