Satellite observations of atmospheric methane plumes offer a means for global mapping of methane point sources. Here we use the GHGSat-D satellite instrument with 50 m effective spatial resolution ...and 9–18% single-pass column precision to quantify mean source rates for three coal mine vents (San Juan, United States; Appin, Australia; and Bulianta, China) over a two-year period (2016–2018). This involves averaging wind-rotated observations from 14 to 24 overpasses to achieve satisfactory signal-to-noise. Our wind rotation method optimizes the wind direction information for individual plumes to account for error in meteorological databases. We derive source rates from the time-averaged plumes using integrated mass enhancement (IME) and cross-sectional flux (CSF) methods calibrated with large eddy simulations. We find time-averaged source rates ranging from 2320 to 5850 kg h–1 for the three coal mine vents, with 40–45% precision (1σ), and generally consistent with previous estimates. The IME and CSF methods agree within 15%. Our results demonstrate the potential of space-based monitoring for annual reporting of methane emissions from point sources and suggest that future satellite instruments with similar pixel resolution but better precision should be able to constrain a wide range of point sources.
Observations of surface ozone available from ∼1,000 sites across China for the past 5 years (2013–2017) show severe summertime pollution and regionally variable trends. We resolve the effect of ...meteorological variability on the ozone trends by using a multiple linear regression model. The residual of this regression shows increasing ozone trends of 1–3 ppbv a−1 in megacity clusters of eastern China thatwe attribute to changes in anthropogenic emissions. By contrast, ozone decreased in some areas of southern China. Anthropogenic NOₓ emissions in China are estimated to have decreased by 21% during 2013–2017, whereas volatile organic compounds (VOCs) emissions changed little. Decreasing NOₓ would increase ozone under the VOC-limited conditions thought to prevail in urban China while decreasing ozone under rural NOₓ-limited conditions. However, simulations with the Goddard Earth Observing System Chemical Transport Model (GEOS-Chem) indicate that a more important factor for ozone trends in the North China Plain is the ∼40% decrease of fine particulate matter (PM2.5) over the 2013–2017 period, slowing down the aerosol sink of hydroperoxy (HO₂) radicals and thus stimulating ozone production.
Atmospheric oxidation of isoprene, the most abundantly emitted non-methane hydrocarbon, affects the abundances of ozone (O3), the hydroxyl radical (OH), nitrogen oxide radicals (NOx), carbon monoxide ...(CO), oxygenated and nitrated organic compounds, and secondary organic aerosol (SOA). We analyze these effects in box models and in the global GEOS-Chem chemical transport model using the new reduced Caltech isoprene mechanism (RCIM) condensed from a recently developed explicit isoprene oxidation mechanism. We find many similarities with previous global models of isoprene chemistry along with a number of important differences. Proper accounting of the isomer distribution of peroxy radicals following the addition of OH and O2 to isoprene influences the subsequent distribution of products, decreasing in particular the yield of methacrolein and increasing the capacity of intramolecular hydrogen shifts to promptly regenerate OH. Hydrogen shift reactions throughout the mechanism lead to increased OH recycling, resulting in less depletion of OH under low-NO conditions than in previous mechanisms. Higher organonitrate yields and faster tertiary nitrate hydrolysis lead to more efficient NOx removal by isoprene and conversion to inorganic nitrate. Only 20 % of isoprene-derived organonitrates (excluding peroxyacyl nitrates) are chemically recycled to NOx. The global yield of formaldehyde from isoprene is 22 % per carbon and less sensitive to NO than in previous mechanisms. The global molar yield of glyoxal is 2 %, much lower than in previous mechanisms because of deposition and aerosol uptake of glyoxal precursors. Global production of isoprene SOA is about one-third from each of the following: isoprene epoxydiols (IEPOX), organonitrates, and tetrafunctional compounds. We find a SOA yield from isoprene of 13 % per carbon, much higher than commonly assumed in models and likely offset by SOA chemical loss. We use the results of our simulations to further condense RCIM into a mini Caltech isoprene mechanism (Mini-CIM) for less expensive implementation in atmospheric models, with a total size (108 species, 345 reactions) comparable to currently used mechanisms.
