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Concentrations of 5 gases (HCl, HNO3, HONO, NH3, SO2) and 8major inorganic ions in particles (Cl−, NO3−,SO42−, NH4+, Na+, K+, Mg2+, Ca2+)were measured with an online monitor MARGA 2S in two size ranges, Dp<2.5 μm and Dp < 10 μm, in Helsinki,Finland from November 2009 to May 2010. The results were compared withfilter sampling, mass concentrations obtained from particle number sizedistributions, and a conventional SO2 monitor. The MARGA yielded lowerconcentrations than those analyzed from the filter samples for most ions.Linear regression yielded the following MARGA vs. filter slopes: 0.72 forCl−, 0.90 for NO3−, 0.85 for SO42−, 0.91 forNH4+ , 0.49 for Na+, 3.0 for Mg2+, and 3.0 for Ca2+and 0.90 for the MARGA vs. SO2 monitor. For K+ there were notenough data points to calculate a statistically significant linearregression. There were clear seasonal cycles in the concentrations of thenitrogen-containing gases: the median concentrations of HNO3, HONO, andNH3 were 0.09 ppb, 0.37 ppb, and 0.01 ppb in winter, respectively, and0.15, 0.15, and 0.14 in spring, respectively. The gas-phase fraction ofnitrogen decreased roughly with decreasing temperature, so that in thecoldest period from January to February the median contribution was 28%but in April to May was 53%. There were also large fractionation variationsthat temperature alone cannot explain. HONO correlated well with NOxbut a large fraction of the HONO-to-NOx ratios were larger thanpublished ratios in a road traffic tunnel, suggesting that a large amount ofHONO had other sources than vehicle exhaust. Aerosol acidity was estimatedby calculating ion equivalent ratios. The sources of acidic aerosols werestudied with trajectory statistics that showed that continental aerosol ismainly neutralized and marine aerosol acidic

atmospheric chemistry and physics

Semi-continuous gas and inorganic aerosol measurements at a Finnish urban site: comparisons with filters, nitrogen in aerosol and gas phases, and aerosol acidity