Showing papers by "Climate Monitoring and Diagnostics Laboratory published in 2005"

Towards Aerosol Light-Absorption Measurements with a 7-Wavelength Aethalometer: Evaluation with a Photoacoustic Instrument and 3-Wavelength Nephelometer

Abstract: Two extreme cases of aerosol optics from the Reno Aerosol Optics Experiment are used to develop a model-based calibration scheme for the 7-wavelength aethalometer. The cases include those of very white and very dark aerosol samples. The former allows fo ra nassessment of the scattering offset associated with this filterbased method, with the wavelength-dependent scattering measured from a 3-wavelength nephelometer, and interpolated and extrapolated to the 7 wavelengths of the aethalometer. A photoacoustic instrument operating at 532 nm is used to evaluate the filter loading effect caused by aerosol light absorption. Multiple scattering theory is used to analytically obtain a filter-loading correction function. This theory shows that the exponential behavior of light absorption in the strong multiple scattering limit scales as the square root of the total absorption optical depth rather than linearly with optical depth as is commonly assumed with Beer’s law. The multiple scattering model also provides a theoretical justification for subtracting a small fraction of aerosol light scattering away from measured apparent light absorption by the filter method. The model is tested against ambient measurements and is found to require coefficients that are situation specific. Several hypotheses are given for this specificity, and suggested methods for reducing it are discussed. Specific findings are as follows. Simultaneous aerosol lightscattering measurements are required for accurate interpretation of aethalometer data for high aerosol single-scattering albedo. Instantaneous errors of up to ±50% are possible for uncorrected data, depending on filter loading. The aethalometer overpredicts black carbon (BC) concentration on a fresh filter and underpredicts BC on a loaded filter. BC and photoacoustic light absorption can be

Conversion of NOAA atmospheric dry air CH4 mole fractions to a gravimetrically prepared standard scale

Abstract: [1] Sixteen mixtures of methane (CH4) in dry air were prepared using a gravimetric technique to define a CH4 standard gas scale covering the nominal range 300–2600 nmol mol−1. It is designed to be suitable for measurements of methane in air ranging from those extracted from glacial ice to contemporary background atmospheric conditions. All standards were prepared in passivated, 5.9 L high-pressure aluminum cylinders. Methane dry air mole fractions were determined by gas chromatography with flame ionization detection, where the repeatability of the measurement is typically better than 0.1% (≤1.5 nmol mol−1) for ambient CH4 levels. Once a correction was made for 5 nmol mol−1 CH4 in the diluent air, the scale was used to verify the linearity of our analytical system over the nominal range 300–2600 nmol mol−1. The gravimetrically prepared standards were analyzed against CH4 in air standards that define the Climate Monitoring and Diagnostics Laboratory (CMDL) CMDL83 CH4 in air scale, showing that CH4 mole fractions in the new scale are a factor of (1.0124 ± 0.0007) greater than those expressed in the CMDL83 scale. All CMDL measurements of atmospheric CH4 have been adjusted to this new scale, which has also been accepted as the World Meteorological Organization (WMO) CH4 standard scale; all laboratories participating in the WMO Global Atmosphere Watch program should report atmospheric CH4 measurements to the world data center on this scale.

Daily global maps of carbon monoxide from NASA's Atmospheric Infrared Sounder

Abstract: Received 24 October 2004; revised 19 January 2005; accepted 4 March 2005; published 1 June 2005. [1] We present the first observations of tropospheric carbon monoxide (CO) by the Atmospheric Infrared Sounder (AIRS) onboard NASA’s Aqua satellite. AIRS daily coverage of 70% of the planet represents a significant evolutionary advance in satellite trace gas remote sensing. Tropospheric CO abundances are retrieved from AIRS 4.55 mm spectral region using the full AIRS retrieval algorithm run in a research mode. The presented AIRS daily global CO maps from 22– 29 September 2002 show large-scale, long-range transport of CO from anthropogenic and natural sources, most notably from biomass burning. The sequence of daily maps reveal CO advection from Brazil to the South Atlantic in qualitative agreement with previous observations. Forward trajectory analysis confirms this scenario and indicates much longer range transport into the southern Indian Ocean. Preliminary comparisons to in situ aircraft profiles indicate AIRS CO retrievals are approaching the 15% accuracy target set by pre-launch simulations. Citation: McMillan, W. W., C. Barnet, L. Strow, M. T. Chahine, M. L. McCourt, J. X. Warner, P. C. Novelli, S. Korontzi, E. S. Maddy, and S. Datta (2005), Daily global maps of carbon monoxide from NASA’s Atmospheric Infrared Sounder, Geophys. Res. Lett., 32, L11801, doi:10.1029/ 2004GL021821.

