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

Abstract. Concentrations of 5 gases (HCl, HNO₃, HONO, NH₃, SO₂) and 8 major inorganic ions in particles (Cl⁻, NO₃⁻, SO₄²⁻, NH₄⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺) were measured with an online monitor MARGA 2S in two size ranges, D_p <2.5 μm and D_p < 10 μm, in Helsinki, Finland from November 2009 to May 2010. The results were compared with filter sampling, mass concentrations obtained from particle number size distributions, and a conventional SO₂ monitor. The MARGA yielded lower concentrations than those analyzed from the filter samples for most ions. Linear regression yielded the following MARGA vs. filter slopes: 0.72 for Cl⁻, 0.90 for NO₃⁻, 0.85 for SO₄²⁻, 0.91 for NH₄⁺ , 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 not enough data points to calculate a statistically significant linear regression. There were clear seasonal cycles in the concentrations of the nitrogen-containing gases: the median concentrations of HNO₃, HONO, and NH₃ were 0.09 ppb, 0.37 ppb, and 0.01 ppb in winter, respectively, and 0.15, 0.15, and 0.14 in spring, respectively. The gas-phase fraction of nitrogen decreased roughly with decreasing temperature, so that in the coldest period from January to February the median contribution was 28% but in April to May was 53%. There were also large fractionation variations that temperature alone cannot explain. HONO correlated well with NO_x but a large fraction of the HONO-to-NO_x ratios were larger than published ratios in a road traffic tunnel, suggesting that a large amount of HONO had other sources than vehicle exhaust. Aerosol acidity was estimated by calculating ion equivalent ratios. The sources of acidic aerosols were studied with trajectory statistics that showed that continental aerosol is mainly neutralized and marine aerosol acidic.