Seasonal variations of monosaccharide anhydrides in PM1 and PM2.5 aerosol in urban areas

Abstract

The concentrations of monosaccharide anhydrides (levoglucosan, mannosan, galactosan) in PM1 and PM2.5 aerosol samples were measured in Brno and Šlapanice in the Czech Republic in winter and summer 2009. 56 aerosol samples were collected together at both sites to investigate the different sources that contribute to aerosol composition in studied localities. Daily PM1 and PM2.5 aerosol samples were collected on pre-fired quartz fibre filters.

The sum of average atmospheric concentration of levoglucosan, mannosan and galactosan in PM1 aerosol in Šlapanice and Brno during winter was 513 and 273 ng m⁻³, while in summer the sum of average atmospheric concentration of monosaccharide anhydrides (MAs) was 42 and 38 ng m⁻³, respectively. The sum of average atmospheric concentration of MAs in PM1 aerosol formed 71 and 63% of the sum of MA concentration in PM2.5 aerosol collected in winter in Šlapanice and Brno, whereas in summer the sum of average atmospheric concentration of MAs in PM1 aerosol formed 45 and 43% of the sum of MA concentration in PM2.5 aerosol in Šlapanice and Brno, respectively.

In winter, the sum of MAs contributed significantly to PM1 mass ranging between 1.37% and 2.67% of PM1 mass (Brno – Šlapanice), while in summer the contribution of the sum of MAs was smaller (0.28–0.32%). Contribution of the sum of MAs to PM2.5 mass is similar both in winter (1.37–2.71%) and summer (0.44–0.55%).

The higher concentrations of monosaccharide anhydrides in aerosols in Šlapanice indicate higher biomass combustion in this location than in Brno during winter season. The comparison of levoglucosan concentration in PM1 and PM2.5 aerosol shows prevailing presence of levoglucosan in PM1 aerosol both in winter (72% on average) and summer (60% on average).

The aerosol samples collected in Šlapanice and Brno in winter and summer show comparable contributions of levoglucosan, mannosan and galactosan to the total amount of monosaccharide anhydrides in both aerosol size fractions. Levoglucosan was the most abundant monosaccharide anhydride with a relative average contribution to the total amount of MAs in the range of 71–82% for PM1 aerosols and 52–79% for PM2.5 aerosols.

Introduction

Atmospheric aerosols play an important role in atmospheric chemistry (Seinfeld and Pandis, 1998, Finlayson-Pitts and Pitts, 2000). They disperse and absorb solar radiation, create cloud nuclei and influence visibility. A number of recent epidemiological studies showed an association between high concentration of aerosol particles and adverse health effects (Pope et al., 1995, de Hartog et al., 2003, Dockery and Stone, 2007).

The effects of atmospheric aerosols depend on particle size and on their chemical composition. Aerosols contain huge amounts of both inorganic and organic compounds emitted from various sources. To reveal major aerosol emission sources, a detailed characterization of chemical composition of aerosol particles is requested.

Biomass burning is known as a significant source of aerosol particles in atmosphere. Characterization of chemical composition of aerosol particles with respect to markers of biomass burning is necessary for the assessment of contribution of biomass burning to aerosol composition and for the understanding of the impact of biomass burning products on the global climate as well as on local and regional air quality. Several organic compounds have been utilized for monitoring of biomass burning emissions. For example, monosaccharide anhydrides (levoglucosan, mannosan and galactosan), diterpenoids, triterpenoids, phytosterols, and methoxyphenols have been successfully used as molecular markers for identification of different sources of biomass combustion (Simoneit, 2002, Zdráhal et al., 2002).

Levoglucosan (1,6-anhydro-β-d-glucopyranose) is a product of cellulose combustion (Simoneit, 2002). Combustion of other materials (e.g., fossil fuels) or biodegradation and hydrolysis of cellulose do not produce levoglucosan. Levoglucosan is relatively stable in the atmosphere, showing no decay over 10 days in acidic conditions, similar to those of atmospheric liquid droplets (Schkolnik and Rudich, 2006). However, recent papers indicate that the atmospheric stability of levoglucosan in aerosols may be rather small under certain conditions, for example levoglucosan decays when exposed to hydroxyl radicals (Hennigan et al., 2010, Hoffmann et al., 2010). Regardless, levoglucosan is utilized as a specific molecular marker for the presence of emissions from a biomass burning source containing cellulose in atmospheric particulate matter. Levoglucosan is accompanied in atmospheric aerosols with other stereoisomeric monosaccharide anhydrides, mannosan (1,6-anhydro-β-d-mannopyranose) and galactosan (1,6-anhydro-β-d-galactopyranose) that result from the pyrolysis of hemicellulose. Emitted amounts of mannosan and galactosan are substantially lower than those of levoglucosan (e.g., Zdráhal et al., 2002).

Monosaccharide anhydrides are analysed mostly by gas chromatography coupled with mass spectrometry (GC/MS) (e.g., Zdráhal et al., 2002, Pashynska et al., 2002, Jordan et al., 2006, Medeiros and Simoneit, 2007, Hsu et al., 2007). Monosaccharide anhydrides are polar compounds and they have to be derivatized to form more volatile compounds prior to their analysis by GC/MS. N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA) containing trimethylchlorosilane (TMCS) (e.g., Zdráhal et al., 2002, Medeiros and Simoneit, 2007, Hsu et al., 2007) and N-methyl-N-trimethylsilyl trifluoroacetamide (MSTFA) with TMCS (e.g., Zdráhal et al., 2002, Pashynska et al., 2002, Hsu et al., 2007) are mostly used as silylation reagents. Levoglucosan has been also analysed by high-performance liquid chromatography coupled with mass spectrometry (LC/MS) (e.g., Dye and Yttri, 2005, Wan and Yu, 2007), anion-exchange high-performance liquid chromatography with pulsed amperometric detection (PAD) (e.g., Engling et al., 2006, Caseiro et al., 2007, Iinuma et al., 2009) or by microchip capillary electrophoresis with PAD (Garcia et al., 2005) and by ion-exclusion chromatography with photodiode array detection at 194 nm (Schkolnik et al., 2005).

This paper presents the comparison of concentrations of monosaccharide anhydrides (MAs) in urban aerosols in a large city (Brno) and a small town (Šlapanice) in the Czech Republic. The seasonal variations as well as concentration differences of MAs in PM1 and PM2.5 aerosols were studied. The contribution of biomass burning to aerosol composition and emission sources of aerosols containing monosaccharide anhydrides in both localities is discussed.