Inverse modelling of national and European CH4 emissions using the atmospheric zoom model TM5

Abstract: A synthesis inversion based on the atmospheric zoom model TM5 is used to derive top-down estimates of CH 4 emissions from individual European countries for the year 2001. We employ a model zoom over Europe with 1° × 1° resolution that is two-way nested into the global model domain (with resolution of 6° × 4°. This approach ensures consistent boundary conditions for the zoom domain and thus European top-down estimates consistent with global CH 4 observations. The TM5 model, driven by ECMWF analyses, simulates synoptic scale events at most European and global sites fairly well, and the use of high-frequency observations allows exploiting the information content of individual synoptic events. A detailed source attribution is presented for a comprehensive set of 56 monitoring sites, assigning the atmospheric signal to the emissions of individual European countries and larger global regions. The available observational data put significant constraints on emissions from different regions. Within Europe, in particular several Western European countries are well constrained. The inversion results suggest up to 50-90% higher anthropogenic CH 4 emissions in 2001 for Germany, France and UK compared to reported UNFCCC values (EEA, 2003). A recent revision of the German inventory, however, resulted in an increase of reported CH 4 emissions by 68.5% (EEA, 2004), being now in very good agreement with our top-down estimate. The top-down estimate for Finland is distinctly smaller than the a priori estimate, suggesting much smaller CH 4 emissions from Finnish wetlands than derived from the bottom-up inventory. The EU-15 totals are relatively close to UNFCCC values (within 4-30%) and appear very robust for different inversion scenarios.

Maximum likelihood estimation of covariance parameters for Bayesian atmospheric trace gas surface flux inversions

Abstract: [1] This paper introduces a Maximum Likelihood (ML) approach for estimating the statistical parameters required for the covariance matrices used in the solution of Bayesian inverse problems aimed at estimating surface fluxes of atmospheric trace gases. The method offers an objective methodology for populating the covariance matrices required in Bayesian inversions, thereby resulting in better estimates of the uncertainty associated with derived fluxes and minimizing the risk of inversions being biased by unrealistic covariance parameters. In addition, a method is presented for estimating the uncertainty associated with these covariance parameters. The ML method is demonstrated using a typical inversion setup with 22 flux regions and 75 observation stations from the National Oceanic and Atmospheric Administration-Climate Monitoring and Diagnostics Laboratory (NOAA-CMDL) Cooperative Air Sampling Network with available monthly averaged carbon dioxide data. Flux regions and observation locations are binned according to various characteristics, and the variances of the model-data mismatch and of the errors associated with the a priori flux distribution are estimated from the available data.

Intercomparison of shortwave radiative transfer codes and measurements

Abstract: [1] Computation of components of shortwave (SW) or solar irradiance in the surface-atmospheric system forms the basis of intercomparison between 16 radiative transfer models of varying spectral resolution ranging from line-by-line models to broadband and general circulation models. In order of increasing complexity the components are: direct solar irradiance at the surface, diffuse irradiance at the surface, diffuse upward flux at the surface, and diffuse upward flux at the top of the atmosphere. These components allow computation of the atmospheric absorptance. Four cases are considered from pure molecular atmospheres to atmospheres with aerosols and atmosphere with a simple uniform cloud. The molecular and aerosol cases allow comparison of aerosol forcing calculation among models. A cloud-free case with measured atmospheric and aerosol properties and measured shortwave radiation components provides an absolute basis for evaluating the models. For the aerosol-free and cloud-free dry atmospheres, models agree to within 1% (root mean square deviation as a percentage of mean) in broadband direct solar irradiance at surface; the agreement is relatively poor at 5% for a humid atmosphere. A comparison of atmospheric absorptance, computed from components of SW radiation, shows that agreement among models is understandably much worse at 3% and 10% for dry and humid atmospheres, respectively. Inclusion of aerosols generally makes the agreement among models worse than when no aerosols are present, with some exceptions. Modeled diffuse surface irradiance is higher than measurements for all models for the same model inputs. Inclusion of an optically thick low-cloud in a tropical atmosphere, a stringent test for multiple scattering calculations, produces, in general, better agreement among models for a low solar zenith angle (SZA = 30°) than for a high SZA (75°). All models show about a 30% increase in broadband absorptance for 30° SZA relative to the clear-sky case and almost no enhancement in absorptance for a higher SZA of 75°, possibly due to water vapor line saturation in the atmosphere above the cloud.

Aerosol Direct Radiative Effects Over the Northwest Atlantic, Northwest Pacific, and North Indian Oceans: Estimates Based on In-situ Chemical and Optical Measurements and Chemical Transport Modeling

Abstract: The largest uncertainty in the radiative forcing of climate change over the industrial era is that due to aerosols, a substantial fraction of which is the uncertainty associated with scattering and absorption of shortwave (solar) radiation by anthropogenic aerosols in cloud-free conditions (IPCC, 2001). Quantifying and reducing the uncertainty in aerosol influences on climate is critical to understanding climate change over the industrial period and to improving predic- tions of future climate change for assumed emission scenar- ios. Measurements of aerosol properties during major field campaigns in several regions of the globe during the past decade are contributing to an enhanced understanding of at- mospheric aerosols and their effects on light scattering and climate. The present study, which focuses on three regions

