Fine-mode organic mass concentrations and sources in the Amazonian wet season (AMAZE-08)

Abstract

Abstract. Real-time mass spectra of non-refractory species in submicron aerosol particles were recorded in a tropical rainforest in the central Amazon Basin during the wet season from February to March 2008, as a part of the Amazonian Aerosol Characterization Experiment (AMAZE-08). Organic material accounted on average for more than 80% of the non-refractory submicron particle mass concentrations during the period of measurements. Ammonium was present in sufficient quantities to partially neutralize sulfate. In this acidic, isoprene-rich, HO2-dominant environment positive-matrix factorization (PMF) of the time series of particle mass spectra identified four statistical factors to account for the 99% variance of the signal intensities of the organic constituents: an HOA factor having a hydrocarbon-like signature and identified as regional and local pollution, an OOA-1 factor associated with long-range transport, an OOA-2 factor implicated as associated with the reactive uptake of isoprene oxidation products, especially of epoxydiols to acidic haze, fog or cloud droplets, and an OOA-3 factor consistent with the fresh production of secondary organic material (SOM) by a mechanism of gas-phase oxidation of biogenic volatile organic compounds (BVOC) followed by gas-to-particle conversion of the oxidation products. The OOA-1, -2, and -3 factors had progressively less oxidized signatures. Aqueous-phase oxidation of water-soluble products of gas-phase photochemistry might have been also involved in the formation of the OOA-2 factor. The campaign-average mass concentrations were in a ratio of 7 : 5 for the OOA-2 compared to the OOA-3 pathway, suggesting the comparable importance of particle-phase compared to gas-phase pathways for the production of SOM during the study period.