Atmospheric Chemistry and Physics Discussions - Aktuelle Forschungsartikel
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Atmospheric Chemistry and Physics Discussions - Verlag: Copernicus Publications
ACPD ist ein interaktives open access journal der Europäischen Geowissenschaftlichen Union.
New parameterization of dust emissions in the global atmospheric chemistry-climate model EMAC
Atmospheric Chemistry and Physics Discussions, 12, 13237-13298, 2012
Author(s): M. Astitha, J. Lelieveld, M. Abdel Kader, A. Pozzer, and A. de Meij
Airborne desert dust influences radiative transfer, atmospheric
chemistry and dynamics, as well as nutrient transport and
deposition. It directly and indirectly affects climate on regional
and global scales. We present two versions of a parameterization
scheme to compute desert dust emissions, incorporated into the
atmospheric chemistry general circulation model EMAC
(ECHAM5/MESSy2.41 Atmospheric Chemistry). One uses a globally
uniform soil particle size distribution, whereas the other
explicitly accounts for different soil textures worldwide. We have
tested these schemes and investigated the sensitivity to input
parameters, using remote sensing data from the Aerosol Robotic
Network (AERONET) and dust concentrations and deposition
measurements from the AeroCom dust benchmark database (and
others). The two schemes are shown to produce similar atmospheric
dust loads in the N-African region, while they deviate in the Asian,
Middle Eastern and S-American regions. The dust outflow from Africa
over the Atlantic Ocean is accurately simulated by both schemes, in
magnitude, location and seasonality. The modelled dust
concentrations and deposition fluxes compare well with observations
at (island) stations in the Atlantic Ocean and Asia, and are
underestimated in the Pacific Ocean where annual means are
relatively low (<1 ?g m?3). The two schemes
perform similarly well, even though the total annual source differs
by ~50%, indicating the importance of transport and
deposition processes (being the same for the two schemes). Our
results emphasize the need to represent arid regions individually
and explicitly in global models according to their unique land
characteristics and meteorological conditions.
Methyl hydroperoxide (CH3OOH) in urban, suburban and rural atmosphere: ambient concentration, budget, and contribution to the atmospheric oxidizing capacity
Atmospheric Chemistry and Physics Discussions, 12, 13089-13118, 2012
Author(s): X. Zhang, S. Z. He, Z. M. Chen, Y. Zhao, and W. Hua
Methyl hydroperoxide (MHP), one of the most important organic peroxides in the atmosphere, contributes to the tropospheric
oxidizing capacity either directly as an oxidant or indirectly as a free radical precursor. In this study we report measurements
of MHP from seven field campaigns at urban, suburban and rural sites in China in winter 2007 and summer 2006/2007/2008. MHP was usually
present in the order of several hundreds of pptv level, but the average mixing ratios have shown a wide range depending on the
season and measuring site. Primary sources and sinks of MHP are investigated to understand the impact of meteorological and
chemical parameters on the atmospheric MHP budget. The MHP/(MHP+H2O2) ratio is also presented here to examine
different sensitivities of MHP and H2O2 to certain atmospheric processes. The diurnal cycle of
MHP/(MHP+H2O2), which is out of phase with the diurnal cycle of both H2O2 and MHP, could imply that MHP
production is more sensitive to the ambient NO concentration, while H2O2 is more strongly influenced by the wet
deposition and the subsequent aqueous chemistry. It is interesting to note that our observation at urban Beijing site in winter
2007 provides evidence for the occasional transport of MHP-containing air masses from the marine boundary layer to the
continent. Highly constrained box model is performed to study the influence of MHP on the free radical cycle. The simulation
shows that MHP has a significant impact on the CH3O2 racial budget in the atmosphere. Furthermore, the contribution
of MHP as an atmospheric oxidant to the overall tropospheric oxidizing capacity is also assessed based on the "Counter
Species" concept.
Sensitivity studies of dust ice nuclei effect on cirrus clouds with the Community Atmosphere Model CAM5
Atmospheric Chemistry and Physics Discussions, 12, 13119-13160, 2012
Author(s): X. Liu, X. Shi, K. Zhang, E. J. Jensen, A. Gettelman, D. Barahona, A. Nenes, and P. Lawson
In this study the effect of dust aerosol on upper tropospheric
cirrus clouds through heterogeneous ice nucleation is
investigated in the Community Atmospheric Model version 5
(CAM5) with two ice nucleation parameterizations. Both
parameterizations consider homogeneous and heterogeneous
nucleation and the competition between the two mechanisms in
cirrus clouds, but differ significantly in the number
concentration of heterogeneous ice nuclei (IN) from
dust. Heterogeneous nucleation on dust aerosol reduces the
occurrence frequency of homogeneous nucleation and thus the
ice crystal number concentration in the Northern Hemisphere
(NH) cirrus clouds compared to simulations with pure
homogeneous nucleation. Global and annual mean shortwave and
longwave cloud forcing are reduced by up to
2 W m?2 due to the presence of dust IN, with the
net cloud forcing change of ?0.2 to ?0.4 W m?2
(cooling). Comparison of model simulations with in situ
aircraft data obtained in NH mid-latitudes suggests that
homogeneous ice nucleation may play an important role in the
ice nucleation at these regions with temperatures of
205–230 K. However, simulations overestimate observed
ice crystal number concentrations in the tropical tropopause
regions with temperatures of 190–205 K, and
overestimate the frequency of occurrence of high ice crystal
number concentration (>200 l?1) and
underestimate the frequency of low ice crystal number
concentration (<30 l?1) at NH
mid-latitudes. These results highlight the importance of
quantifying the number concentrations and properties of
heterogeneous IN (including dust aerosol) in the upper
troposphere from the global perspective.
