Calibration system generating defined concentrations of gaseous sulfuric acid

Sulfuric acid (H₂SO₄) is one of the most important trace gases responsible for the formation of new particles (nucleation) from the gas phase. Field and laboratory observations generally report strong dependencies for the formation rate of new particles (1 to 2 nm in diameter) with the sulfuric acid concentration. Therefore, the accurate measurement of the gas-phase sulfuric acid concentration is essential. We perform such measurements using a nitrate Chemical Ionization-Atmospheric Pressure interface-Time Of Flight mass spectrometer (CI-APi-TOF). However, accurate and reproducible measurements require regular calibrations with defined concentrations of sulfuric acid. For this reason a method has been developed that can be used to generate sulfuric acid concentrations between ~1´10⁷ and 5´10⁹ cm^-3 covering the atmospherically relevant range (Kürten et al., 2012). This is achieved by mixing defined amounts of nitrogen, oxygen, sulfur dioxide and water vapor; the gas mixture flows through a quartz glass tube that can be illuminated with filtered UV light (185 nm from a Hg-lamp) of known intensity. A simple chemistry model is used to calculate the generated sulfuric acid concentration from reactions initiated by the photolysis of water (producing OH radicals). The calibration set-up is portable and can be used during field and laboratory experiments. The estimated systematic error of the calibration factor derived for the nitrate CI-APi-TOF is ~30%.

The calibration factor for sulfuric acid can also be applied for the measurement of highly oxygenated organic molecules (HOM). Since the HOM measured by the mass spectrometer cover a wide mass range (from ca. 200 to 650 Th) a second calibration needs be performed, which takes into account that the transmission efficiency of the mass spectrometer depends on the mass-to-charge ratio of the measured ions. This requires the calibration method described by Heinritzi et al. (2016). It is emphasized, however, that this second calibration is independent of the sulfuric acid calibration and only needed for HOM measurements.