24 research outputs found
Surface ozone over the Tibetan Plateau controlled by stratospheric intrusion
The Tibetan Plateau is a global hotspot of stratospheric intrusion, and elevated
surface ozone was observed at ground monitoring sites. Still, links between
the variability of surface ozone and stratospheric intrusion at the regional
scale remain unclear. This study synthesized ground measurements of surface
ozone over the Tibetan Plateau and analyzed their seasonal variations. The
monthly mean surface ozone concentrations over the Tibetan Plateau peaked
earlier in the south in April and May and later in the north in June and July. The
migration of monthly surface ozone peaks was coupled with the synchronous
movement of tropopause folds and the westerly jet that created
conditions conducive to stratospheric ozone intrusion. Stratospheric ozone intrusion
significantly contributed to surface ozone across the Tibetan Plateau,
especially in the areas with high surface ozone concentrations during their
peak-value month. We demonstrated that monthly variation of surface ozone
over the Tibetan Plateau is mainly controlled by stratospheric intrusion,
which warrants proper consideration in understanding the atmospheric chemistry
and the impacts of ozone over this highland region and beyond.</p
Observation and analysis of spatiotemporal characteristics of surface ozone and carbon monoxide at multiple sites in the Kathmandu Valley, Nepal
Residents of the Kathmandu Valley experience severe particulate
and gaseous air pollution throughout most of the year, even during much of
the rainy season. The knowledge base for understanding the air pollution in
the Kathmandu Valley was previously very limited but is improving rapidly
due to several field measurement studies conducted in the last few years.
Thus far, most analyses of observations in the Kathmandu Valley have been
limited to short periods of time at single locations. This study extends the
past studies by examining the spatial and temporal characteristics of two
important gaseous air pollutants (CO and O3) based on simultaneous
observations over a longer period at five locations within the valley and on
its rim, including a supersite (at Bode in the valley center, 1345 m above
sea level) and four satellite sites: Paknajol (1380 m a.s.l.) in the Kathmandu
city center; Bhimdhunga (1522 m a.s.l.), a mountain pass on the valley's
western rim; Nagarkot (1901 m a.s.l.), another mountain pass on the eastern
rim; and Naikhandi (1233 m a.s.l.), near the valley's only river outlet. CO and
O3 mixing ratios were monitored from January to July 2013, along with
other gases and aerosol particles by instruments deployed at the Bode
supersite during the international air pollution measurement campaign
SusKat-ABC (Sustainable Atmosphere for the Kathmandu Valley – endorsed by
the Atmospheric Brown Clouds program of UNEP). The monitoring of O3 at
Bode, Paknajol and Nagarkot as well as the CO monitoring at Bode were
extended until March 2014 to investigate their variability over a complete
annual cycle. Higher CO mixing ratios were found at Bode than at the outskirt
sites (Bhimdhunga, Naikhandi and Nagarkot), and all sites except Nagarkot
showed distinct diurnal cycles of CO mixing ratio, with morning peaks and
daytime lows. Seasonally, CO was higher during premonsoon (March–May) season
and winter (December–February) season than during monsoon season
(June–September) and postmonsoon (October–November) season. This is
primarily due to the emissions from brick industries, which are only
operational during this period (January–April), as well as increased domestic
heating during winter, and regional forest fires and agro-residue burning
during the premonsoon season. It was lower during the monsoon due to
rainfall, which reduces open burning activities within the valley and in the
surrounding regions and thus reduces sources of CO. The meteorology of the
valley also played a key role in determining the CO mixing ratios. The wind
is calm and easterly in the shallow mixing layer, with a mixing layer height
(MLH) of about 250 m, during the night and early morning. The MLH slowly
increases after sunrise and decreases in the afternoon. As a
result, the westerly wind becomes active and reduces the mixing ratio during
the daytime. Furthermore, there was evidence of an increase in the O3
mixing ratios in the Kathmandu Valley as a result of emissions in the
Indo-Gangetic Plain (IGP) region, particularly from biomass burning
including agro-residue burning. A top-down estimate of the CO emission flux
was made by using the CO mixing ratio and mixing layer height measured at
Bode. The estimated annual CO flux at Bode was 4.9 µg m−2 s−1,
which is 2–14 times higher than that in widely used emission
inventory databases (EDGAR HTAP, REAS and INTEX-B). This difference in CO
flux between Bode and other emission databases likely arises from large
uncertainties in both the top-down and bottom-up approaches to estimating the
emission flux. The O3 mixing ratio was found to be highest during the
premonsoon season at all sites, while the timing of the seasonal minimum
