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The distribution and variations of atmospheric CO2 from 1981 to 1992 were determined by measuring CO2 mixing ratios in samples collected weekly at a cooperative global air sampling network. The results constitute the most geographically extensive, carefully calibrated, internally consistent CO2 data set available. Analysis of the data reveals that the global CO2 growth rate has declined from a peak of ~2.5 ppm yr-1 in 1987-1988 to ~0.6 ppm yr-1 in 1992. In 1992 we find no increase in atmospheric CO2 from 30° to 90°N. Variations in fossil fuel CO2 emissions cannot explain this result. The north pole-south pole CO2 difference increased from ?3 ppm during 1981-1987 to ~4 ppm during 1988-1991. In 1992 the difference was again ~3 ppm. A two-dimensional model analysis of the data indicates that the low CO2 growth rate in 1992 is mainly due to an increase in the northern hemisphere CO2 sink from 3.9 Gt C yr-1 in 1991 to 5.0 Gt C yr-1 in 1992. The increase in the north pole-south pole CO2 difference appears to result from an increase in the southern hemisphere CO2 sink from ~0.5 to ~1.5 Gt C yr-1.
 Time series of cloud solar transmission and cloud occurrence frequency are developed for the past 27 years at four globally remote and climatically diverse surface locations. A new methodology is developed that objectively segregates times of cloud-free conditions from those times when clouds are detected in high-time-resolution total solar irradiance observations that are obtained from pyranometers. The methodology for cloud detection depends on the magnitude and short-term variability of observed departures from clear-sky conditions. Expected clear-sky irradiances are based on interpolated clear-sky observations. Results of the new cloud detection methodology are compared to four independent cloud detection methods. An effective cloud transmission is determined as the ratio of observed irradiance in the presence of clouds to that expected in the absence of clouds. Selective forward scattering by clouds toward the observation site results in computed effective transmissions frequently being >1.0. It is shown that conditional temporal averaging of effective cloud transmission over periods of three days or more virtually eliminates the unrealistic cloud transmissions exceeding 1.0. Such temporal averaging of the surface measurements is advantageous for comparing against other area-wide cloud transmission estimates, such as those determined from satellite or by numerical climate models. The cloud occurrence frequency and the effective solar transmission for long-term observational records are summarized into monthly and annual averages, and their long-term variability is investigated. Temporal variations in frequency distributions of transmission are used to determine which clouds are responsible for changes in mean cloudiness. A statistically significant upward trend in cloud occurrence frequency, from 76 to 82% between 1976 and 2001, is detected at Barrow, Alaska, where clouds having solar transmission near 0.2 exhibit the largest increase. At the South Pole, decadal timescale oscillations in both cloud characteristics are detected, but no particular cloud category is identified as the source of that oscillation.
Abstract. We measured the global distribution of tropospheric N2O mixing ratios during the NASA airborne Atmospheric Tomography (ATom) mission. ATom measured concentrations of ∼ 300 gas species and aerosol properties in 647 vertical profiles spanning the Pacific, Atlantic, Arctic, and much of the Southern Ocean basins, nearly from pole to pole, over four seasons (2016–2018). We measured N2O concentrations at 1 Hz using a quantum cascade laser spectrometer (QCLS). We introduced a new spectral retrieval method to account for the pressure and temperature sensitivity of the instrument when deployed on aircraft. This retrieval strategy improved the precision of our ATom QCLS N2O measurements by a factor of three (based on the standard deviation of calibration measurements). Our measurements show that most of the variance of N2O mixing ratios in the troposphere is driven by the influence of N2O-depleted stratospheric air, especially at mid- and high latitudes. We observe the downward propagation of lower N2O mixing ratios (compared to surface stations) that tracks the influence of stratosphere–troposphere exchange through the tropospheric column down to the surface. The highest N2O mixing ratios occur close to the Equator, extending through the boundary layer and free troposphere. We observed influences from a complex and diverse mixture of N2O sources, with emission source types identified using the rich suite of chemical species measured on ATom and the geographical origin calculated using an atmospheric transport model. Although ATom flights were mostly over the oceans, the most prominent N2O enhancements were associated with anthropogenic emissions, including from industry (e.g., oil and gas), urban sources, and biomass burning, especially in the tropical Atlantic outflow from Africa. Enhanced N2O mixing ratios are mostly associated with pollution-related tracers arriving from the coastal area of Nigeria. Peaks of N2O are often associated with indicators of photochemical processing, suggesting possible unexpected source processes. In most cases, the results show how difficult it is to separate the mixture of different sources in the atmosphere, which may contribute to uncertainties in the N2O global budget. The extensive data set from ATom will help improve the understanding of N2O emission processes and their representation in global models.
