Analysis of the air pollution climate at a background site in the Po valley

Abstract

The Po valley in northern Italy is renowned for its high air pollutant concentrations. Measurements of air pollutants from a background site in Modena, a town of 200 thousand inhabitants within the Po valley, are analysed. These comprise hourly data for CO, NO, NO₂, NO_x, and O₃, and daily gravimetric equivalent data for PM₁₀ from 1998–2010. The data are analysed in terms of long-term trends, annual, weekly and diurnal cycles, and auto-correlation and cross-correlation functions. CO, NO and NO₂ exhibit a strongly traffic-related pattern, with daily peaks at morning and evening rush hour and lower concentrations over the weekend. Ozone shows an annual cycle with a peak in July due to local production; notwithstanding the diurnal cycle dominated by titration by nitrogen oxide, the decreasing long term trend in NO concentration did not affect the long term trend in O₃, whose mean concentration remained steady over the sampling period. PM₁₀ shows a strong seasonality with higher concentration in winter and lower concentration in summer and spring. Both PM₁₀ and ozone show a marked weekly cycle in summer and winter respectively. Regressions of PM₁₀ upon NO_x show a consistently greater intercept in winter, representing higher secondary PM₁₀ in the cooler months of the year. There is a seasonal pattern in primary PM₁₀ to NO_x ratios, with lower values in winter and higher values in summer, but the reasons are unclear.