Significant spatial gradients in new particle formation frequency in Greece during summer
Authors: Aktypis Andreas , Kaltsonoudis Christos , Matrali Angeliki , Vasilakopoulou Christina N. , Mihalopoulos Nikolaos , Kalkavouras Panayiotis , Bougiatioti Aikaterini , Kalivitis Nikos , Eleftheriadis Konstantinos , Vratolis Stergios , Gini Maria I. , Kouras Athanasios , Lazaridis Mihalis , Chatoutsidou Sofia Eirini , Nenes Athanasios , Pandis Spyros N. 
Published: February 2023

Homogeneous nucleation is an important source of new particles in the atmosphere worldwide. The resulting newly formed stable nuclei can grow to larger sizes and affect air quality and climate. Unexpected significant spatial variability of the nucleation frequency has been observed in Greece in the only previous relative study: very high frequency in Thessaloniki, intermediate in Eastern Crete and low in Patras (Patoulias et al., 2018). Our hypothesis is that Greece may be an excellent natural laboratory to investigate the factors affecting nucleation and to understand the reasons behind this surprising variability.Extensive continuous aerosol size distribution measurements took place during two summers (2020 and 2021) as part of the PANACEA project in 11 different locations: Patras, Xanthi, Ioannina, Finokalia, Athens, Thessaloniki, Sifnos, Chania, Costa Navarino (NEO), Lesvos and Mt. Helmos. The instrumentation used included a number of scanning mobility particle sizers (SMPS) for the measurement of the particles and a suite of gas monitors for measuring SO2, NOx, NH3 and CO in selected sites. A particle size magnifier (PSM) was deployed in the Patras site during the 2021 campaign, providing valuable information regarding nanoparticles with diameter down to 1 nm.The observations suggest that indeed the nucleation frequency during summer in Greece varies from close to zero in the southwestern parts of the country to more than 70% in the northern central and eastern regions. The analysis of the measurements in the various sites shows that the proximity to coal-fired power plants is a major factor affecting the nucleation frequency. North-eastern and northern airmasses passing over such locations in the Balkans and Western Turkey where strongly associated with nucleation. Also, the emissions of ammonia during summer, suggest that it exhibits similar spatial gradients with the observed nucleation frequency and may be controlling nucleation in Greece. The corresponding measurements in each site, were also used to estimate the corresponding particle growth and formation rates and the condensation and coagulation sinks.The detailed analysis of the measurements in Patras, Western Greece, suggests that nucleation was infrequent in this location (12%), but particles that were formed a few hours earlier over central Greece are often transported to this area after they have grown to sizes of 20-30 nm. The air mass history suggested that new particle formation often took place in the vicinity of an area 100-150 km northeast of Patras, with significant agricultural activity and therefore, high emissions of ammonia and amines. 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.