Ten years of ozonesonde data at the south pole are used to investigate trends and search for indicators that can be used to detect Antarctic ozone recovery in the future. These data indicate that there have been no systematic winter temperature trends at altitudes of 7–25 km and thus no expected changes in stratospheric cloud particle surface area, which would affect heterogeneous chemistry. Springtime ozone depletion has been very severe since about 1992, with near-total loss of ozone in the 14- to 18-km region, but has lessened somewhat in 1994 and 1995. probably because of the decay of the sulfate aerosol from the Mount Pinatubo eruption which was present at 10–16 km. Sulfate aerosol particles from the Pinatubo eruption resulted in new ozone depletion in 1992 and 1993 in the 10- to 12-km region where it is too warm for polar stratospheric clouds (PSCs) to form. The volcanic aerosol also augmented depletion related to PSCs at 12–16 km. Although ozone depletion was not as severe in 1995 as in 1993, the depleted region remained intact longer than ever, with record low values throughout December in 1995. Since about 1992, a pseudo-equilibrium seems to have been reached in which springtime ozone depletion, as measured by the total column or the ozone in the 12- to 20-km main stratospheric cloud region, has remained relatively constant. Independent of volcanic aerosol, ozone depletion has extended into the upper altitudes at 22–24 km since about 1992. There is some indication that ozone depletion has also worsened at the bottom of the depletion region at 12–14 km. Extensions of the ozone hole in the vertical dimension are believed to be the result of increases in man-made halogens and not due to changes in particle surface area or dynamics. A quasi-biennial component in the ozone destruction rate in September, especially above 18 km, is believed to be related to variations in the transport of halogen-bearing molecules to the polar region. A number of indicators for recovery of the ozone hole have been identified. They include an end to springtime ozone depletion at 22–24 km, a 12- to 20-km mid-September column ozone loss rate of less than about 3 Dobson Units (DU) per day, and a 12- to 20-km ozone column value of more than about 70 DU on September 15. It is estimated that if the Montreal protocol and its amendments, banning and/or limiting substances that deplete the ozone layer, is adhered to, recovery of the Antarctic ozone hole may be conclusively detected from the aforementioned changes in the vertical profile of ozone as early as the year 2008. Future volcanic eruptions would affect ozone at 10–16 km, making detection more difficult, but indicators such as depletion in the 22- to 24-km region will be immune to these effects.
Abstract. The detection of increasing global CFC-11 emissions after 2012 alerted society to a possible violation of the Montreal Protocol on Substances that Deplete the Ozone Layer (MP). This alert resulted in parties to the MP taking urgent actions. As a result, atmospheric measurements made in 2019 suggest a sharp decline in global CFC-11 emissions. Despite the success in the detection and mitigation of part of this problem, regions fully responsible for the recent global emission changes in CFC-11 have not yet been identified. Roughly two thirds (60 ± 40 %) of the emission increase between 2008–2012 and 2014–2017 and two thirds (60 ± 30 %) of the decline between 2014–2017 and 2019 were explained by regional emission changes in eastern mainland China. Here, we used atmospheric CFC-11 measurements made from two global aircraft surveys – the HIAPER (High-performance Instrumented Airborne Platform for Environmental Research) Pole-to-Pole Observations (HIPPO) in November 2009–September 2011 and the Atmospheric Tomography Mission (ATom) in August 2016–May 2018, in combination with the global CFC-11 measurements made by the US National Oceanic and Atmospheric Administration during these two periods – to derive global and regional emission changes in CFC-11. Our results suggest Asia accounted for the largest fractions of global CFC-11 emissions in both periods: 43 (37–52) % during November 2009–September 2011 and 57 (49–62) % during August 2016–May 2018. Asia was also primarily responsible for the emission increase between these two periods, accounting for 86 (59–115) % of the global CFC-11 emission rise between the two periods. Besides eastern mainland China, temperate western Asia and tropical Asia also contributed significantly to global CFC-11 emissions during both periods and likely to the global CFC-11 emission increase. The atmospheric observations further provide strong constraints on CFC-11 emissions from North America and Europe, suggesting that each of them accounted for 10 %–15 % of global CFC-11 emissions during the HIPPO period and smaller fractions in the ATom period. For South America, Africa, and Australia, the derived regional emissions had larger dependence on the prior assumptions of emissions and emission changes due to a lower sensitivity of the observations considered here to emissions from these regions. However, significant increases in CFC-11 emissions from southern hemispheric lands were not likely due to the observed increase of north-to-south interhemispheric gradients in atmospheric CFC-11 mole fractions from 2012–2017.
Abstract. Bromine released from the decomposition of short-lived brominated source gases contributes as a sink of ozone in the lower stratosphere. The two major contributors are CH2Br2 and CHBr3. In this study, we investigate the global seasonal distribution of these two substances, based on four High Altitude and Long Range Research Aircraft (HALO) missions, the HIAPER Pole-to-Pole Observations (HIPPO) mission, and the Atmospheric Tomography (ATom) mission. Observations of CH2Br2 in the free and upper troposphere indicate a pronounced seasonality in both hemispheres, with slightly larger mixing ratios in the Northern Hemisphere (NH). Compared to CH2Br2, CHBr3 in these regions shows larger variability and less clear seasonality, presenting larger mixing ratios in winter and autumn in NH midlatitudes to high latitudes. The lowermost stratosphere of SH and NH shows a very similar distribution of CH2Br2 in hemispheric spring with differences well below 0.1 ppt, while the differences in hemispheric autumn are much larger with substantially smaller values in the SH than in the NH. This suggests that transport processes may be different in both hemispheric autumn seasons, which implies that the influx of tropospheric air (“flushing”) into the NH lowermost stratosphere is more efficient than in the SH. The observations of CHBr3 support the suggestion, with a steeper vertical gradient in the upper troposphere and lower stratosphere in SH autumn than in NH autumn. However, the SH database is insufficient to quantify this difference. We further compare the observations to model estimates of TOMCAT (Toulouse Off-line Model of Chemistry And Transport) and CAM-Chem (Community Atmosphere Model with Chemistry, version 4), both using the same emission inventory of Ordóñez et al. (2012). The pronounced tropospheric seasonality of CH2Br2 in the SH is not reproduced by the models, presumably due to erroneous seasonal emissions or atmospheric photochemical decomposition efficiencies. In contrast, model simulations of CHBr3 show a pronounced seasonality in both hemispheres, which is not confirmed by observations. The distributions of both species in the lowermost stratosphere of the Northern and Southern hemispheres are overall well captured by the models with the exception of southern hemispheric autumn, where both models present a bias that maximizes in the lowest 40 K above the tropopause, with considerably lower mixing ratios in the observations. Thus, both models reproduce equivalent flushing in both hemispheres, which is not confirmed by the limited available observations. Our study emphasizes the need for more extensive observations in the SH to fully understand the impact of CH2Br2 and CHBr3 on lowermost-stratospheric ozone loss and to help constrain emissions.
