The Biogenic Aerosols - Effects on Clouds and Climate (BAECC) experiment provided a bridge from a 19-year in situ observation record to the impact of biogenic aerosol on clouds, precipitation, and climate. This was achieved by simultaneous observations of precursor vapor emission, aerosol, cloud and precipitation microstructure. This data set was used to: 1) link precursor emissions and aerosol, 2) link aerosol at the surface to aerosol in the mixed layer, and 3) investigate the aerosol indirect effect on clouds and precipitation. To achieve those goals, the Atmospheric Radiation Measurement Mobile Facility (AMF) was deployed in Hyytiälä, Finland, for a period of 7.5 months, starting in January 2014, to capture the seasonal variability of biogenic aerosol at this boreal environment site. The experiment took place at the University of Helsinki SMEAR II research station, which has been monitoring biosphere-atmosphere interactions continuously since 1996.

The AMF observations were supplemented by tower- and surface-based measurements of aerosol and precursor gases. During intensive observational periods, aircraft observations of aerosol microphysics were performed. The experiment also benefited from existing measurements of precipitation provided by the Finnish Meteorological Institute observational network. The 7.5-month data set was placed in perspective with the long-time series available from Hyytiälä. It was utilized in modeling efforts ranging from process models to global climate models, capitalizing on the ability to perform radiative transfer calculations with full closure.

In Finland, a high-latitude country, the majority of precipitation is initiated by ice-phase processes (Mason 1971). Since part of the BAECC campaign took place during winter, surface-based observations of solid precipitation microphysics, that is, particle size distribution, particle habit, and their physical properties, can be used to infer what cloud-to-precipitation processes take place and guide retrievals based on remote sensing observations. To take advantage of this opportunity, an intensive observation period termed the BAECC Snowfall Experiment (SNEX), focusing on snowfall, was undertaken from 1 February through to 30 April 2014.