The observational part of our research is largely based on measurements carried out at the SMEAR III station located in Helsinki, Finland. We have two active sites, where the fluxes of CO2, H2O, heat and aerosol particles are continuosly measured using the eddy covariance (EC) technique (Fig. 1). The two sites represent different land uses: the semi-urban Kumpula (blue) has been running since 2004 (Vesala et al. 2008, Järvi et al. 2009) and the urban site, Hotel Torni (red, break in the measurements in 2020 due to building renovation),  since 2009 in the city centre representing a highly built-up land cover (Nordbo et al. 2013, Kurppa et al. 2015). Within the approximate footprint of the Kumpula site, half of the surface is covered with vegetation, whereas in city centre the fraction is only 22%. Kumpula is also ICOS (Integrated Carbon Observation System) associated ecosystem station. In addition to the long-term measurements, we have conducted several EC measurement campaigns related to other compounds like COS, VOCs, CO and N2O.

With the aid of the observations, we try to understand how different urban land uses affect the surface exchange processes with particular emphasis on the role of vegetation. We have also extensive ecophysiological observations made in Helsinki area to understand in detail the gas exchange and functioning of urban vegetation. The observations are also used in urban land surface model development.

The current and past projects are

• Research Infrastructures Services Reinforcing Air Quality Monitoring Capacities in European Urban & Industrial AreaS (RI-Urbans), 2021-2025
• CO-CARBON, 2020-2023
• CarboCity, 2019-2023
• Pan-Eurasian Experiment (PEEX), 2015 - present
• Integrated Carbon Observations System (ICOS), 2008 - present
• The exchange of carbon dioxide, heat and water in urban areas, PI: Leena Järvi, The Academy of Finland post-doctoral fellow, 2010 - 2013
• The FP7 project BRIDGE (sustainaBle uRban plannIng Decision support accountinG for urban mEtabolism), PI: Timo Vesala, 2007 - 2011
• Micrometeorology and air quality in urban areas, PI: Leena Järvi, Maj and Tor Nessling Foundation, 2007 - 2010

Figure: The locations of the two eddy covariance (EC) stations in Helsinki (Karsisto et al. 2015). The 1 km circular areas around the station approximate the source areas for the EC measurements.

We actively participate to the development of the Surface Urban Energy and Water Balance Scheme SUEWS (Järvi et al. 2011, 2019, Ward et al. 2016) in collaboration with the Prof. Sue Grimmond a the University of Reading. The model simulates the energy and water balance components and carbon dioxide surface exchange through the use of commonly measured meteorological variables and information on the surface cover in a neighborhood scale. The model is part of the Urban Multi-Scale Environmental Prediction Tool (UMEP, Lindberg et al. 2017). Besides model development, we use SUEWS to examine both long- and short-term behavior of the surface balances in different urban areas. Current study interests are on  carbon dioxide surface exchange  and improved parametererisation of particularly the biogenic components in urban areas. The current and past research projects are

• Pilot Application in Urban Landscapes - towards integrated city observatories for greenhouse gases (ICOS-Cities), PI: Werner Kutch, EU H2020,
• Individuals, communities and municipalities mitigating climate change by carbon smart green space (CO-CARBON), PI: Leena Järvi, The Academy of Finland, 2020 - 2023
• Urban green space solutions in carbon neutral cities (CarboCity), PI: Leena Järvi, The Academy of Finland, 2019 - 2022
• Modelling of the surface energy and water balances in high-latitude cities, PI: Leena Järvi, Maj and Tor Nessling foundation, 2014 - 2017
• The exchange of carbon dioxide, heat and water in urban areas, PI: Leena Järvi, The Academy of Finland post-doctoral fellow, 2010 - 2013

Figure: Modelled energy balance at the urban site (Rl) in Montreal
during (a) cold snow, (b) warm snow and (c) snow-free periods in 2007-2008 using SUEWS (Järvi et al. 2014).

We develop and use the Large Eddy Simulation model PALM to examine the flow fields and air quality in Helsinki. We have added the sectional aerosol particle model (SALSA) in PALM so that it can be used to simulate aerosol particle size distribution within complex urban landscapes. Current research topics include the impact of urban planning on local air quality in different cities, structure of coastal urban boundary layer, impact of thermal turbulence on local air quality. We also further develop the model to include more comprehensive aerosol dynamics. Current and past reseach projects are

• Sustainable urban development emerging from the merger of cutting edge Climate, Social and Computer Sciences (CouSCOUS), PI: Laura Ruotsalainen, Academy of Finland, 2020-2024
• Hienon resoluution ilmanlaatumallinnus kaupunkisuunnittelun tukena (Fine resolution air quality modelling as support of urban planning), PI: Leena Järvi, Helsinki Metropolitan Region Urban Research Program, 2017-2018
• Towards reliable prediction of urban climate using novel methodology development, PI: Mikko Auvinen, Maj and Tor Nessling foundation, 2017-2018
• Modelling air quality and ventilation inside city blocks using large-eddy simulations, PI: Mona Kurppa, Doctoral School in Atmospheric Sciences of the UHEL, 2017-2020
• Effect of building structures on the ventilation of Helsinki city boulevards, PI: Antti Hellsten (FMI), Helsinki City Planning Department, 2016
• Large eddy simulations of a realistic urban surface, PI: Timo Vesala, CSC Grand Challenge, 2014 - 2015

Figure: Flow field in central Helsinki as simulated using PALM model using detailed surface model (M. Auvinen).