Abstract
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, Dp <2.5 μm and Dp < 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 NOx but a large fraction of the HONO-to-NOx 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.
Highlights
Concentrations of major inorganic ions in aerosols have been measured for decades by sampling on filters and subsequently analyzing them with ion chromatography (e.g., Mulik et al, 1976; Stevens et al, 1978; Mulik and Sawicki, 1979)
In order to study atmospheric processes, methods have been developed for measuring aerosol chemical composition at a higher time resolution and to avoid the artifacts associated with filter sampling
Sample air was first drawn through the Wet Rotating Denuder (WRD) where water-soluble gases diffused to the absorption solution, particles were collected in a steam-jet aerosol collector (SJAC) (Slanina et al, 2001)
Summary
Concentrations of major inorganic ions in aerosols have been measured for decades by sampling on filters and subsequently analyzing them with ion chromatography (e.g., Mulik et al, 1976; Stevens et al, 1978; Mulik and Sawicki, 1979). Another semicontinuous method that uses a SJAC is the ambient ion monitor (AIM, URG Corporation, USA) that determines the concentrations of several anions and cations in aerosol (e.g., Wu and Wang, 2007; Nie et al, 2010) and exists in four different configurations (URG 9000A–D) that have different performances Another method that is based on the same principle is the Dionex Gas Particle Ion Chromatography (GPIC) system that measures concentrations of water-soluble inorganic aerosol constituents (Cl−, NO−3 , SO24−, and NH+4 ) and their associated precursor gases (HCl, SO2, HNO3, NH3) (Godri et al, 2009). A commercially available MARGA (Applikon Analytical BV, Netherlands) was operated by the Finnish Meteorological Institute Air Quality Research (FMI/AQR) at the urban background station in Kumpula, Helsinki, from November 2009 to May 2010 These were the first semi-continuous measurements of sulphur dioxide (SO2), nitric acid (HNO3), nitrous acid (HNO2 or HONO), hydrochloric acid (HCl) and ammonia (NH3) using a rotating wet annular denuder in combination with an on-line ion chromatograph in Finland. The observations on nitrogen compounds (both in the gas and aerosol phases) are discussed, and the aerosol data are used to study aerosol acidity and its sources
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