On the relationship between total ozone and atmospheric dynamics and chemistry at mid-latitudes – Part 1: Statistical models and spatial fingerprints of atmospheric dynamics and chemistry
Atmospheric Chemistry and Physics Discussions, 12, 13161-13199, 2012
Author(s): L. Frossard, H. E. Rieder, M. Ribatet, J. Staehelin, J. A. Maeder, S. Di Rocco, A. C. Davison, and T. Peter
We use models for mean and extreme values of total column ozone on
spatial scales to analyze "fingerprints" of atmospheric dynamics
and chemistry on long-term ozone changes at northern and southern
mid-latitudes. The r-largest order statistics method is used for
pointwise analysis of extreme events in low and high total ozone
(termed ELOs and EHOs, respectively). For the corresponding mean
value analysis a pointwise autoregressive moving average model
(ARMA) is used. The statistical models include important atmospheric
covariates to describe the dynamical and chemical state of the
atmosphere: the solar cycle, the Quasi-Biennial Oscillation (QBO),
ozone depleting substances (ODS) in terms of equivalent effective
stratospheric chlorine (EESC), the North Atlantic Oscillation (NAO),
the Antarctic Oscillation (AAO), the El~Niño/Southern
Oscillation (ENSO), and aerosol load after the volcanic eruptions of
El Chichón and Mt. Pinatubo. The influence of the individual
covariates on mean and extreme levels in total column ozone is
derived on a grid cell basis. The results show that
"fingerprints", i.e., significant influence, of dynamical and
chemical features are captured in both the "bulk" and the tails of
the ozone distribution, respectively described by means and
EHOs/ELOs. While results for the solar cycle, QBO and EESC are in
good agreement with findings of earlier studies, unprecedented
spatial fingerprints are retrieved for the dynamical covariates.
Mid-tropospheric ?D observations from IASI/MetOp at high spatial and temporal resolution
Atmospheric Chemistry and Physics Discussions, 12, 13053-13087, 2012
Author(s): J.-L. Lacour, C. Risi, L. Clarisse, S. Bony, D. Hurtmans, C. Clerbaux, and P.-F. Coheur
In this paper we present a method to retrieve HDO,
H2O and δD from IASI radiances
spectra. It relies on an existing radiative transfer model
(Atmosphit) and optimal estimation inversion scheme, but goes
further than our previous work by explicitly considering
correlations between the two species. Global (fixed)
HDO and H2O a priori profiles together with
a covariance matrix were built from daily model simulations of
HDO and H2O profiles (the LMDz-iso model is
used) over the whole globe and a whole year. The retrieval
parameters are described and characterized in terms of
errors. We show that IASI is mostly sensitive to
δD in the middle troposphere and allows
retrieving δD for an integrated
3–6 km column with an error of 38‰ on an
individual measurement basis. We examine the performance of the retrieval to capture the temporal (seasonal and
short-term) and spatial variations of δD by
analyzing one year of measurement at two dedicated sites
(Darwin and Izaña) and a latitudinal band from ?60°
to 60° for a 15 days period in January. The
performances are compared with LMDz-iso simulations. We report
a general excellent agreement between IASI and the model and
demonstrate the capabilities of IASI to reproduce the large
scale variations of δD (seasonal cycle and
latitudinal gradient) with good accuracy. In particular, we
show that there is no systematic significant bias in the
retrieved δD values in comparison with the
model, and that the retrieved variability is similar to the
modeled one although there are certain significant differences
depending on the location. Moreover the noticeable differences
between IASI and the model reported and briefly examined tend
to suggest modeling issues instead of retrieval
effects. Finally, we strengthen the unprecedented capabilities
of IASI to capture short-term variations in
δD, further highlighting the added value of
the sounder for monitoring hydrological processes.
