17,051 research outputs found
Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature)
Reactive gases and aerosols are produced by terrestrial ecosystems, processed within plant canopies, and can then be emitted into the above-canopy atmosphere. Estimates of the above-canopy fluxes are needed for quantitative earth system studies and assessments of past, present and future air quality and climate. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) is described and used to quantify net terrestrial biosphere emission of isoprene into the atmosphere. MEGAN is designed for both global and regional emission modeling and has global coverage with ~1 km<sup>2</sup> spatial resolution. Field and laboratory investigations of the processes controlling isoprene emission are described and data available for model development and evaluation are summarized. The factors controlling isoprene emissions include biological, physical and chemical driving variables. MEGAN driving variables are derived from models and satellite and ground observations. Tropical broadleaf trees contribute almost half of the estimated global annual isoprene emission due to their relatively high emission factors and because they are often exposed to conditions that are conducive for isoprene emission. The remaining flux is primarily from shrubs which have a widespread distribution. The annual global isoprene emission estimated with MEGAN ranges from about 500 to 750 Tg isoprene (440 to 660 Tg carbon) depending on the driving variables which include temperature, solar radiation, Leaf Area Index, and plant functional type. The global annual isoprene emission estimated using the standard driving variables is ~600 Tg isoprene. Differences in driving variables result in emission estimates that differ by more than a factor of three for specific times and locations. It is difficult to evaluate isoprene emission estimates using the concentration distributions simulated using chemistry and transport models, due to the substantial uncertainties in other model components, but at least some global models produce reasonable results when using isoprene emission distributions similar to MEGAN estimates. In addition, comparison with isoprene emissions estimated from satellite formaldehyde observations indicates reasonable agreement. The sensitivity of isoprene emissions to earth system changes (e.g., climate and land-use) demonstrates the potential for large future changes in emissions. Using temperature distributions simulated by global climate models for year 2100, MEGAN estimates that isoprene emissions increase by more than a factor of two. This is considerably greater than previous estimates and additional observations are needed to evaluate and improve the methods used to predict future isoprene emissions
Which processes drive observed variations of HCHO columns over India?
We interpret HCHO column variations observed by the Ozone
Monitoring Instrument (OMI), aboard the NASA Aura satellite, over India
during 2014 using the GEOS-Chem atmospheric chemistry and transport model. We
use a nested version of the model with a horizontal resolution of
approximately 25 km. HCHO columns are related to local emissions of volatile
organic compounds (VOCs) with a spatial smearing that increases with the VOC
lifetime. Over India, HCHO has biogenic, pyrogenic, and anthropogenic VOC
sources. Using a 0-D photochemistry model, we find that isoprene has the
largest molar yield of HCHO which is typically realized within a few hours. We also
find that forested regions that neighbour major urban conurbations are
exposed to high levels of nitrogen oxides. This results in depleted hydroxyl
radical concentrations and a delay in the production of HCHO from isoprene
oxidation. We find that propene is the only anthropogenic VOC emitted in
major Indian cities that produces HCHO at a comparable (but slower) rate to
isoprene. The GEOS-Chem model reproduces the broad-scale annual mean HCHO
column distribution observed by OMI (<i>r</i> = 0.6), which is dominated by a
distinctive meridional gradient in the northern half of the country, and by
localized regions of high columns that coincide with forests. Major
discrepancies are noted over the Indo-Gangetic Plain (IGP) and Delhi. We find that the
model has more skill at reproducing observations during winter (JF) and
pre-monsoon (MAM) months with Pearson correlations <i>r</i> > 0.5 but with a
positive model bias of <mo form="infix">≃</mo> 1×10<sup>15</sup> molec cm<sup>−2</sup>. During the
monsoon season (JJAS) we reproduce only a diffuse version of the observed
meridional gradient (<i>r</i> = 0.4). We find that on a continental scale most of
the HCHO column seasonal cycle is explained by monthly variations in surface
temperature (<i>r</i> = 0.9), suggesting a role for biogenic VOCs, in agreement with
the 0-D and GEOS-Chem model calculations. We also find that the seasonal
cycle during 2014 is not significantly different from the 2008 to 2015 mean
seasonal variation. There are two main loci for biomass burning (the states of
Punjab and Haryana, and northeastern India), which we find makes a significant contribution
(up to 1×10<sup>15</sup> molec cm<sup>−2</sup>) to
observed HCHO columns only during March and April over northeastern India.