Ozone trends, derived from 1979-1996 Dobson spectrophotometer total ozone data obtained at five U.S. mainland midlatitude stations, averaged -3.4, -4.9, -2.6, -1.9, and -3.3%/decade for winter, spring, summer, and autumn months, and on an annual basis, respectively. At the lower latitude stations of Mauna Loa and Samoa, corresponding-period annual ozone trends were -0.4 and -1.3%/decade, respectively, while at Huancayo, Peru, the 1979-1991 annual trend was -0.9%/decade. A linear trend approximation to ozone changes that occurred since 1978 during austral daylight times at Amundsen-Scott (South Pole) station, Antarctica, yielded a value of -12%/decade. By combining 1979-1996 annual trend data for three U.S. mainland stations with trends for the sites derived from 1963-1978 data, it is estimated that the ozone decrease at U.S. midlatitudes through 1996, relative to ozone present in the mid-1960s, was -6.7%. Similar analyses incorporating South Pole data obtained since 1963 yielded an ozone change at South Pole (daylight observations) through 1996 of approximately -25%. South Pole October total ozone values in 1996 were lower than mid-1960s October ozone values by a factor of two. Trend data are also presented for several shorter record period stations, including the foreign cooperative stations of Haute Provence, France; Lauder, New Zealand; and Perth, Australia.
The significant climate feedback of stratospheric water vapor (SWV) necessitates quantitative estimates of SWV budget changes. Model simulations driven by the newest European Centre for Medium-Range Weather Forecast reanalysis ERA5, satellite observations from the Stratospheric Water and OzOne Satellite Homogenized data set, Microwave Limb Sounder, and in situ frost point hygrometer observations from Boulder all show substantial and persistent stratospheric moistening after a sharp drop in water vapor at the turn of the millennium. This moistening occurred mainly during 2000–2006 and SWV abundances then remained high over the last decade. We find strong positive trends in the Northern Hemisphere and weak negative trends over the South Pole, mainly during austral winter. Moistening of the tropical stratosphere after 2000 occurred during late boreal winter/spring, reached values of ∼0.2 ppm/decade, was well correlated with a warming of the cold point tropopause by ∼0.4 K/decade and can only be partially attributed to El Nino-Southern Oscillation and volcanic eruptions.
The Antarctic ozone hole is a seasonal depletion of the ozone layer over Antarctica occurring every austral spring since the early 1980s. The depletion depends on the amount of ozone depleting substances (ODS) and the meteorological conditions in the Antarctic lower stratosphere. The structure and evolution of the 2018 Antarctic ozone hole and its relation to previous years were studied using global reanalysis temperatures (from MERRA-2), the NOAA ozonesonde record collected at Amundsen–Scott South Pole Station, satellite observations, and insights derived from a global chemical transport model.
Concern for stratospheric ozone depletion has prompted a phase out in production and use of fully halogenated chlorofluorocarbons (CFCs). Partially hydrogenated CFCs (HCFCs) are among the compounds used as replacements. HCFCs are preferred to CFCs because model calculations predict that HCFCs will have shorter atmospheric lifetimes and release less reactive chlorine to the stratosphere. The major HCFC in use today is HCFC-22. This study reports the results of measurements of HCFC-22 in air samples collected at the South pole. A mean atmospheric mixing ration of 95 ppt was determined for HCFC-22. 13 refs., 3 figs.
The Antarctic ozone hole is a severe ozone depletion that regularly appears in austral spring. In 2016, Antarctic stratospheric ozone depletion was less severe compared to the 1991–2006 average (a period of peak chlorine and bromine over Antarctica), but ozone levels were still low compared to pre-1990 levels. Figure 6.11a displays the ozone column between 12 and 20 km derived from NOAA South Pole balloon profiles averaged over 21 September to 16 October (the period with the largest ozone depletion). The 2016 South Pole ozone column was ~6 Dobson units (DU) higher than the 1991–2006 average (horizontal dashed line in the figure), and all ozone column means through the ozone minimum seasons since 2009 have been higher than this 1991–2006 average.
The Antarctic ozone hole is showing weak evidence of a decrease in area, based upon the last 15 years of ground and satellite observations. The 2014 Antarctic stratospheric ozone depletion was less severe compared to the 1995–2005 average, but ozone levels were still low compared to pre-1990 levels. Figure 6.12a displays the average column ozone between 12 and 20 km derived from NOAA South Pole balloon profiles. The 2014 South Pole ozone column inventory was relatively high with respect to a 1991–2006 average (horizontal line), and in fact, all of the 2009–2014 ozone column inventories were higher than the 1991–2006 average. The 1998–2014 period shows a positive secular trend (blue line), excluding 2002 (the year with a major stratospheric sudden warming; Roscoe et al. 2005).
