Development of a sensitive long path absorption photometer to quantify peroxides in aerosol particles (Peroxide-LOPAP)

Atmos. Meas. Tech., 5, 2339–2348, 2012 www.atmos-meas-tech.net/5/2339/2012/ doi:10.5194/amt-5-2339-2012 © Author(s) 2012. CC Attribution 3.0 License. Atmospheric Measurement Techniques Development of a sensitive long path absorption photometer to quantify peroxides in aerosol particles (Peroxide-LOPAP) P. Mertes1 , L. Pfaffenberger1 , J. Dommen1 , M. Kalberer2 , and U. Baltensperger1 1 Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, Villigen, Switzerland 2 Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, UK Correspondence to: J. Dommen () Received: 16 January 2012 – Published in Atmos. Meas. Tech. Discuss.: 13 February 2012 Revised: 25 July 2012 – Accepted: 31 July 2012 – Published: 2 October 2012 Abstract. A new off-line instrument to quantify peroxides in aerosol particles using iodometry in long path absorption spectroscopy has been developed and is called peroxide long path absorption photometer (Peroxide-LOPAP). The new analytical setup features important technical innovations compared to hitherto published iodometric peroxide measurements. Firstly, the extraction, chemical conversion and measurement of the aerosol samples are performed in a closed oxygen-free (∼ 1 ppb) environment. Secondly, a 50-cm optical detection cell is used for an increased photometric sensitivity. The limit of detection was 0.1 µM peroxide in solution or 0.25 nmol m−3 with respect to an aerosol sample volume of 1 m3 . The test reaction was done at a constant elevated temperature of 40 ◦ C and the reaction time was 60 min. Calibration experiments showed that the test reaction with all reactive peroxides, i.e. hydrogen peroxide (H2 O2 ), peracids and peroxides with vicinal carbonyl groups (e.g. lauroyl peroxide) goes to completion and their sensitivity (slope of calibration curve) varies by only ±5 %. However, very inert peroxides have a lower sensitivity. For example, tert-butyl hydroperoxide shows only 37 % sensitivity compared to H2 O2 after 1 h. A kinetic study revealed that even after 5 h only 85 % of this inert compound had reacted. The time trends of the peroxide content in secondary organic aerosol (SOA) from the ozonolysis and photooxidation of α-pinene in smog chamber experiments were measured. The highest mass fraction of peroxides with 34 % (assuming a molecular weight of 300 g mol−1 ) was found in freshly generated SOA from α-pinene ozonolysis. Mass fractions decreased with increasing NO levels in the photooxidation experiments. A decrease of the peroxide content was also observed with aging of the aerosol, indicating a decomposition of peroxides in the particles. 1 Introduction Beside their decisive role in atmospheric processes ambient fine and ultrafine particles have also an important impact on human health, predominantly on respiratory and cardiovascular systems (Pope and Dockery, 2006; Pope et al., 2009). Up to about 70 % of these ambient particles are composed of organic material (Jimenez et al., 2009). However, the highly complex organic mixture is chemically still poorly characterized (Hallquist et al., 2009). The reaction of volatile organic compounds with ozone and OH radicals in the polluted troposphere generates a variety of oxygenated organic compounds like aldehydes, ketones, carboxylic acids, nitrates and organic hydroperoxides of low volatility, which can partition into aerosols (Atkinson, 2000; Atkinson and Arey, 2003; Kroll and Seinfeld, 2008). Organic hydroperoxides are generated in the atmosphere in three different ways: (1) by gas phase reactions of HO2 radicals with organic peroxy radicals RO2 (e.g. Atkinson, 2000), (2) via the reaction of water with “Criegee intermediates” from alkene ozonolysis (e.g. Hasson et al., 2001) and (3) by aqueous-phase photochemical reactions in atmospheric water (e.g. Faust et al., 1993). Depending on the volatility and solubility of these peroxides they partition more or less into the particle phase. Model simulations predict organic hydroperoxides to be major contributors of secondary organic aerosol mass (Bonn et al., 2004; Johnson et al., 2004). The contribution of peroxides to health risk is assumed to be important because of their high reactivity and oxidation potential (Morio et al., 2001). Besides the interest in the formation and chemical composition of aerosols this is another important motivation to analyse the content of hydrogen peroxide and organic peroxides in secondary organic aerosol (SOA). Published by Copernicus Publications on behalf of the European Geosciences Union. 2340 P. Mertes et al.: Development of a sensitive long path absorption photometer Along with several methods to analyse peroxides, the most widely used techniques are based on electrochemical reduction (e.g. Qi and Baldwin, 1993) and fluorometric detection of a fluorescing dimer, which is produced from the derivatization of a peroxide catalysed by the enzyme horseradish peroxidase (e.g. Lazrus et al., 1985; Kok et al., 1986; Wang and Glaze, 1998). However, the relevant drawbacks of these detection methods are different sensitivities for different peroxide species as well as the missing accessibility for dialkyl peroxides, which need to be hydrolysed first. Moreover, an aqueous solvent is required to sustain the catalytic activity of the enzyme. This hinders the quantitative extraction of less water soluble peroxides from the particles. We used iodometry for quantification of the total peroxide content (Banerjee and Budke, 1964). This method was already applied in other studies to measure the peroxide content of aerosols (Docherty et al., 2005; Ziemann, 2005; Surratt et al., 2006; Nguyen et al., 2010). Herein peroxy-groups oxidize iodide ions (I− ) to molecular iodine (I2 ) in solution which subsequently forms yellow coloured triiodide ions (I− 3 ). However, molecular oxygen reacts in the same way with iodide ions, which is a major drawback of this method. We minimised the influence of oxygen by constructing a closed oxygen-free instrument. Furthermore, due to insensitive spectrophotometry with 1-cm cuvettes, the experiments in former studies had to be performed at very high aerosol concentrations and the time resolution was rather low (2–4 h) (e.g. Surratt et al., 2006). Peroxide mass fractions from 20 % to over 100 % were reported in SOA from various precursors assuming a molecular weight of 300 g mol−1 for peroxides (Docherty et al., 2005; Ziemann, 2005; Surratt et al., 2006; Nguyen et al., 2010). To enhance the sensitivity we developed a long path absorption spectroscopy system. According to Lambert-Beer’s law, the sensitivity of spectrophotometry can be enhanced by increasing the optical path length. However, this requires that the background from the reagents can be kept at a low level. This sensitive analytical method has already been successfully applied in different colorimetric detection studies (e.g. Yao et al., 1998; Heland et al., 2001; Callahan et al., 2002). In analogy t (...truncated)