Effects of continental boundary layer evolution, convection, turbulence and entrainment, on aerosol formation

Aerosol nucleation events occurring in the continental boundary layer over the boreal forest region in Finland, during the BIOFOR experiment, have bee n examined to elucidate the role of micrometeorology in promoting such even ts. Invariably, during the spring campaign of 1999, nucleation events occur red in Arctic and polar air masses during cold air outbreaks. Under clear-s ky conditions, typical of these synoptic meteorological patterns, the bound ary layer evolution was characterized by the rapid growth of a mixed layer, convection and strong entrainment, first from the residual later and later from the free troposphere. It was found that the freshly nucleated particl es were detected within two hours from the onset of strong turbulent kineti c energy, independent of how fast the boundary layer evolved. When consider ing the growth time from cluster size of approximate to 1 nm to detectable sizes of 3 nm, the nucleation and onset of strong turbulence coincided almo st exactly. The most likely site for nucleation to take place was the mixed layer or the entrainment zone, while the forest canopy and the free tropos phere could be excluded as the nucleation region. There are several possibl e explanations for the correlation between the onset of turbulence and nucl eation: (1) new aerosols or clusters may have been entrained from the resid ual layer into the mixed layer where they then (in the case of clusters) un derwent growth to detectable sizes; (2) two or more precursor gases may hav e been mixed with each other over the entrainment zone; (3) the adiabatic c ooling in the rising convective plumes and the turbulent fluctuation in tem perature and vapors by the entrainment flux may have enhanced aerosol forma tion; (4) a sudden decrease in preexisting aerosol due to dilution of the m ixed layer aerosol by entrained air may have reduced the vapor sink enough to initiate nucleation. However, the lack of vertical profile measurements of nucleation mode aerosols, precursor vapors and turbulent fluctuations th roughout and above the mixed-layer results in it remaining an open question as to which one of these processes dominates.