Interest in stratospheric aerosol and its role in climate have increased over the last decade due to the observed increase in stratospheric aerosol since 2000 and the potential for changes in the ...sulfur cycle induced by climate change. This review provides an overview about the advances in stratospheric aerosol research since the last comprehensive assessment of stratospheric aerosol was published in 2006. A crucial development since 2006 is the substantial improvement in the agreement between in situ and space-based inferences of stratospheric aerosol properties during volcanically quiescent periods. Furthermore, new measurement systems and techniques, both in situ and space based, have been developed for measuring physical aerosol properties with greater accuracy and for characterizing aerosol composition. However, these changes induce challenges to constructing a long-term stratospheric aerosol climatology. Currently, changes in stratospheric aerosol levels less than 20% cannot be confidently quantified. The volcanic signals tend to mask any nonvolcanically driven change, making them difficult to understand. While the role of carbonyl sulfide as a substantial and relatively constant source of stratospheric sulfur has been confirmed by new observations and model simulations, large uncertainties remain with respect to the contribution from anthropogenic sulfur dioxide emissions. New evidence has been provided that stratospheric aerosol can also contain small amounts of nonsulfatematter such as black carbon and organics. Chemistry-climate models have substantially increased in quantity and sophistication. In many models the implementation of stratospheric aerosol processes is coupled to radiation and/or stratospheric chemistry modules to account for relevant feedback processes.
We present a multiyear time series of column abundances of carbon monoxide (CO), hydrogen cyanide (HCN), and ethane (C2H6) measured using Fourier-transform infrared (FTIR) spectrometers at 10 sites ...affiliated with the Network for the Detection of Atmospheric Composition Change (NDACC). Six are high-latitude sites: Eureka, Ny-Ålesund, Thule, Kiruna, Poker Flat, and St. Petersburg, and four are midlatitude sites: Zugspitze, Jungfraujoch, Toronto, and Rikubetsu. For each site, the interannual trends and seasonal variabilities of the CO time series are accounted for, allowing background column amounts to be determined. Enhancements above the seasonal background were used to identify possible wildfire pollution events. Since the abundance of each trace gas emitted in a wildfire event is specific to the type of vegetation burned and the burning phase, correlations of CO to the long-lived wildfire tracers HCN and C2H6 allow for further confirmation of the detection of wildfire pollution. A GEOS-Chem tagged CO simulation with Global Fire Assimilation System (GFASv1.2) biomass burning emissions was used to determine the source attribution of CO concentrations at each site from 2003 to 2018. For each detected wildfire pollution event, FLEXPART back-trajectory simulations were performed to determine the transport times of the smoke plume. Accounting for the loss of each species during transport, the enhancement ratios of HCN and C2H6 with respect to CO were converted to emission ratios. We report mean emission ratios with respect to CO for HCN and C2H6 of 0.0047 and 0.0092, respectively, with a standard deviation of 0.0014 and 0.0046, respectively, determined from 23 boreal North American wildfire events. Similarly, we report mean emission ratios for HCN and C2H6 of 0.0049 and 0.0100, respectively, with a standard deviation of 0.0025 and 0.0042, respectively, determined from 39 boreal Asian wildfire events. The agreement of our emission ratios with literature values illustrates the capability of ground-based FTIR measurements to quantify biomass burning emissions. We provide a comprehensive dataset that quantifies HCN and C2H6 emission ratios from 62 wildfire pollution events. Our dataset provides novel emission ratio estimates, which are sparsely available in the published literature, particularly for boreal Asian sources.
We present a consistent intercomparison of the mean age of air (AoA) according to five modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim), the ...Japanese Meteorological Agency's Japanese 55-year Reanalysis (JRA-55), the National Centers for Environmental Prediction Climate Forecast System Reanalysis (CFSR) and the National Aeronautics and Space Administration's Modern Era Retrospective analysis for Research and Applications version 1 (MERRA) and version 2 (MERRA-2). The modeling tool is a kinematic transport model driven only by the surface pressure and wind fields. It is validated for ERA-I through a comparison with the AoA computed by another transport model.
