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.

中文翻译：

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2-甲基吡咯在4-氧戊醛和柠檬烯二次有机气溶胶褐变中的作用

已知氨或硫酸铵（AS）与柠檬烯在次级有机气溶胶（SOA）中的羰基反应形成了在505 nm处具有明显吸收带的棕色碳（BrC）。这项研究检查了AS和4-oxopentanal（4-OPA）水溶液中的褐变过程，该溶液在许多柠檬烯SOA化合物中均具有1,4-二羰基结构。发现4-OPA与AS的水反应产生2-甲基吡咯（2-MP），其通过气相色谱法检测。尽管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。通过使2-MP与4-OPA或SO烯SOA反应来证明这一点，两者均产生了BrC，其吸收带分别在475和505 nm处。溶液的蒸发极大地促进了4-OPA与AS和硝酸铵反应中BrC的形成，表明脱水过程在BrC形成中的重要作用。还发现4-OPA在水性反应中可产生具有宽范围氨基酸和胺的BrC。这些结果表明4-OPA可能是最小的与大气相关的化合物，其能够通过与柠檬烯SOA相同的机理进行褐变。