Sensitivity of atmospheric CO2 inversions to seasonal and interannual variations in fossil fuel emissions

Abstract: [1] Estimates of fossil fuel CO2 are a critical component in atmospheric CO2 inversions. Rather than solving for this portion of the atmospheric CO2 budget, inversions typically include estimates of fossil fuel CO2 as a known quantity. However, this assumption may not be appropriate, particularly as inversions continue to solve for fluxes at reduced space and timescales. In this study, two different alterations are made to widely used fossil fuel CO2 emissions estimates, and these altered emissions are run through a series of atmospheric inversion experiments. The first alteration is the inclusion of a seasonal cycle which depends upon both season and latitude. The other alteration is the inclusion of year-by-year changes in the spatial distribution of fossil fuel CO2 emissions. All but the interannual inversion experiments are run with three models from the TransCom 3 atmospheric inversion intercomparison. These three models span the key components of atmospheric transport and hence can be expected to capture the range of potential bias caused by assumed fossil fuel CO2 emission estimates when interacting with transport processes. Key findings include the lack of seasonal rectification of the seasonally varying fossil fuel CO2 emissions in the annual mean. Examination of monthly fluxes in the seasonal inversion, however, indicates that significant bias is likely occurring and may be as large as 50% of the residual flux during certain times of the year. In this study, interannual variations were little effected by shifts in the spatial pattern of fossil fuel CO2 emissions. However, as the spatial scale of inversions is reduced, potential bias will likely increase.

Impact of particulate organic matter on the relative humidity dependence of light scattering: A simplified parameterization

Abstract: [1] Measurementsduringrecentfieldcampaignsdownwind of the Indian subcontinent, Asia, and the northeastern United States reveal a substantial decrease in the relative humidity dependence of light scattering, fssp(RH), with increasing mass fraction of particulate organic matter (POM) for submicrometer aerosol. Using data from INDOEX (INDian Ocean EXperiment), ACE Asia (Aerosol Characterization Experiment – Asia), and ICARTT (International Consortium for Atmospheric Research on Transport and Transformation), we have identified, within measurement limitations, the impact of POM on the fssp(RH) of accumulation mode sulfate-POM mixtures. The result is a parameterization that quantifies the POM mass fraction - fssp(RH) relationship for use in radiative transfer and air quality models either as input or as validation. The parameterization is valid where the aerosol consists of an internally mixed sulfatecarbonaceous accumulation mode and other externally mixed components (e.g. sea salt, dust) and is applicable on both global and regional scales. Citation: Quinn, P. K., et al. (2005), Impact of particulate organic matter on the relative humidity dependence of light scattering: A simplified parameterization, Geophys. Res. Lett., 32, L22809, doi:10.1029/ 2005GL024322.

Four decades of ozonesonde measurements over Antarctica

Abstract: [1] Ozonesonde observations from Syowa and the South Pole over more than 40 years are described and intercompared. Observations from the two sites reveal remarkable agreement, supporting and extending the understanding gained from either individually. Both sites exhibit extensive Antarctic ozone losses in a relatively narrow altitude range from about 12 to 24 km in October, and the data are consistent with temperature-dependent chemistry involving chlorine on polar stratospheric clouds as the cause of the ozone hole. The maximum October ozone losses at higher altitudes near 18 km (70 hPa) appear to be transported to lower levels near the tropopause on a timescale of a few months, which is likely to affect the timing of the effects of ozone depletion on possible tropospheric climate changes. Both sites also show greater ozone losses in the lowermost stratosphere after the volcanic eruption of Mt. Pinatubo, supporting the view that surface chemistry can be enhanced by volcanic perturbations and that the very deep ozone holes observed in the early 1990s reflected such enhancements. Sparse data from the Syowa station in the early 1980s also suggest that enhanced ozone losses due to the El Chichon eruption may have contributed to the beginning of a measurable ozone hole. Observations at both locations show that some ozone depletion now occurs during much if not all year at lower altitudes near 12–14 km. Correlations between temperature and ozone provide new insights into ozone losses, including its nonlinear character, maximum effectiveness, and utility as a tool to distinguish dynamical effects from chemical processes. These data also show that recent changes in ozone do not yet indicate ozone recovery linked to changing chlorine abundances but provide new tools to probe observations for the first such future signals.

Direct transport of midlatitude stratospheric ozone into the lower troposphere and marine boundary layer of the tropical Pacific Ocean

