3.4. Isoprene SOA marker, polyol, and monosaccharide concentrations
Table 1 presents the median and mean concentrations of the selected isoprene SOA marker compounds, polyols, and monosaccharidic compounds. All together the isoprene SOA marker compounds contributed to 0.11% and 0.12% of the OC in 2012 and 2013, which is lower than the percentages obtained for the anhydrosugars (i.e., 2.0% and 2.2%). It is worth noting that the median concentration of 2MG is higher (8.6 ng m3 and 15.8 ng m3 in 2012 and 2013) than the medians obtained for the other major isoprene SOA markers, i.e., 2MT (2.0 ng m3 and 1.93 ng m3) and 2ME (4.9 ng m3 and 5.2 ng m3). This could be explained by the fact that 2MG is preferentially formed under polluted conditions in the
presence of NOx, while 2MT and 2ME are formed under clean (lowNOx) conditions (Surratt et al., 2006). Also, the C5-alkene triols showed very low concentrations, as could be expected since they are preferentially formed under clean (low-NOx) conditions (Surratt et al., 2006). The values obtained for the 2-methyltetrols 2MT and 2ME can be compared with values obtained for Rondonia at a pasture site during a 2002 winter episode, where the median concentrations of 2MT and 2ME were estimated at 51 ng m3 and 145 ng m3 (Claeys et al., 2010). One of the possible reasons for the higher values in Rondonia are the higher isoprene emissions from tropical rainforest vegetation.