Abstract. We present the laboratory results of immersion freezing efficiencies of
cellulose particles at supercooled temperature (T) conditions. Three types of
chemically homogeneous cellulose samples are used as surrogates that
represent supermicron and submicron ice-nucleating plant structural
polymers. These samples include microcrystalline cellulose (MCC), fibrous
cellulose (FC) and nanocrystalline cellulose (NCC). Our immersion freezing
dataset includes data from various ice nucleation measurement techniques
available at 17 different institutions, including nine dry dispersion
and 11 aqueous suspension techniques. With a total of 20 methods, we
performed systematic accuracy and precision analysis of measurements from
all 20 measurement techniques by evaluating T-binned (1 ∘C)
data over a wide T range (−36 ∘C <T<-4 ∘C). Specifically, we intercompared the geometric surface
area-based ice nucleation active surface site (INAS) density data derived from
our measurements as a function of T, ns,geo(T). Additionally, we also
compared the ns,geo(T) values and the freezing spectral slope parameter
(Δlog(ns,geo)/ΔT) from our measurements to previous
literature results. Results show all three cellulose materials are
reasonably ice active. The freezing efficiencies of NCC samples agree
reasonably well, whereas the diversity for the other two samples spans
≈ 10 ∘C. Despite given uncertainties within each
instrument technique, the overall trend of the ns,geo(T) spectrum traced
by the T-binned average of measurements suggests that predominantly
supermicron-sized cellulose particles (MCC and FC) generally act as more
efficient ice-nucleating particles (INPs) than NCC with about 1 order of
magnitude higher ns,geo(T).
Phenolic compounds are common constituents
of atmospheric aerosols.
They form by pyrolysis of lignin and by biodegradation of plant material
and are commonly found in biomass burning plumes, resuspended soil
dust, and in anthropogenic secondary organic aerosols (SOA). In this
study, we show that reactions of Fe(III), a major constituent of mineral
dust, with several phenolic compounds (guaiacol, catechol, syringol,
o- and p-cresol) that are common in atmospheric aerosols, result in
the formation of water insoluble light-absorbing compounds and reduced
Fe(II). The study was conducted under acidic conditions (pH = 1–2),
relevant for areas impacted by biomass burning, anthropogenic emissions,
and mineral dust. The reaction products have been characterized using
a high-performance liquid chromatography coupled to photodiode array
and high-resolution mass spectrometry detectors, UV–visible
spectroscopy, X-ray photoelectron spectroscopy, and thermal gravimetric
analysis. The major identified chromophores are oligomers of the reaction
precursors that efficiently absorb light between 300 and 500 nm. The
amounts of oligomers vary significantly between the systems studied.
The highest amount was observed for guaiacol and catechol, and the
least were detected in the syringol experiments, suggesting that the
oligomerization proceeds through carbon–carbon coupling preferred
at para- and ortho- positions, coupled to the reduction of Fe(III)
to Fe(II). The results suggest that aqueous-phase radical reactions
of phenolic compounds may be an efficient source of light-absorbing
atmospheric organic compounds (brown carbon) that play important roles
in Earth’s radiative forcing on global and regional scales
and of quinones that can affect health.
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