Abstract: [1] The detailed survey of midlatitude stratospheric intrusions penetrating into the Northern Hemisphere tropics was one goal of the Pacific Sub-Tropical Jet Study 2004, conducted from Honolulu, Hawaii, during 19–29 January and 28 February to 15 March. Using the National Oceanic and Atmospheric Administration G-IV jet aircraft, instrumented with dropsondes and a 1-s resolution ozone instrument, we targeted an intrusion above Hawaii on 29 February. The data describe the strongest tropospheric ozone enhancements ever measured above Hawaii (in comparison to a 22 year ozonesonde record) and illustrate the mixing of stratospheric ozone into the midtroposphere as a result of convection triggered by the advection of relatively cold midlatitude air into the tropics. Measurements from the G-IV and Mauna Loa Observatory (3.4 km) show enhanced ozone in the lower troposphere, indicating that the remnants of the intrusion reached these levels. This conclusion is supported by a study using a stratospheric ozone tracer generated by the FLEXPART Lagrangian particle dispersion model. This paper also describes a similar intrusion that enhanced ozone at Mauna Loa on 10 March, as well as Honolulu, which is located in the marine boundary layer. G-IV flights in and out of Honolulu measured enhanced ozone associated with this event on several occasions. The 10 March event transported an estimated 1.75 Tg of ozone into the tropical troposphere, and we suggest that stratospheric intrusions that break away from the polar jet stream as they advect into the tropics are more effective at transporting ozone into the troposphere than intrusions that remain close to the polar jet stream in midlatitudes. Analysis of the dynamic conditions indicates that the frequency of stratospheric intrusions was not anomalous during January–March 2004. While the 10 March event was by itself an extreme event, strong stratospheric intrusions can be expected to influence the tropical lower troposphere in any year.

An improved Kalman Smoother for atmospheric inversions

Abstract: . We explore the use of a fixed-lag Kalman smoother for sequential estimation of atmospheric carbon dioxide fluxes. This technique takes advantage of the fact that most of the information about the spatial distribution of sources and sinks is observable within a few months to half of a year of emission. After this period, the spatial structure of sources is diluted by transport and cannot significantly constrain flux estimates. We therefore describe an estimation technique that steps through the observations sequentially, using only the subset of observations and modeled transport fields that most strongly constrain the fluxes at a particular time step. Estimates of each set of fluxes are sequentially updated multiple times, using measurements taken at different times, and the estimates and their uncertainties are shown to quickly converge. Final flux estimates are incorporated into the background state of CO2 and transported forward in time, and the final flux uncertainties and covariances are taken into account when estimating the covariances of the fluxes still being estimated. The computational demands of this technique are greatly reduced in comparison to the standard Bayesian synthesis technique where all observations are used at once with transport fields spanning the entire period of the observations. It therefore becomes possible to solve larger inverse problems with more observations and for fluxes discretized at finer spatial scales. We also discuss the differences between running the inversion simultaneously with the transport model and running it entirely off-line with pre-calculated transport fields. We find that the latter can be done with minimal error if time series of transport fields of adequate length are pre-calculated.

A comparison and summary of aerosol optical properties as observed in situ from aircraft, ship, and land during ACE‐Asia

Abstract: [1] During the ACE-Asia campaign in March–May 2001, in situ measurements of aerosol optical properties were made from multiple airborne and land- or ship-based platforms. Using a suite of direct interplatform comparisons and a campaign-wide statistical comparison, we test the precision of these measurements, and we determine whether the platforms sampled similar aerosol. Data included in the study are from the National Center for Atmospheric Research C-130 aircraft; the CIRPAS Twin Otter aircraft; the National Oceanographic and Atmospheric Administration (NOAA) ship R.V. Ronald H. Brown; and the Gosan surface station on Jeju Island, located off the southern tip of South Korea. Comparisons were made of total and submicron light scattering at 450, 550, and 700 nm; total and submicron absorption at 550 nm; the Angstrom exponent; single scatter albedo of the total aerosol, submicron and supermicron aerosol at 550 nm; hemispheric backscatter fraction at 550 nm; and light scattering hygroscopic growth at 550 nm. For the campaign-wide comparison, the data are broken down by light scattering fine mode fraction since the aerosol in the ACE-Asia study region were a variable mix of pollution, dust, and sea salt. Finally, we calculate how the observed uncertainties in the aerosol optical properties propagate to uncertainties in top-of-atmosphere radiative forcing. Single scatter albedo showed excellent agreement among all platforms other than the Twin Otter, with discrepancies generally <0.02. These data sets combine to give campaign-wide values of single scatter albedo of 0.885 ± 0.023 for the submicron aerosol (i.e. pollution) and 0.957 ± 0.031 for the supermicron aerosol (which, for these data, was predominantly dust). The data also indicated that, as expected, the Low Turbulent Inlet on the C-130 produced enhanced concentrations of coarse mode aerosol. There also may have been significant coarse mode particle losses on the other platforms. These effects combined to produce generally lower fine mode fractions and Angstrom exponents on the C-130 than on the other platforms. Large discrepancies in hemispheric backscatter fraction and light scattering hygroscopic growth were observed in both the side-by-side and statistical comparisons. We are not able to explain these differences, though possible causes are discussed. Studies of the TSI, Inc. nephelometer backscatter measurement and of the two methods used here to measure hygroscopic growth are needed to clarify the source of these observed discrepancies. A better understanding of the effects of nonsphericity on hemispheric backscatter fraction is also needed.