varied across the sites. The daily maximum 8 h average O3 exceeded
the WHO recommended guideline of 50 ppb on more days at the hilltop station
of Nagarkot (159 out of 357Â days) than at the urban valley bottom sites of Paknajol
(132 out of 354Â days) and Bode (102 out of 353Â days), presumably due to the influence of
free-tropospheric air at the high-altitude site (as also indicated by Putero
et al., 2015, for the Paknajol site in the Kathmandu Valley) as well as to
titration of O3 by fresh NOx emissions near the urban sites. More than
78 % of the exceedance days were during the premonsoon period at all
sites. The high O3 mixing ratio observed during the premonsoon periodÂ
is of a concern for human health and ecosystems, including agroecosystems in
the Kathmandu Valley and surrounding regions.</p
The impact of residential combustion emissions on atmospheric aerosol, human health, and climate
Combustion of fuels in the residential sector for cooking and heating results in the emission of aerosol and aerosol precursors impacting air quality, human health, and climate. Residential emissions are dominated by the combustion of solid fuels. We use a global aerosol microphysics model to simulate the impact of residential fuel combustion on atmospheric aerosol for the year 2000. The model underestimates black carbon (BC) and organic carbon (OC) mass concentrations observed over Asia, Eastern Europe, and Africa, with better prediction when carbonaceous emissions from the residential sector are doubled. Observed seasonal variability of BC and OC concentrations are better simulated when residential emissions include a seasonal cycle. The largest contributions of residential emissions to annual surface mean particulate matter (PM2.5) concentrations are simulated for East Asia, South Asia, and Eastern Europe. We use a concentration response function to estimate the human health impact due to long-term exposure to ambient PM2.5 from residential emissions. We estimate global annual excess adult (>30 years of age) premature mortality (due to both cardiopulmonary disease and lung cancer) to be 308 000 (113 300–497 000, 5th to 95th percentile uncertainty range) for monthly varying residential emissions and 517 000 (192 000–827 000) when residential carbonaceous emissions are doubled. Mortality due to residential emissions is greatest in Asia, with China and India accounting for 50% of simulated global excess mortality. Using an offline radiative transfer model we estimate that residential emissions exert a global annual mean direct radiative effect between −66 and +21 mW m−2, with sensitivity to the residential emission flux and the assumed ratio of BC, OC, and SO2 emissions. Residential emissions exert a global annual mean first aerosol indirect effect of between −52 and −16 mW m−2, which is sensitive to the assumed size distribution of carbonaceous emissions. Overall, our results demonstrate that reducing residential combustion emissions would have substantial benefits for human health through reductions in ambient PM2.5 concentrations
STUDY OF AEROSOL OPTICAL PROPERTIES OVER TWO SITES IN THE FOOTHILLS OF THE CENTRAL HIMALAYAS
Atmospheric aerosol possesses impacts on climate system and ecological environments, human health and agricultural productivity. The environment over Himalayas and Tibetan Plateau region are continuously degraded due to the transport of pollution from the foothills of the Himalayas; mostly the Indo-Gangetic Plain (IGP). Thus, analysis of aerosol optical properties over two sites; Lumbini and Kathmandu (the southern slope of central Himalayas) using AERONET’s CIMEL sun photometer were conducted in this study. Aerosol optical depth (AOD at 500 nm), angstrom exponent (α or AE), volume size distribution (VSD), single scattering albedo (SSA) and asymmetry parameter (AP) were studied for 2013–2014 and the average AOD was found to be: 0.64 ± 0.41 (Lumbini) and 0.45 ± 0.30 (Kathmandu). The average AE was found to be: 1.25 ± 0.24 and 1.26 ± 0.18 respectively for two sites. The relation between AOD and AE was used to discriminate the aerosol types over these sites which indicated anthropogenic, mixed and biomass burning origin aerosol constituted the major aerosol types in Lumbini and Kathmandu. A clear bi-modal distribution of aerosol volume size was observed with highest volume concentration during the post-monsoon season in fine mode and pre-monsoon season in coarse mode (Lumbini) and highest value over both modes during pre-monsoon season in Kathmandu. The single scattering albedo (SSA) and asymmetry parameter (AP) analyses suggested aerosols over the Himalayan foothills sites are dominated by absorbing and anthropogenic aerosols from urban and industrial activities and biomass burning. Long-term studies are essential to understand and characterize the nature of aerosol over this research gap zone
Overview of VOC emissions and chemistry from PTR-TOF-MS measurements during the SusKat-ABC campaign: high acetaldehyde, isoprene and isocyanic acid in wintertime air of the Kathmandu Valley
The Kathmandu Valley in Nepal suffers from severe wintertime air pollution.
