Letters to the Editors
Letters to the editor of South China Morning Post - Response to a news report on roadside Air Pollution Index
I wish to make a correction to your report "Air pollution worst in 18 years in shopping areas" (24 February). Your report was based on research by Friends of the Earth (FoE).
FoE says in 2013 there are 184 days where the roadside air pollution index (API) exceeded 100, and last year was the worst on record. The number of such days was in fact 132 days. The number of days with roadside API exceeding 100 in 2010, 2011 and 2012 are 141, 172 and 142 days respectively.
Having understood FoE's methodology, we have explained to them their two errors, and we also issued our own press release for clarification.
FoE's first error was they averaged out the hourly API and counted those days with "daily average" greater than 100. As the API was reported on an hourly basis, it is inappropriate to calculate and use the "daily averages" for determining the number of days with API exceeding 100. The second error was FoE added up all individual roadside station's figures to give the overall roadside figure. FoE overlooked that API greater than 100 would normally also be observed simultaneously at more than one stations and hence adding up all individual station's figures would double or even triple count some of these daily figures.
We welcome both non-governmental organizations and also reporters to check their calculations with us ahead of time so as to avoid errors. We recognize air science is complicated.
Yours sincerely,
Christine Loh
Under Secretary for the Environment