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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 0.1 Mm-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. [ABSTRACT FROM AUTHOR]

The Arctic absorbing aerosols have a high potential to accelerate global warming. Accurate and sensitive measurements of their concentrations, variability and atmospheric mixing are needed. Filter-based aerosol light absorption measurement methods are the most widely applied in the Arctic. Those will be the focus of this study. Aerosol light absorption was measured during one month field campaign in June–July 2019 at the Pallas Global AtmosphericWatch (GAW) station in northern Finland. The campaign provided a real-world test for different absorption measurement techniques supporting the goals of the EMPIR BC metrology project in developing aerosol absorption standard and reference methods. Very low aerosol concentrations prevailed during the campaign which imposed a challenge for the instruments detection. In this study we compare the results from five filter-based absorption techniques: Aethalometer models AE31 and AE33, Particle Soot Absorption Photometer (PSAP), Multi Angle Absorption Photometer (MAAP) and Continuous Soot Monitoring System (COSMOS), and from one indirect method called Extinction Minus Scattering (EMS). The sensitivity of the filter-based techniques was adequate to measure aerosol light absorption coefficients down to around 0.05 Mm-1 levels. The average value measured during the campaign using MAAP was 0.09 Mm-1 (at wavelength of 637 nm). When data were averaged for >1h, an agreement of around 20% was obtained between instruments. COSMOS measured systematically the lowest absorption coefficient values, which was expected due to the sample pre-treatment in COSMOS inlet. PSAP showed the best linear correlation with MAAP (R2 = 0.85), followed by AE33 and COSMOS (R2 = 0.84). The noisy data from AE31 resulted in a slightly lower, yet a significant, correlation with MAAP (R2 = 0.46). In contrast to the filter-based techniques, the sensitivity of the indirect EMS method to measure aerosol absorption was not adequate at such low concentrations levels. An absorption coefficient on the order of >1 Mm-1 was estimated as the lowest limit, to reliably distinguish the signal from the noise. Throughout the campaign the aerosol was highly scattering with an average single-scattering albedo of 0.97. Two different air-mass origins could be identified: the north-east and from the north-west. The north-eastern air masses contained higher fraction of thickly coated light absorbing particles than the westerly air masses. Aerosol scattering, absorption and the particle coating thickness increased on the last ten days of the campaign during the north-eastern air flow. The simultaneous changes in aerosol source region, mixing state, concentration and particle optical size were reflected in the instruments' response in a complex way. The observed decrease in aerosol size suggested additional activation of secondary particle formation mechanisms. The results demonstrate the challenges encountered in the Arctic absorbing aerosol measurements. The applicability and uncertainties of different techniques are discussed and new knowledge on the absorbing aerosol characteristics in summer Arctic air masses reference to the source region is provided. [ABSTRACT FROM AUTHOR]

The performance of the Measurements of Pollution in the Troposphere (MOPITT) retrievals over urban regions has not been validated systematically, even though MOPITT observations are widely used to study CO over urban regions. Here we validate MOPITT products over urban regions using aircraft measurements from DISCOVER-AQ, SEAC4RS, ARIAs, A-FORCE, and KORUS-AQ campaigns. Overall, MOPITT performs reasonably well over both urban and non-urban regions, overall biases for V8J and V8T vary from -0.7% to 0.0%, and from 2.0% to 3.5%, respectively. The evaluation statistics of MOPITT V8J and V8T over non-urban regions are better than that over urban regions with smaller biases and higher correlation coefficients. We find that the performance of MOPITT V8J and V8T at high CO concentrations is not as good as that at low CO concentrations, although CO variability may tend to exaggerate retrieval biases in heavily-polluted scenes. We test the sensitivities of validation results to assumptions and data filters applied during the comparisons of MOPITT retrievals and in-situ profiles. The results at the surface are insensitive to the model-based profile extension (required due to aircraft altitude limitations) whereas the results at levels with limited aircraft observations are more sensitive to the model-based profile extension. The validation results are insensitive to the allowed maximum time difference as criteria for co-location (12 hours, 6 hours, 3 hours, and 1 hour), and are generally insensitive to the radius for co-location, except for the case where the radius is small (25 km) and hence the MOPITT retrievals included in the validation become very small. Daytime MOPITT products have overall smaller biases than nighttime MOPITT products when comparing both MOPITT daytime and nighttime retrievals to the daytime aircraft observations. However, it would be premature to draw conclusions on the performance of MOPITT nighttime retrievals without nighttime aircraft observations. Applying signal-to-noise ratio (SNR) filters does not necessarily improve the overall agreement between MOPITT retrievals and in-situ profiles, likely due to the reduced number of MOPITT retrievals that result for comparison. Comparisons of MOPITT retrievals and in-situ profiles over complex urban or polluted regimes are inherently challenging due to spatial and temporal variabilities of CO within MOPITT retrieval pixels (i.e., footprints). We demonstrate the some of that errors are due to CO representativeness with these sensitivity tests, but further quantification of validation errors due to CO variability within the MOPITT footprint will require future work. [ABSTRACT FROM AUTHOR]