High levels of fine particulate matter (PM_2.5) pollution in East Asia often exceed local air quality standards. Observations from the Korea–United States Air Quality (KORUS-AQ) field campaign in May and June 2016 showed that development of extreme pollution (haze) occurred through a combination of long-range transport and favorable meteorological conditions that enhanced local production of PM_2.5. Atmospheric models often have difficulty simulating PM_2.5 chemical composition during haze, which is of concern for the development of successful control measures. We use observations from KORUS-AQ to examine the ability of the GEOS-Chem chemical transport model to simulate PM_2.5 composition throughout the campaign and identify the mechanisms driving the pollution event. At the surface, the model underestimates sulfate by −64 % but overestimates nitrate by +36 %. The largest underestimate in sulfate occurs during the pollution event, for which models typically struggle to generate elevated sulfate concentrations due to missing heterogeneous chemistry in aerosol liquid water in the polluted boundary layer. Hourly surface observations show that the model nitrate bias is driven by an overestimation of the nighttime peak. In the model, nitrate formation is limited by the supply of nitric acid, which is biased by +100 % against aircraft observations. We hypothesize that this is due to a large missing sink, which we implement here as a factor of 5 increase in dry deposition. We show that the resulting increased deposition velocity is consistent with observations of total nitrate as a function of photochemical age. The model does not account for factors such as the urban heat island effect or the heterogeneity of the built-up urban landscape, resulting in insufficient model turbulence and surface area over the study area that likely results in insufficient dry deposition. Other species such as NH₃ could be similarly affected but were not measured during the campaign. Nighttime production of nitrate is driven by NO₂ hydrolysis in the model, while observations show that unexpectedly elevated nighttime ozone (not present in the model) should result in N₂O₅ hydrolysis as the primary pathway. The model is unable to represent nighttime ozone due to an overly rapid collapse of the afternoon mixed layer and excessive titration by NO. We attribute this to missing nighttime heating driving deeper nocturnal mixing that would be expected to occur in a city like Seoul. This urban heating is not considered in air quality models run at large enough scales to treat both local chemistry and long-range transport. Key model failures in simulating nitrate, mainly overestimated daytime nitric acid, incorrect representation of nighttime chemistry, and an overly shallow and insufficiently turbulent nighttime mixed layer, exacerbate the model's inability to simulate the buildup of PM_2.5 during haze pollution. To address the underestimate in sulfate most evident during the haze event, heterogeneous aerosol uptake of SO₂ is added to the model, which previously only considered aqueous production of sulfate from SO₂ in cloud water. Implementing a simple parameterization of this chemistry improves the model abundance of sulfate but degrades the SO₂ simulation, implying that emissions are underestimated. We find that improving model simulations of sulfate has direct relevance to determining local vs. transboundary contributions to PM_2.5. During the haze pollution event, the inclusion of heterogeneous aerosol uptake of SO₂ decreases the fraction of PM_2.5 attributable to long-range transport from 66 % to 54 %. Locally produced sulfate increased from 1 % to 25 % of locally produced PM_2.5, implying that local emissions controls could have a larger effect than previously thought. However, this additional uptake of SO₂ is coupled to the model nitrate prediction, which affects the aerosol liquid water abundance and chemistry driving sulfate–nitrate–ammonium partitioning. An additional simulation of the haze pollution with heterogeneous uptake of SO₂ to aerosol and simple improvements to the model nitrate simulation results in 30 % less sulfate due to 40 % less nitrate and aerosol water, and this results in an underestimate of sulfate during the haze event. Future studies need to better consider the impact of model physical processes such as dry deposition and nighttime boundary layer mixing on the simulation of nitrate and the effect of improved nitrate simulations on the overall simulation of secondary inorganic aerosol (sulfate + nitrate + ammonium) in East Asia. Foreign emissions are rapidly changing, increasing the need to understand the impact of local emissions on PM_2.5 in South Korea to ensure continued air quality improvements.

