Intercontinental Chemical Transport Experiment–North America Research Articles

Evaluation of updated nitric acid chemistry on ozone precursors and radiative effects

Abstract. This study shows that revising the reaction rate of NO2 + HO· → HNO3 improves simulated nitrogen partitioning and changes the simulated radiative effects of several short-lived climate forcers (SLCF). Both laboratory and field study analyses have found that the reaction rate should be reduced by 13–30% from current recommendations. We evaluate the GEOS-Chem model over North America with and without the recommended update using observations from the Intercontinental Chemical Transport Experiment – North America (INTEX-NA) Phase A campaign. Revising the NO2 + HO· → HNO3 rate coefficient improves model performance of oxidized nitrogen partitioning by increasing NOx concentrations in the upper troposphere and decreasing HNO3 throughout the troposphere. The increase in NOx concentrations has a corresponding global increase in O3 concentrations and local increases in sulfate aerosols, causing a perturbation in simulated radiative effects. These findings demonstrate the positive influence the mechanism update has on the partitioning of oxidized nitrogen species, the benefits it provides when compared to aircraft observations, and the simulated radiative effects that the reduction induces.

Mapping of North American methane emissions with high spatial resolution by inversion of SCIAMACHY satellite data

We estimate methane emissions from North America with high spatial resolution by inversion of Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite observations using the Goddard Earth Observing System Chemistry (GEOS-Chem) chemical transport model and its adjoint. The inversion focuses on summer 2004 when data from the Intercontinental Chemical Transport Experiment-North America (INTEX-A) aircraft campaign over the eastern U.S. are available to validate the SCIAMACHY retrievals and evaluate the inversion. From the INTEX-A data we identify and correct a water vapor-dependent bias in the SCIAMACHY data. We conduct an initial inversion of emissions on the horizontal grid of GEOS-Chem (1/2° × 2/3°) to identify correction tendencies relative to the Emission Database for Global Atmospheric Research (EDGAR) v4.2 emission inventory used as a priori. We then cluster these grid cells with a hierarchical algorithm to extract the maximum information from the SCIAMACHY observations. A 1000 cluster ensemble can be adequately constrained, providing ~100 km resolution across North America. Analysis of results indicates that the Hudson Bay Lowland wetlands source is 2.1 Tg a−1, lower than the a priori but consistent with other recent estimates. Anthropogenic U.S. emissions are 30.1 ± 1.3 Tg a−1, compared to 25.8 Tg a−1 and 28.3 Tg a−1 in the EDGAR v4.2 and Environmental Protection Agency (EPA) inventories, respectively. We find that U.S. livestock emissions are 40% greater than in these two inventories. No such discrepancy is apparent for overall U.S. oil and gas emissions, although this may reflect some compensation between overestimate of emissions from storage/distribution and underestimate from production. We find that U.S. livestock emissions are 70% greater than the oil and gas emissions, in contrast to the EDGAR v4.2 and EPA inventories where these two sources are of comparable magnitude.

Combining Bayesian methods and aircraft observations to constrain the HO<sup>.</sup> + NO<sub>2</sub> reaction rate

Abstract. Tropospheric ozone is the third strongest greenhouse gas, and has the highest uncertainty in radiative forcing of the top five greenhouse gases. Throughout the troposphere, ozone is produced by radical oxidation of nitrogen oxides (NOx = NO + NO2). In the upper troposphere (8–10 km), current chemical transport models under-estimate nitrogen dioxide (NO2) observations. Improvements to simulated NOx production from lightning have increased NO2 predictions, but the predictions in the upper troposphere remain biased low. The upper troposphere has low temperatures (T < 250 K) that increase the uncertainty of many important chemical reaction rates. This study constrains uncertain reaction rates by combining model predictions with measurements from the Intercontinental Chemical Transport Experiment-North America observational campaign. The results show that the nitric acid formation rate, which is the dominant sink of NO2 and radicals, is currently over-estimated by 22% in the upper troposphere. The results from this study suggest that the temperature sensitivity of nitric acid formation is lower than currently recommended. Since the formation of nitric acid removes nitrogen dioxide and radicals that drive the production of ozone, the revised reaction rate will affect ozone concentrations in upper troposphere impacting climate and air quality in the lower troposphere.

