Atmospheric Sciences Division Seminar

Forests are one of the major global sources of secondary aerosols and thus affect climate and human health, but the particle formation mechanisms over forest areas are poorly understood. While it has been generally believed that oxidation products of biogenic volatile organic compounds (BVOCs) emitted from trees can contribute to secondary aerosol formation, a recent plant chamber experiment has shown that isoprene suppresses new particle formation (NPF) over trees and thereby reduces aerosols global cooling effect over forests [Kiendler-Scharr et al., 2009]. Our observations made in a rural, mixed deciduous forest in Michigan during the summer show surprisingly rare occurrence of NPF in a forest with high emission rates of isoprene. No NPF events were observed during the 6 weeks of measurements, except two brief ultrafine particle events that took place during the early evening as opposed to the noontime where NPF was typically observed elsewhere. Sulfuric acid concentrations were in the 106 cm-3 range, and under very low preexisting aerosol particles, this should be a sufficient condition for NPF even during the summer. The ratio of emitted isoprene carbon to monoterpene carbon at this site was similar to that in Amazon rainforest (ratio > 10 at both sites), where NPF was never observed at the ground level, compared with a ratio < 0.2 in Finland boreal forests where frequent NPF was reported. The two early evening ultrafine particle events were associated with the transported anthropogenic sulfur plumes and the ultrafine particles formed via ion induced nucleation. These results imply that NPF does not occur in a mixed deciduous forest during the summer due to isoprene suppression effects and occurs only under the influence of anthropogenic plumes. Changes in land cover and environmental conditions could modify the isoprene suppression of NPF in some forest regions resulting in a radiative forcing that could influence climate. In this talk, I will also bri