The Accuracy of Solar Irradiance Calculations Used in Mesoscale Numerical Weather Prediction

Abstract: In this paper, solar irradiance forecasts made by mesoscale numerical weather prediction models are compared with observations taken during three air-quality experiments in various parts of the United States. The authors evaluated the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) and the National Centers for Environmental Prediction (NCEP) Eta Model. The observations were taken during the 2000 Texas Air Quality Experiment (TexAQS), the 2000 Central California Ozone Study (CCOS), and the New England Air Quality Study (NEAQS) 2002. The accuracy of the model forecast irradiances show a strong dependence on the aerosol optical depth. Model errors on the order of 100 W m−2 are possible when the aerosol optical depth exceeds 0.1. For smaller aerosol optical depths, the climatological attenuation used in the models yields solar irradiance estimates that are in good agreement with the observations.

Extensive observations of CO2 carbon isotope content in and above a high-elevation subalpine forest

Abstract: [1] The dynamics of forest-atmosphere CO2 carbon isotope exchange were examined in a coniferous forest in Colorado, United States. Tunable diode laser absorption spectrometry provided extensive characterization of the carbon isotope content (δ13C) of CO2. Observed patterns in δ13C of forest air were associated with photosynthesis, respiration, and atmospheric boundary layer dynamics. Similar relationships between δ13C and CO2 were observed at all forest heights and confined to a relatively narrow envelope. Substantial variation was observed in the isotope ratio of nocturnal ecosystem respiration (δ13CR, calculated from isotopic mixing lines). A systematic bias was identified when estimating δ13CR from data sets with small range in CO2 in the samples, leading us to restrict analysis of δ13CR to periods with CO2 range >40 μmol mol−1. Values of δ13CR varied from −28.1 to −25.2‰, with variation from one night to the next as large as 1.7‰. A consistent difference was observed between δ13CR calculated near the forest floor (<2 m height) versus the upper canopy (5–11 m) on the same nights. δ13CR was more enriched in the upper canopy than near the ground on 34 of 43 nights, with a mean enrichment of 0.6‰ and a maximum of 2.3‰. A similar pattern was observed comparing δ13CR at night with the analogous quantity calculated during daytime, but only a few daytime periods met the 40 μmol mol−1 criterion. Comparisons between air samples measured (1) 10 m above the forest canopy, (2) 3 km away, and (3) within the convective boundary layer 125 km distant showed CO2 differences between sites as large as 5–6 μmol mol−1 even at midday. These results suggest that attempts to use flask measurements at remote monitoring stations as a proxy for the air directly interacting with a vegetation canopy should be made with caution. However, our results also suggest that substantial information about biosphere-atmosphere isotopic exchange can be obtained by simultaneous examination of CO2 and δ13C at multiple spatial scales.

Trajectory model sensitivity to differences in input data and vertical transport method

Abstract: [1] In this study, trajectory model sensitivity to the input meteorological data and vertical transport method is quantified. ERA-40 and NCEP/NCAR reanalysis data sets are used to compare isentropic and kinematic 3-D tropospheric trajectories. Expanding on previous trajectory sensitivity studies, we accumulate deviation statistics for an entire year and for three geographically diverse sites. The horizontal trajectory deviations are summarized as a percentage of average distance traveled. These results allow ranking from least to greatest among the five causes of trajectory uncertainty investigated here: minor differences in computational methodology, 3–4%; time interpolation, 9–25%; vertical transport method, 18–34%; meteorological input data, 30–40%; and combined two-way differences in vertical transport method and meteorological input data, 39–47%. Although the deviations are somewhat dependent on starting location because of the influence of meteorology, at all three sites, 3-D trajectories attained higher elevations and wind speeds than isentropic trajectories. In addition, deviation statistics for 3-D trajectories exceeded those for isentropic trajectories. The reasons for this derive from uncertainties in the supplied vertical wind fields and the higher wind speeds in 3-D trajectories on the one hand and the vertical constraints imposed by the isentropic assumption on the other.

On the distribution and variability of ozone in the tropical upper troposphere: Implications for tropical deep convection and chemical-dynamical coupling

Abstract: [1] Tropical ozonesonde measurements display events of substantially reduced or near-zero ozone in the upper troposphere that can be coherent over broad spatial scales. Available observations indicate that these events occur most frequently between about 300 and 100 mbar in the tropical southwest Pacific region. The spatial structure of the events suggests linkages to deep convection as the primary cause, with the potential for long-range transport from the southwest Pacific to other locations. Observations are sparse in time as well as space, but suggest possible long-term changes in tropical ozone transport and the frequency of deep convection there since the 1980s.