Longer-term (i.e., 20-40 years) tropospheric ozone (O3) time series obtained from surface and ozonesonde observations have been analyzed to assess possible changes with time through 2010. The time series have been selected to reflect relatively broad geographic regions and where possible minimize local scale influences, generally avoiding sites close to larger urban areas. Several approaches have been used to describe the changes with time, including application of a time series model, running 15-year trends, and changes in the distribution by month in the O3 mixing ratio. Changes have been investigated utilizing monthly averages, as well as exposure metrics that focus on specific parts of the distribution of hourly average concentrations (e.g., low-, mid-, and high-level concentration ranges). Many of the longer time series (˜30 years) in mid-latitudes of the Northern Hemisphere, including those in Japan, show a pattern of significant increase in the earlier portion of the record, with a flattening over the last 10-15 years. It is uncertain if the flattening of the O3 change over Japan reflects the impact of O3 transported from continental East Asia in light of reported O3 increases in China. In the Canadian Arctic, declines from the beginning of the ozonesonde record in 1980 have mostly rebounded with little overall change over the period of record. The limited data in the tropical Pacific suggest very little change over the entire record. In the southern hemisphere subtropics and mid-latitudes, the significant increase observed in the early part of the record has leveled off in the most recent decade. At the South Pole, a decline observed during the first half of the 35-year record has reversed, and O3 has recovered to levels similar to the beginning of the record. Our understanding of the causes of the longer-term changes is limited, although it appears that in the mid-latitudes of the northern hemisphere, controls on O3 precursors have likely been a factor in the leveling off or decline from earlier O3 increases.
Using a set of selected surface ozone (nine stations) and ozone vertical profile measurements (from six stations), we have documented changes in tropospheric ozone at a number of locations. From two stations at high northern hemisphere (NH) latitudes there has been a significant decline in ozone amounts throughout the troposphere since the early 1980s. At midlatitudes of the NH where data are the most abundant, on the other hand, important regional differences prevail. The two stations in the eastern United States show that changes in ozone concentrations since the early 1970s have been relatively small. At the two sites in Europe, however, ozone amounts increased rapidly into the mid?1980s, but have increased less rapidly (or in some places not at all) since then. Increases at the Japanese ozonesonde station have been largest in the lower troposphere, but have slowed in the recent decade. The tropics are sparsely sampled but do not show significant changes. Small increases are suggested at southern hemisphere (SH) midlatitudes by the two surface data records. In Antarctica large declines in the ozone concentration are noted in the South Pole data, and like those at high latitudes of the NH, seem to parallel the large decreases in the stratosphere.
Surface ozone measurements made in Antarctica during the 1960's show a pronounced annual variation with a summer minimum and winter to late winter maximum. Furthermore, the maximum in surface ozone precedes that in total ozone by from 3 to 5 months, the indication being a loose coupling between the Antarctic stratosphere and troposphere. The annual cycle in surface ozone, instead of reflecting changes in the Antarctic stratosphere, may be a consequence of the variation in low-level meridional transport of ozone from higher latitudes into the Antarctic continent by synoptic scale disturbances. As might be expected from a consideration of Antarctic geography and meteorology, no significant diurnal variations occur in surface ozone. The nonperiodic surface ozone fluctuations observed during the late spring and summer months at South Pole station are most likely caused by sporadic breakdowns of the low-level inversion layer. At the lower latitude stations the day-to-day variations in surface ozone are in all likelihood associated with changes in weather systems.
Thirteen years of ozone soundings at the Antarctic Belgrano II station (78° S, 34.6° W) have been analysed to establish a climatology of stratospheric ozone and temperature over the area. The station is inside the polar vortex during the period of development of chemical ozone depletion. Weekly periodic profiles provide a suitable database for seasonal characterization of the evolution of stratospheric ozone, especially valuable during wintertime, when satellites and ground-based instruments based on solar radiation are not available. The work is focused on ozone loss rate variability (August–October) and its recovery (November–December) at different layers identified according to the severity of ozone loss. The time window selected for the calculations covers the phase of a quasi-linear ozone reduction, around day 220 (mid-August) to day 273 (end of September). Decrease of the total ozone column over Belgrano during spring is highly dependent on the meteorological conditions. Largest depletions (up to 59%) are reached in coldest years, while warm winters exhibit significantly lower ozone loss (20%). It has been found that about 11% of the total O3 loss, in the layer where maximum depletion occurs, takes place before sunlight has arrived, as a result of transport to Belgrano of air from a somewhat lower latitude, near the edge of the polar vortex, providing evidence of mixing inside the vortex. Spatial homogeneity of the vortex has been examined by comparing Belgrano results with those previously obtained for South Pole station (SPS) for the same altitude range and for 9 yr of overlapping data. Results show more than 25% higher ozone loss rate at SPS than at Belgrano. The behaviour can be explained taking into account (i) the transport to both stations of air from a somewhat lower latitude, near the edge of the polar vortex, where sunlight reappears sooner, resulting in earlier depletion of ozone, and (ii) the accumulated hours of sunlight, which become much greater at the South Pole after the spring equinox. According to the variability of the ozone hole recovery, a clear connection between the timing of the breakup of the vortex and the monthly ozone content was found. Minimum ozone concentration of 57 DU in the 12–24 km layer remained in November, when the vortex is more persistent, while in years when the final stratospheric warming took place "very early", mean integrated ozone rose by up to 160–180 DU.
