[Paper Review] 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 Impacts4 citations

TL;DR

Despite a 75.9% reduction in NOx and 53.1% reduction in VOCs from traffic during Beijing's COVID-19 lockdown, haze pollution persisted due to an imbalanced emission reduction that increased atmospheric oxidizing capacity, particularly in surrounding regions, leading to enhanced secondary aerosol formation. The study concludes that local traffic controls alone are insufficient for haze mitigation without coordinated regional VOC and NH3 emission reductions.

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.

Motivation & Objective

• To investigate the impact of drastically reduced on-road vehicle emissions during the COVID-19 lockdown on Beijing’s winter haze pollution.
• To assess the role of imbalanced NOx and VOC reductions in altering atmospheric oxidizing capacity and secondary aerosol formation.
• To evaluate the effectiveness of local traffic control policies in reducing fine particulate matter (PM2.5) under real-world emission changes.
• To identify key drivers of haze persistence despite significant traffic emission reductions.

Proposed method

• Developed a real-time on-road vehicle emission inventory to quantify daily traffic emissions before and during the lockdown.
• Integrated in-situ air quality monitoring data with a localized chemical transport model (CTM) to simulate atmospheric chemistry and aerosol formation.
• Applied a nonlinear chemical response framework to analyze the sensitivity of secondary organic aerosols (SOA) and secondary inorganic aerosols (SIA) to changes in NOx and VOC precursors.
• Used a regional modeling system to isolate the contributions of local versus upwind emissions to secondary particle formation.
• Tracked changes in oxidizing capacity (e.g., OH radical levels) to assess their role in enhancing secondary aerosol production.
• Conducted sensitivity simulations to evaluate the impact of synchronized VOC and NH3 emission controls across regions.

Experimental results

Research questions

• RQ1 To what extent did the reduction in on-road vehicle emissions during the lockdown affect PM2.5 levels in Beijing?
• RQ2 How did the imbalance between NOx and VOC reductions influence atmospheric oxidizing capacity and secondary aerosol formation?
• RQ3 Why did haze persist despite significant local traffic emission reductions?
• RQ4 What role did regional emissions, particularly from surrounding areas, play in sustaining secondary particle production during the lockdown?
• RQ5 How would coordinated regional control of VOCs and NH3 affect the effectiveness of local traffic emission policies?

Key findings

• Traffic emissions in Beijing dropped by 75.9% for NOx (125.3 Mg/day) and 53.1% for VOCs (52.9 Mg/day) during the lockdown period.
• The imbalanced reduction in NOx and VOCs led to a significant increase in atmospheric oxidizing capacity, particularly in surrounding regions.
• Enhanced oxidizing capacity in upwind regions drove increased formation of secondary aerosols, which contributed to persistent haze in Beijing.
• Local reductions in vehicular emissions had minimal impact on secondary aerosol levels due to the nonlinear chemical response of the atmosphere.
• The study found that secondary aerosol formation was more sensitive to regional precursor availability than to local traffic controls.
• The results suggest that simultaneous regional control of VOCs and NH3 is essential to maximize the benefits of local traffic emission reductions.