Nitrogen dioxide in the atmosphere : a study on the distribution of the air pollutant in the Maltese Islands

Abstract

Air pollution has long been linked to adverse health and environmental problems. Such realisation has lead to formulation of strategies and actions at various community scales, aimed at reducing air pollution. One of the air pollutants of concern today is the gaseous nitrogen dioxide (NO2). This study uses Geographical Information Systems (GIS) to characterise the spatial distribution of nitrogen dioxide in the Maltese Islands. The research is based on pollution concentration data collected by means of diffusion tube samplers at 134 locations across Malta and Gozo. Various contributing factors such as transport attributes, meteorological variables, land use characteristics and population data were used during analysis to explain the spatial variation in the concentration of nitrogen dioxide. The accuracy of the diffusion tube data was also assessed through a co-location study where diffusion tube samplers were exposed alongside the reference method for measuring the concentration of nitrogen dioxide in ambient air. The results revealed that wind speed, land use type, population density and traffic volumes are all variables that have an impact on the concentration of NO2. However, the effect of such factors depends on the characteristics of the site in which air quality monitoring is undertaken. While population density, traffic volumes and land use type are significant predictors of NO2 at traffic sites, wind speed and land us type are important in shaping the concentration of the pollutant at urban background locations. Incorporating such contributing factors in the spatial modelling of NO2 levels was shown to increase the accuracy of the prediction. In addition land use regression followed by kriging of the residuals proved to be the most accurate and best suited m Diffusion tube samplers were shown to be precise in measuring the ambient concentration of NO2. This makes such measurement technique suitable for indicative monitoring of the pollutant concentration. However, since the results of this study have revealed that diffusion tubes overestimate the concentration of NO2, a correction factor needs to be applied to any data acquired through such passive measurement. Pollution maps showing the distribution of nitrogen dioxide in the Maltese Islands indicate that the highest concentrations of NO2 at urban background levels occur in the Harbour region. Traffic is a major source of nitrogen dioxide in Malta and as a result locations characterised by high traffic volumes are also sites of high pollutant concentrations. While a number of actions and policies aimed at reducing nitrogen dioxide emissions are already in place, this study has identified other areas where research can support the decision-making process in further limiting the adverse effects of pollution from nitrogen dioxide and attaining sustainability goals.