Text S1
Aerosol size distributions between 10 – 100 nm were measured with a
custom-built nano-Scanning Mobility Particle Sizer (nSMPS) consisting of
a nano Differential Mobility Analyzer (TSI 3085) and a Condensation
Particle Counter (TSI 3010) [Ortega et al. , 2019]. Scan times
were 30s and continuous measurements were made during both ascending and
descending voltage ramps. Aerosol size distributions were also measured
by three optical probes. Two Ultra High Sensitivity Aerosol
Spectrometers (UHSAS; Droplet Measurement Technologies) [Kupc et
al. , 2018], one mounted on a wing and one mounted in a rack inside
the plane, measured aerosol particles, with effective size ranges
between 0.07 - 1.0 µm for the rack-mounted and 0.08 - 1.0 µm for the
wing-mounted unit. For Figs. 3a and S1, nSMPS and UHSAS measurements
were combined to create continuous size distributions by averaging
dN/dlogDp values in the overlapping size ranges. A wing mounted Passive
Cavity Aerosol Spectrometer (PCASP; Droplet Measurement Technologies)
also measured particles between 0.1 and 3.0 µm.
A single column Cloud Condensation Nuclei Counter (CCNC; Droplet
Measurement Technologies) was used to measure CCN number concentrations
(NCCN) at set supersaturations between ≈ 0.1 – 0.6%
[Roberts and Nenes , 2005] for at least 5 min at each
supersaturation. Because supersaturation is a function of pressure, the
CCNC sampled from a constant pressure inlet at 500 mb, and laboratory
calibrations using size selected ammonium sulfate were performed at this
pressure. Refractory black carbon (rBC) number and mass concentrations
were measured with a Single Particle Soot Photometer (SP2; Droplet
Measurement Technologies) [Schwarz et al. , 2006] which uses a
1064 nm Nd:YAG laser to heat absorbing material > 70 nm in
diameter to its vaporization temperature and measures the resulting
incandescence. Scattered laser light is used to determine total aerosol
size and incandescence to determine black carbon mass. The SP2 sample
line was diluted with HEPA-filtered ambient air to prevent signal
saturation. The single scattering albedo (SSA) of the aerosol was
calculated by dividing the difference of particulate absorption measured
by a photoacoustic absorption spectrometer (PAS) [Foster et
al. , 2019] and the extinction measured by a CAPS
PMSSA [Onasch et al. , 2015] by the CAPS
extinction. This approach was taken to avoid potential errors in the
scattering channel of the CAPS PMSSA due to truncation
errors from larger particles or nonlinearity that can occur in the
optically thick plumes encountered during this study.
Submicron nonrefractory aerosol mass and composition were measured with
a High-Resolution time-of-flight Aerosol Mass Spectrometer (HR-AMS;
Aerodyne Inc.) with a pressure controlled inlet [Garofalo et
al. , 2019]. For single particle composition, ambient particles were
collected with the NCAR Solid Diffuser Inlet (SDI) (based on the
University of Hawaii design of McNaughton et al. [2007]), and
a counterflow virtual impactor (CVI) was used to collect evaporated
cloud droplet residual particles [Noone et al. , 1988;Twohy et al. , 2003]. These individual aerosol and droplet
residual particles were periodically collected onto carbon-coated
electron microscope grids using a custom two-stage impactor. The
impactor’s 50% cut sizes were about 0.5 and 0.1 µm dry diameter
(assuming spherical particle densities of 1.5 g cm-3at typical sampling pressures of 600 mb).
The single-particle elemental composition of a subset of samples was
measured via analytical Scanning Transmission Electron Microscopy (STEM)
and X-ray spectroscopy at Colorado State University (JEOL JEM-2100F 200
kV microscope with an Oxford Max 80 Energy Dispersive Spectroscopy
system). Particles were dividing into categories based on their
elements and morphology. Organic particles were identified as those
containing carbon and oxygen detectable above the background substrate,
as well as sometimes potassium, nitrogen, sulfur and sodium. Because of
their ubiquity in recent smoke plumes, these organic particles were
presumed to be of biomass burning origin. They were also distinguishable
from soot aggregates and biological entities by their quasi-round shape.
A mineral dust, ash and metals category was for irregular particles with
elevated levels of silicon, aluminum, iron, magnesium, zinc, titanium,
often with elevated oxygen. Particles in the sulfate category contained
quasi-spherical particles with sulfur and oxygen and without carbon
detectable above background levels. Sea-salt based sea-spray particles
were identified as those containing sodium with chlorine and/or sulfur
with minor potassium, magnesium, and other elements present in seawater.
Cloud droplet number concentrations Nd were measured
with a Droplet Measurement Technologies cloud droplet probe (CDP, 2-50
µm) modified with a pinpoint aperture to reduce errors due to
coincidence [Lance et al. , 2010]. This modification is
expected to minimize errors in droplet number concentration up to about
1850 cm-3, while measurements in clouds with higher
droplet concentrations may under-report number concentrations and
over-report droplet size [McFarquhar et al. , 2017]. In-cloud
particles larger than 75 µm physical diameter were measured with a
Particle Measuring Systems 2D-C optical array probe, modified with
faster electronics for better response at high airspeeds, mounted on the
left outboard wingpod location. Bulk cloud liquid water content was
measured using the CSIRO/King hot-wire probe [King et al. ,
1978].