The functional-group composition of organic aerosol (OA) in urban PM2.5 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 PM2.5 in Beijing. Functional groups were identified via fourth-order derivative spectra and Gaussian deconvolution. The method demonstrates high linearity (R2 = 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-, R2 = 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 (OMFTIR) 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.
Wang et al. (Wed,) studied this question.