[论文解读] Significant reduced traffic in Beijing failed to relieve haze pollution during the COVID-19 lockdown: implications for haze mitigation

Lv Z, Xianfeng Wang+13 more|arXiv (Cornell University)|Jun 12, 2020

Air Quality and Health Impacts被引用 4

一句话总结

尽管北京新冠疫情期间交通排放的NOx减少了75.9%，VOCs减少了53.1%，但由于减排不平衡导致大气氧化能力增强，特别是在周边地区，进而加剧了二次气溶胶的形成，因此霾污染依然持续。研究结论指出，若无区域性的VOC和NH3排放协同控制，仅靠本地交通管控无法有效缓解霾污染。

ABSTRACT

The COVID-19 outbreak greatly limited human activities and reduced primary emissions particularly from urban on-road vehicles, but coincided with Beijing experiencing pandemic haze, raising the public concerns of the validity and effectiveness of the imposed traffic policies to improve the air pollution. Here, we explored the relationship between local vehicle emissions and the winter haze in Beijing before and during the COVID-19 lockdown period based on an integrated analysis framework, which combines a real-time on-road emission inventory, in-situ air quality observations and a localized chemical transport modeling system. We found that traffic emissions decreased substantially affected by the pandemic, with a higher reduction for NOx (75.9%, 125.3 Mg/day) compared to VOCs (53.1%, 52.9 Mg/day). Unexpectedly, our results show that the imbalanced emission abatement of NOx and VOCs from vehicles led to a significant rise of the atmospheric oxidizing capacity in urban areas, but only resulting in modest increases in secondary aerosols due to the inadequate precursors. However, the enhanced oxidizing capacity in the surrounding regions greatly increased the secondary particles with relatively abundant precursors, which is mainly responsible for Beijing haze during the lockdown period. Our results indicate that the winter haze in Beijing was insensitive to the local vehicular emissions reduction due to the complicated nonlinear response of the fine particle and air pollutant emissions. We suggest mitigation policies should focus on accelerating VOC and NH3 emissions reduction and synchronously controlling regional sources to release the benefits on local traffic emission control.

研究动机与目标

• 调查新冠疫情期间北京道路交通车辆排放大幅减少对冬季霾污染的影响。
• 评估NOx与VOC减排不平衡对大气氧化能力及二次气溶胶形成的影响。
• 在真实排放变化背景下，评估本地交通管控政策对细颗粒物（PM2.5）减排的有效性。
• 识别尽管交通排放显著减少，霾污染仍持续存在的关键驱动因素。

提出的方法

• 开发了实时道路交通车辆排放清单，以量化封禁前后的每日交通排放。
• 将现场空气质量监测数据与本地化的化学传输模型（CTM）相结合，模拟大气化学过程与气溶胶形成。
• 应用非线性化学响应框架，分析二次有机气溶胶（SOA）和二次无机气溶胶（SIA）对NOx和VOC前体物变化的敏感性。
• 利用区域模型系统，分离本地排放与上风向排放对二次颗粒物形成的贡献。
• 追踪氧化能力（如OH自由基浓度）的变化，评估其在增强二次气溶胶生成中的作用。
• 开展敏感性模拟，评估跨区域同步控制VOC和NH3排放的影响。

实验结果

研究问题

• RQ1 封禁期间道路交通车辆排放的减少在多大程度上影响了北京的PM2.5水平？
• RQ2 NOx与VOC减排的不平衡如何影响大气氧化能力及二次气溶胶的形成？
• RQ3 为何尽管本地交通排放显著减少，霾污染仍持续存在？
• RQ4 区域排放，特别是周边地区排放，在封禁期间对维持二次颗粒物生成起到了何种作用？
• RQ5 若在区域范围内协同控制VOC和NH3，将如何影响本地交通排放政策的减排效果？

主要发现

• 封禁期间，北京交通排放的NOx减少了75.9%（从125.3 Mg/day），VOCs减少了53.1%（从52.9 Mg/day）。
• NOx与VOC减排的不平衡导致大气氧化能力显著增强，特别是在周边地区。
• 上风向地区氧化能力的增强推动了二次气溶胶的增加，进而导致北京持续出现霾污染。
• 由于大气具有非线性化学响应特性，本地机动车排放减少对二次气溶胶水平影响甚微。
• 研究发现，二次气溶胶的形成对区域前体物的可利用性比对本地交通管控更为敏感。
• 结果表明，同步实施区域性的VOC和NH3控制是最大化本地交通减排效益的关键。