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   Partial lockdown measures took effect in Thailand from March 22, 2020 to prevent the spread of the coronavirus. People widely believed that the air quality in Bangkok would improve during the lockdown. This study aims to understand the effects of the traffic on Bangkok PM2.5 and gas concentrations, by comparing the air quality before and during the lockdown. Some pollutant concentrations in pre-lockdown period were higher, but the differences were not significant, except for O3. When results in the full lockdown month, April 2020, were compared to the same period in 2019, the average PM2.5 concentrations at the road sites decreased by 11.1%. On the other hand, it increased by 16.7% at the business area sites. No clear relation of the PM2.5 change to the reduction of traffic and diesel fuel consumption was observed. The reduction of NO2 was clear, caused by the significant drop in traffic and fuel consumption: in turn, it contributed to an increase of O3. The increment of PM2.5 levels during the lockdown was an external effect, even though significant change of local sources occurred. The values of OC/EC ratios in fine particles and the backward trajectory simulation confirmed that the peaks of PM2.5 levels were affected by both transboundary and local aerosol transport from open biomass burning. Hence, it is clear that road traffic, as well as industries or other human activities, were not the most influential factors on high PM2.5 levels in Bangkok in normal conditions. Possible solutions to reduce sources of Bangkok air pollutants include introduction of small-scale machinery for sugarcane harvesting to reduce biomass burning, adoption of higher standards to control diesel engine emission, and mutual agreement and action among ASEAN members in transboundary haze reduction.

Effects of Coronavirus Induced City Lockdown on PM2.5 and Gaseous Pollutant Concentrations in Bangkok