Using non-negative matrix factorization for the identification of daily patterns of particulate air pollution in Beijing during 2004–2008
Atmospheric Chemistry and Physics Discussions, 12, 13015-13052, 2012
Author(s): A. Thiem, U. Schlink, X.-C. Pan, M. Hu, A. Peters, A. Wiedensohler, S. Breitner, J. Cyrys, B. Wehner, C. Rösch, and U. Franck
Increasing traffic density and a changing car fleet on the one hand
as well as various reduction measures on the other hand may
influence the composition of the particle population and, hence, the
health risks for residents of megacities like Beijing. A suitable
tool for identification and quantification of source group-related
particle exposure compositions is desirable in order to derive
optimal adaptation and reduction strategies and therefore, is
presented in this paper.
Particle number concentrations have been measured in high time- and
space-resolution at an urban background monitoring site in Beijing,
China, during 2004–2008. In this study a new pattern recognition
procedure based on non-negative matrix factorization (NMF) was
introduced to extract characteristic diurnal air pollution patterns
of particle number and volume size distributions for the study
period. Initialization and weighting strategies for NMF applications
were carefully considered and a scaling procedure for ranking of
obtained patterns was implemented. In order to account for varying
particle sizes in the full diameter range [3 nm; 10 μm]
two separate NMF applications (a) for diurnal particle number
concentration data (NMF-N) and (b) volume concentration data (NMF-V)
have been performed.
Five particle number concentration-related NMF-N factors were
assigned to patterns mainly describing the development of ultrafine
(particle diameter Dp < 100 nm instead of DP) as well as fine particles (Dp
< 2.5 μm), since absolute number concentrations are
highest in these diameter ranges. The factors are classified into
primary and secondary sources. Primary sources mostly involved
anthropogenic emission sources such as traffic emissions or
emissions of nearby industrial plants, whereas secondary sources
involved new particle formation and accumulation (particle growth)
processes. For the NMF-V application the five extracted factors
mainly described coarse particle (2.5 μm <
Dp < 10 μm) variations, generated by
processes like dust storm events. Because particle volume depends
on particle diameter in a cubic manner, larger particles are
emphasized in the latter application.
In order to gain insight in the day-by-day varying source-associated
composition of the particle burden non-negative linear combinations
of individual source-associated pollution patterns were used to
approximate the original particle data. Consequently, this NMF-based
procedure provides a reasonable numerical-statistical tool for the
description of daily patterns of particle pollution, source
identification and reconstruction of daily patterns by summarizing
weighted factors.
Factor analysis of combined organic and inorganic aerosol mass spectra from high resolution aerosol mass spectrometer measurements
Atmospheric Chemistry and Physics Discussions, 12, 13299-13335, 2012
Author(s): Y. L. Sun, Q. Zhang, J. J. Schwab, T. Yang, N. L. Ng, and K. L. Demerjian
The high resolution mass spectra of organic and inorganic aerosols from
aerosol mass spectrometer (AMS) measurements were first combined into
positive matrix factorization (PMF) analysis to investigate the sources and
evolution processes of atmospheric aerosols. The new approach is able to
study the mixing of organic aerosols (OA) and inorganic species, the acidity
of OA factors, and the fragment ion patterns related to photochemical
processing. In this study, PMF analysis of the unified AMS spectral matrices
resolved 8 factors for the submicron aerosols measured at Queens College in
New York City in summer 2009. The hydrocarbon-like OA (HOA) and cooking OA
(COA) contain very minor inorganic species, indicating the different sources
and mixing characteristics between primary OA and secondary species. The two
factors that are primarily ammonium sulfate (SO4-OA) and
ammonium nitrate (NO3-OA), respectively, are overall
neutralized, of which the OA in SO4-OA shows the highest
oxidation state (O/C = 0.69) among OA factors. The semi-volatile
oxygenated OA comprises two components, i.e., a less oxidized (LO-OOA) and
a more oxidized (MO-OOA). The MO-OOA represents a local photochemical product
with the diurnal profile exhibiting a pronounced noon peak, consistent with
those of formaldehyde (HCHO) and Ox (= O3+NO2). The much higher
NO+/NO2+ fragment ion ratio in
MO-OOA than that from ammonium nitrate alone provides evidence for the
formation of organic nitrates. The amine-related nitrogen-enriched OA (NOA)
contains ~25% of acidic inorganic salts, elucidating the
formation of secondary OA from amines in acidic environments. The size
distributions derived from 3-dimensional size-resolved mass spectra show
distinct diurnal evolving behaviors for different OA factors, but overall
a progressing evolution from smaller to larger particle mode as a function of
oxidation states. Our results demonstrate that PMF analysis by incorporating
inorganic aerosols is of importance for gaining more insights into the
sources and processes, mixing characteristics, and acidity of OA.
