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. Global observations and model studies indicate that new particle formation(NPF) in the upper troposphere (UT) and subsequent particles supply 40 %–60 % of cloud condensation nuclei (CCN) in the lower troposphere, thusaffecting the Earth's radiative budget. There are several plausiblenucleation mechanisms and precursor species in this atmospheric region,which, in the absence of observational constraints, lead to uncertainties inmodeled aerosols. In particular, the type of nucleation mechanism andconcentrations of nucleation precursors, in part, determine the spatialdistribution of new particles and resulting spatial distribution of CCN fromthis source. Although substantial advances in understanding NPF have beenmade in recent years, NPF processes in the UT in pristine marine regions arestill poorly understood and are inadequately represented in global models. Here, we evaluate commonly used and state-of-the-art NPF schemes in aLagrangian box model to assess which schemes and precursor concentrationsbest reproduce detailed in situ observations. Using measurements of aerosolsize distributions (0.003 &lt; Dp &lt; 4.8 µm) in the remotemarine troposphere between ∼0.18 and 13 km altitude obtainedduring the NASA Atmospheric Tomography (ATom) mission, we show that highconcentrations of newly formed particles in the tropical UT over both theAtlantic and Pacific oceans are associated with outflow regions of deepconvective clouds. We focus analysis on observations over the remote PacificOcean, which is a region less perturbed by continental emissions than theAtlantic. Comparing aerosol size distribution measurements over the remotePacific with box model simulations for 32 cases shows that none of the NPFschemes most commonly used in global models, including binary nucleation ofsulfuric acid and water (neutral and ion-assisted) and ternary involvingsulfuric acid, water, and ammonia, are consistent with observations,regardless of precursor concentrations. Through sensitivity studies, we findthat the nucleation scheme among those tested that is able to explain mostconsistently (21 of 32 cases) the observed size distributions is that ofRiccobono et al. (2014), which involves both organic species and sulfuricacid. The method of Dunne et al. (2016), involving charged sulfuricacid–water–ammonia nucleation, when coupled with organic growth of thenucleated particles, was most consistent with the observations for 5 of 32cases. Similarly, the neutral sulfuric acid–water–ammonia method of Napari (2002), when scaled with a tuning factor and with organic growth added, wasmost consistent for 6 of 32 cases. We find that to best reproduce bothnucleation and growth rates, the mixing ratios of gas-phase organicprecursors generally need to be at least twice that of SO2, a proxy fordimethyl sulfide (DMS). Unfortunately, we have no information on the natureof oxidized organic species that participated in NPF in this region. Globalmodels rarely include organic-driven nucleation and growth pathways in UTconditions where globally significant NPF takes place, which may result inpoor estimates of NPF and CCN abundance and contribute to uncertainties inaerosol–cloud–radiation effects. Furthermore, our results indicate that theorganic aerosol precursor vapors may be important in the tropical UT abovemarine regions, a finding that should guide future observational efforts.

The potential role of organics in new particle formation and initial growth in the remote tropical upper troposphere