Meteorologisches Observatorium Hohenpeißenberg, Sulfuric acid is a key components in new particle formation in the atmosphere. Nucleation rate and the number concentration of freshly nucleated particles are both observed to strongly depend on sulfuric acid concentration (Kerminen et al., 2010, Paasonen et al., 2010). The only available technique to measure sulfuric acid in atmosphere (some 10 5 –10 7 molecules/cm³) is Chemical Ionization Mass Spectrometry (CIMS) (Fig. 1). Fig. 1: Scheme of a CIMS instrument ((Berresheim et al., 2000) H 2 SO 4 measurement by CIMS needs indirect calibration, generally, photolysis of water vapor is used to produce OH radicals, which are then titrated by excess SO 2 to yield H 2 SO 4 (Eisele and Tanner, 1993). Table 1 gives typical uncertainties of calibration, OH measurement and H 2 SO 4 measurements. Table 1: 2-sigma uncertainties for CIMS measurements Chemical artifacts in photolysis and titration zone in ambient air calibration: Between photolysis, titration, and the addition of an OH-scavenger (rear injector) (several 10 ms each) OH and HO 2 radicals are not in equilibrium and inter-conversion with artifact OH loss or H 2 SO 4 formation may occur –> Periods short (<50ms), chemical model for corrections (typically 10%) Artifacts due to matrix effects using synthetic air flow tube calibrators: Conditions in titration, flow tube and ion-molecule reaction zone are different from ambient air and ion-molecule cluster formation (acid ions with water and neutral acids and other polar compounds) is suppressed which might result in different instrument sensitivity for ambient air matrix and synthetic air matrix -> occasional changes in CIMS sensitivity in ambient air are observed and are not understood Chemical artifacts in ambient air measurements – same as above Matrix effect in ambient air measurements: Neutral molecule clusters of H 2 SO 4 with polar molecules (water, acids, ammonia, amines,…) may form (Kurten et al., 2010). Depending on the clusters, the ionization reaction with nitrate-ion clusters in the ion-molecule reaction zone might be suppressed. Accordingly, cluster bound H 2 SO 4 is (depending on the clusters) not quantitatively detected – but: CIMS is for measurement of non-clustered H 2 SO 4, yet, the question remains which form of H 2 SO 4 prevails in the atmosphere Production of artifact H 2 SO 4 in the CIMS system: Impurities in the used process gases, e.g. t/c-2-pentene in propane, might react with ambient ozone and produce Criegee radicals which then can form H 2 SO 4 from ambient SO 2. Another issue is impurities in the nitric acid added to the sheath gas. Malfunction of valves and flow controllers, plugged injectors … are other sources of problems which are often hard to detect –> multiple problems hard to resolve Sulfuric Acid Measurements by CIMS – Uncertainties and Consistency between Various Data Sets C. PLASS-DÜLMER 1, T. ELSTE 1, P. PAASONEN 2, and T. PETÄJÄ 2 1 Hohenpeissenberg Meteorological Observatory, Deutscher Wetterdienst, Germany, and 2 Department of Physics, University of Helsinki, Finland To check for consistency between different data sets, the calculated H 2 SO 4 from balance equations (assuming stationary state) can be compared to the measured data: d/dt [H 2 SO 4 ] = k OH [OH] [SO 2 ] – CS [H 2 SO 4 ] => [H 2 SO 4 ] = k OH [OH] [SO 2 ] / CS This approach compares measured H 2 SO 4 with calculated H 2 SO 4 from OH measured by the same CIMS, thus, uncertainties are substantially reduced. Data from the DWD- CIMS from various campaigns and from different CIMS in Hyytiälä are compared. Fig. 2: H 2 SO 4 data obtained by the DWD CIMS during EUCAARI Sulfuric acid calculated from balance is generally lower than measured sulfuric acid (Eisele and Tanner, 2003, Petäjä et al., 2009), on average by factors of 1-2, indicating either an overestimation of the CS or a missing production pathway for sulfuric acid. Fig. 4: Median diurnal cycles of k-UV=[H 2 SO 4 ] meas / ([SO 2 ] UV / CS) with UV=integrated irradiation nm (left) and of k-OH-calc.=[H 2 SO 4 ] meas / ([OH] meas [SO 2 ] / CS); CIMS by UHEL , MPI , DWD  Using the UV radiation instead of measured OH in k-UV (Fig. 4) allows to compare H 2 SO 4-meas with independently measured quantities. At noon-time, k-UV varies on average between and m²/J, e.g. factor 2.5 between min and max (Forest Fire data were excluded for contaminated conditions). The same comparison for k- OH-calc yields (min) and cm³/(molec.s) (max), which is more variable probably due to additional uncertainties in OH measurements. H 2 SO 4 data sets are comparable (median, noon) by better than factor 2 (k-UV) other H 2 SO 4 sources than OH + SO 2 in the morning/evening (Criegee+SO 2 ?) overestimation of condensation sink? (H 2 SO 4 +H2O+… clusters / accomodation) ACKNOWLEDGEMENTS We wish to thank many contributors at DWD, UHEL, Melpitz (IFT), San Pietro Capofiume (CNR), and NCAR for support, data provision and cooperation. Financial support was given by the EUCAARI project. REFERENCES Berresheim et al., Int. J. Mass Spectrom., 202, (2000) 91.Boy et al., Atmos. Chem. Phys. 5, 863, Eisele and Tanner, J. Geophys. Res. 98 (1993) Kerminen et al., Atmos. Chem. Phys., 10, 10829, Kurten et al., Atmos. Chem. Phys. Disc., 10, 30539, Paasonen et al., Atmos. Chem. Phys., 10, 11223, 2010 Petäjä et al., Atmos. Chem. Phys., 9, 7435, DWD - 10/2010 Melpitz Fig. 3: Sulfuric acid concentration calculated versus measured H 2 SO 4 for different CIMS instruments in different campaigns. Upper row by DWD-CIMS: San Pietro Capofiume, Italy Melpitz, Germany Hohenpeissenberg lower row all Hyytiälä by: MPI-Heidelberg CIMS (Boy et al., 2005) UHEL-CIMS NCAR-CIMS oper. by UHEL (Petäjä et al., 2009) San Pietro Capofiume, June/July 2009 Melpitz, May 2008 Hohenpeißenberg, San Pietro Capofiume Melpitz