Molecular Characterization of Atmospheric Organic Particles Collected in the Amazonia

Description

We will show results and preliminary conclusions of the molecular-level analysis of organic particles collected using multi-stage MOUDI impactors deployed in the Amazonia at T3 and ZF2 field sites. The T3 site is directly influenced by the urban emissions from Manaus, and the ZF2 is a pristine jungle site. The samples were analyzed by nanospray-desorption electrospray ionization mass spectrometry with high mass resolution (m/Δm = 100,000). The mass spectra from the MOUDI sampler stages 7-8 (0.18-0.56 µm, accumulation mode aerosol) in both samples display broad envelopes of oligomeric biogenic secondary organic aerosol (SOA), likely formed from isoprene. The spectra are very similar to those observed in our previous smog chamber experiments at UCI on isoprene photooxidation. The mass spectra from MOUDI stages 3-4, which collect larger particles (1.8-5.6 µm), are different and likely correspond to primary aerosol material emitted directly by plants (the observation is also supported by microscopy and micro-spectroscopy studies showing characteristic images and chemical signatures of biological particles). The mass spectra on MOUDI stage 10, which collects the smallest particles (56-100 nm), show very abundant appearances of additional anthropogenic SOA formed in the urban plume. The detected differences in MS spectra from the clean ZF2 samples and T3 samples indicates that smallest 56-100 nm particles sampled at the T3 site contain a unique group of aromatic compounds, which are missing from the ZF2 site. In addition, the overall number of the observed compounds and the average number of carbon atoms is substantially higher at the T3 site indicating higher extent of oligomeric products in the urban influenced plume. The molecular composition of OA at the T3 site is thus a complex mixture of isoprene SOA constituents, anthropogenic SOA constituents, and possible products of their cross reactions. The higher degree of unsaturation and the larger sizes of molecules characteristic of the aerosol from the T3 site may have a number of important consequences on aerosol properties, such as higher particle viscosity, lower volatility of SOA, and larger fraction of light-absorbing (brown carbon) compounds in SOA.