On the relationship between total ozone and atmospheric dynamics and chemistry at mid-latitudes – Part 2: The effects of the El Niño/Southern Oscillation, volcanic eruptions and contributions of atmospheric dynamics and chemistry to long-term total ozone changes
Atmospheric Chemistry and Physics Discussions, 12, 13201-13236, 2012
Author(s): H. E. Rieder, L. Frossard, M. Ribatet, J. Staehelin, J. A. Maeder, S. Di Rocco, A. C. Davison, T. Peter, P. Weihs, and F. Holawe
We present the first spatial analysis of "fingerprints" of the El
Niño/Southern Oscillation (ENSO) and atmospheric aerosol load
after major volcanic eruptions (El Chichón and Mt. Pinatubo) in
extreme low and high (termed ELOs and EHOs, respectively) and mean
values of total ozone for the northern and southern mid-latitudes
(defined as the region between 30° and 60° north and
south, respectively). Significant influence on ozone extremes was
found for the warm ENSO phase in both hemispheres during spring,
especially towards low latitudes, indicating the enhanced ozone
transport from the tropics to the extra-tropics. Further, the
results confirm findings of recent work on the connection between
the ENSO phase and the strength and extent of the southern ozone
"collar". For the volcanic eruptions the analysis confirms
findings of earlier studies for the northern mid-latitudes and gives
new insights for the Southern Hemisphere. The results provide
evidence that the negative effect of the eruption of El Chichón
might be partly compensated by a strong warm ENSO phase in 1982–83
at southern mid-latitudes. The strong west-east gradient in the
coefficient estimates for the Mt. Pinatubo eruption and the analysis
of the relationship between the AAO and ENSO phase, the extent and
the position of the southern ozone "collar" and the polar vortex
structure provide clear evidence for a dynamical "masking" of the
volcanic signal at southern mid-latitudes. The paper also analyses
the contribution of atmospheric dynamics and chemistry to long-term
total ozone changes. Here, quite heterogeneous results have been
found on spatial scales. In general the results show that EESC and
the 11-yr solar cycle can be identified as major contributors to
long-term ozone changes. However, a strong contribution of dynamical
features (El Niño/Southern Oscillation (ENSO), North Atlantic
Oscillation (NAO), Antarctic Oscillation (AAO), Quasi-Biennial
Oscillation (QBO)) to ozone variability and trends is found at
a regional level. For the QBO (at 30 and 50 hPa), strong
influence on total ozone variability and trends is found over large
parts of the northern and southern mid-latitudes, especially towards
equatorial latitudes. Strong influence of ENSO is found over the
Northern and Southern Pacific, Central Europe and central southern
mid-latitudes. For the NAO, strong influence on column ozone is
found over Labrador/Greenland, the Eastern United States, the
Euro-Atlantic Sector and Central Europe. For the NAO's southern
counterpart, the AAO, strong influence on ozone variability and
long-term changes is found at lower southern mid-latitudes,
including the southern parts of South America and the Antarctic
Peninsula, and central southern mid-latitudes.
Global emission estimates and radiative impact of C4F10, C5F12, C6F14, C7F16 and C8F18
Atmospheric Chemistry and Physics Discussions, 12, 12987-13014, 2012
Author(s): D. J. Ivy, M. Rigby, M. Baasandorj, J. B. Burkholder, and R. G. Prinn
Global emission estimates based on new atmospheric
observations are presented for the acylic high molecular
weight perfluorocarbons (PFCs): decafluorobutane
(C4F10), dodecafluoropentane (C5F12),
tetradecafluorohexane (C6F14),
hexadecafluoroheptane (C7F16) and
octadecafluorooctane (C8F18). Emissions are
estimated using a 3-dimensional chemical transport model and an inverse
method that includes a growth constraint on emissions. The
observations used in the inversion are based on newly measured
archived air samples that cover a 39-yr period, from 1973 to
2011, and include 36 Northern Hemispheric and 46 Southern Hemispheric samples (Ivy et al., 2012). The derived emission
estimates show that global emission rates were largest in the
1980s and 1990s for C4F10 and C5F12, and
in the 1990s for C6F14,C7F16 and
C8F18. After a subsequent decline, emissions have
remained relatively stable, within 20%, for the last
5 yr. Bottom-up emission estimates are available from the
Emission Database for Global Atmospheric Research version 4.2
(EDGARv4.2) for C4F10, C5F12,
C6F14 and C7F16, and inventories of
C4F10, C5F12 andC6F14 are
reported to the United Nations' Framework Convention on
Climate Change (UNFCCC) by Annex 1 countries that have
ratified the Kyoto Protocol. The atmospheric measurement based
emission estimates are 20 times larger than EDGARv4.2 for
C4F10 and over three orders of magnitude for
C5F12. The derived emission estimates for
C6F14 largely agree with the bottom-up estimates
from EDGARv4.2. Moreover, the C7F16 emission
estimates are comparable to those of EDGARv4.2 at their peak
in the 1990s, albeit significant underestimation for the other
time periods. There are no bottom-up emission estimates for
C8F18, thus the emission rates reported here are
the first for C8F18. The reported inventories for
C4F10, C5F12 and C6F14 to
UNFCCC are five to ten times lower than those estimated in
this study. In addition, we present measured infrared
absorption spectra for C7F16 and C8F18,
and estimate their radiative efficiencies and global warming
potentials (GWPs). We find that C8F18's radiative
efficiency is similar to trifluoromethyl sulfur
pentafluoride's (SF5CF3) at
0.57 W m?2 ppb?1, which is the highest radiative
efficiency of any measured atmospheric species. Using the 100-yr
time horizon GWPs, the high molecular weight perfluorocarbons
studied here contributed up to 15.4% of the total PFC
emissions in CO2 equivalents in 1997 and 6% of
the total PFC emissions in 2009.
