Abstract
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 (H2O2), 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 H2O2 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 photo-oxidation 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 photo-oxidation experiments. A decrease of the peroxide content was also observed with aging of the aerosol, indicating a decomposition of peroxides in the particles.
Highlights
Beside their decisive role in atmospheric processes ambient fine and ultrafine particles have an important impact on human health, predominantly on respiratory and cardiovascular systems (Pope and Dockery, 2006; Pope 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)
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)
Summary
Beside their decisive role in atmospheric processes ambient fine and ultrafine particles have an important impact on human health, predominantly on respiratory and cardiovascular systems (Pope and Dockery, 2006; Pope et al, 2009). 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). According to Lambert-Beer’s law, the sensitivity of spectrophotometry can be enhanced by increasing the optical path length This requires that the background from the reagents can be kept at a low level. In analogy to the original long path absorption photometer (LOPAP) which was developed for HONO measurements (Kleffmann et al, 2002), we call the instrument Peroxide-LOPAP
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