We analyzed seasonality and interannual variability of tropospheric hydrogen cyanide (HCN)
columns in densely populated eastern China for the first time. The results
were derived from solar ...absorption spectra recorded with a ground-based high-spectral-resolution Fourier transform infrared (FTIR) spectrometer in Hefei
(31∘54′ N, 117∘10′ E) between 2015 and
2018. The tropospheric HCN columns over Hefei, China, showed significant
seasonal variations with three monthly mean peaks throughout the year. The
magnitude of the tropospheric HCN column peaked in May, September, and December. The tropospheric HCN column reached a maximum monthly
mean of (9.8±0.78)×1015 molecules cm−2 in May
and a minimum monthly mean of (7.16±0.75)×1015 molecules cm−2 in November. In most cases, the tropospheric HCN columns
in Hefei (32∘ N) are higher than the FTIR observations in Ny-Ålesund (79∘ N), Kiruna (68∘ N), Bremen (53∘ N), Jungfraujoch (47∘ N), Toronto (44∘ N), Rikubetsu
(43∘ N), Izana (28∘ N), Mauna Loa (20∘ N), La
Reunion Maido (21∘ S), Lauder (45∘ S), and Arrival
Heights (78∘ S) that are affiliated with the Network for Detection
of Atmospheric Composition Change (NDACC). Enhancements of tropospheric HCN
column were observed between September 2015 and July 2016 compared to the
same period of measurements in other years. The magnitude of the enhancement
ranges from 5 % to 46 % with an average of 22 %. Enhancement of
tropospheric HCN (ΔHCN) is correlated with the concurrent
enhancement of tropospheric CO (ΔCO), indicating that enhancements
of tropospheric CO and HCN were due to the same sources. The GEOS-Chem tagged CO simulation, the global fire maps, and the potential source
contribution function (PSCF) values calculated using back trajectories
revealed that the seasonal maxima in May are largely due to the influence of
biomass burning in Southeast Asia (SEAS) (41±13.1 %), Europe
and boreal Asia (EUBA) (21±9.3 %), and Africa (AF) (22±4.7 %). The seasonal maxima in September are largely due to the influence
of biomass burnings in EUBA (38±11.3 %), AF (26±6.7 %),
SEAS (14±3.3 %), and North America (NA) (13.8±8.4 %).
For the seasonal maxima in December, dominant contributions are from AF (36±7.1 %), EUBA (21±5.2 %), and NA (18.7±5.2 %). The tropospheric HCN enhancement between September 2015 and July
2016 at Hefei (32∘ N) was attributed to an elevated influence of
biomass burnings in SEAS, EUBA, and Oceania (OCE) in this period. In
particular, an elevated number of fires in OCE in the second half of 2015
dominated the tropospheric HCN enhancement between September and December 2015. An
elevated number of fires in SEAS in the first half of 2016 dominated the
tropospheric HCN enhancement between January and July 2016.
Changes of atmospheric methane total columns (CH4) since 2005 have been evaluated using Fourier transform infrared (FTIR) solar observations carried out at 10 ground-based sites, affiliated to the ...Network for Detection of Atmospheric Composition Change (NDACC). From this, we find an increase of atmospheric methane total columns of 0.31 ± 0.03 % year−1 (2σ level of uncertainty) for the 2005–2014 period. Comparisons with in situ methane measurements at both local and global scales show good agreement. We used the GEOS-Chem chemical transport model tagged simulation, which accounts for the contribution of each emission source and one sink in the total methane, simulated over 2005–2012. After regridding according to NDACC vertical layering using a conservative regridding scheme and smoothing by convolving with respective FTIR seasonal averaging kernels, the GEOS-Chem simulation shows an increase of atmospheric methane total columns of 0.35 ± 0.03 % year−1 between 2005 and 2012, which is in agreement with NDACC measurements over the same time period (0.30 ± 0.04 % year−1, averaged over 10 stations). Analysis of the GEOS-Chem-tagged simulation allows us to quantify the contribution of each tracer to the global methane change since 2005. We find that natural sources such as wetlands and biomass burning contribute to the interannual variability of methane. However, anthropogenic emissions, such as coal mining, and gas and oil transport and exploration, which are mainly emitted in the Northern Hemisphere and act as secondary contributors to the global budget of methane, have played a major role in the increase of atmospheric methane observed since 2005. Based on the GEOS-Chem-tagged simulation, we discuss possible cause(s) for the increase of methane since 2005, which is still unexplained.