Fine particulate matter (PM2.5) is a severe air pollution
problem in China. Observations of PM2.5 have been available since 2013
from a large network operated by the China National Environmental ...Monitoring
Center (CNEMC). The data show a general 30 %–50 % decrease in annual mean
PM2.5 across China over the 2013–2018 period, averaging at −5.2 µg m−3 a−1. Trends in the five megacity cluster regions targeted by
the government for air quality control are -9.3±1.8 µg m−3 a−1 (±95 % confidence interval) for Beijing–Tianjin–Hebei,
-6.1±1.1 µg m−3 a−1 for the Yangtze River Delta, -2.7±0.8 µg m−3 a−1 for the Pearl River Delta, -6.7±1.3 µg m−3 a−1 for the Sichuan Basin, and -6.5±2.5 µg m−3 a−1 for the Fenwei Plain (Xi'an). Concurrent 2013–2018
observations of sulfur dioxide (SO2) and carbon monoxide (CO) show that
the declines in PM2.5 are qualitatively consistent with drastic
controls of emissions from coal combustion. However, there is also a large
meteorologically driven interannual variability in PM2.5 that
complicates trend attribution. We used a stepwise multiple linear regression
(MLR) model to quantify this meteorological contribution to the PM2.5 trends across China. The MLR model correlates the 10 d PM2.5
anomalies to wind speed, precipitation, relative humidity, temperature, and
850 hPa meridional wind velocity (V850). The meteorology-corrected
PM2.5 trends after removal of the MLR meteorological contribution can
be viewed as being driven by trends in anthropogenic emissions. The mean
PM2.5 decrease across China is −4.6 µg m−3 a−1 in the
meteorology-corrected data, 12 % weaker than in the original data, meaning
that 12 % of the PM2.5 decrease in the original data is
attributable to meteorology. The trends in the meteorology-corrected data
for the five megacity clusters are -8.0±1.1 µg m−3 a−1 for Beijing–Tianjin–Hebei (14 % weaker than in the original
data), -6.3±0.9 µg m−3 a−1 for the Yangtze River Delta
(3 % stronger), -2.2±0.5 µg m−3 a−1 for the Pearl River
Delta (19 % weaker), -4.9±0.9 µg m−3 a−1 for
the Sichuan Basin (27 % weaker), and -5.0±1.9 µg m−3 a−1 for the Fenwei Plain (Xi'an; 23 % weaker); 2015–2017 observations of
flattening PM2.5 in the Pearl River Delta and increases in the Fenwei
Plain can be attributed to meteorology rather than to relaxation of emission
controls.
Surface ozone data from the Chinese Ministry of Ecology
and Environment (MEE) network show sustained increases across the country
over the 2013–2019 period. Despite Phase 2 of the Clean Air Action ...Plan targeting
ozone pollution, ozone was higher in 2018–2019 than in previous years. The
mean summer 2013–2019 trend in maximum 8 h average (MDA8) ozone was 1.9 ppb a−1 (p<0.01) across China and 3.3 ppb a−1 (p<0.01) over the North China Plain (NCP). Fitting ozone to meteorological
variables with a multiple linear regression model shows that meteorology
played a significant but not dominant role in the 2013–2019 ozone trend,
contributing 0.70 ppb a−1 (p<0.01) across China and 1.4 ppb a−1 (p=0.02) over the NCP. Rising June–July temperatures over the NCP
were the main meteorological driver, particularly in recent years
(2017–2019), and were associated with increased foehn winds. NCP data for
2017–2019 show a 15 % decrease in fine particulate matter (PM2.5)
that may be driving the continued anthropogenic increase in ozone, as well as
unmitigated emissions of volatile organic compounds (VOCs). VOC emission
reductions, as targeted by Phase 2 of the Chinese Clean Air Action Plan, are
needed to reverse the increase in ozone.
We use a model of agricultural sources of ammonia (NH3) coupled to a chemical transport model to estimate the impact of U.S. food export on particulate matter concentrations (PM2.5). We find that ...food export accounts for 11% of total U.S. NH3 emissions (13% of agricultural emissions) and that it increases the population-weighted exposure of the U.S. population to PM2.5 by 0.36 μg m–3 on average. Our estimate is sensitive to the proper representation of the impact of NH3 on ammonium nitrate, which reflects the interplay between agricultural (NH3) and combustion emissions (NO, SO2). Eliminating NH3 emissions from food export would achieve greater health benefits than the reduction of the National Ambient Air Quality Standards for PM2.5 from 15 to 12 μg m–3. Valuation of the increased premature mortality associated with PM2.5 from food export (36 billion US$ (2006) per year) amounts to 50% of the gross food export value. Livestock operations in densely populated areas have particularly large health costs. Decreasing SO2 and NO x emissions will indirectly reduce health impact of food export as an ancillary benefit.
Pseudomonas aeruginosa colonizes the airways of cystic fibrosis (CF) patients, causing infections that can last for decades. During the course of these infections, P. aeruginosa undergoes a number of ...genetic adaptations. One such adaptation is the loss of swimming motility functions. Another involves the formation of the rugose small colony variant (RSCV) phenotype, which is characterized by overproduction of the exopolysaccharides Pel and Psl. Here, we provide evidence that the two adaptations are linked. Using random transposon mutagenesis, we discovered that flagellar mutations are linked to the RSCV phenotype. We found that flagellar mutants overexpressed Pel and Psl in a surface-contact dependent manner. Genetic analyses revealed that flagellar mutants were selected for at high frequencies in biofilms, and that Pel and Psl expression provided the primary fitness benefit in this environment. Suppressor mutagenesis of flagellar RSCVs indicated that Psl overexpression required the mot genes, suggesting that the flagellum stator proteins function in a surface-dependent regulatory pathway for exopolysaccharide biosynthesis. Finally, we identified flagellar mutant RSCVs among CF isolates. The CF environment has long been known to select for flagellar mutants, with the classic interpretation being that the fitness benefit gained relates to an impairment of the host immune system to target a bacterium lacking a flagellum. Our new findings lead us to propose that exopolysaccharide production is a key gain-of-function phenotype that offers a new way to interpret the fitness benefits of these mutations.