AIRS views transport from 12 to 22 July 2004 Alaskan/Canadian fires: Correlation of AIRS CO and MODIS AOD with forward trajectories and comparison of AIRS CO retrievals with DC‐8 in situ measurements during INTEX‐A/ICARTT

We present observations of transport of tropospheric carbon monoxide (CO) obtained by the Atmospheric Infrared Sounder (AIRS) on board NASA's Aqua satellite during the Intercontinental Chemical Transport Experiment–North America (INTEX‐A) field campaign in the summer of 2004, part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). In situ measurements from NASA's DC‐8 provide crucial assessment of AIRS midtropospheric (400–500 mbar) CO retrievals. For the nine best INTEX‐A profiles, convolution of the in situ profiles with AIRS verticality functions demonstrates version 4 AIRS CO retrievals between 400 and 500 mbar are biased high by approximately 8% with a standard deviation slightly less than 5%. The 400–500 mbar region is the only portion of the version 4 AIRS CO retrievals that can be validated as presented here. In some cases, AIRS CO retrievals may be sensitive to CO in the lower to midtroposphere (800–500 mbar). Focusing on one major episode, we investigate transport of CO from a large fire outbreak in the Alaskan/Canadian Yukon region from 11 to 14 July 2004 and follow it downwind to the southeastern United States and Europe by 22 July 2004. Comparison of AIRS CO maps and forward trajectories from fire locations reveals substantial variations in fire emissions especially emission injection height. Any useful forecast model must control for such fire emission variabilities to predict correctly the downwind impact. To match the forward trajectory analyses with AIRS CO observations requires some fires to have directly injected emissions to at least 500 mbar and perhaps as high as 300 mbar. Ground‐based lidar observations show smoke plume altitudes from 3 to 11 km over Wisconsin and from 1 to 4 km over Maryland in agreement with the forward trajectories. The Wisconsin lidar profiles on the afternoon of 18 July 2004 constrain the altitude of CO‐rich smoke observed by AIRS and MODIS to lie between 2 and 5.5 km above the surface, roughly 800 to 500 mbar. We find that changes in the correlation between AIRS CO and MODIS AOD reflect changes in the CO vertical distribution during this event. This finding is confirmed by in situ measurements, meteorological analyses, and forward trajectory analyses.

Long‐range convective ozone transport during INTEX

Ozone transport across the tropopause near a convective region in the central Pacific is calculated for a 1‐day period using a nested grid, high‐resolution simulation with the University of Wisconsin Nonhydrostatic Modeling System (UWNMS). This ozone is then tracked into the region where DC‐8 lidar ozone observations on 6 July 2004 were taken over the United States during the Intercontinental Chemical Transport Experiment‐North America (INTEX‐NA). On 2 July a convective complex developed north of the Midway Islands and intensified as it propagated eastward. Satellite observations captured this event well, and the UWNMS simulation placed the convection just east of an upper level trough along the subtropical jet. The proximity of the strong convection to the lowered tropopause resulted in significant stratospheric ozone contribution (SOC) to the troposphere. During the 24 h period starting at 1200 UTC 2 July, total net ozone flux into the troposphere was calculated to be about 0.2 Tg across the nested grid domain. Because the convective outflow was feeding the subtropical jet, the SOC was quickly transported across the eastern Pacific and into the INTEX‐NA region. Using an “SOC tracer” we show that a small but significant percentage of observed ozone on the DC8 flight can be attributed to SOC from this one convective event. This supports the idea that the stratosphere over the Pacific is an important source of ozone for air entering North America from the west.

Carbonyl sulfide (OCS): Large‐scale distributions over North America during INTEX‐NA and relationship to CO2

An extensive set of carbonyl sulfide (OCS) observations were made as part of the NASA Intercontinental Chemical Transport Experiment–North America (INTEX‐NA) study, flown from 1 July to 14 August 2004 mostly over the eastern United States and Canada. These data show that summertime OCS mixing ratios at low altitude were dominated by surface drawdown and were highly correlated with CO2. Although local plumes were observed on some low‐altitude flight legs, anthropogenic OCS sources were small compared to this sink. These INTEX‐NA observations were in marked contrast to the early springtime 2001 Transport and Chemical Evolution over the Pacific experiment, which sampled Asian outflow dominated by anthropogenic OCS emissions. To test the gridded OCS fluxes used in past models, the INTEX‐NA observations were combined with the sulfur transport Eulerian model (STEM) regional atmospheric chemistry model for a top‐down assessment of bottom‐up OCS surface fluxes for North America. Initial STEM results suggest that the modeled fluxes underestimate the OCS plant sink by more than 200%.

