The atmospheric measurements of the radicals OH, XO2 (XO2 = HO2, RO2) and sulphuric acid H2SO4 are of great importance for the studies related to the atmospheric chemistry. The OH and XO2 radical play a central role in practically all atmospheric chemical conversion processes including the ozone formation, degradation of pollutants, formation of secondary organics and aerosols and others. As well, the sulphuric acid is of major importance in the processes of the formation of new atmospheric particles. The measurements of the radicals OH, XO2 and sulphuric acid present a significant challenge due to their high reactivity and low atmospheric concentrations. Presently there exist only two methods for the measurements of OH radicals: FAGE and CIMS. The H2SO4 can presently be measure only with the CIMS method. These measurements are quite complex and presently there are few instruments which can be used for the atmospheric field measurements. The SAMU instrument developed in LATMOS during 2004-2010 and presently being utilized and further developed in LPC2E is a chemical ionisation mass spectrometer based on the conversion of the radicals OH and XO2 into the H2SO4. Although this approach is common for the OH measurements with CIMS, the SAMU is quite different in some aspects, for example, capability of measuring OH radicals in polluted environment at high concentration of NO or possibility of quasi-simultaneous measurements of OH, XO2 and H2SO4.
DESCRIPTION / PRINCIPE / REALISATION / PERFORMANCES
OH radicals are measured by titrating OH radicals with SO2 to form H2SO4 in a chemical conversion reactor (CRR). H2SO4 is detected as the HSO4─ ion produced by chemical ionisation with NO3─ in an ion-molecule reactor (IMR) following the CCR. To distinguish for atmospheric H2SO4 the chemical titration can be performed using 34SO2. Total RO2 are measured by converting RO2 into OH radicals via reactions with NO injected in the CCR followed by conversion of OH into sulfuric acid.
Air is sampled at a flow rate of 10 SLM creating turbulent flow in the CCR (Re = 2100). The turbulent flow conditions minimize possible influence of wind speed on the measurements and ensure fast mixing of reactants (34SO2 and NO) and the radical quencher (NO2). NO2 used as a scavenger removes not only the OH radicals, but also peroxy radicals converting them into HO2NO2 and RO2NO2. Switching the reactants between different injectors allows measurements in four different modes: background, two different OH measurement modes and RO2 mode. The two OH modes differ by the times of chemical conversion, 3.5 and 36 ms. Ratio of the signals with the short and the long conversion times may be used as an indicator of an artificial OH formation in the reactor.
The concentration of the radicals is derived from the measured ratio of the H34SO4¯ and NO3¯ ion peak intensities. Calibration coefficient is determined using N2O actinometry and OH/RO2 generation in a turbulent flow reactor by photolysis of N2O or H2O at 184.9 nm. The calibration of HO2, CH3O2 and other RO2 is performed by adding into the calibration cell photolysis reactor CO, CH4 (or other RO2 precursors) converting any OH radical to RO2. The overall estimated calibration accuracy (2σ) for OH is about 25%. The uncertainty of the RO2 measurements is typically higher due to uncertainty in RO2 composition in air.
Usually during the field measurements the instrument is installed in a shipping container with the CCR fixed to the roof of the container via an interface cap covered with a PTFE sheet. The sampling aperture of the CCR (3 mm diameter) is positioned 50 cm above the roof and about 3 m above the ground surface.
Performance data – Accounting for the calibration uncertainties and measurement precision, the overall 2σ uncertainty of the 15 min averaged measurements of OH is estimated to be 20%. For RO2 the uncertainty depends on RO2 composition in ambient air. – The lower limits of detection for OH and RO2 radicals at S/N=3 and a 2 minute integration time is 5×105 molecule cm-3 and 2×106 molecule cm-3, respectively.- Time resolution: 2 min for one point of OH (typically averaged to 15 min time steps) – Interferences: – artificial OH generation in the CCR at high NO ([NO] > 5-10 ppb) (correction can be applied); – interference from H2SO4 formed via Criegee+SO2 reactions (studies in progress);
HISTORIQUE / UTILISATION
History of utilization :
– 2009, SIRTA, Palaiseau, MEGAPOLI, FP7 (”Megacities : Emissions, urban, regional and Global Atmospheric POLlutionand climate effects, and Integrated tools for assessment and mitigation”)
– 2009-10, DDU, Antarctic, OPALE, ANR (“Oxidant Production over Antarctica Land and its Export”, ANR)
– 2010-11, Dôme C, Antarctic, OPALE, ANR
– 2013, Cap Corse, CHARMEX, ANR (« Chemistry-Aerosol Mediterranean Experiment »)
– 2014 –present, Laboratory studies: COGNAC, ANR (« Chemistry of Organic biradicals : GeNesis of AtmospheriC aerosols”), CRASA, LEFE (« Criegee Radicals As a source of Sulfuric Acid in the atmosphere”), …