The slow production of HCHO from propene oxidation results in a smeared
hotspot over Delhi that we resolve only on an annual mean timescale by using
a temporal oversampling method. Using a linear regression model to relate
GEOS-Chem isoprene emissions to HCHO columns we infer seasonal isoprene
emissions over two key forest regions from the OMI HCHO column data. We find
that the a posteriori emissions are typically lower than the a priori
emissions, with a much stronger reduction of emissions during the monsoon
season. We find that this reduction in emissions during monsoon months
coincides with a large drop in satellite observations of leaf phenology that
recovers in post monsoon months. This may signal a forest-scale response to
monsoon conditions
Seasonal distribution and drivers of surface fine particulate matter and organic aerosol over the Indo-Gangetic Plain
The Indo-Gangetic Plain (IGP) is home to 9 % of the global population and is responsible for a
large fraction of agricultural crop production in Pakistan, India, and Bangladesh. Levels of fine particulate matter (mean diameter <2.5 µm, PM2.5)
across the IGP often exceed human health recommendations, making
cities across the IGP among the most polluted in the world. Seasonal
changes in the physical environment over the IGP are dominated by the
large-scale south Asian monsoon system that dictates the timing of
agricultural planting and harvesting. We use the WRF-Chem model to study the seasonal anthropogenic,
pyrogenic, and biogenic influences on fine particulate matter and its
constituent organic aerosol (OA) over the IGP
that straddles Pakistan, India, and Bangladesh during 2017–2018. We find that surface air quality
during pre-monsoon (March–May) and monsoon (June–September) seasons is
better than during post-monsoon (October–December) and winter
(January–February) seasons, but all seasonal mean values of PM2.5
still exceed the recommended levels, so that air pollution is a year-round problem. Anthropogenic
emissions influence the magnitude and distribution of PM2.5 and
OA throughout the year, especially over urban sites, while pyrogenic
emissions result in localised contributions over the central and upper
parts of IGP in all non-monsoonal seasons, with the highest impact during
post-monsoon seasons that correspond to the post-harvest season in the
agricultural calendar. Biogenic emissions play an important role in
the magnitude and distribution of PM2.5 and OA during the monsoon
season, and they show a substantial contribution to secondary OA (SOA),
particularly over the lower IGP. We find that the OA contribution to
PM2.5 is significant in all four seasons (17 %–30 %), with primary
OA generally representing the larger fractional contribution. We find
that the volatility distribution of SOA is driven mainly by the mean
total OA loading and the washout of aerosols and gas-phase aerosol
precursors that result in SOA being less volatile during the
pre-monsoon and monsoon season than during the post-monsoon and winter
seasons.</p
Sources and budgets for CO and O-3 in the northeastern Pacific during the spring of 2001: Results from the PHOBEA-II Experiment
Abstract. Ground and airborne measurements of CO, ozone, and aerosols were obtained in th
Mapping isoprene emissions over North America using formaldehyde column observations from space
We present a methodology for deriving emissions of volatile organic compounds (VOC) using space-based column observations of formaldehyde (HCHO) and apply it to data from the Global Ozone Monitoring Experiment (GOME) satellite instrument over North America during July 1996. The HCHO column is related to local VOC emissions, with a spatial smearing that increases with the VOC lifetime. Isoprene is the dominant HCHO precursor over North America in summer, and its lifetime (≃1 hour) is sufficiently short that the smearing can be neglected. We use the Goddard Earth Observing System global 3-D model of tropospheric chemistry (GEOS-CHEM) to derive the relationship between isoprene emissions and HCHO columns over North America and use these relationships to convert the GOME HCHO columns to isoprene emissions. We also use the GEOS-CHEM model as an intermediary to validate the GOME HCHO column measurements by comparison with in situ observations. The GEOS-CHEM model including the Global Emissions Inventory Activity (GEIA) isoprene emission inventory provides a good simulation of both the GOME data (r2 = 0.69, n = 756, bias = +11%) and the in situ summertime HCHO measurements over North America (r2 = 0.47, n = 10, bias = −3%). The GOME observations show high values over regions of known high isoprene emissions and a day-to-day variability that is consistent with the temperature dependence of isoprene emission. Isoprene emissions inferred from the GOME data are 20% less than GEIA on average over North America and twice those from the U.S. EPA Biogenic Emissions Inventory System (BEIS2) inventory. The GOME isoprene inventory when implemented in the GEOS-CHEM model provides a better simulation of the HCHO in situ measurements than either GEIA or BEIS2 (r2 = 0.71, n = 10, bias = −10%)