Volatile organic compounds (VOCs) are key constituents of air pollution,
though their specific role in the valley is poorly understood due to
insufficient data. During the SusKat-ABC (Sustainable Atmosphere for the
Kathmandu Valley–Atmospheric Brown Clouds) field campaign conducted in Nepal
in the winter of 2012–2013, a comprehensive study was carried out to
characterise the chemical composition of ambient Kathmandu air, including the
determination of speciated VOCs, by deploying a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) – the first such deployment in South
Asia. In the study, 71 ion peaks (for which measured ambient concentrations exceeded the
2<i>σ</i> detection limit) were detected in the PTR-TOF-MS mass scan
data, highlighting the chemical complexity of ambient air in the valley. Of
the 71 species, 37 were found to have campaign average concentrations greater
than 200 ppt and were identified based on their spectral
characteristics, ambient diel profiles and correlation with specific emission
tracers as a result of the high mass resolution (<i>m</i> ∕ Δ<i>m</i>  >  4200) and temporal resolution (1 min) of the PTR-TOF-MS. The
concentration ranking in the average VOC mixing ratios during our wintertime
deployment was acetaldehyde (8.8 ppb)  >  methanol (7.4 ppb)
 >  acetone + propanal (4.2 ppb)  >  benzene (2.7 ppb)  > 
toluene (1.5 ppb)  >  isoprene (1.1 ppb)  >  acetonitrile
(1.1 ppb)  >  C8-aromatics ( ∼ 1 ppb)  >  furan
( ∼ 0.5 ppb)  >  C9-aromatics (0.4 ppb). Distinct diel
profiles were observed for the nominal isobaric compounds isoprene
(<i>m</i> ∕ <i>z</i>  =  69.070) and furan (<i>m</i> ∕ <i>z</i>  =  69.033).
Comparison with wintertime measurements from several locations elsewhere in
the world showed mixing ratios of acetaldehyde ( ∼  9 ppb),
acetonitrile ( ∼  1 ppb) and isoprene ( ∼  1 ppb) to
be among the highest reported to date. Two "new" ambient compounds,
namely formamide (<i>m</i> ∕ <i>z</i>  =  46.029) and acetamide
(<i>m</i> ∕ <i>z</i>  =  60.051), which can photochemically produce isocyanic
acid in the atmosphere, are reported in this study along with nitromethane (a
tracer for diesel exhaust), which has only recently been detected in ambient
studies. Two distinct periods were selected during the campaign for detailed
analysis: the first was associated with high wintertime emissions of biogenic
isoprene and the second with elevated levels of ambient acetonitrile,
benzene and isocyanic acid from biomass burning activities. Emissions from
biomass burning and biomass co-fired brick kilns were found to be the
dominant sources for compounds such as propyne, propene, benzene and
propanenitrile, which correlated strongly with acetonitrile (<i>r</i><sup>2</sup> > 0.7), a
chemical tracer for biomass burning. The calculated total VOC OH reactivity
was dominated by acetaldehyde (24.0 %), isoprene (20.2 %)
and propene (18.7 %), while oxygenated VOCs and isoprene
collectively contributed to more than 68 % of the total ozone
production potential. Based on known secondary organic aerosol (SOA) yields and measured ambient concentrations in the Kathmandu Valley, the relative SOA production potential of VOCs were benzene  >  naphthalene  >  toluene  >  xylenes  >  monoterpenes  >  trimethylbenzenes  >  styrene  >  isoprene.