Bias in Dobson total ozone measurements at high latitudes due to approximations in calculations of ozone absorption coefficients and air mass

Abstract: [1] The Dobson spectrophotometer is the primary standard instrument for ground-based measurements of total column ozone. The accuracy of its data depends on the knowledge of ozone absorption coefficients used for data reduction. We document an error in the calculations that led to the set of absorption coefficients currently recommended by the World Meteorological Organisation (WMO). This error has little effect because an empirical adjustment was applied to the original calculations before the coefficients were adopted by WMO. We provide evidence that this adjustment was physically sound. The coefficients recommended by WMO are applied in the Dobson network without correction for the temperature dependence of the ozone absorption cross sections. On the basis of data measured by Dobson numbers 80 and 82, which were operated by the National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring and Diagnostics Laboratory at the South Pole, we find that omission of temperature corrections may lead to systematic errors in Dobson ozone data of up to 4%. The standard Dobson ozone retrieval method further assumes that the ozone layer is located at a fixed height. This approximation leads to errors in air mass calculations, which are particularly relevant at high latitudes where ozone measurements are performed at large solar zenith angles (SZA). At the South Pole, systematic errors caused by this approximation may exceed 2% for SZAs larger than 80°. The bias is largest when the vertical ozone distribution is distorted by the “ozone hole” and may lead to underestimation of total ozone by 4% at SZA = 85° (air mass 9). Dobson measurements at the South Pole were compared with ozone data from a collocated SUV-100 UV spectroradiometer and Version 8 overpass data from NASA's Total Ozone Mapping Spectrometer (TOMS). Uncorrected Dobson ozone values tend to be lower than data from the two other instruments when total ozone is below 170 Dobson units or SZAs are larger than 80°. When Dobson measurements are corrected for the temperature dependence of the ozone absorption cross section and accurate air mass calculations are implemented, data from the three instruments agree with each other to within ±2% on average and show no significant dependence on SZA or total ozone.

A springtime comparison of tropospheric ozone and transport pathways on the east and west coasts of the United States

Abstract: [1] We have conducted a study to determine the influence of Asian pollution plumes on free tropospheric ozone above the west coast of the United States during spring. We also explored the additional impact of North American emissions on east coast free tropospheric ozone. Long-term ozone monitoring sites in the United States are few, but we obtained ozonesonde profiles from Trinidad Head on the west coast, Huntsville, Alabama, in the southeast, and Wallops Island, Virginia, on the east coast. Additional east coast ozone profiles were measured by the MOZAIC commercial aircraft at Boston, New York City, and Philadelphia. Kilometer-averaged ozone was compared between Trinidad Head and the three east coast sites (MOZAIC, Wallops Island, and Huntsville). Only in the 0–1 km layer did the MOZAIC site have a statistically significant greater amount of ozone than Trinidad Head. Likewise only the 0–1 and 1–2 km layers had greater ozone at Wallops Island and Huntsville in comparison to Trinidad Head. While Wallops Island did show greater ozone than Trinidad Head at 6–9 km, this excess ozone was attributed to a dry air mass sampling bias. A particle dispersion model was used to determine the surface source regions for each case, and the amount of anthropogenic NOx tracer that would have been emitted into each air mass. Transport times were limited to 20 days to focus on the impact of direct transport of pollution plumes from the atmospheric boundary layer. As expected, the amount of NOx tracer emitted into the east coast profiles was much greater in the lower and mid troposphere than at the west coast. At various altitudes at both coasts there existed a significant positive correlation between ozone and the NOx tracer, but the explained variance was generally less than 30%. On the east coast, Wallops Island had the weakest relationship between ozone and the NOx tracer, while Huntsville had the strongest. During spring, differences in photochemistry and transport pathways in the lowest 2 km of the troposphere results in an extra 5–14 ppbv of ozone on the east coast in comparison to Trinidad Head. However, despite differing amounts of NOx tracer from Asia and North America in the free troposphere, we found no significant difference in free tropospheric ozone between the east and west coasts of the United States during spring.

Large decadal scale changes of polar ozone suggest solar influence

Abstract: Long-term measurements of polar ozone show an unexpectedly large decadal scale variability in the mid-stratosphere during winter. Negative ozone anomalies are strongly correlated with the flux of energetic electrons in the radiation belt, which is modulated by the 11-year solar cycle. The magnitude of the observed decadal ozone changes (?20%) is much larger than any previously reported solar cycle effect in the atmosphere up to this altitude. The early-winter ozone anomalies subsequently propagate downward into the lower stratosphere and may even influence total ozone and meteorological conditions during spring. These findings suggest a previously unrecognized mechanism by which solar variability impacts on climate through changes in polar ozone.

Long-term record of atmospheric CO2 and stable isotopic ratios at Waliguan Observatory : Background features and possible drivers, 1991-2002