Identification of mercury emissions from forest fires, lakes, regional and local sources using measurements in Milwaukee and an inverse method
Atmospheric Chemistry and Physics Discussions, 12, 12935-12986, 2012
Author(s): B. de Foy, C. Wiedinmyer, and J. J. Schauer
Gaseous elemental mercury is a global pollutant that can lead to
serious health concerns via deposition to the biosphere and
bio-accumulation in the food chain. Hourly measurements between June
2004 and May 2005 in an urban site (Milwaukee, WI) show elevated
levels of mercury in the atmosphere with numerous short-lived peaks as
well as longer-lived episodes. The measurements are analyzed with an
inverse model to obtain information about mercury emissions. The model
is based on high resolution meteorological simulations (WRF), hourly
back-trajectories (WRF-FLEXPART) and forward grid simulations (CAMx).
The hybrid formulation combining back-trajectories and grid
simulations is used to identify potential source regions as well as
the impacts of forest fires and lake surface emissions. Uncertainty
bounds are estimated using a bootstrap method on the inversions.
Comparison with the US Environmental Protection Agency's National
Emission Inventory (NEI) and Toxic Release Inventory (TRI) shows that
emissions from coal-fired power plants are properly characterized, but
emissions from local urban sources, waste incineration and metal
processing could be significantly under-estimated. Emissions from the
lake surface and from forest fires were found to have significant
impacts on mercury levels in Milwaukee, and to be underestimated by
a factor of two or more.
Impact of HONO on global atmospheric chemistry calculated with an empirical parameterization in the EMAC model
Atmospheric Chemistry and Physics Discussions, 12, 12885-12934, 2012
Author(s): Y. F. Elshorbany, B. Steil, C. Brühl, and J. Lelieveld
The photolysis of HONO is important for the atmospheric
HOx (OH+HO2) radical budget and ozone
formation, especially in polluted air. Nevertheless, owing to
the incomplete knowledge of HONO sources, realistic HONO
mechanisms have not yet been implemented in global models. We
investigated measurement data sets from 15 field measurement
campaigns conducted in different countries worldwide. It
appears that the HONO/NOx ratio is a good proxy
predictor for HONO mixing ratios under different atmospheric
conditions. From the robust relationship between HONO and
NOx, a representative mean HONO/NOx ratio of
0.02 has been derived. Using a global chemistry-climate model
and employing this HONO/NOx ratio, realistic HONO
levels are simulated, being about one order of magnitude
higher than the reference calculations, which only consider
the reaction OH+NO-> HONO. The resulting enhancement
of HONO significantly impacts HOx levels and
photo-oxidation products (e.g, O3, PAN), mainly in
polluted regions. Furthermore, the relative enhancements in OH
and secondary products were higher in winter than in summer,
thus enhancing the oxidation capacity in polluted regions,
especially in winter, when the other photolytic OH sources are
of minor importance. Our results underscore the need to
improve the understanding of HONO chemistry and its
representation in atmospheric models.
Model evaluation of marine primary organic aerosol emission schemes
Atmospheric Chemistry and Physics Discussions, 12, 12853-12883, 2012
Author(s): B. Gantt, M. S. Johnson, N. Meskhidze, J. Sciare, J. Ovadnevaite, D. Ceburnis, and C. D. O'Dowd
In this study, five different marine primary organic aerosol (POA) emission schemes have been
evaluated using the GEOS-Chem chemical transport model in order to provide guidance for their
implementation in air quality and climate models. These emission schemes, categorized in two
groups based on varying dependences of chlorophyll a concentration ([chl a]) and 10 m wind
speed (U10), have large differences in their magnitude, spatial
distribution, and seasonality. Model comparison with weekly and monthly mean values of the organic
aerosol mass concentration at two coastal sites shows that the source function exclusively related
to [chl a] does a better job replicating surface observations. Sensitivity simulations of the
sea spray-based parameterizations show that improved predictions of the seasonality of the marine
POA concentrations can be achieved by varying the U10 and [chl a]
dependence of the organic mass fraction of sea spray aerosol. A top-down estimate of submicron
marine POA emissions based on the parameterization that compares best to the observed weekly and
monthly mean values of marine organic aerosol surface concentrations has a global average emission
rate of 6.3 Tg yr−1. Evaluation of existing marine POA source functions
against a case study during which marine POA contributed the major fraction of submicron aerosol
mass shows that none of the existing parameterizations are able to reproduce the hourly-averaged
observations. Our calculations suggest that in order to capture episodic events in submicron
marine POA concentration over the ocean, new source functions need to be developed that are
grounded in the physical processes unique to the organic fraction of sea spray aerosol.
