Formation of water-soluble dicarboxylic acids, oxoacids and a-dicarbonyls by ozone oxidation of isoprene

Abstract:

Water-soluble dicarboxylic acids such as oxalic acid (C₂) are the dominant organic compound class in atmospheric aerosols. They can act as cloud condensation nuclei and affect on the Earth climate. Diacids can be primary emitted from fossil fuel combustion and biomass burning and secondarily produced by photochemical oxidations of biogenic and anthropogenic hydrocarbons. However, their sources and formation processes are still not well understood. Recently model and observation studies suggested the importance of isoprene as a precursor of oxalic acid. Isoprene is the most abundant BVOC emitted from terrestrial plants and can serve as important precursors of diacids. We conducted a laboratory oxidation of isoprene (2.0 ppm) with ozone (4.3 ppm) in a Teflon bag for 10 to 480 min. The formed particles were collected with quartz fiber filters and analyzed for diacids, oxoacids and a-dicarbonyls employing water extraction and butyl ester derivatization and using GC and GC/MS techniques. Here, we report the analytical results to better understand the formation process of diacids and related compounds from isoprene.

We detected homologous series of saturated diacids (C₂-C₆), unsaturated diacids (maleic and methylmaleic acids), w-oxocarboxylic acids (C₂-C₉), pyruvic acid, glyoxal and methylglyoxal. We found that oxalic acid (3000-9700 ngm^-3) is the most abundant diacid followed by succinic (C₄) or malonic (C₃) acid. Their concentrations increased with reaction time showing a maximum in 4 hours. Interestingly, C₃/C₄ ratios increased with time. The second most abundant species after oxalic acid was generally methylglyoxal (3600-9600 ngm^-3), except for the 30 min. sample where methylglyoxal was more abundant than oxalic acid. Glyoxylic acid (wC₂) was found as the most abundant oxoacid (1600-3800 ngm^-3) followed by wC₃ and wC₄. Although the concentrations of diacids and related compounds are 1-2 orders magnitude higher than those reported in ambient aerosols, this study demonstrates that isoprene is an important precursor of diacids in atmospheric particles.