Determining temporal variations in nitrogen dioxide air quality in an urban area

Abstract

The assessment of temporal changes in air pollutants to evaluate reduction strategies has to be done while taking into account meteorological variability. In this paper, several years of nitrogen dioxide (NO2) hourly data are analyzed at two sampling sites in Valencia urban area (Spain). Nitric oxide (NO) hourly concentrations are used as an indicator of traffic emissions. The results show that annual levels of NO2 have exceeded the limit values within the Directive of the European Union, in three years of the studied period at one sampling station. The low-pass filter developed by Zurbenko and Kolmogorov is applied to split the logarithm of NO2 and NO hourly concentrations into long-term, seasonal and short-term components. Long-term components represent a small amount in the overall variability of air pollution data. Seasonal and short-term components mask the underlying relationship between NO2 and emissions if studied as a whole. The pollutant temporal components have to be studied separately due to their different physical and explanatory mechanisms. Meteorological effects are analyzed and removed from filtered pollutants time series using forward stepwise multiple regressions. The dependencies of urban NO2 on the corresponding vehicular emissions and relevant meteorological parameters are non-linear. The percentage of temporal variations of NO2 above 24 hours explained by candidate meteorological variables is 26.6% at P.Silla and 31.4% at Viveros. The joint evolution of the long-term components of NO2 and NO time series is analyzed. Long-term trend of hourly NO can explain 81.3 % of the total variance of long-term NO2 changes at one site, indication that traffic is a major factor in NO2 formation at this location.