Department of Chemistry, University of California , Irvine, California 92697, United States.
Department of Chemistry, Purdue University , West Lafayette, Indiana 47907, United States.
Environ Sci Technol. 2017 Oct 3;51(19):11048-11056. doi: 10.1021/acs.est.7b02293. Epub 2017 Sep 15.
Reactions of ammonia or ammonium sulfate (AS) with carbonyls in secondary organic aerosol (SOA) produced from limonene are known to form brown carbon (BrC) with a distinctive absorption band at 505 nm. This study examined the browning processes in aqueous solutions of AS and 4-oxopentanal (4-OPA), which has a 1,4-dicarbonyl structural motif present in many limonene SOA compounds. Aqueous reactions of 4-OPA with AS were found to produce 2-methyl pyrrole (2-MP), which was detected by gas chromatography. While 2-MP does not absorb visible radiation, it can further react with 4-OPA eventually forming BrC compounds. This was demonstrated by reacting 2-MP with 4-OPA or limonene SOA, both of which produced BrC with absorption bands at 475 and 505 nm, respectively. The formation of BrC in the reaction of 4-OPA with AS and ammonium nitrate was greatly accelerated by evaporation of the solution suggesting an important role of the dehydration processes in BrC formation. 4-OPA was also found to produce BrC in aqueous reactions with a broad spectrum of amino acids and amines. These results suggest that 4-OPA may be the smallest atmospherically relevant compound capable of browning by the same mechanism as limonene SOA.
氨或硫酸铵（AS）与柠檬烯二次有机气溶胶（SOA）中的羰基反应已知会形成具有 505nm 特征吸收带的棕色碳（BrC）。本研究考察了 AS 和 4-氧戊醛（4-OPA）在水溶液中的褐变过程，4-OPA 具有存在于许多柠檬烯 SOA 化合物中的 1,4-二羰基结构基序。发现 4-OPA 与 AS 的水溶液反应会生成 2-甲基吡咯（2-MP），通过气相色谱法检测到。虽然 2-MP 不吸收可见光，但它可以与 4-OPA 进一步反应，最终形成 BrC 化合物。这通过将 2-MP 与 4-OPA 或柠檬烯 SOA 反应来证明，这两种物质分别在 475nm 和 505nm 处产生了 BrC 吸收带。溶液蒸发大大加速了 4-OPA 与 AS 和硝酸铵反应中 BrC 的形成，这表明脱水过程在 BrC 形成中起着重要作用。还发现 4-OPA 可以与广泛的氨基酸和胺在水溶液中反应生成 BrC。这些结果表明，4-OPA 可能是最小的与大气相关的化合物，能够通过与柠檬烯 SOA 相同的机制产生褐变。
