The authors define SR as SR = ΔC / Cseason × 100%, where Cseason is the seasonal mean concentration. This normalization choice is problematic. The scavenging effect of a single rain event should be evaluated against its immediate pre-event conditions, not a long-term seasonal average.
This raises two key questions:
How can a rain event’s efficiency be meaningfully assessed using a baseline (seasonal mean) that may be heavily influenced by many other rain events and long-term pollution trends?
Doesn’t this formula artificially inflate the SR in seasons with low background pollution (e.g., summer) and suppress it in high-pollution seasons (e.g., winter), making cross-seasonal comparisons invalid?
This methodological flaw could significantly skew the reported disparities in scavenging rates for different PM2.5 components, rainfall intensities, and seasons.
A more robust and physically meaningful calculation of the scavenging rate should normalize the concentration change by the pre-rain concentration. The following adjusted formula is recommended:
Recommended Scavenging Rate (SR’):
SR’ = (Cb – Ca) / Cb × 100%
Cb = Average concentration in the 3 hours before the precipitation event.
Ca = Average concentration in the 3 hours after the precipitation event.
Formula with Diurnal Adjustment:
SR’ = (Ćb – Ća) / Ćb × 100%
Where Ćb and Ća are the diurnally-adjusted values as defined in the paper. This would isolate the precipitation’s effect from the normal daily cycle while maintaining a physically sound normalization.