Table 1.

Some of the unresolved issues about COVID-19 and corresponding follow-up questions suggested in the article.

Unresolved Issues	Follow-Up Questions
Microphysically, there is a controversy over whether airborne transmission of SARS-CoV-2 takes place, although it is increasingly evident that it is the case.	How do the SARS-CoV-2 virus containing particles interact with other ambient aerosol particles? How does the SARS-CoV-2 virus interact with other coexisting components in the same particles? What are the factors determining the atmospheric lifetime and fate of SARS-CoV-2 virus containing particles? What is the minimum amount SARS-CoV-2 virus required to cause COVID-19? Consequently, how do those factors affect the infectiousness of the SARS-CoV-2 virus? How does the effectiveness of airborne transmission of SARS-CoV-2 virus compare with transmission through close physical contact and contaminated surfaces (also known as fomite)? How do we compile science-based infectious control measures and policies and recommendations to the general public in both indoor and outdoor public areas?
2.Macrophysically, there is a seemingly positive correlation between the COVID-19 death rate and pollution level in many cities. It is uncertain if such a positive correlation is prevalent worldwide. Also, the underlying mechanisms responsible for that remain to be explored.
3.The impacts on the global climate from the drastically reduced greenhouse gases and particulate matter (PM) emissions during lockdown remain to be examined.	5.How does the reduced emissions during lockdown affect air quality and global climate quantitatively and qualitatively? What are the mechanisms behind those impacts? 6.How do we leverage the testbed offered by the lockdown for compiling a future emission policy for air quality improvement and global warming mitigation?
4.The impacts on air quality from the drastically reduced gaseous pollutants and PM emissions during lockdown remain to be examined.