Influence of meteorology on PM₁₀ trends and variability in Switzerland from 1991 to 2008

Abstract. Measurements of airborne particles with aerodynamic diameter of 10 μm or less (PM₁₀) and meteorological observations are available from 13 stations distributed throughout Switzerland and representing different site types. The effect of all available meteorological variables on PM₁₀ concentrations was estimated using Generalized Additive Models. Data from each season were treated separately. The most important variables affecting PM₁₀ concentrations in winter, autumn and spring were wind gust, the precipitation rate of the previous day, the precipitation rate of the current day and the boundary layer depth. In summer, the most important variables were wind gust, Julian day and afternoon temperature. In addition, temperature was important in winter. A "weekend effect" was identified due to the selection of variable "day of the week" for some stations. Thursday contributes to an increase of 13% whereas Sunday contributes to a reduction of 12% of PM₁₀ concentrations compared to Monday on average over 9 stations for the yearly data. The estimated effects of meteorological variables were removed from the measured PM₁₀ values to obtain the PM₁₀ variability and trends due to other factors and processes, mainly PM₁₀ emissions and formation of secondary PM₁₀ due to trace gas emissions. After applying this process, the PM₁₀ variability was much lower, especially in winter where the ratio of adjusted over measured mean squared error was 0.27 on average over all considered sites. Moreover, PM₁₀ trends in winter were more negative after the adjustment for meteorology and they ranged between −1.25 μg m⁻³ yr⁻¹ and 0.07 μg m⁻³ yr⁻¹. The adjusted trends for the other seasons ranged between −1.34 μg m⁻³ yr⁻¹ and −0.26 μg m⁻³ yr⁻¹ in spring, −1.40 μg m⁻³ yr⁻¹ and −0.28 μg m⁻³ yr⁻¹ in summer and −1.28 μg m⁻³ yr⁻¹ and −0.11 μg m⁻³ yr⁻¹ in autumn. The estimated trends of meteorologically adjusted PM₁₀ were in general non-linear. The two urban street sites considered in the study, Bern and Lausanne, experienced the largest reduction in measured and adjusted PM₁₀ concentrations. This indicates a verifiable effect of traffic emission reduction strategies implemented during the past two decades. The average adjusted yearly trends for rural, urban background and urban street stations were −0.37, −0.53 and −1.2 μg m⁻³ yr⁻¹ respectively. The adjusted yearly trends for all stations range from −0.15 μg m⁻³ yr⁻¹ to −1.2 μg m⁻³ yr⁻¹ or −1.2% yr⁻¹ to −3.3% yr⁻¹.