4 research outputs found
Exploration of the influence of environmental conditions on secondary organic aerosol formation and organic species properties using explicit simulations: development of the VBS-GECKO parameterization
Atmospheric chambers have been widely used to study secondary organic aerosol
(SOA) properties and formation from various precursors under different
controlled environmental conditions and to develop parameterization to
represent SOA formation in chemical transport models (CTMs). Chamber
experiments are however limited in number, performed under conditions that
differ from the atmosphere and can be subject to potential artefacts from
chamber walls. Here, the Generator for Explicit Chemistry and Kinetics of
Organics in the Atmosphere (GECKO-A) modelling tool has been used in a box
model under various environmental conditions to (i) explore the sensitivity
of SOA formation and properties to changes on physical and chemical
conditions and (ii) develop a volatility basis set (VBS)-type parameterization.
The set of parent hydrocarbons includes n-alkanes and 1-alkenes with 10, 14,
18, 22 and 26 carbon atoms, α-pinene, β-pinene and limonene,
benzene, toluene, o-xylene, m-xylene and p-xylene. Simulated SOA yields and
their dependences on the precursor structure, organic aerosol load,
temperature and NOx levels are consistent with the
literature. GECKO-A was used to explore the distribution of molar mass,
vaporization enthalpy, OH reaction rate and Henry's law coefficient of the
millions of secondary organic compounds formed during the oxidation of the
different precursors and under various conditions. From these explicit
simulations, a VBS-GECKO parameterization designed to be implemented in 3-D
air quality models has been tuned to represent SOA formation from the 18
precursors using GECKO-A as a reference. In evaluating the ability of
VBS-GECKO to capture the temporal evolution of SOA mass, the mean relative
error is less than 20 % compared to GECKO-A. The optimization procedure
has been automated to facilitate the update of the VBS-GECKO on the basis of
the future GECKO-A versions, its extension to other precursors and/or its
modification to carry additional information.</p
Origins and characterization of CO and O<sub>3</sub> in the African upper troposphere
Between December 2005 and 2013, the In-service Aircraft for a Global Observing System (IAGOS) program produced almost daily in situ measurements of CO and O3 between Europe and southern Africa. IAGOS data combined with measurements from the Infrared Atmospheric Sounding Interferometer (IASI) instrument aboard the Metop-A satellite (2008–2013) are used to characterize meridional distributions and seasonality of CO and O3 in the African upper troposphere (UT). The FLEXPART particle dispersion model and the SOFT-IO model which combines the FLEXPART model with CO emission inventories are used to explore the sources and origins of the observed transects of CO and O3.We focus our analysis on two main seasons: December to March (DJFM) and June to October (JJASO). These seasons have been defined according to the position of Intertropical Convergence Zone (ITCZ), determined using in situ measurements from IAGOS. During both seasons, the UT CO meridional transects are characterized by maximum mixing ratios located 10∘ from the position of the ITCZ above the dry regions inside the hemisphere of the strongest Hadley cell (132 to 165 ppb at 0–5∘ N in DJFM and 128 to 149 ppb at 3–7∘ S in JJASO) and decreasing values southward and northward. The O3 meridional transects are characterized by mixing ratio minima of ∼42–54 ppb at the ITCZ (10–16∘ S in DJFM and 5–8∘ N in JJASO) framed by local maxima (∼53–71 ppb) coincident with the wind shear zones north and south of the ITCZ. O3 gradients are strongest in the hemisphere of the strongest Hadley cell. IASI UT O3 distributions in DJFM have revealed that the maxima are a part of a crescent-shaped O3 plume above the Atlantic Ocean around the Gulf of Guinea.CO emitted at the surface is transported towards the ITCZ by the trade winds and then convectively uplifted. Once in the upper troposphere, CO-enriched air masses are transported away from the ITCZ by the upper branches of the Hadley cells and accumulate within the zonal wind shear zones where the maximum CO mixing ratios are found. Anthropogenic and fires both contribute, by the same order of magnitude, to the CO budget of the African upper troposphere.Local fires have the highest contribution and drive the location of the observed UT CO maxima. Anthropogenic CO contribution is mostly from Africa during the entire year, with a low seasonal variability. There is also a large contribution from Asia in JJASO related to the fast convective uplift of polluted air masses in the Asian monsoon region which are further westward transported by the tropical easterly jet (TEJ) and the Asian monsoon anticyclone (AMA).O3 minima correspond to air masses that were recently uplifted from the surface where mixing ratios are low at the ITCZ. The O3 maxima correspond to old high-altitude air masses uplifted from either local or long-distance area of high O3 precursor emissions (Africa and South America during all the year, South Asia mainly in JJASO) and must be created during transport by photochemistry
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Impact of chamber wall loss of gaseous organic compounds on secondary organic aerosol formation: explicit modeling of SOA formation from alkane and alkene oxidation
Recent studies have shown that low volatility gas-phase species can be lost
onto the smog chamber wall surfaces. Although this loss of organic vapors to
walls could be substantial during experiments, its effect on secondary
organic aerosol (SOA) formation has not been well characterized and
quantified yet. Here the potential impact of chamber walls on the loss of
gaseous organic species and SOA formation has been explored using the
Generator for Explicit Chemistry and Kinetics of the Organics in the
Atmosphere (GECKO-A) modeling tool, which explicitly represents SOA formation
and gas–wall partitioning. The model was compared with 41 smog chamber
experiments of SOA formation under OH oxidation of alkane and alkene series
(linear, cyclic and C12-branched alkanes and terminal, internal and 2-methyl
alkenes with 7 to 17 carbon atoms) under high NOx conditions. Simulated
trends match observed trends within and between homologous series. The loss
of organic vapors to the chamber walls is found to affect SOA yields as well
as the composition of the gas and the particle phases. Simulated
distributions of the species in various phases suggest that nitrates,
hydroxynitrates and carbonylesters could substantially be lost onto walls.
The extent of this process depends on the rate of gas–wall mass transfer,
the vapor pressure of the species and the duration of the experiments. This
work suggests that SOA yields inferred from chamber experiments could be
underestimated up a factor of 2 due to the loss of organic vapors to chamber
walls
Impact of chamber wall loss of gaseous organic compounds on secondary organic aerosol formation: explicit modeling of SOA formation from alkane and alkene oxidation
Recent studies have shown that low volatility gas-phase species can be lost
onto the smog chamber wall surfaces. Although this loss of organic vapors to
walls could be substantial during experiments, its effect on secondary
organic aerosol (SOA) formation has not been well characterized and
quantified yet. Here the potential impact of chamber walls on the loss of
gaseous organic species and SOA formation has been explored using the
Generator for Explicit Chemistry and Kinetics of the Organics in the
Atmosphere (GECKO-A) modeling tool, which explicitly represents SOA formation
and gas–wall partitioning. The model was compared with 41 smog chamber
experiments of SOA formation under OH oxidation of alkane and alkene series
(linear, cyclic and C<sub>12</sub>-branched alkanes and terminal, internal and 2-methyl
alkenes with 7 to 17 carbon atoms) under high NO<sub><i>x</i></sub> conditions. Simulated
trends match observed trends within and between homologous series. The loss
of organic vapors to the chamber walls is found to affect SOA yields as well
as the composition of the gas and the particle phases. Simulated
distributions of the species in various phases suggest that nitrates,
hydroxynitrates and carbonylesters could substantially be lost onto walls.
The extent of this process depends on the rate of gas–wall mass transfer,
the vapor pressure of the species and the duration of the experiments. This
work suggests that SOA yields inferred from chamber experiments could be
underestimated up a factor of 2 due to the loss of organic vapors to chamber
walls