A Stainless Steel Filter-Based Infrared Spectroscopy Method for Monitoring Hourly Evolution of Functional Groups in PM _2.5

The functional-group composition of organic aerosol (OA) in urban PM_2.5 is complicated and varies rapidly, yet its characterization with hourly resolution remains challenging by using traditional Fourier Transform Infrared Spectroscopy (FTIR) techniques. We introduce a new method coupling a reusable stainless-steel filter (SSF) sampler with diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS) for hourly analysis of PM_2.5 in Beijing. Functional groups were identified via fourth-order derivative spectra and Gaussian deconvolution. The method demonstrates high linearity (<i>R</i>² = 0.96-0.98) between the absorbance of feature bands and the sample mass for key groups (alkane CH, nonacid carbonyl C═O, alcohol COH, and carboxylic COOH) and good sensitivity with absorptivities 8-18 times higher than those reported using a polytetrafluoroethylene (PTFE) filter coupled with transmission-mode FTIR. Good correlations of inorganic ions (NH₄⁺ and NO₃^-, <i>R</i>² = 0.86-0.91) measured by the SSF-DRIFTS and a Monitoring AeRosol and Gases in ambient Air (MARGA) further validate the reliability of this method. During a pollution episode on March 11, 2025, two-dimensional correlation spectroscopy (2D-COS) revealed the evolution of functional groups of OA, i.e., an early response of carbonyl, followed by midday alkane peaks from enhanced daytime primary emissions, then increases in oxygenated groups (C═O and COOH) related to photochemistry, concluding with evening carboxylic acid rises. Reconstructed organic mass (OM_FTIR) explained ∼88% of bulk OM. This study provides an approach to monitor hourly functional-group composition of OA and new insight into OA dynamics in the atmosphere.