Abstract: [1] This paper describes background characteristics of atmospheric CO2 and stable isotopic ratios (δ13C and δ18O) as well as their possible drivers at Waliguan Baseline Observatory (WLG) (36°17′N, 100°54′E, 3816 m above sea level) in the inland plateau of western China. The study is based on observational CO2 data (NOAA Climate Monitoring and Diagnostics Laboratory discrete and WLG continuous measurements) obtained at WLG for the period from May 1991 to December 2002. Over this period the change in monthly means is ∼+16 ppm for CO2, ∼−0.2‰ for δ13C, and ∼−0.5‰ for δ18O. The overall increase of CO2 and subsequent decline of δ13C, with a Δδ13C/ΔCO2 ratio (−0.011 ± 0.105) ‰ ppm−1 at WLG, reflect the persistent worldwide influence of fossil fuel emissions. The negative secular trend of δ18O at WLG is probably due to vigorous 18O exchange with soils in the Northern Hemisphere (NH) and conversion from C3 to C4 plants via land use change. The CO2, δ13C, and δ18O mean annual cycles with peak-to-peak annual amplitudes of ∼10.5 ppm, ∼0.499 ‰, and ∼0.819‰, respectively, at WLG show typical middle-to-high NH continental features that correspond to the seasonal cycle of the terrestrial biosphere. The significant CO2 and δ13C interannual variability at WLG is very likely caused by worldwide climate anomalies and associated regional fluctuation in biospheric CO2 uptake in the Asian inland plateau as well as long-range air mass transport. The results of this study help to provide a basic understanding of the individual sources and sinks of carbon in this area and help us to better address the role of the Asian inland terrestrial biosphere in the global carbon cycle.

The effect of systematic measurement errors on atmospheric CO 2 inversions: a quantitative assessment

Abstract: Surface-atmosphere exchange fluxes of CO 2, es- timated by an interannual atmospheric transport inversion from atmospheric mixing ratio measurements, are affected by several sources of errors, one of which is experimental errors. Quantitative information about such measurement er- rors can be obtained from regular co-located measurements done by different laboratories or using different experimen- tal techniques. The present quantitative assessment is based on intercomparison information from the CMDL and CSIRO atmospheric measurement programs. We show that the ef- fects of systematic measurement errors on inversion results are very small compared to other errors in the flux estima- tion (as well as compared to signal variability). As a prac- tical consequence, this assessment justifies the merging of data sets from different laboratories or different experimental techniques (flask and in-situ), if systematic differences (and their changes) are comparable to those considered here. This work also highlights the importance of regular intercompari- son programs.

Comparisons of aerosol optical depth and surface shortwave irradiance and their effect on the aerosol surface radiative forcing estimation

Abstract: [1] Column aerosol optical depth (AOD) and surface shortwave irradiance (SSI) measurements relevant to computation of the aerosol surface radiative forcing (ΔF) and forcing efficiency (β) were taken as part of Aerosol Characterization Experiment-Asia (ACE-Asia) at the Gosan surface site in Korea in April 2001. We compare the AOD and SSI derived from three different types of Sun photometers and three sets of radiometers. We also estimate the ΔF and β using two methods and quantify the observational uncertainties of these parameters. A comparison of the AOD at 500 nm shows that the three Sun photometers generally agreed within 0.014 (mean), 0.0142 (bias), and 0.0298 (root mean square) for coincident observations. Over the course of the comparison, the mean differences between the SSI measurements were 1.6, 11.7, and 10.1 Wm−2 for direct, diffuse and global irradiances, respectively. However, for both the AOD and the SSI comparisons, relatively high instantaneous differences between the instruments were apparent on days with heavy dust at the surface. The mean β and associated deviations, which were estimated by the combinations of different instrument-derived AODs and SSIs, for simultaneous observation data at a 60° solar zenith angle are −79.50 ± 3.92 and −82.57 ± 5.70 Wm−2/τ500 for globalshaded (sum of direct and diffuse irradiances) and globalunshaded (measured by the unshaded pyranometer) irradiances, respectively. The uncertainties in β associated with uncertainties in the AOD and SSI (in parentheses) for globalshaded and globalunshaded irradiance are about 8.6% and 3.2% (10.7%), respectively. A 2% difference between the measured global irradiances for a given 9 days was translated into an uncertainty of 19% in ΔF. This difference in ΔF between instruments caused a 14% deviation in β.

Toward the development of a diffuse horizontal shortwave irradiance working standard

Abstract: [1] The first intensive observation period (IOP) to simultaneously measure diffuse horizontal shortwave irradiance (scattered solar radiation that falls on a horizontal surface) with a wide array of shaded pyranometers suggested that a consensus might be reached that would permit the establishment of a standard with a smaller uncertainty than previously achieved. A second IOP has been held to refine the first IOP measurements using a uniform calibration protocol, offset corrections for all instruments and validation of those corrections, improvements in some of the instruments, and better data acquisition. The venue for both IOPs was the Department of Energy's Atmospheric Radiation Measurement central facility in northern Oklahoma. The 9 days of measurements in October 2003 included a better mixture of clear and overcast conditions than during the first IOP and revealed considerable differences among the instruments' responses for different cloud conditions. Four of the 15 instruments were eliminated as candidates to be included in the standard because of noisy signals, inadequate offset correction, or instability with respect to the majority of the measurements. Eight pyranometers agreed to within ±2% for clear-sky conditions. Three others have a high bias on clear days relative to these eight, but all 11 agree within ±2% on overcast days. The differences and causes of this behavior under clear and cloudy skies are examined.