The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere1, 2, 3. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane4, 5, 6. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7,8,9,10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004–2011 the model predicts an annual mean NH–SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns11, 12, 13. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.
Molecular hydrogen (H2) is an abundant and reactive constituent of Earth's atmosphere, with links to climate and air quality. Anthropogenic emissions of H2 are expected to rise as the use of H2 as an energy source increases. Documenting past variations in atmospheric H2 will help to validate current understanding of the global H2 budget. The modern instrumental record begins in the 1980s; there is little information about atmospheric H2 prior to that time. Here, we use firn air measurements from a 2001 South Pole campaign to reconstruct atmospheric H2 levels over the 20th century. Inversion of the measurements indicates that H2 over South Pole has increased from 350–540 ppb from 1910–2000. A biogeochemical box model indicates that the atmospheric burden of H2 increased by 37% over that time. The rise in H2 is consistent with increasing H2 emissions from fossil fuel combustion and increasing atmospheric production from methane oxidation.
The atmospheric history of molecular hydrogen (H ) from 1852 to 2003 was reconstructed from measurements of firn air collected at Megadunes, Antarctica. The reconstruction shows that H levels in the southern hemisphere were roughly constant near 330 parts per billion (ppb; nmol H mol air) during the mid to late 1800s. Over the twentieth century, H levels rose by about 70% to 550 ppb. The reconstruction shows good agreement with the H atmospheric history based on firn air measurements from the South Pole. The broad trends in atmospheric H over the twentieth century can be explained by increased methane oxidation and anthropogenic emissions. The H rise shows no evidence of deceleration during the last quarter of the twentieth century despite an expected reduction in automotive emissions following more stringent regulations. During the late twentieth century, atmospheric CO levels decreased due to a reduction in automotive emissions. It is surprising that atmospheric H did not respond similarly as automotive exhaust is thought to be the dominant source of anthropogenic H The monotonic late twentieth century rise in H levels is consistent with late twentieth-century flask air measurements from high southern latitudes. An additional unknown source of H is needed to explain twentieth-century trends in atmospheric H and to resolve the discrepancy between bottom-up and top-down estimates of the anthropogenic source term. The firn air–based atmospheric history of H provides a baseline from which to assess human impact on the H cycle over the last 150 y and validate models that will be used to project future trends in atmospheric composition as H becomes a more common energy source.
The Atmospheric Research Observatory (ARO), part of the National Science Foundation’s (NSF’s) Amundsen-Scott South Pole Station, is located at one of the cleanest and most remote sites on earth. NOAA has been making atmospheric baseline measurements at South Pole since the mid-1970's. The pristine conditions and high elevation make the South Pole a desirable location for many types of research projects and since the early 2000's there have been multiple construction projects to accommodate both a major station renovation and additional research activities and their personnel. The larger population and increased human activity at the station, located in such close proximity to the global baseline measurements conducted at the ARO, calls into question the potential effects of local contamination of the long-term background measurements. In this work, the long-term wind and aerosol climatologies were updated and analyzed for trends. Winds blow toward the ARO from the Clean Air Sector ~88% of the time and while there is some year-to-year variability in this number, the long-term wind speed and direction measurements at South Pole have not changed appreciably in the last 35 years. Several human activity markers including station population, aircraft flights and fuel usage were used as surrogates for local aerosol emissions; peak human activity (and thus likely local emissions) occurred in the 2006 and 2007 austral summer seasons. The long-term aerosol measurements at ARO do not peak during these seasons, suggesting that the quality control procedures in place to identify and exclude continuous sources of local contamination are working and that the NSF’s sector management plan for the Clean Air Sector is effective. No significant trends over time were observed in particle number concentration, aerosol light scattering coefficient, or any aerosol parameter except scattering Ångström exponent, which showed a drop of ~0.02 yr–1 over the 36-year record. The effect of discrete local contamination events in the Clean Air Sector is discussed using one well-documented example.
We present a comparison of chemistry-transport models (TransCom-N2O) to examine the importance of atmospheric transport and surface fluxes on the variability of N2O mixing ratios in the troposphere. Six different models and two model variants participated in the inter-comparison and simulations were made for the period 2006 to 2009. In addition to N2O, simulations of CFC-12 and SF6 were made by a subset of four of the models to provide information on the models' proficiency in stratosphere–troposphere exchange (STE) and meridional transport, respectively. The same prior emissions were used by all models to restrict differences among models to transport and chemistry alone. Four different N2O flux scenarios totalling between 14 and 17 TgN yr−1 (for 2005) globally were also compared. The modelled N2O mixing ratios were assessed against observations from in situ stations, discrete air sampling networks and aircraft. All models adequately captured the large-scale patterns of N2O and the vertical gradient from the troposphere to the stratosphere and most models also adequately captured the N2O tropospheric growth rate. However, all models underestimated the inter-hemispheric N2O gradient by at least 0.33 parts per billion (ppb), equivalent to 1.5 TgN, which, even after accounting for an overestimate of emissions in the Southern Ocean of circa 1.0 TgN, points to a likely underestimate of the Northern Hemisphere source by up to 0.5 TgN and/or an overestimate of STE in the Northern Hemisphere. Comparison with aircraft data reveal that the models overestimate the amplitude of the N2O seasonal cycle at Hawaii (21° N, 158° W) below circa 6000 m, suggesting an overestimate of the importance of stratosphere to troposphere transport in the lower troposphere at this latitude. In the Northern Hemisphere, most of the models that provided CFC-12 simulations captured the phase of the CFC-12, seasonal cycle, indicating a reasonable representation of the timing of STE. However, for N2O all models simulated a too early minimum by 2 to 3 months owing to errors in the seasonal cycle in the prior soil emissions, which was not adequately represented by the terrestrial biosphere model. In the Southern Hemisphere, most models failed to capture the N2O and CFC-12 seasonality at Cape Grim, Tasmania, and all failed at the South Pole, whereas for SF6, all models could capture the seasonality at all sites, suggesting that there are large errors in modelled vertical transport in high southern latitudes.
