Photochemical Pollution Indicators in the Subtropics

Abstract

Indicators evaluated from the long-term hourly averages of ozone volume fractions at air monitor- ing sites are proposed as measures of the photochemical pollution exposure at those sites. These indicators are based on the average of the daily maximum-to-minimum ratio during the period of maximum ozone pro- duction and are corrected a), for the average maximum to average total ozone ratio (indicator P1) and b), for the number of hours the limit ozone volume ratio limit of 80 ppb was exceeded (indicator P2). These indica- tors are then combined into P3 as their geometrical mean. The rationale for the introduction of a new set of indicators is based on the suspicion that ozone volume fractions do not provide information either on the total daily ozone that is produced or on the fraction of it that has produced other photochemical pollution compo- nents despite that ozone correlates quite well with some of them. Unlike the European stations, where every case in excess of the 80 ppb limit occurs within the April to September growth period, stations in the sub- tropics have longer periods (e.g. at Greater Baton Rouge (GBR), USA, for 2001 to 2008) are shifted towards later months June to November (e.g. the Pearl River Delta (PRD) in China for 2006). While GBR and the ru- ral PRD stations exhibit indicators close to those of polluted stations in northern Italy (Po Valley), the (sub)urban PRD stations have high photochemical pollution indicators. However, a surprisingly low indicator level occurs for the coastal Hong Kong stations for reasons possibly attributable to the prevailing easterly winds which bring fresh air and airborne sea-salt particulates. (doi: 10.5562/cca1807)