Investigating diesel engines as an atmospheric source of isocyanic acid in urban areas Atmospheric Chemistry and Physics

Isocyanic acid (HNCO), an acidic gas found in tobacco smoke, urban environments, and biomass-burning-affected regions, has been linked to adverse health outcomes. Gasoline-and diesel-powered engines and biomass burning are known to emit HNCO and hypothesized to emit precursors such as amides that can photochemically react to produce HNCO in the atmosphere. Increasingly, diesel engines in developed countries like the United States are required to use selective catalytic reduction (SCR) systems to reduce tailpipe emissions of oxides of nitrogen. SCR chemistry is known to produce HNCO as an intermediate product, and SCR systems have been implicated as an atmospheric source of HNCO. In this work, we measure HNCO emissions from an SCR system-equipped diesel engine and, in combination with earlier data, use a three-dimensional chemical transport model (CTM) to simulate the ambient concentrations and source/pathway contributions to HNCO in an urban environment. Engine tests were conducted at three different engine loads, using two different fuels and at multiple operating points. HNCO was measured using an acetate chemical ionization mass spectrometer. The diesel engine was found to emit primary HNCO (3-90 mg kg fuel(-1)) but we did not find any evidence that the SCR system or other aftertreatment devices (i.e., oxidation catalyst and particle filter) produced or enhanced HNCO emissions. The CTM predictions compared well with the only available observational datasets for HNCO in urban areas but underpredicted the contribution from secondary processes. The comparison implied that diesel-powered engines were the largest source of HNCO in urban areas. The CTM also predicted that daily-averaged concentrations of HNCO reached a maximum of similar to 110 pptv but were an order of magnitude lower than the 1 ppbv level that could be associated with physiological effects in humans. Precursor contributions from other combustion sources (gasoline and biomass burning) and wintertime conditions could enhance HNCO concentrations but need to be explored in future work.

异氰酸（HNCO）是一种存在于烟草烟雾、城市环境及受生物质燃烧影响区域的酸性气体，与多种不良健康效应存在关联。汽油发动机、柴油发动机及生物质燃烧过程均已被证实会排放HNCO，同时被推测会释放酰胺类前体物，此类前体物可在大气中经光化学反应生成HNCO。当前，美国等发达国家愈发普遍要求柴油发动机安装选择性催化还原（Selective Catalytic Reduction, SCR）系统，以降低尾气中的氮氧化物排放。现有研究表明，SCR反应过程会生成HNCO作为中间产物，因此SCR系统被认定为大气中HNCO的一个排放源。本研究中，我们对搭载SCR系统的柴油发动机的HNCO排放开展了测量，并结合既往观测数据，利用三维化学传输模型（three-dimensional chemical transport model, CTM）模拟了城市环境中HNCO的环境浓度及其各来源与传输路径的贡献占比。本次发动机测试设置了三种不同负荷，采用两种燃料，并在多个运行工况下进行实验。本研究采用乙酸盐化学电离质谱仪对HNCO进行定量检测。实验结果显示，该柴油发动机会排放一次源HNCO（排放因子为3~90 mg·kg⁻¹燃料），但未发现任何证据表明SCR系统或其他后处理装置（即氧化催化剂与颗粒捕集器）会生成或加剧HNCO排放。三维化学传输模型的预测结果与现有仅有的城市HNCO观测数据集吻合度较好，但对二次生成过程的贡献占比存在低估。对比分析结果表明，柴油发动机是城市区域HNCO的最大排放源。该模型还预测，HNCO的日平均浓度峰值约为110 pptv（万亿分之一体积浓度），较可能引发人体生理效应的1 ppbv（十亿分之一体积浓度）水平低一个数量级。其他燃烧源（汽油燃烧与生物质燃烧）及冬季气象条件带来的前体物贡献，可能会提升HNCO的大气浓度，但相关结论仍需在未来研究中进一步探索。