Aerosol Absorbance Spectra Analysis and Detection

Aerosols are liquid or solid very fine particles in the air composed with various chemical composition. They influence both climate dynamics and human health through their physical and chemical properties. Nowadays, accurate continuous chemical characterization of particulate matter (PM) remains challenging, especially for autonomous field deployment leading to a lack of data. This study presents a technical characterization of the Aerosol InfraRed Monitor (AIRMon), the first world instrument for near-real-time PM measurement using aerosol deposition on zinc selenite (ZnSe) crystals followed by Fourier-transform infrared (FTIR) spectroscopy. The objective of this work are to (i) validate a measurement scheduling to reduce transient effects while not affecting too much the noise, (ii) identify critical wavenumbers for assessing cleaning efficiency and detecting contamination, (iii) evaluate mass retrieval accuracy through field mass closure analysis, and (iv) calibrate AIRMon’s response for common aerosol types under laboratory conditions. The study combine a measurement campaign in Athens (February - March 2025) operated alongside the air pollution measurement station in the national observatory of Athens providing PM2.5 reference, with laboratory experiments using ammonium sulfate (AS) ammonium nitrate (AN) and glucose aerosols. Data analysis is organised into three complementary component : (1) transient effect analysis on raw spectra, leading to an optimal truncation period; (2) cleaning efficiency assessment using selected wavenumbers, with baseline correction and vapor/CO2 removal applied where required; and (3) mass closure and laboratory calibration, both using truncated and fully corrected spectra together with OPC-based deposition loss estimates. The results show that excluding the first 60s of each spectral acquisition of the crystal reduces transient effect noise on H2O andCO2 wavenumbers without affecting the measurement stability. Cleaning efficiency is found to have some random variability at some wavenumbers without constant tendency (no accumulation). Some functional groups - particularly nitrates, carbonylCO, CH, alcoholCOH1 and, organics - are more subject to residuals contamination. Applying OPC-based charger loss correction and Airpec decomposition to go from absorbance spectra to mass, the AIRMon mass closure obtained is 83% which is a high results compared to current achievement. Laboratory calibration demonstrated strong linearity for the three tested compound. Overall, this study validate several AIRMon operating variables providing some advice on the measurement scheduling and on the data quality control. It reminds the needs for installation standardization and filling monitoring of the external CO2 bottle as well as some further calibration of the integrated OPC. Finally, this study only validates some operating variables but does not look for best optimized process.