Fast screening compositions of PM_2.5 by ATR-FTIR: Comparison with results from IC andOC/EC analyzers

Chemical speciation of fine particles or PM_2.5 collected on filters is still a costly and time-consuming task. In this study, filter-based PM_2.5 samples were collected during November–December 2013 at four sites in Guangzhou, and the major components were fast screened (~ 7 min per filter sample) by Attenuated Total Reflectance (ATR)-Fourier Transform Infrared Spectroscopic (FTIR) in comparison with that measured by Organic carbon/Element carbon (OC/EC) analyzer and Ion Chromatography (IC). The concentrations of nitrate, ammonium, sulfate, primary organic carbon (POC) and secondary organic carbon (SOC) measured by OC/EC and IC analyzers were better correlated with their infrared absorption peak heights at 1320 cm^-1 for nitrate, 1435, 3045 and 3215 cm^-1 for ammonium, 615 cm^-1 for sulfate, 690, 760 and 890 cm^-1 for POC and 1640 and 1660 cm^-1 for SOC respectively, during polluted days (PM_2.5 > 75 μg/m³) than during clean days (PM_2.5 ≤ 75 μg/m³). With the evolution of a haze episode during our field campaign, the concentrations of the major PM_2.5 components displayed consistent variations with their infrared absorption peak heights, suggesting ATR-FTIR could be a fast and useful technique to characterize filter-based PM_2.5 compositions particularly during pollution events although cautions should be taken when PM_2.5 levels are low. Notably, elevated PM_2.5 mass concentrations occurred with enhanced ratios of [NO₃ ⁻]/[SO₄ ^{2 −}] and [NH₄ ⁺]/[SO₄ ^{2 −}], implying that nitrogenous components play vital roles in the PM_2.5 pollution events in the study region.