The first ambient measurements from any site in South Asia of compounds with
significant health effects such as isocyanic acid, formamide, acetamide,
naphthalene and nitromethane have been reported in this study. Our results
suggest that mitigation of intense wintertime biomass burning activities, in
particular point sources such biomass co-fired brick kilns, would be
important to reduce the emission and formation of toxic VOCs (such as benzene
and isocyanic acid) in the Kathmandu Valley
Organic molecular tracers in the atmospheric aerosols from Lumbini, Nepal, in the northern Indo-Gangetic Plain: influence of biomass burning
To better understand the characteristics of biomass burning in the
northern Indo-Gangetic Plain (IGP), total suspended particles were collected
in a rural site, Lumbini, Nepal, during April 2013 to March 2014 and analyzed
for the biomass burning tracers (i.e., levoglucosan, mannosan, vanillic acid). The annual average concentration of levoglucosan was
734 ± 1043 ng m−3 with the maximum seasonal mean concentration
during post-monsoon season (2206 ± 1753 ng m−3), followed by
winter (1161 ± 1347 ng m−3), pre-monsoon
(771 ± 524 ng m−3) and minimum concentration during monsoon
season (212 ± 279 ng m−3). The other biomass burning tracers
(mannosan, galactosan, p-hydroxybenzoic acid, vanillic acid, syringic acid
and dehydroabietic acid) also showed the similar seasonal variations. There
were good correlations among levoglucosan, organic carbon (OC) and elemental
carbon (EC), indicating significant impact of biomass burning activities on
carbonaceous aerosol loading throughout the year in Lumbini area. According
to the characteristic ratios, levoglucosan ∕ mannosan (lev ∕ man) and
syringic acid ∕ vanillic acid (syr ∕ van), we deduced that the high
abundances of biomass burning products during non-monsoon seasons were mainly
caused by the burning of crop residues and hardwood while the softwood had
less contribution. Based on the diagnostic tracer ratio (i.e., lev ∕ OC),
the OC derived from biomass burning constituted large fraction of total OC,
especially during post-monsoon season. By analyzing the MODIS fire spot
product and 5-day air-mass back trajectories, we further demonstrated that
organic aerosol composition was not only related to the local agricultural
activities and residential biomass usage but also impacted by the
regional emissions. During the post-monsoon season, the emissions from rice
residue burning in western India and eastern Pakistan could impact
particulate air pollution in Lumbini and surrounding regions in southern
Nepal. Therefore, our finding is meaningful and has a great importance for
adopting the appropriate mitigation measures, not only at the local level but
also by involving different regions and nations, to reduce the biomass
burning emissions in the broader IGP region nations
Statistical mechanics of polarizable force fields based on classical Drude oscillators with dynamical propagation by the dual-thermostat extended Lagrangian
Optimized Lennard-Jones Parameters for Druglike Small Molecules
Meaningful
efforts in computer-aided drug design (CADD) require
accurate molecular mechanical force fields to quantitatively characterize
protein–ligand interactions, ligand hydration free energies,
and other ligand physical properties. Atomic models of new compounds
are commonly generated by analogy from the predefined tabulated parameters
of a given force field. Two widely used approaches following this
strategy are the General Amber Force Field (GAFF) and the CHARMM General
Force Field (CGenFF). An important limitation of using pretabulated
parameter values is that they may be inadequate in the context of
a specific molecule. To resolve this issue, we previously introduced
the General Automated Atomic Model Parameterization (GAAMP) for automatically
generating the parameters of atomic models of small molecules, using
the results from ab initio quantum mechanical (QM) calculations as
target data. The GAAMP protocol uses QM data to optimize the bond,
valence angle, and dihedral angle internal parameters, and atomic
partial charges. However, since the treatment of van der Waals interactions
based on QM is challenging and may often be unreliable, the Lennard-Jones
6–12 parameters are kept unchanged from the initial atom types
assignments (GAFF or CGenFF), which limits the accuracy that can be
achieved by these models. To address this issue, a new set of Lennard-Jones
6–12 parameters was systematically optimized to reproduce experimental
neat liquid densities and enthalpies of vaporization for a large set
of 430 compounds, covering a wide range of chemical functionalities.