Continental outflow from the US to the upper troposphere over the North Atlantic during the NASA INTEX-NA Airborne Campaign

Abstract. A case of continental outflow from the United States (US) was examined using airborne measurements from NASA DC-8 flight 13 during the Intercontinental Chemical Transport Experiment – North America (INTEX-NA). Mixing ratios of methane (CH4) and carbon monoxide (CO) at 8–11 km altitude over the North Atlantic were elevated to 1843 ppbv and 134 ppbv respectively, while those of carbon dioxide (CO2) and carbonyl sulfide (COS) were reduced to 372.4 ppmv and 411 pptv respectively. In this region, urban and industrial influences were evidenced by elevated mixing ratios and good linear relationships between urban and industrial tracers compared to North Atlantic background air. Moreover, low mixing ratios and a good correlation between COS and CO2 showed a fingerprint of terrestrial uptake and minimal dilution during rapid transport over a 1–2 day time period. Analysis of synoptic conditions, backward trajectories, and photochemical aging estimates based on C3H8/C2H6 strongly suggested that elevated anthropogenic tracers in the upper troposphere of the flight region were the result of transport via convection and warm conveyor belt (WCB) uplifting of boundary layer air over the southeastern US. This mechanism is supported by the similar slope values of linear correlations between long-lived (months) anthropogenic tracers (e.g., C2Cl4 and CHCl3) from the flight region and the planetary boundary layer in the southeastern US. In addition, the aircraft measurements suggest that outflow from the US augmented the entire tropospheric column at mid-latitudes over the North Atlantic. Overall, the flight 13 data demonstrate a pervasive impact of US anthropogenic emissions on the troposphere over the North Atlantic.

Characteristics of the atmospheric CO2 signal as observed over the conterminous United States during INTEX‐NA

High resolution in situ measurements of atmospheric CO2 were made from the NASA DC‐8 aircraft during the Intercontinental Chemical Transport Experiment–North America (INTEX‐NA) campaign, part of the wider International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). During the summer of 2004, eighteen flights comprising 160 h of measurements were conducted within a region bounded by 27 to 53°N and 36 to 139°W over an altitude range of 0.15 to 12 km. These large‐scale surveys provided the opportunity to examine the characteristics of the atmospheric CO2 signal over sparsely sampled areas of North America and adjacent ocean basins. The observations showed a high degree of variability (≤18%) due to the myriad source and sink processes influencing the air masses intercepted over the INTEX‐NA sampling domain. Surface fluxes had strong effects on continental scale concentration gradients. Clear signatures of CO2 uptake were seen east of the Mississippi River, notably a persistent CO2 deficit in the lowest 2–3 km. When combining the airborne CO2 measurements with LANDSAT and MODIS data products, the lowest CO2 mixing ratios observed during the campaign (337 ppm) were tied to mid‐continental agricultural fields planted in corn and soybeans. We used simultaneous measurements of CO, O3, C2Cl4, C2H6, C2H2 and other unique chemical tracers to differentiate air mass types. Coupling these distinct air mass chemical signatures with transport history permitted identification of convection, stratosphere‐troposphere exchange, long‐range transport from Eastern Asia, boreal wildfires, and continental outflow as competing processes at multiple scales influencing the observed concentrations. Our results suggest these are important factors contributing to the large‐scale distribution in CO2 mixing ratios thus these observations offer new constraints in the computation of the North American carbon budget.

