Diurnal variation in the water-soluble inorganic ions, organic carbon and isotopic compositions of total carbon and nitrogen in biomass burning aerosols from the LBA-SMOCC campaign in Rondônia, Brazil

Abstract

Aerosol particles (PM 2.5) were collected during the day ( n=6) and nighttime ( n=9) from a tropical pasture site in Rondônia, Brazil during an intensive biomass burning period (16–26 September, 2002). Higher normalized (by K +, levoglucosan, or apparent elemental carbon, EC a) mass concentrations of SO 4 2− and CH 3SO 3 − in daytime suggest their photochemical production, while the opposite trend for NO 3 − suggests its transfer to the aerosol phase at lower temperatures and higher humidities, as well as possibly production through hydrolysis of N 2O 5 on aqueous aerosol particles. About 4.2–7.5% of OC (5–13% of water-soluble organic carbon (WSOC)) could be characterized at the molecular level using GC-MS and GC-FID. Among the detected organic compound classes, abundances of anhydrosugars and aromatics were higher in night samples, but sugars/sugar alcohols, diacids, oxoacids and α-dicarbonyls were more abundant in day samples. Consecutive day and night samples showed that δ 13C values of total carbon (TC) were lower in daytime samples, which can be interpreted as resulting from higher contributions of refractory TC depleted in 13C due to predominantly flaming combustion. The δ 15N values of total nitrogen (TN) ranged from +23.5‰ to +25.7‰, however, there was no trend in day and night samples. Higher values of δ 13C and δ 15N for biomass burning particles than those of unburned vegetation reflect positive isotopic enrichment either during the formation of particles or after the emission of particles in the atmosphere.