Reduced ozone loss at the upper edge of the Antarctic Ozone Hole during 2001-2004

Abstract: [1] The top edge (20–22 km) of the Antarctic ozone hole has been identified as a good place to detect ozone recovery (D. J. Hofmann et al., 1997). During the first six POAM observation years (1994–1996, 1998–2000), the early October ozone mixing ratio at the top edge of the ozone hole was nearly constant, while during the last four years (2001–2004) it was noticeably greater. The estimated ozone photochemical loss for air sampled at 20–22 km in 2001–2004 was likewise smaller. During 2001–2004, there were fewer PSCs and generally higher temperatures during August and September at 20–22 km than prior years. The increased ozone was due to both reduced photochemical ozone loss (2001, 2003, and 2004) and enhanced adiabatic descent of the ozone profile (2002). Because of the changing meteorological conditions, the ozone changes at the top edge of the hole cannot be easily attributed to changes in chlorine abundance.

Halocarbon emissions estimated from Advanced Global Atmospheric Gases Experiment measured pollution events at Trinidad Head, California

Abstract: [1] The emissions of halogenated gases from the West Coast region of the United States are estimated from measurements from 1995 to 2003 at the Advanced Global Atmospheric Gases Experiment site at Trinidad Head, California. The emissions estimation procedure uses pollution events combined with population densities integrated along back trajectories, and the estimates are constrained by independent estimates of CH4 and N2O emissions from the U.S. West Coast region. The best fit, average emissions of CH4 and N2O and the average chloroform emissions in California, Oregon, and Washington combined from 1996 to 2002 are 44, 3.7, and 0.07 kg person−1 yr−1, respectively. The emissions per person of CFC-11 (CCl3F), CFC-12 (CCl2F2), CFC-113 (CCl2FCClF2), and methyl chloroform (CH3CCl3) from California in 1996–1998 are calculated to be factors of approximately 2.2, 1.3, 0.7, and 1.6, respectively, less (more for CFC-113) than those reported for the northeastern United States by Barnes et al. (2003). The emission per person of all these gases in the U.S. West Coast region decreased from 1998 to 1999 by a factor of 2 or more, but from 1999 to 2002 the estimated emissions of all four gases have remained fairly constant and are 0.016, 0.048, 0.002, and 0.006 kg person−1 yr−1, respectively. The methyl chloroform estimates suggest a delay of up to 1 year in the decline of the emissions from 1996 to 1998, but otherwise, and in 1999–2000, in contrast to the Millet and Goldstein (2004) results, they are in agreement with the average methyl chloroform emissions per person for the United States based on the UNEP country by country consumption figures (A. McCulloch, private communication, 2004). Averaging the Trinidad Head and the Barnes et al. (2003) per person estimates and multiplying by the U.S. population suggests average methyl chloroform emissions in the United States of 18 Gg yr−1 in 1996 to 1998. In 2001–2002, if the ratio of the emissions per person in these two regions was the same as in 1996–1998, we estimate U.S. emissions of 2.2 Gg yr−1, which is one half of the Millet and Goldstein (2004) estimate.

Ozone loss derived from balloon-borne tracer measurements in the 1999/2000 Arctic winter

Abstract: . Balloon-borne measurements of CFC11 (from the DIRAC in situ gas chromatograph and the DESCARTES grab sampler), ClO and O3 were made during the 1999/2000 Arctic winter as part of the SOLVE-THESEO 2000 campaign, based in Kiruna (Sweden). Here we present the CFC11 data from nine flights and compare them first with data from other instruments which flew during the campaign and then with the vertical distributions calculated by the SLIMCAT 3D CTM. We calculate ozone loss inside the Arctic vortex between late January and early March using the relation between CFC11 and O3 measured on the flights. The peak ozone loss (~1200ppbv) occurs in the 440-470K region in early March in reasonable agreement with other published empirical estimates. There is also a good agreement between ozone losses derived from three balloon tracer data sets used here. The magnitude and vertical distribution of the loss derived from the measurements is in good agreement with the loss calculated from SLIMCAT over Kiruna for the same days.

Retrieval of ice crystal effective diameters from ground‐based near‐infrared spectra of optically thin cirrus

Abstract: [1] The microphysical properties of an optically thin (τ ≤ 0.12) cirrus layer formed by dispersing contrails above Boulder, Colorado, have been examined in a ground-based study. Backscatter and depolarization lidar measurements at 0.532 μm were used to characterize the cloud morphology, while near-infrared (0.9 to 1.7 μm) spectroscopy was used to measure zenith scattered sunlight from the ice particles. The spectra are used to infer the particle effective diameter from published size and shape dependent scattering calculations. The measurements indicate growth in particle size from average effective diameters less than 20 μm in isolated contrails, to more than 40 μm as the contrails coalesced to form a 1–2 km thick cirrus layer. The retrieved size distributions are consistent with published measurements of contrail-cirrus size distributions from in situ observations, and suggest that the lidar/near-infrared spectroscopy combination can provide a useful method for deriving radiatively important information regarding optically thin (τ ≤ 0.1) cirrus under conditions where the single-scattering approximation is valid.