NAST-I tropospheric CO retrieval validation during INTEX-NA and EAQUATE

Troposphere carbon monoxide (CO), as well as other trace species retrieved with advanced ultraspectral remote sensors of Earth observing satellites, is critical in air quality observation, modelling, and forecasting. The retrieval algorithm and the accuracy of the parameters retrieved from passive satellite remote sounders must be validated. The Intercontinental Chemical Transport Experiment - North America (INTEXNA) and the European Aqua Thermodynamic Experiment (EAQUATE) provide important validation of satellite observations with ongoing satellite measurement programmes such as Terra, Aura, and Aqua. One of the experimental objectives is to validate chemical species observed from ultraspectral sounders with aircraft in situ measurements, such as the NPOESS Airborne Sounder Testbed-Interferometer (NAST-I). Detailed intercomparisons between aircraft in situ measured and NAST-I retrieved CO profiles were performed to assess the retrieval capability of a passive infrared spectral remote sounder. Validation results illustrate that the CO vertical structure can be obtained by the NAST-I. The thermal radiances are most sensitive to CO emissions from the free troposphere. However, the profile retrieval accuracy depends on the CO uncertainty in the terrestrial boundary layer. It is shown here that the CO distribution in the terrestrial boundary layer over the sea cannot be obtained with reliable accuracy where there is little contrast between the surface air and surface skin temperature. Copyright © 2007 Royal Meteorological Society

Impact of multiscale dynamical processes and mixing on the chemical composition of the upper troposphere and lower stratosphere during the Intercontinental Chemical Transport Experiment–North America

We use high‐frequency in situ observations made from the DC8 to examine fine‐scale tracer structure and correlations observed in the upper troposphere and lower stratosphere during INTEX‐NA. Two flights of the NASA DC‐8 are compared and contrasted. Chemical data from the DC‐8 flight on 18 July show evidence for interleaving and mixing of polluted and stratospheric air masses in the vicinity of the subtropical jet in the upper troposphere, while on 2 August the DC‐8 flew through a polluted upper troposphere and a lowermost stratosphere that showed evidence of an intrusion of polluted air. We compare data from both flights with RAQMS 3‐D global meteorological and chemical model fields to establish dynamical context and to diagnose processes regulating the degree of mixing on each day. We also use trajectory mapping of the model fields to show that filamentary structure due to upstream strain deformation contributes to tracer variability observed in the upper troposphere. An Eulerian measure of strain versus rotation in the large‐scale flow is found useful in predicting filamentary structure in the vicinity of the jet. Higher‐frequency (6–24 km) tracer variability is attributed to buoyancy wave oscillations in the vicinity of the jet, whose turbulent dissipation leads to efficient mixing across tracer gradients.

Chemical data assimilation estimates of continental U.S. ozone and nitrogen budgets during the Intercontinental Chemical Transport Experiment–North America

Global ozone analyses, based on assimilation of stratospheric profile and ozone column measurements, and NOy predictions from the Real‐time Air Quality Modeling System (RAQMS) are used to estimate the ozone and NOy budget over the continental United States during the July–August 2004 Intercontinental Chemical Transport Experiment–North America (INTEX‐A). Comparison with aircraft, satellite, surface, and ozonesonde measurements collected during INTEX‐A show that RAQMS captures the main features of the global and continental U.S. distribution of tropospheric ozone, carbon monoxide, and NOy with reasonable fidelity. Assimilation of stratospheric profile and column ozone measurements is shown to have a positive impact on the RAQMS upper tropospheric/lower stratosphere ozone analyses, particularly during the period when SAGE III limb scattering measurements were available. Eulerian ozone and NOy budgets during INTEX‐A show that the majority of the continental U.S. export occurs in the upper troposphere/lower stratosphere poleward of the tropopause break, a consequence of convergence of tropospheric and stratospheric air in this region. Continental U.S. photochemically produced ozone was found to be a minor component of the total ozone export, which was dominated by stratospheric ozone during INTEX‐A. The unusually low photochemical ozone export is attributed to anomalously cold surface temperatures during the latter half of the INTEX‐A mission, which resulted in net ozone loss during the first 2 weeks of August. Eulerian NOy budgets are shown to be very consistent with previously published estimates. The NOy export efficiency was estimated to be 24%, with NOx + PAN accounting for 54% of the total NOy export during INTEX‐A.