This paper studies the seasonal variation of surface and column CO at three different sites (Paris, Jungfraujoch and Wollongong), with an emphasis on establishing a link between the CO vertical ...distribution and the nature of CO emission sources. We find the first evidence of a time lag between surface and free tropospheric CO seasonal variations in the Northern Hemisphere. The CO seasonal variability obtained from the total columns and free tropospheric partial columns shows a maximum around March–April and a minimum around September–October in the Northern Hemisphere (Paris and Jungfraujoch). In the Southern Hemisphere (Wollongong) this seasonal variability is shifted by about 6 months. Satellite observations by the IASI–MetOp (Infrared Atmospheric Sounding Interferometer) and MOPITT (Measurements Of Pollution In The Troposphere) instruments confirm this seasonality. Ground-based FTIR (Fourier transform infrared) measurements provide useful complementary information due to good sensitivity in the boundary layer. In situ surface measurements of CO volume mixing ratios at the Paris and Jungfraujoch sites reveal a time lag of the near-surface seasonal variability of about 2 months with respect to the total column variability at the same sites. The chemical transport model GEOS-Chem (Goddard Earth Observing System chemical transport model) is employed to interpret our observations. GEOS-Chem sensitivity runs identify the emission sources influencing the seasonal variation of CO. At both Paris and Jungfraujoch, the surface seasonality is mainly driven by anthropogenic emissions, while the total column seasonality is also controlled by air masses transported from distant sources. At Wollongong, where the CO seasonality is mainly affected by biomass burning, no time shift is observed between surface measurements and total column data.
The Measurements of Pollution in the Troposphere (MOPITT) satellite instrument provides the longest continuous dataset of carbon monoxide (CO) from space. We perform the first validation of MOPITT ...version 6 retrievals using total column CO measurements from ground-based remote-sensing Fourier transform infrared spectrometers (FTSs). Validation uses data recorded at 14 stations, that span a wide range of latitudes (80° N to 78° S), in the Network for the Detection of Atmospheric Composition Change (NDACC). MOPITT measurements are spatially co-located with each station, and different vertical sensitivities between instruments are accounted for by using MOPITT averaging kernels (AKs). All three MOPITT retrieval types are analyzed: thermal infrared (TIR-only), joint thermal and near infrared (TIR–NIR), and near infrared (NIR-only). Generally, MOPITT measurements overestimate CO relative to FTS measurements, but the bias is typically less than 10 %. Mean bias is 2.4 % for TIR-only, 5.1 % for TIR–NIR, and 6.5 % for NIR-only. The TIR–NIR and NIR-only products consistently produce a larger bias and lower correlation than the TIR-only. Validation performance of MOPITT for TIR-only and TIR–NIR retrievals over land or water scenes is equivalent. The four MOPITT detector element pixels are validated separately to account for their different uncertainty characteristics. Pixel 1 produces the highest standard deviation and lowest correlation for all three MOPITT products. However, for TIR-only and TIR–NIR, the error-weighted average that includes all four pixels often provides the best correlation, indicating compensating pixel biases and well-captured error characteristics. We find that MOPITT bias does not depend on latitude but rather is influenced by the proximity to rapidly changing atmospheric CO. MOPITT bias drift has been bound geographically to within ±0.5 % yr−1 or lower at almost all locations.