Ozone deposition into a boreal forest over a decade of observations: evaluating deposition partitioning and driving variables
Atmospheric Chemistry and Physics Discussions, 12, 12715-12758, 2012
Author(s): Ü. Rannik, N. Altimir, I. Mammarella, J. Bäck, J. Rinne, T. M. Ruuskanen, P. Hari, T. Vesala, and M. Kulmala
This study scrutinizes a decade-long series of ozone
deposition measurements in a boreal forest in search for the
signature and relevance of the different deposition
processes. Canopy-level ozone flux measurements were analysed
for deposition characteristics and partitioning into stomatal
and non-stomatal fractions, focusing on growing season
day-time data. Ten years of measurements enabled the analysis
of ozone deposition variation at different time- scales,
including daily to inter-annual variation as well as the
dependence on environmental variables and concentration of
biogenic volatile organic compounds (BVOC-s). Stomatal
deposition was estimated by using multi-layer canopy
dispersion and optimal stomatal control modelling from
simultaneous carbon dioxide and water vapour flux
measurements, non-stomatal was inferred as residual. Also,
utilising big-leaf assumption stomatal conductance was
inferred from water vapour fluxes for dry canopy
conditions. The total ozone deposition was highest during the
peak growing season (4 mm s−1) and lowest during
winter dormancy (1 mm s−1). During the course of
the growing season the fraction of the non-stomatal deposition
of ozone was determined to vary from 26 to 44% during
day time, increasing from the start of the season until the
end of the growing season. By using multi-variate analysis it
was determined that day-time total ozone deposition was mainly
driven by photosynthetic capacity of the canopy, vapour
pressure deficit (VPD), photosynthetically active radiation
and monoterpene concentration. The multi-variate linear model
explained high portion of ozone deposition variance on daily
average level (R2 = 0.79). The explanatory power of
the multi-variate model for ozone non-stomatal deposition was
much lower (R2 = 0.38). Model calculation was
performed to evaluate the potential sink strength of the
chemical reactions of ozone with sesquiterpenes in the canopy
air space, which revealed that sesquiterpenes in typical
amounts at the site were unlikely to cause significant ozone
loss in canopy air space. This was also confirmed by the
statistical analysis that did not link measured sesquiterpene
concentration with ozone deposition. It was concluded that
chemical reactions with monoterpenes, or other removal
mechanisms such as surface reactions, play a role as ozone
non-stomatal sink inside canopy.
In-canopy gas-phase chemistry during CABINEX 2009: sensitivity of a 1-D canopy model to vertical mixing and isoprene chemistry
Atmospheric Chemistry and Physics Discussions, 12, 12801-12852, 2012
Author(s): A. M. Bryan, S. B. Bertman, M. A. Carroll, S. Dusanter, G. D. Edwards, R. Forkel, S. Griffith, A. B. Guenther, R. F. Hansen, D. Helmig, B. T. Jobson, F. N. Keutsch, B. L. Lefer, S. N. Pressley, P. B. Shepson, P. S. Stevens, and A. L. Steiner
Vegetation emits large quantities of biogenic volatile organic compounds
(BVOC). At remote sites, these compounds are the dominant precursors to ozone
and secondary organic aerosol (SOA) production, yet current field studies
show that atmospheric models have difficulty in capturing the observed
HOx cycle and concentrations of BVOC oxidation products. In this
manuscript, we simulate BVOC chemistry within a forest canopy using a
one-dimensional canopy-chemistry model (Canopy Atmospheric CHemistry Emission
model; CACHE) for a mixed deciduous forest in northern Michigan during the
CABINEX 2009 campaign. We find that the base-case model, using
fully-parameterized mixing and the simplified biogenic chemistry of the
Regional Atmospheric Chemistry Model (RACM), underestimates daytime in-canopy
vertical mixing by 50–70% and by an order of magnitude at night, leading
to discrepancies in the diurnal evolution of HOx, BVOC, and BVOC
oxidation products. Implementing observed micrometeorological data from above
and within the canopy substantially improves the diurnal cycle of modeled
BVOC, particularly at the end of the day, and also improves the
observation-model agreement for some BVOC oxidation products and OH
reactivity. We compare the RACM mechanism to a version that includes the
Mainz isoprene mechanism (RACM-MIM) to test the model sensitivity to enhanced
isoprene degradation. RACM-MIM simulates higher concentrations of both
primary BVOC (isoprene and monoterpenes) and oxidation products (HCHO,
MACR + MVK) compared with RACM simulations. Additionally, the revised
mechanism alters the OH concentrations and increases HO2. These
changes generally improve agreement with HOx observations yet
overestimate BVOC oxidation products, indicating that this isoprene mechanism
does not improve the representation of local chemistry at the site. Overall,
the revised mechanism yields smaller changes in BVOC and BVOC oxidation
product concentrations and gradients than improving the parameterization of
vertical mixing with observations, suggesting that uncertainties in vertical
mixing parameterizations are an important component in understanding observed
BVOC chemistry.