Seasonal variations in Greenland Ice Sheet motion : Inland extent and behaviour at higher elevations
Peer reviewedPreprin
Irradiated Interfaces in the Ara OB1, Carina, Eagle Nebula, and Cyg OB2 Massive Star Formation Regions
Regions of massive star formation offer some of the best and most
easily-observed examples of radiation hydrodynamics. Boundaries where
fully-ionized H II regions transition to neutral/molecular photodissociation
regions (PDRs) are of particular interest because marked temperature and
density contrasts across the boundaries lead to evaporative flows and fluid
dynamical instabilities that can evolve into spectacular pillar-like
structures. When detached from their parent clouds, pillars become ionized
globules that often harbor one or more young stars. H2 molecules at the
interface between a PDR and an H II region absorb ultraviolet light from
massive stars, and the resulting fluoresced infrared emission lines are an
ideal way to trace this boundary independent of obscuring dust. This paper
presents H2 images of four regions of massive star formation that illustrate
different types of PDR boundaries. The Ara OB1 star formation region contains a
striking long wall that has several wavy structures which are present in H2,
but the emission is not particularly bright because the ambient UV fluxes are
relatively low. In contrast, the Carina star formation region shows strong H2
fluorescence both along curved walls and at the edges of spectacular pillars
that in some cases have become detached from their parent clouds. The
less-spectacular but more well-known Eagle Nebula has two regions that have
strong fluorescence in addition to its pillars. While somewhat older than the
other regions, Cyg OB2 has the highest number of massive stars of the regions
surveyed and contains many isolated, fluoresced globules that have head-tail
morphologies which point towards the sources of ionizing radiation. These
images provide a collection of potential astrophysical analogs that may relate
to ablated interfaces observed in laser experiments of radiation hydrodynamics
Gravitational Laser Back-Scattering
A possible way of producing gravitons in the laboratory is investigated. We
evaluate the cross section electron + photon electron + graviton
in the framework of linearized gravitation, and analyse this reaction
considering the photon coming either from a laser beam or from a Compton
back-scattering process.Comment: 11 pages, 2 figures (available upon request), RevTeX, IFT-P.03/9
Relativistic Particle-In-Cell Simulation Studies of Prompt and Early Afterglows from GRBs
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks e.g. gamma-ray bursts (GRBs) active galactic
nuclei (AGNs) and microquasars commonly exhibit power-law emission spectra.
Recent PIC simulations of relativistic electron-ion (or electron-positron) jets
injected into a stationary medium show that particle acceleration occurs within
the downstream jet. In collisionless relativistic shocks particle (electron,
positron and ion) acceleration is due to plasma waves and their associated
instabilities (e.g. the Weibel (filamentation) instability) created in the
shock region. The simulations show that the Weibel instability is responsible
for generating and amplifying highly non-uniform small-scale magnetic fields.
These fields contribute to the electron's transverse deflection behind the jet
head. The resulting ``jitter'' radiation from deflected electrons has different
properties compared to synchrotron radiation which assumes a uniform magnetic
field. Jitter radiation may be important for understanding the complex time
evolution and/or spectra in gamma-ray bursts, relativistic jets in general and
supernova remnants.Comment: 19 pages,7 figures, contributed talk at Seventh European Workshop on
Collisionless Shocks, Paris, 7- 9 November 2007. High resolution version can
be obtained at http://gammaray.nsstc.nasa.gov/~nishikawa/shockws07.pd
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