An updated set of time series of derived aerosoloptical depth (AOD) and Ångström’s exponent α from a number of Arctic and Antarctic stations was analyzed to determine the long-term variations of these two parameters. The Arctic measurements were performed at Ny-Ålesund (1991–2010), Barrow (1977–2010) and some Siberian sites (1981–1991). The data were integrated with Level 2.0 AERONET sun-photometer measurements recorded at Hornsund, Svalbard, and Barrow for recent years, and at Tiksi for the summer 2010. The Antarctic data-set comprises sun-photometer measurementsperformed at Mirny (1982–2009), Neumayer (1991–2004), and Terra Nova Bay (1987–2005), and at South Pole (1977–2010). Analyses of daily mean AOD were made in the Arctic by (i) adjusting values to eliminate volcanic effects due to the El Chichón, Pinatubo, Kasatochi and Sarychev eruptions, and (ii) selecting the summer background aerosol data from those affected by forest fire smoke. Nearly null values of the long-term variation of summer background AOD were obtained at Ny-Ålesund (1991–2010) and at Barrow (1977–2010). No evidence of important variations in AOD was found when comparing the monthly mean values of AOD measured at Tiksi in summer 2010 with those derived from multi-filter actinometer measurementsperformed in the late 1980s at some Siberian sites. The long-term variations of seasonal mean AOD for Arctic Haze (AH) conditions and AH episode seasonal frequency were also evaluated, finding that these parameters underwent large fluctuations over the 35-year period at Ny-Ålesund and Barrow, without presenting well-defined long-term variations. A characterization of chemical composition, complex refractive index and single scattering albedo of ground-level aerosol polydispersions in summer and winter–spring is also presented, based on results mainly found in the literature.
The long-term variation in Antarctic AOD was estimated to be stable, within ±0.10% per year, at the three coastal sites, and nearly null at South Pole, where a weak increase was only recently observed, associated with an appreciable decrease in α, plausibly due to the formation of thin stratospheric layers of ageing volcanic particles. The main characteristics of chemical composition, complex refractive index and single scattering albedo of Antarctic aerosols are also presented for coastal particles sampled at Neumayer and Terra Nova Bay, and continental particles at South Pole.
The stable isotopic composition of atmospheric CO2 is being monitored via measurements made at the University of Colorado-Institute of Arctic and Alpine Research, using air samples collected weekly by the Global Air Sampling Network of the NOAA Climate Monitoring and Diagnostics Laboratory. These measurements, in concert with the monitoring of atmospheric CO2 mixing ratios, offer the potential to characterize quantitatively the mechanisms operating in the global carbon cycle, by recording the isotopic signatures imparted to CO2 as it moves among the atmosphere, biosphere, and oceans. This data set increases the number of measurements of atmospheric CO2 isotopes by nearly an order of magnitude over those previously available. We describe the analytical techniques used to obtain and calibrate these data and report measurements from 25 land-based sites, and two ships in the Pacific Ocean, from samples collected during 1990-1993. The typical precision of our mass spectrometric technique is 0.03‰ for δ13C and 0.05‰ for δ18O. Collecting the flask samples without drying leads to loss of δ18O information at many sites. The seasonal cycle in δ13C at sites in the northern hemisphere is highly correlated with that of the CO2 mixing ratio, with amplitudes approaching 1‰ at high latitudes. The seasonal cycle in δ18O is of similar amplitude, though variable from year to year and lags the other species by 2-4 months. Interhemispheric differences of the 1992 and 1993 means of the isotopic tracers are in strong contrast: the north pole-south pole difference for δ13C is ~0.20‰, which though highly quantitatively significant is dwarfed by the ~2‰ difference for δ18O. In contrast to the record of atmospheric δ13C during the 1980s we observe no significant temporal trend in annual mean δ13C during 1990-1993.