Evaluation of atmosphere-biosphere exchange estimations with TCCON measurements
Atmospheric Chemistry and Physics Discussions, 12, 12759-12800, 2012
Author(s): J. Messerschmidt, N. Parazoo, N. M. Deutscher, C. Roehl, T. Warneke, P. O. Wennberg, and D. Wunch
Three estimates of the atmosphere-biosphere exchange are evaluated
using Total Carbon Column Observing Network (TCCON) measurements. We
investigate the Carnegie-Ames-Stanford Approach (CASA), the Simple
Biosphere (SiB) and the GBiome-BGC models transported by the GEOS-Chem
model to simulate atmospheric CO2 concentrations for the time
period between 2006 and 2010. The CO2 simulations are highly
dependent on the choice of the atmosphere-biosphere model and
large-scale errors in the estimates are identified through a
comparison with TCCON data. Enhancing the CO2 uptake in the
boreal forest by 40% and shifting the onset of the growing season
significantly improve the simulated seasonal CO2 cycle using
CASA estimates. The SiB model gives the best estimate for the
atmosphere-biosphere exchange in the comparison with TCCON
measurements.
Mineral dust variability in central West Antarctica associated with ozone depletion
Atmospheric Chemistry and Physics Discussions, 12, 12685-12714, 2012
Author(s): M. Cataldo, H. Evangelista, J. C. Simões, R. H. M. Godoi, I. Simmonds, M. H. Hollanda, I. Wainer, F. E. Aquino, and R. van Grieken
Here we show that mineral dust retrieved from an ice core in the central West Antarctic sector, spanning the last five decades,
provides evidence that northerly air mass incursions into Antarctica, tracked by dust microparticles, have slightly
declined. This result contrasts with dust in ice core records reported in West/coastal Antarctica, which show significant
increases to the present day. We attribute that difference, in part, to changes in the regional climate regime triggered by the
ozone depletion and its consequences for the polar vortex intensity. The vortex maintains the Antarctic central region
relatively isolated from mid-latitude air mass incursions with implications to the intensification of the Westerlies and to
a persistent positive phase of the Southern Annular Mode. We also show that variability of the diameter of insoluble
microparticles in central West Antarctica can be modeled by linear/quadratic functions of both cyclone depth (energy) and wind
intensity around Antarctica.
Stable water isotopologue ratios in fog and cloud droplets are not size-dependent
Atmospheric Chemistry and Physics Discussions, 12, 12663-12684, 2012
Author(s): J. K. Spiegel, F. Aemisegger, M. Scholl, F. G. Wienhold, J. L. Collett Jr., T. Lee, D. van Pinxteren, S. Mertes, A. Tilgner, H. Herrmann, R. A. Werner, N. Buchmann, and W. Eugster
In this work, we present the first observations of stable water isotopologue
ratios in cloud droplets of different sizes collected simultaneously. We
address the question whether the isotope ratio of droplets in a cloud varies
as a function of droplet size. Samples were collected from a ground
intercepted cloud (=fog) during the Hill Cap Cloud Thuringia 2010 campaign
(HCCT-2010) using a three-stage Caltech Active Strand Cloud water Collector
(CASCC). An instrument test revealed that no artificial isotopic
fractionation occurs during sample collection with the CASCC. Furthermore, we
could experimentally confirm the hypothesis that the ? values of cloud
droplets of the relevant droplet sizes (?m-range) were not
significantly different and thus can be assumed to be in isotopic equilibrium
immediately with the surrounding water vapor. However, at the dissolution
period of the cloud differences in isotope ratios of the different droplet sizes
tended to be larger. This is likely to result from the cloud's heterogeneity,
implying that larger and smaller cloud droplets have been collected at
different moments in time, delivering isotope ratios from different
collection times.
