The potential role of organics in new particle formation and initial growth in the remote tropical upper troposphere
Authors: Kupc Agnieszka , Williamson Christina J. , Hodshire Anna L. , Kazil Jan , Ray Eric , Bui T. Paul , Dollner Maximilian , Froyd Karl D. , McKain Kathryn , Rollins Andrew , Schill Gregory P. , Thames Alexander , Weinzierl Bernadett B. , Pierce Jeffrey R. , Brock Charles A. 
Published: December 2020

Abstract. 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, thus
affecting the Earth's radiative budget. There are several plausible
nucleation mechanisms and precursor species in this atmospheric region,
which, in the absence of observational constraints, lead to uncertainties in
modeled aerosols. In particular, the type of nucleation mechanism and
concentrations of nucleation precursors, in part, determine the spatial
distribution of new particles and resulting spatial distribution of CCN from
this source. Although substantial advances in understanding NPF have been
made in recent years, NPF processes in the UT in pristine marine regions are
still poorly understood and are inadequately represented in global models. Here, we evaluate commonly used and state-of-the-art NPF schemes in a
Lagrangian box model to assess which schemes and precursor concentrations
best reproduce detailed in situ observations. Using measurements of aerosol
size distributions (0.003 < Dp < 4.8 µm) in the remote
marine troposphere between ∼0.18 and 13 km altitude obtained
during the NASA Atmospheric Tomography (ATom) mission, we show that high
concentrations of newly formed particles in the tropical UT over both the
Atlantic and Pacific oceans are associated with outflow regions of deep
convective clouds. We focus analysis on observations over the remote Pacific
Ocean, which is a region less perturbed by continental emissions than the
Atlantic. Comparing aerosol size distribution measurements over the remote
Pacific with box model simulations for 32 cases shows that none of the NPF
schemes most commonly used in global models, including binary nucleation of
sulfuric acid and water (neutral and ion-assisted) and ternary involving
sulfuric acid, water, and ammonia, are consistent with observations,
regardless of precursor concentrations. Through sensitivity studies, we find
that the nucleation scheme among those tested that is able to explain most
consistently (21 of 32 cases) the observed size distributions is that of
Riccobono et al. (2014), which involves both organic species and sulfuric
acid. The method of Dunne et al. (2016), involving charged sulfuric
acid–water–ammonia nucleation, when coupled with organic growth of the
nucleated particles, was most consistent with the observations for 5 of 32
cases. Similarly, the neutral sulfuric acid–water–ammonia method of Napari (2002), when scaled with a tuning factor and with organic growth added, was
most consistent for 6 of 32 cases. We find that to best reproduce both
nucleation and growth rates, the mixing ratios of gas-phase organic
precursors generally need to be at least twice that of SO2, a proxy for
dimethyl sulfide (DMS). Unfortunately, we have no information on the nature
of oxidized organic species that participated in NPF in this region. Global
models rarely include organic-driven nucleation and growth pathways in UT
conditions where globally significant NPF takes place, which may result in
poor estimates of NPF and CCN abundance and contribute to uncertainties in
aerosol–cloud–radiation effects. Furthermore, our results indicate that the
organic aerosol precursor vapors may be important in the tropical UT above
marine regions, a finding that should guide future observational efforts.