Simultaneous detection of gaseous sulfuric acid and OH radicals using chemical ionization mass spectrometry

Hydroxyl radical (OH) is the dominant oxidation agent in the atmosphere due to its high reactivity. OH plays a decisive role in the degradation of primary air pollutants and the formation of secondary air pollutants (e.g., SOA and O₃). For the same reason, OH atmospheric abundance is extremely low, and its atmospheric measurement usually presents a challenging job. In this work, we have developed a new method to detect OH radicals using chemical ionization mass spectrometry (CIMS). The working principle of the new method is that OH was first completely converted into gaseous sulfuric acid (SA) and was then detected by a nitrate-CIMS. Potential interference from ambient SA and other reactive intermediates, such as Criegee Intermediates (CIs), was also dealt with by introducing free radical scavengers. A high-resolution time-of-flight mass spectrometer was also used to explore potential isotopic interferences. The new instrument was calibrated with artificially generated SA standards, quantified with the N₂O-actinnometry. A field test was conducted in the Summer of 2023. The results demonstrated that OH concentration ranged from a few 10⁶ molecules cm^-3 up to ~2×10⁷ molecules cm^-3, basically in line with OH level reported in other areas of China. The ambient SA level was significantly lower than OH, likely due to the high condensation loss near the ground surface. These observation results were further verified by a master chemical mechanism (MCM) box model simulation.