Infrequent new particle formation in a coastal Mediterranean city during the summer

Two field campaigns were conducted in Patras during the summers of 2020 and 2021 in order to quantify the nucleation frequency in this area. Out of the 120 days with available measurements only 15 (12%) new particle formation events occurred. This low frequency is surprising given the high sunlight intensity, sulfur availability and low particle matter levels in the area during the study period. On the other hand, nucleation mode particles with average diameters of 20–40 nm appeared during the afternoon on 31% of the days. These particles did not grow, and the initial stages of their formation could not be observed. The corresponding days are traditionally classified as undefined events in similar studies. Our analysis suggests that these nucleation mode particles were not associated with emissions close to the measurement site but rather they had been formed several hours earlier in an area 100–150 km northeast of the field site and had grown significantly by the time they reached Patras. The air mass history suggested that new particle formation often took place in the vicinity of an area with significant agricultural activity and therefore high emissions of ammonia and amines. The term “transported events” can be used to better characterize these observed undefined days. The relatively high emissions of biogenic volatile organic compounds in western Greece, where Patras is located, did not appear to assist in the local formation of new particles.