A comparison of satellite tropospheric carbon monoxide measurements from AIRS and MOPITT during INTEX‐A

Satellite CO measurements from Measurements of Pollution in the Troposphere (MOPITT) and Atmospheric Infrared Sounder (AIRS) were used in the Intercontinental Chemical Transport Experiment–North America (INTEX‐A) by the flight planning team to monitor local emissions and the transport of polluted air masses. Because simultaneous measurements of tropospheric CO from both AIRS and MOPITT were used by different investigators during this experiment, a cross reference and comparison are necessary to understand these two data sets and their impacts to the scientific conclusions developed from them. The global CO mixing ratios at 500 mbar, as well as the CO total column amount, are compared between the two instruments for both direct comparison and the comparison using the same a priori profile for the period from 15 June to 14 August 2004. Also presented are the comparisons of the remotely sensed profiles by AIRS, MOPITT, and the in situ profiles collected by the DACOM. In summary, both sensors agree very well on the horizontal distributions of CO represented by the high correlation coefficients (0.7–0.98), and they agree on the CO concentrations to within an average of 10–15 ppbv. Over land, the CO variability is higher, and the correlations between the two data sets are relatively lower than over ocean; however, there is no evidence of a systematic bias. Over the oceans where the CO concentration is smaller in the lower atmosphere, AIRS‐MOPITT show a positive bias of 15–20 ppbv and the details are presented.

Source characteristics of oxygenated volatile organic compounds and hydrogen cyanide

Airborne trace gas measurements from Transport and Chemical Evolution over the Pacific (TRACE‐P), Pacific Exploratory Mission (PEM)‐Tropics B, and Intercontinental Chemical Transport Experiment‐North America (INTEX‐NA) experiments are analyzed to examine the major source factors contributing to the observed variabilities of oxygenated volatile organic compounds and cyanides. The positive matrix factorization method is applied to coincident measurements of 11 chemicals including CH3OH, CH3COCH3, CH3CHO, C2H2, C2H6, i‐C5H12, CO, CH3Cl, and CHBr3. Measurements of HCN and CH3CN are available for TRACE‐P and INTEX‐NA. We identify major source contributions from the terrestrial biosphere, biomass burning, industry/urban regions, and oceans. Spatial and back trajectory characteristics of these factors are examined. On the basis of TRACE‐P and PEM‐Tropics B data, we find a factor that explains 80–88% of the CH3OH variability, 20–40% of CH3COCH3, 7–35% of CH3CHO, and 41% of HCN, most likely representing the emissions from terrestrial biosphere. Our analysis suggested that biogenic emissions of HCN may be significant. Cyanogenesis in plants is likely a major emission process for HCN, which was not fully accounted for previously. Larger contributions than previous global estimations to CH3COCH3 and CH3CHO by biomass burning and industry/urban sources likely reflect significant secondary production from volatile organic compound oxidation. No evidence was found for large emissions of CH3COCH3 from the ocean. The oceanic CH3CHO contribution implies large regional variations.

Meteorological conditions and anomalies during the Intercontinental Chemical Transport Experiment–North America

Meteorological conditions are described during the Intercontinental Chemical Transport Experiment–North America (INTEX‐NA) that was conducted over the United States during July and August 2004. Relatively zonal flow dominated the contiguous United States during the first 2 weeks of the mission, while a series of large amplitude troughs traversed the eastern half of the country during the final 4 weeks. These troughs were accompanied by cold fronts reaching the Gulf of Mexico, an uncommon occurrence during August. Frontal passages over the northeast were somewhat above average, but the short time interval between passages precluded the formation of stagnant high‐pressure centers containing abundant pollution. Atmospheric chemistry during INTEX‐NA was heavily influenced by record‐breaking fires over Alaska and western Canada. Persistent high pressure over Alaska provided ideal conditions for the wildfires and for transporting their burning by‐products southeastward toward the INTEX domain where they were sampled frequently by INTEX aircraft. Forward trajectories and satellite imagery showed that the plumes later were carried to parts of Europe, Africa, and even the Arctic. Deep convection and lightning were important factors during INTEX‐NA. Cloud‐to‐ground (CG) lightning data show that horizontal patterns and numbers of lightning flashes during INTEX‐NA are similar to those of 2003 and 2005. Statistics derived from 10 day backward trajectories indicate that the DC‐8 often sampled lightning influenced air.

