An ion trap CIMS instrument for combined measurements of atmospheric OH and H 2SO 4: First test measurements above and inside the planetary boundary layer

Abstract

Gaseous sulfuric acid (GSA) plays an important role in atmospheric secondary aerosol formation. To quantify its contribution to these processes long-term measurements of GSA using a very sensitive detection method are required. OH is the most important oxidant in the atmosphere and oxidizes, e.g., SO 2. A powerful detection method is chemical ionization mass spectrometry (CIMS). We have built a novel CIMS instrument composed of an ion trap mass spectrometer (ITMS) and a selective ion source suitable to measure gaseous H 2SO 4 and OH and to do fragmentation studies. The probe air inlet and sampling line were optimized to minimize wall losses of H 2SO 4 and OH. An ion source was developed to minimize artificial signals and to create reagent ions (NO 3 − (HNO 3) h (H 2O) n ) which react highly selectively with GSA. An H 2SO 4/OH calibration source based on water vapor photolysis was developed to overcome uncertainties in remaining wall losses, ion residence time, reaction rate coefficient and mass discrimination. Field measurements of GSA in the planetary boundary layer (several weeks at northern and middle latitudes) and high-altitude mountain-site measurements of GSA and OH were carried out. The ITMS instrument offers a detection limit as low as 1 × 10 5 molecules per cm 3 and a time resolution of about 2 min .