Organic contribution to sub-micron aerosol evolution over a boreal forest - a case study

During the spring and summer of 1998 and spring of 1999 three major interna tional campaigns have been performed at the SMEAR II Station (Station for M easuring Forest Ecosystem-Atmosphere Relations) in Hyytiala, Finland. The m easurements have been conducted as part of the BIOFOR programme, the object ives of which were to elucidate the main factors leading to nucleation and growth of new particles over forested regions. For typical nucleation event s encountered in this environment, total particle concentration and surface area reduce rapidly due to mixing processes in the late morning prior to a nucleation event. After this reduction in pre-existing aerosol, ultra-fine particles (3-10 nm) appear, and grow rapidly lover several hours) towards accumulation mode sizes ( > 100 nm). Volatility measurements of new particl e composition, once they have grown into accumulation mode sizes, illustrat e a significant organic contribution to aerosol mass. Suggested contributor s to this organic mass are pinic acid and/or cis-pinoic acid.