Abstract
Particle Investigations at a Northern Ozarks Tower: NOx, Oxidant, Isoprene Research (PINOT NOIR) were conducted in a Missouri forest dominated by isoprene emissions from May to October 2012. This study presents results of new particle formation (NPF) and the growth of new particles to cloud condensation nuclei (CCN)-active sizes (∼100 nm) observed during this field campaign. The measured sub-5 nm particles were up to ∼20,000 cm−3 during a typical NPF event. Nucleation rates J1 were relatively high (11.0 ± 10.6 cm−3 s−1), and one order of magnitude higher than formation rates of 5 nm particles (J5). Sub-5 nm particle formation events were observed during 64% of measurement days, with a high preference in biogenic volatile organic compounds (BVOCs)- and SO2-poor northwesterly (90%) air masses than in BVOCs-rich southerly air masses (13%). About 80% of sub-5 nm particle events led to the further growth. While high temperatures and high aerosol loadings in the southerly air masses were not favorable for nucleation, high BVOCs in the southerly air masses facilitated the growth of new particles to CCN-active sizes. In overall, 0.4–9.4% of the sub-5 nm particles grew to CCN-active sizes within each single NPF event. During a regional NPF event period that took place consecutively over several days, concentrations of CCN size particles increased by a factor of 4.7 in average. This enhanced production of CCN particles from new particles was commonly observed during all 13 regional NPF events during the campaign period.Copyright 2014 American Association for Aerosol Research
Highlights
New particle formation (NPF) can account for a significant production of global cloud condensation nuclei (CCN)
This study presents results of new particle formation (NPF) and the growth of new particles to cloud condensation nuclei (CCN)-active sizes (»100 nm) observed during this field campaign
NPF occurred during 85% of the days associated with clean northwesterly air masses, in contrast to only 9% of the days with biogenic volatile organic compounds (BVOCs)-rich southerly air masses
Summary
New particle formation (NPF) can account for a significant production of global cloud condensation nuclei (CCN). Kiendler-Scharr and colleagues have suggested, from their plant chamber experiments, that isoprene likely suppresses NPF when isoprene-to-MT ratios exceed a certain threshold (e.g., >1) (Kiendler-Scharr et al 2009; Kiendler-Scharr et al 2012) Consistent with their laboratory studies, field measurements in a mixed deciduous forest in Michigan showed rare NPF occurrences, under high isoprene-to-MT ratio conditions (Kanawade et al 2011). A large number of observations were made in Amazon rainforests with high emissions of isoprene, and these studies have consistently showed an absence of NPF (Poschl et al 2010; P€ohlker et al 2012) Contrary to such hypothesis on the possible suppression of NPF, Surratt and colleagues suggested that oxidation products of isoprene (e.g., IEPOX) can promote the growth of new particles (Surratt et al 2010; Lin et al 2013). The campaign site was located in the ecologically important transitional zone between the hardwood and grassland regions of the central U.S In the present study, we have investigated survival rates of new particles to CCN-active size particles (»100 nm), and the enhancement of CCN particles during regional NPF events
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