Calculations of the hydration free energy indicate that optimal accuracy
for these models is achieved when the molecule–water van der
Waals dispersion is rescaled by a factor of 1.115. The final optimized
model yields an average unsigned error of 0.79 kcal/mol in the hydration
free energies
Optimized Lennard-Jones Parameters for Druglike Small Molecules
Meaningful
efforts in computer-aided drug design (CADD) require
accurate molecular mechanical force fields to quantitatively characterize
protein–ligand interactions, ligand hydration free energies,
and other ligand physical properties. Atomic models of new compounds
are commonly generated by analogy from the predefined tabulated parameters
of a given force field. Two widely used approaches following this
strategy are the General Amber Force Field (GAFF) and the CHARMM General
Force Field (CGenFF). An important limitation of using pretabulated
parameter values is that they may be inadequate in the context of
a specific molecule. To resolve this issue, we previously introduced
the General Automated Atomic Model Parameterization (GAAMP) for automatically
generating the parameters of atomic models of small molecules, using
the results from ab initio quantum mechanical (QM) calculations as
target data. The GAAMP protocol uses QM data to optimize the bond,
valence angle, and dihedral angle internal parameters, and atomic
partial charges. However, since the treatment of van der Waals interactions
based on QM is challenging and may often be unreliable, the Lennard-Jones
6–12 parameters are kept unchanged from the initial atom types
assignments (GAFF or CGenFF), which limits the accuracy that can be
achieved by these models. To address this issue, a new set of Lennard-Jones
6–12 parameters was systematically optimized to reproduce experimental
neat liquid densities and enthalpies of vaporization for a large set
of 430 compounds, covering a wide range of chemical functionalities.
Calculations of the hydration free energy indicate that optimal accuracy
for these models is achieved when the molecule–water van der
Waals dispersion is rescaled by a factor of 1.115. The final optimized
model yields an average unsigned error of 0.79 kcal/mol in the hydration
free energies
Optimized Lennard-Jones Parameters for Druglike Small Molecules
Meaningful
efforts in computer-aided drug design (CADD) require
accurate molecular mechanical force fields to quantitatively characterize
protein–ligand interactions, ligand hydration free energies,
and other ligand physical properties. Atomic models of new compounds
are commonly generated by analogy from the predefined tabulated parameters
of a given force field. Two widely used approaches following this
strategy are the General Amber Force Field (GAFF) and the CHARMM General
Force Field (CGenFF). An important limitation of using pretabulated
parameter values is that they may be inadequate in the context of
a specific molecule. To resolve this issue, we previously introduced
the General Automated Atomic Model Parameterization (GAAMP) for automatically
generating the parameters of atomic models of small molecules, using
the results from ab initio quantum mechanical (QM) calculations as
target data. The GAAMP protocol uses QM data to optimize the bond,
valence angle, and dihedral angle internal parameters, and atomic
partial charges. However, since the treatment of van der Waals interactions
based on QM is challenging and may often be unreliable, the Lennard-Jones
6–12 parameters are kept unchanged from the initial atom types
assignments (GAFF or CGenFF), which limits the accuracy that can be
achieved by these models. To address this issue, a new set of Lennard-Jones
6–12 parameters was systematically optimized to reproduce experimental
neat liquid densities and enthalpies of vaporization for a large set
of 430 compounds, covering a wide range of chemical functionalities.
Calculations of the hydration free energy indicate that optimal accuracy
for these models is achieved when the molecule–water van der
Waals dispersion is rescaled by a factor of 1.115. The final optimized
model yields an average unsigned error of 0.79 kcal/mol in the hydration
free energies