Polar organic marker compounds in atmospheric aerosols during the LBA-SMOCC 2002 biomass burning experiment in Rondônia, Brazil: sources and source processes, time series, diel variations and size distributions

Abstract

Abstract. Measurements of polar organic marker compounds were performed on aerosols that were collected at a pasture site in the Amazon basin (Rondônia, Brazil) using a high-volume dichotomous sampler (HVDS) and a Micro-Orifice Uniform Deposit Impactor (MOUDI) within the framework of the 2002 LBA-SMOCC (Large-Scale Biosphere Atmosphere Experiment in Amazônia -Smoke Aerosols, Clouds, Rainfall, and Climate: Aerosols From Biomass Burning Perturb Global and Regional Climate) campaign. The campaign spanned the late dry season (biomass burning), a transition period, and the onset of the wet season (clean conditions). In the present study a more detailed discussion is presented compared to previous reports on the behavior of selected polar marker compounds, including levoglucosan, malic acid, isoprene secondary organic aerosol (SOA) tracers and tracers for fungal spores. The tracer data are discussed taking into account new insights that recently became available into their stability and/or aerosol formation processes. During all three periods, levoglucosan was the most dominant identified organic species in the Correspondence to: M. Claeys (magda.claeys@ua.ac.be) PM 2.5 size fraction of the HVDS samples. In the dry period levoglucosan reached concentrations of up to 7.5 µg m −3 and exhibited diel variations with a nighttime prevalence. It was closely associated with the PM mass in the size-segregated samples and was mainly present in the fine mode, except during the wet period where it peaked in the coarse mode. Isoprene SOA tracers showed an average concentration of 250 ng m −3 during the dry period versus 157 ng m −3 during the transition period and 52 ng m −3 during the wet period. Malic acid and the 2-methyltetrols exhibited a different size distribution pattern, which is consistent with different aerosol formation processes (i.e., gas-to-particle partitioning in the case of malic acid and heterogeneous formation from gasphase precursors in the case of the 2-methyltetrols). The 2-methyltetrols were mainly associated with the fine mode during all periods, while malic acid was prevalent in the fine mode only during the dry and transition periods, and dominant in the coarse mode during the wet period. The sum of the fungal spore tracers arabitol, mannitol, and erythritol in the PM 2.5 fraction of the HVDS samples during the dry, transition, and wet periods was, on average, 54 ng m −3 , 34 ng m −3 , and 27 ng m −3 , respectively, and revealed minor day/night variation. The mass size distributions of arabitol and mannitol during all periods showed similar patterns and Published by Copernicus Publications on behalf of the European Geosciences Union. an association with the coarse mode, consistent with their primary origin. The results show that even under the heavy smoke conditions of the dry period a natural background with contributions from bioaerosols and isoprene SOA can be revealed. The enhancement in isoprene SOA in the dry season is mainly attributed to an increased acidity of the aerosols, increased NO x concen...

Airborne observations of IEPOX-derived isoprene SOA in the Amazon during SAMBBA

“…The role of aerosol acidity has long been seen as necessary for uptake (Surratt et al, 2007b;Eddingsaas et al, 2010) and this conclusion was supported by observational data from this region presented by Claeys et al (2010), although recent laboratory results from Nguyen et al (2014) show that it is possible to form SOA with the m/z = 82 marker from IEPOX on a neutral ammonium sulfate seed, providing it is aqueous, as opposed to solid. This occurs through ammonium ions (NH + 4 ) acting to catalyse the uptake in the absence of a proton donor.…”

“…While all average NO concentrations were below the approximate figure of 100 pptv often given for when NO-peroxy reactions become important (pp 330-331, Faraday Discussions, 2013), without the use of a detailed chemical model (which would be difficult to constrain with the data here), it is difficult to say definitively here whether or not NO x concentrations were a significant factor in determining SOA formation. However, it is worth noting that Claeys et al (2010) found that the isoprene SOA markers in Rondônia were highest during the dry season, corresponding to the highest NO x concentrations, so in this context, it would seem unlikely.…”

“…Calibrations were performed using monodisperse ammonium nitrate and ammonium sulfate to generate ionisation efficiency data for nitrate and relative ionisation efficiencies for ammonium and sulfate. Equivalent volume concentrations incorporating black carbon data (see below) and using the densities recommended by Cross et al (2007) were compared against those from a Scanning Mobility Particle Sizer (SMPS) and the comparison was good (85-90 % of SMPS volume) if a collection efficiency (CE) of 1 was used for most flights during the campaign. Certain flights (in particular B749 and B750) had less favourable volume comparisons (around 50 %), however these took place during higher humidity conditions (see below) and given that the SMPS employed no sheath air drying, its data are seen as an unreliable reflection of the "dry" volume measured by the AMS under these conditions, so a CE of 1 is assumed for all flights for the sake of internal consistency.…”

“…These are formed from both natural and anthropogenic precursors and are subject to many complex and often poorly understood processes that control the formation. Of specific interest in recent years is the formation of SOA from isoprene (Claeys et al, 2004;Carlton et al, 2009;Claeys et al, 2010). Because of isoprene's abundance in the atmosphere, this could represent a significant portion of the SOA budget in certain regions, even if its mass yield is low relative to other natural VOCs such as monoterpenes and sesquiterpenes.…”

Isomer-Selective Study of the OH-Initiated Oxidation of Isoprene in the Presence of O2 and NO: 2. The Major OH Addition Channel

“…10 Finally, the oxidation of isoprene leads to US and global biogenic SOA formation of 50% and 58%, respectively. 11,12,13 The addition of OH to isoprene results in four distinct hydroxyl alkyl isomers (Figure 1), each of which ultimately leads to different first-generation end products. Although there has been no direct experimental determination of the branching between channels, theoretical work has predicted branching of 0.67, 0.02, 0.02, and 0.29, respectively, for isomers I, II, III, and IV, 14 with an overall rate of (1.0 ( 0.1) × 10 -10 molecule -1 cm 3 s -1 .…”