Atmospheric carbon monoxide (CO) and methane (CH4) mole fractions are measured by ground-based in
situ cavity ring-down spectroscopy (CRDS) analyzers and Fourier transform
infrared (FTIR) ...spectrometers at two sites (St Denis and Maïdo) on
Reunion Island (21∘ S, 55∘ E) in the Indian Ocean.
Currently, the FTIR Bruker IFS 125HR at St Denis records the direct solar
spectra in the near-infrared range, contributing to the Total Carbon Column
Observing Network (TCCON). The FTIR Bruker IFS 125HR at Maïdo records
the direct solar spectra in the mid-infrared (MIR) range, contributing to the Network for the Detection of Atmospheric
Composition Change (NDACC). In order to understand the atmospheric CO and
CH4 variability on Reunion Island, the time series and seasonal
cycles of CO and CH4 from in situ and FTIR (NDACC and TCCON)
measurements are analyzed. Meanwhile, the difference between the in situ and
FTIR measurements are discussed. The CO seasonal cycles observed from the in situ measurements at Maïdo
and FTIR retrievals at both St Denis and Maïdo are in good agreement
with a peak in September–November, primarily driven by the emissions from
biomass burning in Africa and South America. The dry-air column averaged mole
fraction of CO (XCO) derived from the FTIR MIR spectra (NDACC) is
about 15.7 ppb larger than the CO mole fraction near the surface at
Maïdo, because the air in the lower troposphere mainly comes from the
Indian Ocean while the air in the middle and upper troposphere mainly comes
from Africa and South America. The trend for CO on Reunion Island is unclear
during the 2011–2017 period, and more data need to be collected to get a
robust result. A very good agreement is observed in the tropospheric and stratospheric
CH4 seasonal cycles between FTIR (NDACC and TCCON) measurements, and
in situ and the Michelson Interferometer for Passive Atmospheric Sounding
(MIPAS) satellite measurements, respectively. In the troposphere, the
CH4 mole fraction is high in August–September and low in
December–January, which is due to the OH seasonal variation. In the
stratosphere, the CH4 mole fraction has its maximum in March–April
and its minimum in August–October, which is dominated by vertical transport.
In addition, the different CH4 mole fractions between the in situ,
NDACC and TCCON CH4 measurements in the troposphere are discussed,
and all measurements are in good agreement with the GEOS-Chem model
simulation. The trend of XCH4 is 7.6±0.4 ppb yr−1 from
the TCCON measurements over the 2011 to 2017 time period, which is consistent
with the CH4 trend of 7.4±0.5 ppb yr−1 from the in situ
measurements for the same time period at St Denis.
In this paper, we present an optimized retrieval strategy for carbonyl sulfide (OCS), using Fourier transform infrared (FTIR) solar observations made at the high-altitude Jungfraujoch station in the ...Swiss Alps. More than 200 lines of the ν3 fundamental band of OCS have been systematically evaluated and we selected 4 microwindows on the basis of objective criteria minimizing the effect of interferences, mainly by solar features, carbon dioxide and water vapor absorption lines, while maximizing the information content. Implementation of this new retrieval strategy provided an extended time series of the OCS abundance spanning the 1995–2015 time period, for the study of the long-term trend and seasonal variation of OCS in the free troposphere and stratosphere.
Three distinct periods characterize the evolution of the tropospheric partial columns: a first decreasing period (1995–2002), an intermediate increasing period (2002–2008), and the more recent period (2008–2015) which shows no significant trend. Our FTIR tropospheric and stratospheric time series are compared with new in situ gas chromatography mass spectrometry (GCMS) measurements performed by Empa (Laboratory for Air Pollution/Environmental Technology) at the Jungfraujoch since 2008, and with space-borne solar occultation observations by the ACE-FTS instrument on-board the SCISAT satellite, respectively, and they show good agreement. The OCS signal recorded above Jungfraujoch appears to be closely related to anthropogenic sulfur emissions.
•An approach for retrieving OCS from ground-based infrared spectra is presented.•The OCS trend over 1995–2015 is characterized.•A non-monotonic long-term evolution of OCS with time is observed.•Comparison with satellite and in situ surface measurements is included.
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