Weakness of the weekend effect in aerosol number concentrations

Weekday related anthropogenic aerosol emissions have been suggested to affect regional climate via indirect aerosol effects. I studied the variability of potential cloud condensation nuclei using measurements of number size distributions of Cloud Condensation Nuclei (CCN)-sized aerosol particles and CCNs measured at several European regional background stations, located at a wide variety of environments. With notably rare exceptions, there were no statistically significant difference between concentrations on different weekdays. I further analysed the concentration time-series of four long-period datasets in Germany and Finland with wavelet analysis. Outside of urban areas, very little weekday-connected variability was found. The lack of 7-day variability outside of cities is in contrast of earlier studies in this field, which used mostly particle mass as the representative measure of aerosol concentration. A time-scale and variability analysis showed that PM10 and PM2.5 are more sensitive for the weekly variation than CCN-sized particles. Using mass-based variations as a proxy for short-term variations of CCN particle numbers can thus overestimate the weekend effect for these particles. The results of this study do not support aerosol indirect effects from 50 to 500 nm diameter particles as a major contributor on potential weekday connected variations in European meteorology.

► The weekly variation of cloud condensation nuclei sized aerosol concentrations was studied. ► No clear signal of weekly variation in the number of CCN was found in the European background air. ► Time-scale analysis shows that most mass-based measurements overestimate the weekly variability of CCNs. ► Using PM or optical measurements to evaluate the short-term variability of NCCN is not recommended. ► The indirect aerosol effects are not likely to cause weekly variation in European background.