Nocturnal surface ozone enhancement and trend over urban and suburban sites in Portugal

Long term (2000-2010) analysis of hourly surface ozone data from 6 out of 9 background air pollution monitoring sites in Portugal shows prominent secondary/nocturnal peak in monthly averaged diurnal variation during winter months (Nov-Feb). Of 9 sites: 3 are urban, 5 suburban and 1 rural. The bimodal diurnal variation of surface ozone shows a major peak around 15 h and a minor peak around 03-05 h with minima around 08 h and 20 h in the months of November February. During winter months, more than 50% of observations in Porto and Lisbon region had nocturnal surface ozone concentrations higher than 40 mu g m(-3) compared to monthly averaged early morning/late evening minima (20+/-5 mu g m(-3)). Analysis also shows an increasing trend in nocturnal surface ozone daily maxima over 5 urban/suburban sites and over another site (Alfragide) which exhibits the same trend but only in the last 7 years of the studied period. Also the non-parametric Mann-Kendall test confirms that the trends are statistically significant. The enhancement of nocturnal surface ozone is most likely associated with transport process and prevailing meteorological conditions since photochemical production of ozone ceases after sunset. (C) 2013 Elsevier Ltd. All rights reserved.