A versatile instrument for simultaneous detection of atmospheric H2SO4 and OH radicals

Both sulfuric acid (H₂SO₄) (SA) and hydroxyl radical (OH) play critical roles in the atmospheric chemistry processes. In most environments, from the pristine Tibet plateau to highly populated megacities, SA is the decisive nucleation precursor. OH, on the other hand, dominates the atmospheric oxidation capacity under most circumstances. Therefore, accurate measurements of SA and OH are important for atmospheric chemistry studies. This study developed an instrument based on chemical ionization mass spectrometry (CIMS) to measure SA and OH in the air simultaneously. The working principle was based on the nitrate (NO₃^-) CIMS. SA was ionized by NO₃^- directly to form bisulfate anion (HSO₄^-), which was then detected with a high-resolution time-of-flight mass spectrometer (HR-ToF-MS). OH was first converted into SA by excess sulfur dioxide (SO₂) and then detected as SA total, which is the sum of ambient SA, OH-converted SA, and background signals due to the high concentration of SO₂ (~1ppmv). In order to minimize wall losses, a 3-cm ID, 50-cm long sample inlet was used, and a blower was employed to suck in ambient air at ~100 L min^-1. Two 1/16 in gas injectors were installed at the front end of the inlet and 30 cm downstream of the front injector. The instrument was operated in three sequential modes, i.e., the ambient SA mode, OH mode, and background (BG) mode. No reagent gases were injected in the SA mode, and ambient SA was detected directly. During the OH mode, SO₂ was injected into the sample flow through the first injector. For the BG mode, both SO₂ and pure propane (C₃H₈) were injected into the sample flow through the first injector. Since C₃H₈ concentration was about a few hundred ppmv, nearly two orders of magnitude higher than SO₂, OH was completely scavenged during the BG mode, and only ambient SA and SO₂ BG were detected. A constant stream of propane was injected into the inlet through the second injector to prevent further free radical cycling. Therefore, the difference between the OH mode and the BG mode was the ambient OH signal. Each mode was operated for about 3-min, and the 9-min detection limits of SA and OH were ~2×10⁵ molecules cm^-3 and 4×10⁵ molecules cm^-3 (3σ), respectively. The instrument was calibrated by known concentrations of SA standards generated by a low-pressure Hg-lamp, the intensity of which was determined by the N₂O-actinometry. The instrument was field tested in a mountain site located in Longquan Mountain, Chengdu, China. Both SA and OH showed clear diurnal patterns following the solar radiation, ranging from less than the detection limit to a few 10⁶ molecules cm^-3. Further intercomparison between the measurements and model simulations was also conducted to verify the measurement results.