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 Rondonia, Brazil

摘要

Aerosol particles (PM2.5) were collected during the day (n = 6) and nighttime (n = 9) from a tropical pasture site in Rondonia, 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 CH3SO_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 N2O5 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~(13)C values of total carbon (TC) were lower in daytime samples, which can be interpreted as resulting from higher contributions of refractory TC depleted in ~(13)C due to predominantly flaming combustion. The δ~(15)N 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~(13)C and δ~(15)N 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.