We review the capability of current and scheduled satellite
observations of atmospheric methane in the shortwave infrared (SWIR) to
quantify methane emissions from the global scale down to point ...sources. We
cover retrieval methods, precision and accuracy requirements, inverse and
mass balance methods for inferring emissions, source detection thresholds,
and observing system completeness. We classify satellite instruments as area
flux mappers and point source imagers, with complementary attributes. Area
flux mappers are high-precision (<1 %) instruments with 0.1–10 km
pixel size designed to quantify total methane emissions on regional to
global scales. Point source imagers are fine-pixel (<60 m)
instruments designed to quantify individual point sources by imaging of the
plumes. Current area flux mappers include GOSAT (2009–present), which
provides a high-quality record for interpretation of long-term methane
trends, and TROPOMI (2018–present), which provides global continuous daily
mapping to quantify emissions on regional scales. These instruments already
provide a powerful resource to quantify national methane emissions in
support of the Paris Agreement. Current point source imagers include the
GHGSat constellation and several hyperspectral and multispectral land
imaging sensors (PRISMA, Sentinel-2, Landsat-8/9, WorldView-3), with
detection thresholds in the 100–10 000 kg h−1 range that enable
monitoring of large point sources. Future area flux mappers, including
MethaneSAT, GOSAT-GW, Sentinel-5, GeoCarb, and CO2M, will increase the
capability to quantify emissions at high resolution, and the MERLIN lidar
will improve observation of the Arctic. The averaging times required by area
flux mappers to quantify regional emissions depend on pixel size, retrieval
precision, observation density, fraction of successful retrievals, and
return times in a way that varies with the spatial resolution desired. A
similar interplay applies to point source imagers between detection
threshold, spatial coverage, and return time, defining an observing system
completeness. Expanding constellations of point source imagers including
GHGSat and Carbon Mapper over the coming years will greatly improve
observing system completeness for point sources through dense spatial
coverage and frequent return times.
Mercury (Hg) is a global pollutant that affects human and ecosystem health. We synthesize understanding of sources, atmosphere-land-ocean Hg dynamics and health effects, and consider the implications ...of Hg-control policies. Primary anthropogenic Hg emissions greatly exceed natural geogenic sources, resulting in increases in Hg reservoirs and subsequent secondary Hg emissions that facilitate its global distribution. The ultimate fate of emitted Hg is primarily recalcitrant soil pools and deep ocean waters and sediments. Transfers of Hg emissions to largely unavailable reservoirs occur over the time scale of centuries, and are primarily mediated through atmospheric exchanges of wet/dry deposition and evasion from vegetation, soil organic matter and ocean surfaces. A key link between inorganic Hg inputs and exposure of humans and wildlife is the net production of methylmercury, which occurs mainly in reducing zones in freshwater, terrestrial, and coastal environments, and the subsurface ocean. Elevated human exposure to methylmercury primarily results from consumption of estuarine and marine fish. Developing fetuses are most at risk from this neurotoxin but health effects of highly exposed populations and wildlife are also a concern. Integration of Hg science with national and international policy efforts is needed to target efforts and evaluate efficacy.
Methane is the second strongest anthropogenic greenhouse gas and its atmospheric burden has more than doubled since 1850. Methane concentrations stabilized in the early 2000s and began increasing ...again in 2007. Neither the stabilization nor the recent growth are well understood, as evidenced by multiple competing hypotheses in recent literature. Here we use a multispecies two-box model inversion to jointly constrain 36 y of methane sources and sinks, using ground-based measurements of methane, methyl chloroform, and the C13/C12 ratio in atmospheric methane (δ
13CH₄) from 1983 through 2015. We find that the problem, as currently formulated, is underdetermined and solutions obtained in previous work are strongly dependent on prior assumptions. Based on our analysis, the mathematically most likely explanation for the renewed growth in atmospheric methane, counterintuitively, involves a 25-Tg/y decrease in methane emissions from 2003 to 2016 that is offset by a 7% decrease in global mean hydroxyl (OH) concentrations, the primary sink for atmospheric methane, over the same period. However, we are still able to fit the observations if we assume that OH concentrations are time invariant (as much of the previous work has assumed) and we then find solutions that are largely consistent with other proposed hypotheses for the renewed growth of atmospheric methane since 2007. We conclude that the current surface observing system does not allow unambiguous attribution of the decadal trends in methane without robust constraints on OH variability, which currently rely purely on methyl chloroform data and its uncertain emissions estimates.
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