Atmospheric Chemistry and Physics Discussions, 12, 12623-12662, 2012
Author(s): A. Welti, F. Lüönd, Z. A. Kanji, O. Stetzer, and U. Lohmann
The time dependence of immersion freezing was studied for
temperatures between 236 K and 243 K. Droplets
with single immersed, size-selected 400 nm and
800 nm kaolinite particles were produced at
300 K, cooled down to supercooled temperatures typical
for mixed-phase cloud conditions, and the fraction of frozen
droplets with increasing residence time was detected. To
simulate the conditions of immersion freezing in mixed-phase
clouds we used the Zurich Ice Nucleation Chamber (ZINC) and
its vertical extension, the Immersion Mode Cooling chAmber
(IMCA). We observed that the frozen fraction of droplets
increased with increasing residence time in the chamber. This
suggests that there is a time dependence of immersion freezing
and supports the importance of a stochastic component in the
ice nucleation process. The rate at which droplets freeze was
observed to decrease towards higher temperatures and smaller
particle sizes. Comparison of the laboratory data with four
different ice nucleation models, three based on classical
nucleation theory with different representations of the
particle surface properties and one singular, suggest that the
classical, stochastic approach combined with a distribution of
contact angles is able to reproduce the ice nucleation
observed in these experiments most accurately. Using the
models to calculate the increase in frozen fraction at typical
mixed-phase cloud temperatures over an extended period of
time, yields an equivalent effect of ?1 K
temperature shift and an increase in time scale by a factor of
~10.
Heterogeneous ice nucleation on atmospheric aerosols: a review of results from laboratory experiments
Atmospheric Chemistry and Physics Discussions, 12, 12531-12621, 2012
Author(s): C. Hoose and O. Möhler
A small subset of the atmospheric aerosol population has the ability
to induce ice formation at conditions under which ice would not form
without them (heterogeneous ice nucleation). While no closed
theoretical description of this process and the requirements for good
ice nuclei is available, numerous studies have attempted to quantify
the ice nucleation ability of different particles empirically in
laboratory experiments. In this article, an overview of these results
is provided. Ice nucleation onset conditions for various mineral dust,
soot, biological, organic and ammonium sulphate particles are
summarized. Typical temperature-supersaturation regions can be
identified for the onset of ice nucleation of these different particle
types, but the various particle sizes and activated fractions reported
in different studies have to be taken into account when comparing
results obtained with different methodologies. When intercomparing
only data obtained under the same conditions, it is found that dust
mineralogy is not a consistent predictor of higher or lower ice
nucleation ability. However, the broad majority of studies agrees on
a reduction of deposition nucleation by various coatings on mineral
dust. The ice nucleation active surface site (INAS) density is
discussed as a normalized measure for ice nucleation activity. For
most immersion and condensation freezing measurements on mineral dust,
estimates of the temperature-dependent INAS density agree within about
two orders of magnitude. For deposition nucleation on dust, the spread
is significantly larger, but a general trend of increasing INAS
densities with increasing supersaturation is found. For soot, the
presently available results are divergent. Estimated average INAS
densities are high for ice-nucleation active bacteria at high subzero
temperatures. At the same time, it is shown that some other biological
aerosols, like certain pollen grains and fungal spores, are not
intrinsically better ice nuclei than dust, but owe their high ice
nucleation onsets to their large sizes. Surface-area-dependent
parameterizations of heterogeneous ice nucleation are discussed. For
immersion freezing on mineral dust, fitted INAS densities are
available, but should not be used outside the temperature interval of
the data they were based on. Classical nucleation theory, if employed
with one fitted contact angle, does not reproduce the observed
temperature dependence for immersion nucleation, temperature and
supersaturation dependence for deposition nucleation, and time
dependence.
Mixing of dust and NH3 observed globally over anthropogenic dust sources
Atmospheric Chemistry and Physics Discussions, 12, 12503-12530, 2012
Author(s): P. Ginoux, L. Clarisse, C. Clerbaux, P.-F. Coheur, O. Dubovik, N. C. Hsu, and M. Van Damme
The global distribution of dust column burden derived from
MODIS Deep Blue aerosol products is compared to NH3
column burden retrieved from IASI infrared spectra. We found
similarities in their spatial distributions, in particular
their hot spots are often collocated over croplands and to
a lesser extent pastures. Globally, we found 22% of
dust burden collocated with NH3. This confirms the
importance of anthropogenic dust from agriculture. Regionally,
the Indian subcontinent has the highest amount of dust mixed
with NH3 (26%), mostly over cropland and
during the pre-monsoon season. North Africa represents
50% of total dust burden but accounts for only
4% of mixed dust, which is found over croplands and
pastures in Sahel and the coastal region of the
Mediterranean. In order to evaluate the radiative effect of
this mixing on dust optical properties, we derive the mass
extinction efficiency for various mixtures of dust and
NH3, using AERONET sunphotometers data. We found that
for dusty days the coarse mode mass extinction efficiency
decreases from 0.62 to 0.48 m2 g−1 as NH3
burden increases from 0 to 40 mg m−2. The fine
mode extinction efficiency, ranging from 4 to
16 m2 g−1, does not appear to depend on
NH3 concentration or relative humidity but rather on
mineralogical composition and mixing with other aerosols. Our
results imply that a significant amount of dust is already
mixed with ammonium salt before its long range transport. This
in turn will affect dust lifetime, and its interactions with
radiation and cloud properties.
Posted on 16 May 2012 | 12:00 am
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