Modelling intra- and inter-day variability of NO2 concentrations in Portugal

Abstract. Nitrogen dioxide (NO2), is a pollutant that is toxic by inhalation and there is evidence that long-term exposure to this, at high concentrations, has adverse health effects, namely in respiratory and cardiovascular systems. The goal of this study is to characterize the spatial and temporal evolution of NO2 concentration levels, taking into account that environmental data often incorporate distinct recurring patterns in time, imposed by social habits. We aim at capturing the cyclic nature of these environmental indicators, identifying the intra and inter-day variability. Simultaneously, we aim at modelling the temporal and spatial correlation inherent to this type of data. This study focus on NO2 hourly data collected in Portugal from October 1 to December 31, 2014. An initial exploratory study suggests that there are two main seasonal effects in the data and identifies variables such as the type of site, environment, and the day of the week as possible explanatory variables. Furthermore, the analysis of the correlation between meteorological parameters, as air temperature, wind speed and relative humidity and NO2 levels identifies significant negative associations among them. After describing the trend function, geostatistical approaches are applied to the resulting residuals with the aim of characterizing the space-time variability and deriving the predicted values through the kriging tools. This methodology can be used to complement the current design sampling, where there are districts without monitoring stations or with many missing values. Moreover, as meteorological data are available earlier than NO2 levels, we draw scenarios for NO2 levels for 2015.