Heterogeneous Nitrate Production Mechanisms in Intense Haze Events in the North China Plain
Heterogeneous Nitrate Production Mechanisms in Intense Haze Events in the North China Plain
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DOI：
10.1029/2021jd034688
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发表时间：
2021-05-16
影响因子：
4.4
通讯作者：
Alexander, Becky
中科院分区：
文献类型：
--
作者：
Chan, Yuk-Chun;Evans, Mathew J.;Alexander, Becky
对华北平原冬季空气质量的研究表明，尽管氮氧化物排放量迅速减少，但颗粒物硝酸盐污染仍然存在。这种有趣的氮氧化物-硝酸盐关系可能源于非线性硝酸盐形成化学，但目前还不清楚NCP中占主导地位的反馈机制。本文利用GEOS-Chem（GC）化学传输模型对北京冬季O-17过量硝酸盐（O-17（NO3-）增量）的观测结果进行了重新解释，以估计各种硝酸盐产生途径的重要性以及它们的贡献如何随灰霾事件的强度而变化。我们还分析了NOy化学和[PM2.5]在观测和模型模拟中的其他指标之间的关系。我们发现，该模型平均有一个负偏差的千分之-0.9和增量O-17（NO3-）和[O-x，O-主要]（相当于[O-3] + [NO2] + [p-NO3-]）分别为-36%，而高估了氮氧化率（[NO3-]/（[NO3-] + [NO2]）+0.12在强烈的雾霾。在更强烈的霾中，差异变得更大。我们将模型偏差归因于对气溶胶NO2吸收的高估和对冬季O-3浓度的低估。我们的研究结果强调，需要解决与空气质量模型中NO2的非均相化学相关的不确定性。结果表明，在高NOx-高PM2.5条件下，大气中N2 O 5的吸收是北京冬季硝酸盐生成的主要途径，但其速率受臭氧的限制。尽管[NOx]减少，但只要[O-3]增加，硝酸盐的产生速率可能会继续增加，从而产生负反馈，降低空气污染缓解的有效性。简明语言摘要硝酸盐是城市空气中颗粒物的主要成分，已被确定为华北平原冬季雾霾趋势的重要驱动因素。虽然人们早就知道许多化学反应可以将气相氮氧化物转化为大气中的颗粒硝酸盐，但在强烈雾霾中不同反应的贡献仍然难以捉摸。最近，硝酸盐中氧稳定同位素（O-16，O-17和O-18）的分析已成为了解其化学来源的一种有前途的工具。在这项研究中，我们重新审视了硝酸盐的同位素观测在冬季北京，并将其与空气质量模型的预测。我们对观测结果的分析表明，该模型可能高估了在强烈的雾霾事件期间通过二氧化氮气体（NO2）和颗粒之间的反应产生的硝酸盐。在模型中删除此硝酸盐形成途径后，我们表明，在北京强烈的雾霾事件的硝酸盐的产生强烈调制臭氧，二次污染物的形成是依赖于氮氧化物和挥发性有机化合物（VOCs）。可能通过减少挥发性有机化合物排放来减少臭氧浓度的政策也将减少冬季雾霾事件期间硝酸盐的形成。
Studies of wintertime air quality in the North China Plain (NCP) show that particulate-nitrate pollution persists despite rapid reduction in NOx emissions. This intriguing NOx-nitrate relationship may originate from non-linear nitrate-formation chemistry, but it is unclear which feedback mechanisms dominate in NCP. In this study, we re-interpret the wintertime observations of O-17 excess of nitrate ( increment O-17(NO3-)) in Beijing using the GEOS-Chem (GC) chemical transport model to estimate the importance of various nitrate-production pathways and how their contributions change with the intensity of haze events. We also analyze the relationships between other metrics of NOy chemistry and [PM2.5] in observations and model simulations. We find that the model on average has a negative bias of -0.9 parts per thousand and -36% for increment O-17(NO3-) and [O-x,O-major] (equivalent to [O-3] + [NO2] + [p-NO3-]), respectively, while overestimating the nitrogen oxidation ratio ([NO3-]/([NO3-] + [NO2])) by +0.12 in intense haze. The discrepancies become larger in more intense haze. We attribute the model biases to an overestimate of NO2-uptake on aerosols and an underestimate in wintertime O-3 concentrations. Our findings highlight a need to address uncertainties related to heterogeneous chemistry of NO2 in air-quality models. The combined assessment of observations and model results suggest that N2O5 uptake in aerosols and clouds is the dominant nitrate-production pathway in wintertime Beijing, but its rate is limited by ozone under high-NOx-high-PM2.5 conditions. Nitrate production rates may continue to increase as long as [O-3] increases despite reduction in [NOx], creating a negative feedback that reduces the effectiveness of air pollution mitigation.Plain Language Summary Nitrate, a major component of particles in urban air, has been identified as an important driver for recent trends in wintertime haze in the North China Plain. While it has long been known that many chemical reactions can convert gas-phase nitrogen oxides into particulate nitrate in the atmosphere, the contribution from different reactions in intense haze remains elusive. Recently, analysis of oxygen stable isotopes (O-16, O-17, and O-18) in nitrate has become a promising tool for understanding its chemical origins. In this study, we re-examine the isotopic observations of nitrate in wintertime Beijing and compare them with predictions made by an air-quality model. Our analysis of observations suggests that the model likely overestimates nitrate production via the reactions between nitrogen dioxide gas (NO2) and particles during intense haze events. After removing this nitrate formation pathway in the model, we demonstrate that nitrate production during intense haze events in Beijing is strongly modulated by ozone, a secondary pollutant whose formation is dependent on nitrogen oxides and volatile organic compounds (VOCs). Policies that result in a reduction of ozone concentrations, possibly through reductions in VOC emissions, will also reduce the formation of nitrate during wintertime haze events.