Measurement of HO2NO2 in the free troposphere during the Intercontinental Chemical Transport Experiment–North America 2004

The first direct in situ measurements of HO2NO2 in the upper troposphere were performed from the NASA DC‐8 during the Intercontinental Chemical Transport Experiment–North America 2004 with a chemical ionization mass spectrometer (CIMS). These measurements provide an independent diagnostic of HOx chemistry in the free troposphere and complement direct observations of HOx, because of the dual dependency of HO2NO2 on HOx and NOx. On average, the highest HO2NO2 mixing ratio of 76 pptv (median = 77 pptv, σ = 39 pptv) was observed at altitudes of 8–9 km. Simple steady state calculations of HO2NO2, constrained by measurements of HOx, NOx, and J values, are in good agreement (slope = 0.90, R2 = 0.60, and z = 5.5–7.5 km) with measurements in the midtroposphere where thermal decomposition is the major loss process. Above 8 km the calculated steady state HO2NO2 is in poor agreement with observed values (R2 = 0.20) and is typically larger by a factor of 2.4. Conversely, steady state calculations using model‐derived HOx show reasonable agreement with the observed HO2NO2 in both the midtroposphere (slope = 0.96, intercept = 7.0, and R2 = 0.63) and upper troposphere (slope = 0.80, intercept = 32.2, and R2 = 0.58). These results indicate that observed HO2 and HO2NO2 are in poor agreement in the upper troposphere but that HO2NO2 levels are consistent with current photochemical theory.

Carbon dioxide column abundances at the Wisconsin Tall Tower site

We have developed an automated observatory for measuring atmospheric column abundances of CO2 and O2 using near‐infrared spectra of the Sun obtained with a high spectral resolution Fourier Transform Spectrometer (FTS). This is the first dedicated laboratory in a new network of ground‐based observatories named the Total Carbon Column Observing Network. This network will be used for carbon cycle studies and validation of spaceborne column measurements of greenhouse gases. The observatory was assembled in Pasadena, California, and then permanently deployed to northern Wisconsin during May 2004. It is located in the heavily forested Chequamegon National Forest at the WLEF Tall Tower site, 12 km east of Park Falls, Wisconsin. Under clear sky conditions, ∼0.1% measurement precision is demonstrated for the retrieved column CO2 abundances. During the Intercontinental Chemical Transport Experiment–North America and CO2 Boundary Layer Regional Airborne Experiment campaigns in summer 2004, the DC‐8 and King Air aircraft recorded eight in situ CO2 profiles over the WLEF site. Comparison of the integrated aircraft profiles and CO2 column abundances shows a small bias (∼2%) but an excellent correlation.

Airborne measurements of spectral direct aerosol radiative forcing in the Intercontinental chemical Transport Experiment/Intercontinental Transport and Chemical Transformation of anthropogenic pollution, 2004

As part of the INTEX‐NA (Intercontinental chemical Transport Experiment–North America) and ITCT (Intercontinental Transport and Chemical Transformation of anthropogenic pollution) field studies, the NASA Ames 14‐channel Airborne Tracking Sunphotometer (AATS‐14) and a pair of Solar Spectral Flux Radiometers (SSFR) took measurements from aboard a Sky Research Jet stream 31 (J31) aircraft during 19 science flights over the Gulf of Maine during 12 July to 8 August 2004. The combination of coincident AATS‐14 and SSFR measurements yields plots of net (downwelling minus upwelling) spectral irradiance as a function of aerosol optical depth (AOD) as measured along horizontal flight legs. By definition, the slope of these plots yields the instantaneous change in net irradiance per unit AOD change and is referred to as the instantaneous spectral aerosol radiative forcing efficiency, Ei (W m−2 nm−1). Numerical integration over a given spectral range yields the instantaneous broadband aerosol radiative forcing efficiency (W m−2). This technique for deriving Ei is called the aerosol gradient method. Within 10 case studies considered suitable for our analysis we found a high variability in the derived instantaneous aerosol forcing efficiencies for the visible wavelength range (350–700 nm), with a mean of −79.6 W m−2 and a standard deviation of 21.8 W m−2 (27%). An analytical conversion of the instantaneous forcing efficiencies to 24‐hour‐average values yielded −45.8 ± 13.1 W m−2 (mean ± std). We present spectrally resolved aerosol forcing efficiencies between 350 and 1670 nm, estimates of the midvisible aerosol single scattering albedo and a comparison of observed broadband forcing efficiencies to previously reported values.