Particulate matter time-series and Köppen-Geiger climate classes in North America and Europe

Four years of time-series data on the particulate matter (PM) concentrations from 801 monitoring stations located in Europe and 234 stations in North America were analyzed. Using k-means clustering with distance correlation as a measure for similarity, 5 distinct PM clusters in Europe and 9 clusters across the United States of America (USA) were found. This study shows that meteorology has an important role in controlling PM concentrations, as comparison between Köppen-Geiger climate zones and identified PM clusters revealed very good spatial overlapping. Moreover, the Köppen-Geiger boundaries in Europe show a high similarity to the boundaries as defined by PM clusters. The western USA is much more diverse regarding climate zones; this characteristic was confirmed by cluster analysis, as 6 clusters were identified in the west, and only 3 were identified on the eastern side of the USA. The lowest similarity between PM time-series in Europe was observed between the Iberian Peninsula and the north Europe clusters. These two regions also show considerable differences, as the cold semi-arid climate has a long and hot summer period, while the cool continental climate has a short summertime and long and cold winters. Additionally, intra-continental examination of European clusters showed meteorologically driven phenomena in autumn 2011 encompassing a large European region from Bulgaria in the south, Germany in central Europe and Finland in the north with high PM concentrations in November and a decline in December 2011. Inter-continental comparison between Europe and the USA clusters revealed a remarkable difference between the PM time-series located in humid continental zone. It seems that because of higher shortwave downwelling radiation (≈210 W m−2) over the USA's continental zone, and consequently more intense production of secondary aerosols, a summer peak in PM concentration was observed. On the other hand, Europe's humid continental climate region experiences lower solar radiation (≈180 W m−2); consequently, the elevated summer-time PM concentrations were not detected.