Measurements of HCl and CH4 taken by the aircraft laser infrared absorption spectrometer (ALIAS) on the ER-2 high-altitude research aircraft during the Southern Hemisphere winter of 1994 have been used to examine the abundance of HCl as a fraction of total inorganic chlorine. The fractional abundance of HCl shows a threshold behaviour as a function of temperature history; on a 10 day timescale, the abundance dropped sharply in those air parcels experiencing a temperature < 195 K, but little or no change was seen in parcels which stayed warmer than this temperature. The behaviour mirrors well the temperature behaviour calculated for the transformation of HCl into reactive forms (Cl-2, HOCl) from laboratory studies of sulfate aerosols and polar stratospheric clouds. During the course of the winter, the fractional abundance of HCl outside the vortex decreased from its values in late May by about a third, while inside it dropped to near zero by early August. Some recovery was evident in October. Examples of the peel-off of low-HCl air equatorward of the wind maximum were evident in early June. Meteorological trajectories are used to show, in a case study of a flight in early August, that air parcels which experienced temperatures of < 195 K, and as a result had low fractional HCl abundances, did so largely poleward of the maximum in the polar night jet stream. Encountering temperatures of < 195 K during the previous 10 days was a necessary and sufficient condition for the transformation of HCl into reactive forms by heterogeneous reactions. The trajectories further showed that air arriving from sub-tropical latitudes had higher fractional HCl abundances than the air in the middle latitudes, and much higher fractions than the air at high latitudes. The resulting picture is one in which the fractional abundance of HCl in air at mid latitudes was the result of mixing of air from sub-tropical latitudes with air mainly from polward of the jet stream core which has experienced temperatures < 195 K. The sensitivity of the fractional abundance of HCl to the assumption that no HCl enters the stratosphere via the tropical tropopause is examined in the light of an observed profile near the equator with a volume fraction of 0.4 ppb HCl, zero ClO and tropospheric mixing ratios of CFCs at the tropical tropopause.
Abstract. Optical properties of surface aerosols at Dome C, Antarctica, in 2007–2013 and their potential source areas are presented. Scattering coefficients (σsp) were calculated from measured particle number size distributions with a Mie code and from filter samples using mass scattering efficiencies. Absorption coefficients (σap) were determined with a three-wavelength Particle Soot Absorption Photometer (PSAP) and corrected for scattering by using two different algorithms. The scattering coefficients were also compared with σsp measured with a nephelometer at the South Pole Station (SPO). The minimum σap was observed in the austral autumn and the maximum in the austral spring, similar to other Antarctic sites. The darkest aerosol, i.e., the lowest single-scattering albedo ωo≈0.91, was observed in September and October and the highest ωo≈0.99 in February and March. The uncertainty of the absorption Ångström exponent αap is high. The lowest αap monthly medians were observed in March and the highest in August–October. The equivalent black carbon (eBC) mass concentrations were compared with eBC measured at three other Antarctic sites: the SPO and two coastal sites, Neumayer and Syowa. The maximum monthly median eBC concentrations are almost the same (∼3±1 ng m−3) at all these sites in October–November. This suggests that there is no significant difference in eBC concentrations between the coastal and plateau sites. The seasonal cycle of the eBC mass fraction exhibits a minimum f(eBC) ≈0.1 % in February–March and a maximum ∼4 %–5 % in August–October. Source areas were calculated using 50 d FLEXPART footprints. The highest eBC concentrations and the lowest ωo were associated with air masses coming from South America, Australia and Africa. Vertical simulations that take BC particle removal processes into account show that there would be essentially no BC particles arriving at Dome C from north of latitude 10âˆ˜ S at altitudes <1600 m. The main biomass-burning regions Africa, Australia and Brazil are more to the south, and their smoke plumes have been observed at higher altitudes than that, so they can get transported to Antarctica. The seasonal cycle of BC emissions from wildfires and agricultural burning and other fires in South America, Africa and Australia was calculated from data downloaded from the Global Fire Emissions Database (GFED). The maximum total emissions were in August–September, but the peak of monthly average eBC concentrations is observed 2–3 months later in November, not only at Dome C, but also at the SPO and the coastal stations. The air-mass residence-time-weighted BC emissions from South America are approximately an order of magnitude larger than from Africa and Oceania, suggesting that South American BC emissions are the largest contributors to eBC at Dome C. At Dome C the maximum and minimum scattering coefficients were observed in austral summer and winter, respectively. At the SPO σsp was similar to that observed at Dome C in the austral summer, but there was a large difference in winter, suggesting that in winter the SPO is more influenced by sea-spray emissions than Dome C. The seasonal cycles of σsp at Dome C and at the SPO were compared with the seasonal cycles of secondary and primary marine aerosol emissions. The σsp measured at the SPO correlated much better with the sea-spray aerosol emission fluxes in the Southern Ocean than σsp at Dome C. The seasonal cycles of biogenic secondary aerosols were estimated from monthly average phytoplankton biomass concentrations obtained from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite sensor data. The analysis suggests that a large fraction of the biogenic scattering aerosol observed at Dome C has been formed in the polar zone, but it may take a month for the aerosol to be formed, be grown and get transported from the sea level to Dome C.
Earth system models project that the tropical land carbon sink will decrease in size in response to an increase in warming and drought during this century, probably causing a positive climate feedback. But available data are too limited at present to test the predicted changes in the tropical carbon balance in response to climate change. Long-term atmospheric carbon dioxide data provide a global record that integrates the interannual variability of the global carbon balance. Multiple lines of evidence demonstrate that most of this variability originates in the terrestrial biosphere. In particular, the year-to-year variations in the atmospheric carbon dioxide growth rate (CGR) are thought to be the result of fluctuations in the carbon fluxes of tropical land areas. Recently, the response of CGR to tropical climate interannual variability was used to put a constraint on the sensitivity of tropical land carbon to climate change. Here we use the long-term CGR record from Mauna Loa and the South Pole to show that the sensitivity of CGR to tropical temperature interannual variability has increased by a factor of 1.9 ± 0.3 in the past five decades. We find that this sensitivity was greater when tropical land regions experienced drier conditions. This suggests that the sensitivity of CGR to interannual temperature variations is regulated by moisture conditions, even though the direct correlation between CGR and tropical precipitation is weak. We also find that present terrestrial carbon cycle models do not capture the observed enhancement in CGR sensitivity in the past five decades. More realistic model predictions of future carbon cycle and climate feedbacks require a better understanding of the processes driving the response of tropical ecosystems to drought and warming.
