Isoprene suppression of new particle formation in a mixed deciduous forest
Production of new particles over forests is an im- portant source of cloud condensation nuclei that can affect climate. While such particle formation events have been widely observed, their formation mechanisms over forests are poorly understood. Our observations made in a mixed deciduous forest with large isoprene emissions during the summer displayed a surprisingly rare occurrence of new par- ticle formation (NPF). Typically, NPF events occur around noon but no NPF events were observed during the 5 weeks of measurements. The exceptions were two evening ultra- fine particle events. During the day, sulfuric acid concentra- tions were in the 106 cm−3 range with very low preexisting aerosol particles, a favorable condition for NPF to occur even during the summer. The ratio of emitted isoprene carbon to monoterpene carbon at this site was similar to that in Ama- zon rainforests (ratio >10), where NPF events are also very rare, compared with a ratio <0.5 in Finland boreal forests, where NPF events are frequent. Our results suggest that large isoprene emissions can suppress NPF formation in forests al- though the underlying mechanism for the suppression is un- clear. The two evening ultrafine particle events were asso- ciated with the transported anthropogenic sulfur plumes and ultrafine particles were likely formed via ion-induced nucle- ation. Changes in landcover and environmental conditions could modify the isoprene suppression of NPF in some for- est regions resulting in a radiative forcing that could have influence on the climate.