Measurements of Atmospheric HO₂ Radicals Using Br-CIMS with Elimination of Potential Interferences from Ambient Peroxynitric Acid

As a promising direct measurement method of atmospheric hydroperoxyl radicals (HO₂), bromide chemical ionization mass spectrometry (Br-CIMS) has been first demonstrated by Sanchez et al. (<i>Atmos. Meas. Tech.</i> 2016, <b>9</b>, 3851-3861). However, field application of this method is currently still sparse, and there is still a gap between measured HO₂ concentrations and calculated ones derived from the atmospheric equilibrium between HO₂ and peroxynitric acid (HO₂NO₂). In this work, we constructed an improved Br-CIMS with optimizations of custom-built front-end devices, chamber pressures, and instrumental voltages to achieve a 3σ detection limit of 0.5 ppt at an integration time of 60 s and a sensitivity of 1-3 cps ppt^-1 under a total reagent ion signal of 0.2 MHz for HO₂ detection. HO₂NO₂, a product from atmospheric reactions between HO₂ and NO₂, can also be detected by Br-CIMS, whose interference on the HO₂ measurement was found but nearly eliminated by regulating key CIMS voltages to minimize the decomposition of (BrHO₂NO₂)^- ions in the MS. In addition, a 2 week field campaign was carried out in urban Shanghai, demonstrating that the interference of HO₂ from ambient HO₂NO₂ was less than 10% of the true HO₂ signal under our optimized CIMS voltage setting. Our study suggests that Br-CIMS is a reliable technique for atmospheric HO₂ measurements.