7 research outputs found
Reactive processing of formaldehyde and acetaldehyde in aqueous aerosol mimics: Surface tension depression and secondary organic products
The reactive uptake of carbonyl-containing volatile organic compounds (cVOCs)
by aqueous atmospheric aerosols is a likely source of particulate organic
material. The aqueous-phase secondary organic products of some cVOCs are
surface-active. Therefore, cVOC uptake can lead to organic film formation at
the gas-aerosol interface and changes in aerosol surface tension. We examined
the chemical reactions of two abundant cVOCs, formaldehyde and acetaldehyde, in
water and aqueous ammonium sulfate (AS) solutions mimicking tropospheric
aerosols. Secondary organic products were identified using Aerosol Chemical
Ionization Mass Spectrometry (Aerosol-CIMS), and changes in surface tension
were monitored using pendant drop tensiometry. Hemiacetal oligomers and aldol
condensation products were identified using Aerosol-CIMS. Acetaldehyde
depresses surface tension to 65(\pm2) dyn/cm in pure water (a 10% surface
tension reduction from that of pure water) and 62(\pm1) dyn/cm in AS solutions
(a 20.6% reduction from that of a 3.1 M AS solution). Surface tension
depression by formaldehyde in pure water is negligible; in AS solutions, a 9%
reduction in surface tension is observed. Mixtures of these species were also
studied in combination with methylglyoxal in order to evaluate the influence of
cross-reactions on surface tension depression and product formation in these
systems. We find that surface tension depression in the solutions containing
mixed cVOCs exceeds that predicted by an additive model based on the
single-species isotherms.Comment: Published in Atmospheric Chemistry and Physics 22 November 201
Primary marine aerosol emissions from the Mediterranean Sea during pre-bloom and oligotrophic conditions: Correlations to seawater chlorophyll a from a mesocosm study
Allison N. Schwier et al.© Author(s) 2015. The effect of ocean acidification and changing water conditions on primary (and secondary) marine aerosol emissions is not well understood on a regional or a global scale. To investigate this effect as well as the indirect effect on aerosol that changing biogeochemical parameters can have, ∼ 52 m3 pelagic mesocosms were deployed for several weeks in the Mediterranean Sea during both winter pre-bloom and summer oligotrophic conditions and were subjected to various levels of CO2 to simulate the conditions foreseen in this region for the coming decades. After seawater sampling, primary bubble-bursting aerosol experiments were performed using a plunging water jet system to test both chemical and physical aerosol parameters (10-400 nm). Comparing results obtained during pre-bloom and oligotrophic conditions, we find the same four log-normal modal diameters (18.5 ± 0.6, 37.5 ± 1.4, 91.5 ± 2.0, 260 ± 3.2 nm) describing the aerosol size distribution during both campaigns, yet pre-bloom conditions significantly increased the number fraction of the second (Aitken) mode, with an amplitude correlated to virus-like particles, heterotrophic prokaryotes, TEPs (transparent exopolymeric particles), chlorophyll a and other pigments. Organic fractions determined from kappa closure calculations for the diameter, Dp ∼ 50 nm, were much larger during the pre-bloom period (64 %) than during the oligotrophic period (38 %), and the organic fraction decreased as the particle size increased. Combining data from both campaigns together, strong positive correlations were found between the organic fraction of the aerosol and chlorophyll a concentrations, heterotrophic and autotrophic bacteria abundance, and dissolved organic carbon (DOC) concentrations. As a consequence of the changes in the organic fraction and the size distributions between pre-bloom and oligotrophic periods, we find that the ratio of cloud condensation nuclei (CCN) to condensation nuclei (CN) slightly decreased during the pre-bloom period. The enrichment of the seawater samples with microlayer samples did not have any effect on the size distribution, organic content or the CCN activity of the generated primary aerosol. Partial pressure of CO2, pCO2, perturbations had little effect on the physical or chemical parameters of the aerosol emissions, with larger effects observed due to the differences between a pre-bloom and oligotrophic environment.This work was supported by the MISTRALS/ChArMEx project and by the EC FP7 project “Mediterranean Sea Acidification in a changing climate” (MedSeA; grant agreement 265103).Peer Reviewe
Ammonium Addition (and Aerosol pH) Has a Dramatic Impact on the Volatility and Yield of Glyoxal Secondary Organic Aerosol
Glyoxal
is an important precursor to secondary organic aerosol
(SOA) formed through aqueous chemistry in clouds, fogs, and wet aerosols,
yet the gas-particle partitioning of the resulting mixture is not
well understood. This work characterizes the volatility behavior of
the glyoxal precursor/product mix formed after aqueous hydroxyl radical
oxidation and droplet evaporation under cloud-relevant conditions
for 10 min, thus aiding the prediction of SOA via this pathway (SOA<sub>Cld</sub>). This work uses kinetic modeling for droplet composition,
droplet evaporation experiments and temperature-programmed desorption
aerosol–chemical ionization mass spectrometer analysis of gas-particle
partitioning. An effective vapor pressure (<i>p</i>′<sub>L,eff</sub>) of ∼10<sup>–7</sup> atm and an enthalpy
of vaporization (Δ<i>H</i><sub>vap,eff</sub>) of ∼70
kJ/mol were estimated for this mixture. These estimates are similar
to those of oxalic acid, which is a major product. Addition of ammonium
until the pH reached 7 (with ammonium hydroxide) reduced the <i>p</i>′<sub>L,eff</sub> to <10<sup>–9</sup> atm
and increased the Δ<i>H</i><sub>vap,eff</sub> to >80
kJ/mol, at least in part via the formation of ammonium oxalate. pH
7 samples behaved like ammonium oxalate, which has a vapor pressure
of ∼10<sup>–11</sup> atm. We conclude that ammonium
addition has a large effect on the gas-particle partitioning of the
mixture, substantially enhancing the yield of SOA<sub>Cld</sub> from
glyoxal
Aqueous-Phase Secondary Organic Aerosol and Organosulfate Formation in Atmospheric Aerosols: A Modeling Study
We have examined aqueous-phase secondary organic aerosol
(SOA)
and organosulfate (OS) formation in atmospheric aerosols using a photochemical
box model with coupled gas-phase chemistry and detailed aqueous aerosol
chemistry. SOA formation in deliquesced ammonium sulfate aerosol is
highest under low-NO<i><sub>x</sub></i> conditions, with
acidic aerosol (pH = 1) and low ambient relative humidity (40%). Under
these conditions, with an initial sulfate loading of 4.0 μg
m<sup>–3</sup>, 0.9 μg m<sup>–3</sup> SOA is predicted
after 12 h. Low-NO<i><sub>x</sub></i> aqueous-aerosol SOA
(aaSOA) and OS formation is dominated by isoprene-derived epoxydiol
(IEPOX) pathways; 69% or more of aaSOA is composed of IEPOX, 2-methyltetrol,
and 2-methyltetrol sulfate ester. 2-Methyltetrol sulfate ester comprises
>99% of OS mass (66 ng m<sup>–3</sup> at 40% RH and pH 1).
In urban (high-NO<sub><i>x</i></sub>) environments, aaSOA
is primarily formed via reversible glyoxal uptake, with 0.12 μg
m<sup>–3</sup> formed after 12 h at 80% RH, with 20 μg
m<sup>–3</sup> initial sulfate. OS formation under all conditions
studied is maximum at low pH and lower relative humidities (<60%
RH), i.e., when the aerosol is more concentrated. Therefore, OS species
are expected to be good tracer compounds for aqueous aerosol-phase
chemistry (vs cloudwater processing)