中文翻译：

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在 KORUS-AQ 期间，物理过程表示的限制阻碍了 PM2.5 的成功模拟

东亚的高水平细颗粒物 ( PM _{2.5 ) 污染通常超过当地空气质量标准。}2016 年 5 月和 6 月韩国-美国空气质量 (KORUS-AQ) 实地调查的观察结果表明，极端污染（雾霾）的发展是通过远程运输和有利的气象条件共同促进当地PM _2.5产生的. 大气模型通常难以模拟雾霾期间的PM _2.5化学成分，这对于制定成功的控制措施至关重要。我们使用来自 KORUS-AQ 的观察结果来检查 GEOS-Chem 化学传输模型模拟PM _2.5的能力组成整个运动，并确定驱动污染事件的机制。在表面上，模型低估了硫酸盐- 64 %，但高估了硝酸盐+ 36 %。硫酸盐的最大低估发生在污染事件期间，由于在污染边界层的气溶胶液态水中缺少异质化学，模型通常难以产生升高的硫酸盐浓度。每小时表面观察表明，模型硝酸盐偏差是由对夜间峰值的高估驱动的。在该模型中，硝酸盐的形成受到硝酸供应的限制，该供应受到+100% 反对飞机观察。我们假设这是由于缺少一个大的水槽，我们在这里将其实现为干沉降增加 5 倍。我们表明，由此产生的沉积速度增加与作为光化学年龄函数的总硝酸盐的观察结果一致。该模型没有考虑城市热岛效应或建成城市景观的异质性等因素，导致研究区域的模型湍流和表面积不足，可能导致干沉降不足。其他物种如NH ₃可能会受到类似影响，但在活动期间未进行测量。硝酸盐的夜间生产由NO _2驱动模型中的水解，而观察表明夜间臭氧意外升高（模型中不存在）应导致N ₂ O ₅水解作为主要途径。由于下午混合层的过快塌陷和NO的过度滴定，该模型无法代表夜间臭氧. 我们将此归因于夜间供暖的缺失导致夜间混合更深，预计在首尔这样的城市会发生这种情况。这种城市供暖在空气质量模型中没有考虑到，该模型运行的规模足够大，可以同时处理当地化学和远程传输。模拟硝酸盐的关键模型失败，主要是高估了白天的硝酸，夜间化学的不正确表示，以及夜间混合层过浅和湍流不足，加剧了模型无法模拟雾霾污染期间PM _{2.5的积累。}为了解决在雾霾事件期间最明显的硫酸盐低估问题，SO _{2的异质气溶胶吸收}被添加到模型中，该模型以前只考虑了从云水中的SO _{2生成硫酸盐的水溶液。}对该化学进行简单的参数化改进了硫酸盐的模型丰度，但降低了SO ₂模拟，这意味着排放被低估了。我们发现改进硫酸盐模型模拟与确定本地与跨界对PM _2.5的贡献直接相关。在雾霾污染事件中， SO ₂的异质气溶胶吸收降低了PM _2.5的比例归因于从 66% 到 54% 的远程运输。本地生产的硫酸盐从本地生产的PM _2.5的 1% 增加到 25% ，这意味着本地排放控制可能产生比以前认为的更大的影响。然而，SO ₂的这种额外吸收与模型硝酸盐预测相结合，这会影响气溶胶液态水的丰度和驱动硫酸盐-硝酸盐-铵分配的化学物质。对SO ₂异质吸收的雾霾污染的附加模拟由于硝酸盐和气溶胶水减少了 40%，因此对模型硝酸盐模拟的简单改进导致硫酸盐减少了 30%，这导致在雾霾事件期间低估了硫酸盐。未来的研究需要更好地考虑模型物理过程（例如干沉降和夜间边界层混合）对硝酸盐模拟的影响以及改进的硝酸盐模拟对  东部二次无机气溶胶（硫酸盐+ 硝酸盐 +铵）整体模拟的影响亚洲。外国排放物正在迅速变化，增加了了解当地排放物对韩国PM _{2.5影响的需求，以确保持续改善空气质量。}