Air pollution due to particulate matter with an aerodynamic diameter ≤ 10 mm (PM 10 ) and diameter ≤ 2.5 mm (PM 2.5 ) was investigated using available measurements in Croatia. Hourly and daily PM 10 and PM 2.5 mass concentrations were analysed for urban and rural background stations. Urban and industrial PM 10 measurements from 2006 to 2014 and rural background PM 10 and PM 2.5 measurements from 2011 to 2014 were used to determine the main characteristics of regional and local air pollution related to particles and to assess the level of air pollution according to the defined limit values (CAFÉ Directive 2008/50/EC). Significant difference between coastal and continental urban and rural PM 10 levels was found with high PM 10 concentrations in continental areas and considerably lower PM 10 values at the coast. Substantially different spatial distribution of rural background PM concentrations with relatively high annual average concentration ~ 30 mg/m 3 at e.g. continental station Kopački rit, to only ~ 5 mg/m 3 at Hum located on the island of Vis was found. Strong spatial PM gradients from the continent towards the coast are resulting from the distinct differences in climatological characteristics between Croatia's inlands and the coastline and different physical processes affecting the PM concentrations in the two regions (e.g. sea breeze, deposition, wet removal, resuspension). Furthermore, distinct complex chemical mechanisms that depend on emission rate and PM composition and on meteorological parameters (sun radiation, humidity etc.) have different influence on the formation of secondary organic aerosol in marine and continental boundary layer. Ratio between rural PM 2.5 and PM 10 mass concentrations (PM 2.5 / PM 10 ) for all sites was investigated and compared to the results obtained in other European studies. Rural site-specific PM 2.5 / PM 10 ratios ranged from 0.6 to 0.9 during the warmer part of the year while during the colder part of the year ratios were higher and ranged from 0.85 to 0.98. The spatial gradient of rural PM 2.5 / PM 10 values is orientated from the coast to the continent implying that at the coast the PM concentrations are mainly composed of fine particles. with the corresponding PM concentrations and an initial source identification was conducted based on bivariate polar plots.

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<p><strong>Biochemical responses of oligotrophic Adriatic Sea surface layers to atmospheric deposition inputs</strong></p><p><strong> </strong></p><p>Frka<sup>1</sup>, A. Miliković<sup>1</sup>, A. Penezić<sup>1</sup>, S. Bakija Alempijević<sup>1</sup>, B. Gašparović<sup>1</sup>, S. Skejić<sup>2</sup>, D. Šantić<sup>2</sup>, S. Brzaj<sup>3</sup>, V. Džaja Grgičin<sup>3</sup>, S. Vidič<sup>3</sup>, I. Šimić<sup>4</sup>, I. Bešlić<sup>4</sup>, S. Žužul<sup>4</sup>, R. Godec<sup>4</sup>, G. Pehnec<sup>4</sup></p><p><sup>1</sup>Division for marine and environmental research, Ruđer Bošković Institute, Zagreb, Croatia</p><p><sup>2</sup>Institute of Oceanography and Fisheries, Split, Croatia</p><p><sup>3</sup>Croatian Meteorological and Hydrological Service, Zagreb, Croatia</p><p><sup>4</sup>Institute for Medical Research and Occupational Health, Zagreb, Croatia</p><p> </p><p>The atmosphere is a significant pathway by which both natural and anthropogenic material is transported from continents to both coastal and open seas. Once deposited through atmospheric deposition (AD) processing, atmospheric particulate matter (PM) provides the aqueous ecosystems with an external source of nutrients and pollutants. This, in turn, influences the organic matter (OM) production by the phytoplankton, changes CO<sub>2</sub> uptake and indirectly affects the climate. The input of AD is especially important in oligotrophic environments and it is expected to increase in the future scenarios of a warmer atmosphere with increased PM emissions and deposition rates. While the majority of the data related to the AD impacts generated so far in the Mediterranean have been conducted on its western and eastern regions, the effects of the AD inputs to oligotrophic surface waters of the Adriatic Sea sub-basin are unknown. This work is designed to assess the impact of AD on complex biochemical responses of Adriatic oligotrophic systems, considering the sea surface microlayer (SML) at the air-water interface.</p><p>Field campaign was conducted during the period of retrieval of sea surface oligotrophic conditions (February-July 2019) at the Martinska, Central Adriatic, Croatia. On-line black carbon (BC) concentrations were measured while the PM<sub>10</sub>, wet and total deposition samples as well as the SML and underlying water (ULW; 0.5 m depth) samples were collected simultaneously. The temporal dynamics of the SML biology as well as concentrations of  inorganic and organic constituents enabled the assessment of their sources and the nature of the enrichments taking place within the SML. The first comprehensive insight into concentration levels of macro nutrients (N, P), trace metals (eg. Cu, Pb, Cd, Ni, Zn, Co) and OM (including aromatic pollutants) in atmospheric samples, their transport history, source apportionment and deposition fluxes to the oligotrophic Adriatic area will be presented. Daily and seasonal variations of PM<sub>10</sub> composition were affected by local traffic and open-fire events as well as by local meteorological conditions and long-range transport. The BC contribution of biomass burning versus fossil fuel combustion changed seasonally. Source apportionment module of LOTOS-EUROS chemical transport model enabled identification and quantification of main source areas contributing to deposition of PM. The main PM contributor is a public power sector outside Croatia while other contributing sectors are energy production, traffic, residential combustion as well as shipping. First deposition fluxes estimates show reasonable agreement between model calculations and measured data, and could be used for more general assessments of atmospheric inputs.</p><p> </p><p><strong>Acknowledgment</strong>: This work has been supported by Croatian Science Foundation under the IP-2018-01-3105 project: Biochemical responses of oligotrophic Adriatic surface ecosystems to atmospheric deposition inputs.</p>