Abstract
Abstract. A method is presented to quantify the low-molecular-weight organic acids such as formic, acetic, propionic, butyric, pyruvic, glycolic, oxalic, malonic, succinic, malic, glutaric, and methanesulfonic acid in the atmospheric gas and particle phases, based on a combination of the Monitor for AeRosols and Gases in ambient Air (MARGA) and an additional ion chromatography (Compact IC) instrument. Therefore, every second hourly integrated MARGA gas and particle samples were collected and analyzed by the Compact IC, resulting in 12 values per day for each phase. A proper separation of the organic target acids was initially tackled by a laboratory IC optimization study, testing different separation columns, eluent compositions and eluent flow rates for both isocratic and gradient elution. Satisfactory resolution of all compounds was achieved using a gradient system with two coupled anion-exchange separation columns. Online pre-concentration with an enrichment factor of approximately 400 was achieved by solid-phase extraction consisting of a methacrylate-polymer-based sorbent with quaternary ammonium groups. The limits of detection of the method range between 0.5 ng m−3 for malonate and 17.4 ng m−3 for glutarate. Precisions are below 1.0 %, except for glycolate (2.9 %) and succinate (1.0 %). Comparisons of inorganic anions measured at the TROPOS research site in Melpitz, Germany, by the original MARGA and the additional Compact IC are in agreement with each other (R2 = 0.95–0.99). Organic acid concentrations from May 2017 as an example period are presented. Monocarboxylic acids were dominant in the gas phase with mean concentrations of 306 ng m−3 for acetic acid, followed by formic (199 ng m−3), propionic (83 ng m−3), pyruvic (76 ng m−3), butyric (34 ng m−3) and glycolic acid (32 ng m−3). Particulate glycolate, oxalate and methanesulfonate were quantified with mean concentrations of 26, 31 and 30 ng m−3, respectively. Elevated concentrations of gas-phase formic acid and particulate oxalate in the late afternoon indicate photochemical formation as a source.
Highlights
Low-molecular-weight organic acids have been measured in the gas (Lee et al, 2009; Bao et al, 2012) and particle phases (Boreddy et al, 2017; Miyazaki et al, 2014; van Pinxteren et al, 2014) as well as in precipitation and cloud water (Sun et al, 2016; van Pinxteren et al, 2005)
The present study describes the instrumental development of an online-coupled pre-concentration and ion chromatography (IC) separation system to determine organic acids in the gas and particle phases as an extension of the MARGA
Regarding the MARGA system, these liquid concentrations would correspond to the mass concentrations of 250, 125 and 25 ng m−3, respectively
Summary
Low-molecular-weight organic acids have been measured in the gas (Lee et al, 2009; Bao et al, 2012) and particle phases (Boreddy et al, 2017; Miyazaki et al, 2014; van Pinxteren et al, 2014) as well as in precipitation and cloud water (Sun et al, 2016; van Pinxteren et al, 2005). There are still unknown sources of these short-chained compounds (Millet et al, 2015; Stavrakou et al, 2012). Because of their hygroscopicity (Kawamura and Bikkina, 2016), the organic acids contribute to the acidity of precipitation, dew, fog and clouds (Lee et al, 2009; van Pinxteren et al, 2016). Stieger et al.: MARGA upgrade for organic acid quantification and wet deposition in remote areas, where they can have an influence on the sensitive ecosystem (Friedman et al, 2017; Himanen et al, 2012; Sabbioni et al, 2003)
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