Abstract.As demonstrated in a number of investigations, gaseous sulfuric acid plays a central role in atmospheric aerosol formation. Using chemical ionization mass spectrometer the gas-phase sulfuric acid and OH concentration were measured in Hyytiälä, SMEAR II station, Southern Finland during 24 March to 28 June 2007. Clear diurnal cycles were observed as well as differences between new particle formation event days and non-event days. Typically, the daily maximum concentrations of gas phase sulfuric acid varied from 3×10 5 to 2×10 6 molec cm −3 between non-event and event days. Noon-time OH concentrations varied from 3-6×10 5 molec cm −3 and not a clear difference between event and non-events was detected. The measured time series were also used as a foundation to develop reasonable proxies for sulfuric acid concentration. The proxies utilized source and sink terms, and the simplest proxy is radiation times sulfur dioxide divided by condensation sink. Since it is still challenging to measure sulfuric acid in ambient concentrations, and due to its significant role in atmospheric particle formation, reasonable proxies are needed. We use all together three different proxies and one chemical box model and compared their results to the measured data. The proxies for the sulfuric acid concentration worked reasonably well, and will be used to describe sulfuric acid concentrations in SMEAR II station, when no measured sulfuric acid data is available. With caution the proxies could be applied to other environments as well.

[1] Mass identified ion cluster distributions were measured under ambient atmospheric conditions and compared with model predictions based on laboratory ion cluster thermodynamics data. The results are shown from several days where atmospheric sulfur concentrations were high and thus ion-induced cluster growth was anticipated. Atmospheric gas phase sulfuric acid, temperature, relative humidity, SO 2 , mobility distributions of ions and small charged particles, and aerosol size distributions were also measured in support of the model calculations. The relative agreement of measurement and model for the first and second sulfuric acid clusters (HSO 4 À (H 2 SO 4 ) m ) for m = 1 and 2 is quite good but suggests that sulfuric acid clustering may not occur at the collision rate. Clusters for higher m values were not observed, which is also consistent with model predictions for the conditions under which measurements were performed. The lack of both observed and predicted large ion clusters is also consistent with the independent measurements of ion mobility distributions and particle size distributions, which showed similar numbers of positively and negatively charged ultrafine particles, suggesting that neither positive nor negative ion-induced nucleation processes were likely to have contributed significantly to observed new particle formation rates during this study. The relatively low observed concentrations of the bisulfate ion also suggest that the processes leading to the first sulfuric acid/bisulfate cluster (HSO 4 À H 2 SO 4 ) may be more complicated than simple sulfuric acid clustering or exchange reactions. While nucleation was observed on some days, measurements suggest that ion-induced nucleation did not contribute significantly to new particle production or growth during these events. This does not rule out the possibility that ion-induced nucleation could contribute significantly to atmospheric new particle formation under very different atmosphere conditions such as in areas with much lower temperatures and higher ion concentrations.

[1] HONO was measured by a LOPAP instrument (LOng Path Absorption Photometer) for one month during the OASIS spring 2009 campaign in Barrow, Alaska. HONO concentrations between ≤ 0.4 pptv (DL) and ∼500 pptv were measured. The very high concentrations observed on several days were caused by local direct emissions and were highly correlated with the NO x and CO data. When only "clean days" were considered, average HONO concentrations varied between ≤ 0.4 -10 pptv. Average HONO/NO x and HONO/NO y ratios of ∼6% and ∼1% were observed, respectively, in good agreement with other remote LOPAP measurement data, but lower than measured in most other polar regions by other methods. The strong correlation between sharp peaks of OH and HONO during daytime, which was not observed for any other measured radical precursor, suggested that HONO photolysis was a major source of OH radicals in Barrow. This was supported by calculated net OH radical production by HONO and O 3 photolysis for which the contribution of O 3 (2%) could be neglected compared to that of HONO (98%). A net extra HONO/OH source necessary to explain elevated HONO levels during daytime of up to 90 pptv/h was determined, which was highly correlated with the actinic flux. Accordingly, a photochemical HONO source is proposed here, in good agreement with recent studies.

The first measurements of OH, H2SO4, and MSA performed at the South Pole as part of the Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) study are presented. OH concentrations were found to be quite elevated for such a dry environment, with average values of 2x106 molecule cm−3. Model simulations suggest that much of the observed OH is a result of unexpectedly high NO concentrations. Concentrations of H2SO4 and MSA were generally low with average values of 2.5x105 and 1x105 molecule cm−3, respectively. Major variations in the concentration levels of the above species were found to have a high correlation with changes in the polar mixing layer as estimated from the measured temperature difference from 22 to 2m above the snow surface. Chemical details are discussed.

Laboratory characterizations of the peroxy radical chemical ionization mass spectrometer (PerCIMS) instrument have been performed. The instrument functions by drawing ambient air through a 50-microm-diameter orifice into an inlet held at low pressure. Peroxy radicals (HO2 and RO2) within this air are detected by amplified chemical conversion into a unique ion (HSO4-) via the chemistry initiated by the addition of NO and SO2 to the inlet. HSO4- ions are then quantified by a quadrupole filter mass spectrometer. PerCIMS provides measurements of the sum of peroxy radicals, HO2 + RO2 (HOxROx mode), or the HO2 component only (HO2 mode), achieved through the control of concentration of NO and SO2 added to the instrument. The characterization and response of this instrument have been evaluated through modeling of inlet chemistry and laboratory experiments and have also been demonstrated through successful deployment during field campaigns. The performance of PerCIMS with respect to calibration pressure and relative humidity is reported, as are the sensitivities of the instrument to organic peroxy radicals with different hydrocarbon groups. These data show PerCIMS to be a practical field instrument for the fast and accurate evaluation of the concentration of peroxy radicals over a variety of atmospheric conditions. The estimated accuracy of the derived [HOxROx] concentrations is +/- 35% (at the 95% confidence interval), while [HO2] measurements have accuracies of +/- 41% (at the 95% confidence interval). Typical precision of measurements well above the detection limit is 10%, and typical detection limits are 1 x 10(7) radicals cm(-3) for 15-s averaging times.