Seasonal variations in black and organic carbon wet and dry deposition rates at SMEAR III station (60oN), Finland

Black carbon (BC) and organic carbon (OC, including brown carbon BrC) aerosols in the atmosphere, and their wet and dry deposition, are important for their climatic and cryospheric effects. Seemingly small amounts of BC in snow, of the order of 10–100 parts per billion by mass (ppb), have been shown to decrease its albedo by 1–5 %. Due to the albedo-feedback mechanism, surface darkening accelerates snow and ice melt. In snow, the temporal variability of light absorbing aerosols, such as BC, depends both on atmospheric and cryospheric processes, mostly on sources and atmospheric transport, and dry and wet deposition processes, as well as post-depositional snow processes.

We started a new research activity on BC and OC wet and dry deposition at Helsinki Kumpula SMEAR III station (60°12 N, 24°57 E, Station for Measuring Ecosystem-Atmosphere Relations, https://www.atm.helsinki.fi/SMEAR/index.php/smear-iii). The work included winter, spring, summer and autumn deposition samples during January 2019 - June 2020 (sampling is currently on hold). In winter, wet deposition consisted of snowfall and rainwater samples. Dry deposition samples were separately collected in 2020. For sample collection, a custom-made device, including a heating-system, was applied. The samples were analyzed using the OCEC analyzer of the Finnish Meteorological Institute’s aerosol laboratory, Helsinki, Finland. The special features in our deposition data are: 

• seasonal BC, OC, and TC (total carbon, the sum of BC and OC) deposition data for an urban background station at 60 ^oN
• precipitation received as either water or snow  
• dry deposition samples included (only in 2020)
• data as wet and dry deposition rates [concentration/time/area]
• simultaneous atmospheric measurements of the SMEAR III station

Since our deposition samples are collected manually, the data are non-continuous, yet they allow us to provide deposition rates. Such data can be utilized in various modeling approaches including, for example, climate and long-range transport and deposition modeling. According to our knowledge, these data are the first BC (determined as elemental carbon, EC), OC and TC wet and dry deposition data to represent Finland. Our sampling location, north of 60 deg. N, can be useful for other high-latitude studies and Arctic assessments, too.

Acknowledgements. We gratefully acknowledge support from the Academy of Finland NABCEA-project of Novel Assessment of Black Carbon in the Eurasian Arctic (no. 296302) and the Academy of Finland Flagship funding (grant no. 337552).