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

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, NOx, 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 NOx 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 NOx ratios, with lower values in winter and higher values in summer, but the reasons are unclear.