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

摘要

Measurements of polar organic marker compounds were performed on aerosolsthat were collected at a pasture site in the Amazon basin (Rondônia,Brazil) using a high-volume dichotomous sampler (HVDS) and a Micro-OrificeUniform Deposit Impactor (MOUDI) within the framework of the 2002 LBA-SMOCC(Large-Scale Biosphere Atmosphere Experiment in Amazônia – SmokeAerosols, Clouds, Rainfall, and Climate: Aerosols From Biomass BurningPerturb Global and Regional Climate) campaign. The campaign spanned the late dryseason (biomass burning), a transition period, and the onset of the wet season (clean conditions). In the present study a moredetailed discussion is presented compared to previous reports on thebehavior of selected polar marker compounds, including levoglucosan, malicacid, isoprene secondary organic aerosol (SOA) tracers and tracers forfungal spores. The tracer data are discussed taking into account newinsights that recently became available into their stability and/or aerosolformation processes. During all three periods, levoglucosan was the mostdominant identified organic species in the PM size fraction of theHVDS samples. In the dry period levoglucosan reached concentrations of up to7.5 μg m and exhibited diel variations with a nighttimeprevalence. It was closely associated with the PM mass in thesize-segregated samples and was mainly present in the fine mode, exceptduring the wet period where it peaked in the coarse mode. Isoprene SOAtracers showed an average concentration of 250 ng m during the dryperiod versus 157 ng m during the transition period and 52 ng mduring the wet period. Malic acid and the 2-methyltetrolsexhibited a different size distribution pattern, which is consistent withdifferent aerosol formation processes (i.e., gas-to-particle partitioning inthe case of malic acid and heterogeneous formation from gas-phase precursorsin the case of the 2-methyltetrols). The 2-methyltetrols were mainlyassociated with the fine mode during all periods, while malic acid wasprevalent in the fine mode only during the dry and transition periods, anddominant in the coarse mode during the wet period. The sum of the fungalspore tracers arabitol, mannitol, and erythritol in the PM fractionof the HVDS samples during the dry, transition, and wet periods was, onaverage, 54 ng m, 34 ng m, and 27 ng m, respectively,and revealed minor day/night variation. The mass size distributions ofarabitol and mannitol during all periods showed similar patterns and anassociation with the coarse mode, consistent with their primary origin. Theresults show that even under the heavy smoke conditions of the dry period anatural background with contributions from bioaerosols and isoprene SOA canbe revealed. The enhancement in isoprene SOA in the dry season is mainlyattributed to an increased acidity of the aerosols, increased NOconcentrations and a decreased wet deposition.