Figure 4

The high concentrations of CO: (a) eight-hour average concentration in Jan 2017; (b) eight-hour average concentration in Oct 2017, (c) eight-hour average concentration in Nov 2017; (d) eight-hour average concentration in Dec 2017.
The high concentrations of CO: (a) eight-hour average concentration in Jan 2017; (b) eight-hour average concentration in Oct 2017, (c) eight-hour average concentration in Nov 2017; (d) eight-hour average concentration in Dec 2017.