Furthering information from OH and HO₂ +RO₂ observations using a high resolution time of flight mass spectrometer

Abstract. An instrument has been developed which allows an entire suite of oxidation products (H₂SO₄ and Extremely Low Volatile Organic Compounds, ELVOCs) to be measured along with the concentrations of the key radical species OH and HO₂ +RO₂. The system combines the techniques for chemical conversion of OH or HO₂ +RO₂ into H₂SO₄ together with nitrate ion (NO₃-) Chemical Ionization (CI) and Atmospheric Pressure interface - Time of Flight Mass Spectrometer (NO₃- CI-APi-ToF) which has been previously used for the detection of ELVOCs and a few other oxygenated organic species. The system exhibits the same sensitivity towards OH or HO₂ +RO₂ as previous quadrupole chemical ionization (CIMS) measurements with limits of detection of ~2 x 10⁵ and ~2 x 10⁶ molecule cm^-3 for OH and HO₂+RO₂ respectively. Unlike previous CIMS measurements, the use of a NO₃- CI-APi-ToF allows the acquisition of the entire mass spectrum at high resolution with each measurement, allowing one to see how the organic species behave when the sample flow is perturbed with reagent gasses (SO₂ or NO). While the combination of these measurements into one instrument is of great practical value, it is the combination of these data within the spectra obtained in each mode that extends the information far beyond the measurements themselves.