We describe a new 4D-Var inversion framework for nitrous oxide (N2O) based on the GEOS-Chem chemical transport model and its adjoint, and apply it in a series of observing system simulation experiments to assess how well N2O sources and sinks can be constrained by the current global observing network. The employed measurement ensemble includes approximately weekly and quasi-continuous N2O measurements (hourly averages used) from several long-term monitoring networks, N2O measurements collected from discrete air samples onboard a commercial aircraft (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container; CARIBIC), and quasi-continuous measurements from the airborne HIAPER Pole-to-Pole Observations (HIPPO) campaigns. For a 2-year inversion, we find that the surface and HIPPO observations can accurately resolve a uniform bias in emissions during the first year; CARIBIC data provide a somewhat weaker constraint. Variable emission errors are much more difficult to resolve given the long lifetime of N2O, and major parts of the world lack significant constraints on the seasonal cycle of fluxes. Current observations can largely correct a global bias in the stratospheric sink of N2O if emissions are known, but do not provide information on the temporal and spatial distribution of the sink. However, for the more realistic scenario where source and sink are both uncertain, we find that simultaneously optimizing both would require unrealistically small errors in model transport. Regardless, a bias in the magnitude of the N2O sink would not affect the a posteriori N2O emissions for the 2-year timescale used here, given realistic initial conditions, due to the timescale required for stratosphere–troposphere exchange (STE). The same does not apply to model errors in the rate of STE itself, which we show exerts a larger influence on the tropospheric burden of N2O than does the chemical loss rate over short (< 3 year) timescales. We use a stochastic estimate of the inverse Hessian for the inversion to evaluate the spatial resolution of emission constraints provided by the observations, and find that significant, spatially explicit constraints can be achieved in locations near and immediately upwind of surface measurements and the HIPPO flight tracks; however, these are mostly confined to North America, Europe, and Australia. None of the current observing networks are able to provide significant spatial information on tropical N2O emissions. There, averaging kernels (describing the sensitivity of the inversion to emissions in each grid square) are highly smeared spatially and extend even to the midlatitudes, so that tropical emissions risk being conflated with those elsewhere. For global inversions, therefore, the current lack of constraints on the tropics also places an important limit on our ability to understand extratropical emissions. Based on the error reduction statistics from the inverse Hessian, we characterize the atmospheric distribution of unconstrained N2O, and identify regions in and downwind of South America, central Africa, and Southeast Asia where new surface or profile measurements would have the most value for reducing present uncertainty in the global N2O budget.
Insoluble trace gases are trapped in polar ice at the firn-ice transition, at approximately 50 to 100 m below the surface, depending primarily on the site temperature and snow accumulation. Models of trace gas transport in polar firn are used to relate firn air and ice core records of trace gases to their atmospheric history. We propose a new model based on the following contributions. First, the firn air transport model is revised in a poromechanics framework with emphasis on the non-homogeneous properties and the treatment of gravitational settling. We then derive a nonlinear least square multi-gas optimisation scheme to calculate the effective firn diffusivity (automatic diffusivity tuning). The improvements gained by the multi-gas approach are investigated (up to ten gases for a single site are included in the optimisation process). We apply the model to four Arctic (Devon Island, NEEM, North GRIP, Summit) and seven Antarctic (DE08, Berkner Island, Siple Dome, Dronning Maud Land, South Pole, Dome C, Vostok) sites and calculate their respective depth-dependent diffusivity profiles. Among these different sites, a relationship is inferred between the snow accumulation rate and an increasing thickness of the lock-in zone defined from the isotopic composition of molecular nitrogen in firn air (denoted δ15N). It is associated with a reduced diffusivity value and an increased ratio of advective to diffusive flux in deep firn, which is particularly important at high accumulation rate sites. This has implications for the understanding of δ15N of N2 records in ice cores, in relation with past variations of the snow accumulation rate. As the snow accumulation rate is clearly a primary control on the thickness of the lock-in zone, our new approach that allows for the estimation of the lock-in zone width as a function of accumulation may lead to a better constraint on the age difference between the ice and entrapped gases.
The HIAPER Pole-to-Pole Observations (HIPPO) programme has completed three of five planned aircraft transects spanning the Pacific from 85°N to 67°S, with vertical profiles every approximately 2.2° of latitude. Measurements include greenhouse gases, long-lived tracers, reactive species, O2/N2 ratio, black carbon (BC), aerosols and CO2 isotopes. Our goals are to address the problem of determining surface emissions, transport strength and patterns, and removal rates of atmospheric trace gases and aerosols at global scales and to provide strong tests of satellite data and global models. HIPPO data show dense pollution and BC at high altitudes over the Arctic, imprints of large N2O sources from tropical lands and convective storms, sources of pollution and biogenic CH4 in the Arctic, and summertime uptake of CO2 and sources for O2 at high southern latitudes. Global chemical signatures of atmospheric transport are imaged, showing remarkably sharp horizontal gradients at air mass boundaries, weak vertical gradients and inverted profiles (maxima aloft) in both hemispheres. These features challenge satellite algorithms, global models and inversion analyses to derive surface fluxes. HIPPO data can play a crucial role in identifying and resolving questions of global sources, sinks and transport of atmospheric gases and aerosols.