Abstract. Ambient aerosol size distributions (> 3 nm) and OH, H 2 SO 4 , and terpene concentrations were measured from April 1998 to August 2000 at a rural continental site in southern Germany. New particle formation (NPF) events were detected on 18% of all days, typically during midday hours under sunny and dry conditions. The number of newly formed particles correlated significantly with solar irradiance and ambient levels of H 2 SO 4 . A pronounced anticorrelatation of NPF events with the pre-existing particle surface area was identified in the cold season, often associated with the advection of dry and relatively clean air masses from southerly directions (Alps). Estimates of the particle formation rate based on observations were around 1 cm −3 s −1 , being in agreement with the predictions of ternary homogeneous H 2 SO 4 -NH 3 -H 2 O nucleation within a few orders of magnitude. The experimentally determined nucleation mode particle growth rates were on average 2.6 nm h −1 , with a fraction of 0.7 nm h −1 being attributed to the cocondensation of H 2 SO 4 -H 2 O-NH 3 . The magnitude of nucleation mode particle growth was neither significantly correlated to H 2 SO 4 , nor to the observed particle formation rate. Turn-over rate calculations of measured monoterpenes and aromatic hydrocarbons suggest that especially the oxidation products of monoterpenes have the capacity to contribute to the growth of nucleation mode particles. Although a large number of precursor gases, aerosol and meteorological parameters were measured, the ultimate key factors controlling the occurence of NPF events could not be identified.

[1] Atmospheric concentrations of gaseous sulfuric acid (H 2 SO 4 ), methane sulfonic acid (MSA), and hydroxyl radicals (OH) were measured by chemical ionization mass spectrometry (CIMS) during the second New Particle Formation and Fate in the Coastal Environment (PARFORCE) campaign in June 1999 at Mace Head, Ireland. Overall median concentrations in marine background air were 1.5, 1.2, and 0.12 ϫ 10 6 cm Ϫ3 , respectively. H 2 SO 4 was also present at night indicating significant contributions from nonphotochemical sources. A strong correlation was found between daytime OH and H 2 SO 4 levels in clean marine air suggesting a fast local production of H 2 SO 4 from sulfur precursor gases. Steady state balance calculations of ambient H 2 SO 4 levels agreed with measured concentrations if either very low H 2 SO 4 sticking coefficients (0.02-0.03) or sources in addition to the SO 2 ϩ OH reaction were assumed. Overall, variations in ambient H 2 SO 4 levels showed no correlation with either the tidal cycle or ultrafine particle (UFP) concentrations. However, on particular days an anticorrelation between H 2 SO 4 and UFP levels was occasionally observed providing evidence for the contribution of H 2 SO 4 to new particle formation and/or particle growth. Gaseous MSA concentrations were inversely correlated with dew point temperature reflecting a highly sensitive gas-particle partitioning equilibrium of this compound. The present observations seriously question the general use of MSA as a conservative tracer to infer the relative production yield of H 2 SO 4 from dimethylsulfide (DMS) oxidation. MSA/H 2 SO 4 concentration ratios typically ranged between 0.06 and 1.0 in marine air at ground level. Measured diel OH profiles showed a significant deviation from concurrent variations of the ozone photolysis frequency. They also showed up to 1 order of magnitude lower values compared to OH concentrations calculated with a simple photochemical box model. These differences were most pronounced during particle nucleation events occurring on sunny days around noon and at low tide. The present results suggest that both the oxidation capacity and the particle formation potential in the coastal boundary layer were significantly affected by reactions of unknown compounds prevailing in this type of environment.

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Abstract. Volcanic emissions from the Eyjafjallajökull vol-

Abstract. Hydroxyl radicals (OH) are the major oxidizing species in the troposphere. Because of their central importance, absolute measurements of their concentrations are needed to validate chemical mechanisms of atmospheric models. The extremely low and highly variable concentrations in the troposphere, however, make measurements of OH difficult. Three techniques are currently used worldwide for tropospheric observations of OH after about 30 years of technical developments: Differential Optical Laser Absorption Spectroscopy (DOAS), Laser-Induced Fluorescence Spectroscopy (LIF), and Chemical Ionisation Mass Spectrometry (CIMS). Even though many measurement campaigns with OH data were published, the question of accuracy and precision is still under discussion.Here, we report results of the first formal, blind intercomparison of these techniques. Six OH instruments (4 LIF, 1 CIMS, 1 DOAS) participated successfully in the ground-based, international HOxComp campaign carried out in Jülich, Germany, in summer 2005. Comparisons were performed for three days in ambient air (3 LIF, 1 CIMS) and for six days in the atmosphere simulation chamber SAPHIR (3 LIF, 1 DOAS). All instruments were found to measure tropospheric OH concentrations with high sensitivity and good time resolution. The pairwise correlations between different data sets were linear and yielded high correlation coefficients (r 2 =0.75−0.96). Excellent absolute agreement wasCorrespondence to: H.-P. Dorn (h.p.dorn@fz-juelich.de) observed for the instruments at the SAPHIR chamber, yielding slopes between 1.01 and 1.13 in the linear regressions. In ambient air, the slopes deviated from unity by factors of 1.06 to 1.69, which can partly be explained by the stated instrumental accuracies. In addition, sampling inhomogeneities and calibration problems have apparently contributed to the discrepancies. The absolute intercepts of the linear regressions did not exceed 0.6×10 6 cm −3 , mostly being insignificant and of minor importance for daytime observations of OH. No relevant interferences with respect to ozone, water vapour, NO x and peroxy radicals could be detected. The HOxComp campaign has demonstrated that OH can be measured reasonably well by current instruments, but also that there is still room for improvement of calibrations.