Paul C. Novelli

Distributions and recent changes of carbon monoxide in the lower troposphere

Since 1988, the distribution of carbon monoxide (CO) in the lower troposphere has been determined using a globally distributed air sampling network. Site locations range from 82°N to 90°S, with wide longitudinal coverage, and represent the marine boundary layer, regionally polluted atmospheres, and the free troposphere. These measurements present a unique, intercalibrated, and internally consistent data set that are used to better define the global temporal and spatial distribution of CO. In this paper, times series from 49 sites are discussed. With an average lifetime of ∼2 months, CO showed significant concentration gradients. In the marine boundary layer, mixing ratios were greatest in the northern winter (200-220 ppb) and lowest in the southern summer (35-45 ppb). The interhemispheric gradient showed strong seasonality with a maximum difference between the high latitudes of the northern and southern hemispheres (160-180 ppb) in February and March and a minimum in July and August (10-20 ppb). Higher CO was found in regions near human development relative to those over more remote areas. The distributions provide additional evidence of the widespread pollution of the lower atmosphere. Remote areas in the high northern hemisphere are polluted by anthropogenic activities in the middle latitudes, and those in the southern hemisphere are heavily influenced by the burning of biomass in the tropics. While tropospheric concentrations of CO exhibit periods of increase and decrease, the globally averaged CO mixing ratio over the period from 1990 through 1995 decreased at a rate of approximately 2 ppb yr -1 .

Mixing ratios of carbon monoxide in the troposphere

Carbon monoxide (CO) mixing ratios were measured in air samples collected weekly at eight locations. The air was collected as part of the CMDL/NOAA cooperative flask sampling program (Climate Monitoring and Diagnostics Laboratory, formerly Geophysical Monitoring for Climatic Change, Air Resources Laboratory/National Oceanic and Atmospheric Administration) at Point Barrow, Alaska (71°N), Niwot Ridge, Colorado (40°N), Mauna Loa and Cape Kumakahi, Hawaii (19°N), Guam, Marianas Islands (13°N), Christmas Island (2°N), Ascension Island (8°S) and American Samoa (14°S). Half-liter or 3-L glass flasks fitted with glass piston stopcocks holding teflon O rings were used for sample collection. CO levels were determined within several weeks of collection using gas chromatography followed by mercuric oxide reduction detection, and mixing ratios were referenced against the CMDL/NOAA carbon monoxide standard scale. During the period of study (mid-1988 through December 1990) CO levels were greatest in the high latitudes of the northern hemisphere (mean mixing ratio from January 1989 to December 1990 at Point Barrow was approximately 154 ppb) and decreased towards the south (mean mixing ratio at Samoa over a similar period was 65 ppb). Mixing ratios varied seasonally, the amplitude of the seasonal cycle was greatest in the north and decreased to the south. Carbon monoxide levels were affected by both local and regional scale processes. The difference in CO levels between northern and southern latitudes also varied seasonally. The greatest difference in CO mixing ratios between Barrow and Samoa was observed during the northern winter (about 150 ppb). The smallest difference, 40 ppb, occurred during the austral winter. The annually averaged CO difference between 71°N and 14°S was approximately 90 ppb in both 1989 and 1990; the annually averaged interhemispheric gradient from 71°N to 41°S is estimated as approximately 95 ppb.

Molecular hydrogen in the troposphere: Global distribution and budget

Molecular hydrogen (H2) has been measured since 1989 in air samples collected using a globally distributed sampling network. Time series from 50 locations are used to better define the distribution and recent changes of H2 in the remote lower troposphere. These data show that the globally averaged H2 mixing ratio between 1991 and 1996 was about 531±6 parts per billion (ppb). Hydrogen exhibited well-defined seasonal cycles in each hemisphere, with similar seasonal maxima (530–550 ppb). However, in the Northern Hemisphere the seasonal minimum was 70 ppb deeper than in the Southern Hemisphere (∼450 and 520 ppb, respectively), resulting in ∼3% more H2 in the south than in the north. With these data we have reevaluated the global H2 budget. Methane oxidation is the largest source of H2 to the troposphere, and soil uptake accounts for much of its sink. The global annual turnover is estimated as ∼75 Tg H2 yr−1. The annual turnover, combined with a calculated tropospheric burden of 155 Tg, indicates a lifetime of ∼2 years. While our understanding of the global distribution of the sources and sinks of H2 is still incomplete, the lower annual minimum in the north may be reasonably attributed to hemispheric asymmetry in uptake by soils. The seasonal cycles in the two hemispheres show unusual similarities: the northern and the southern seasonal maxima and minima were offset by only a few months. We suggest that the seasonal cycle in the Southern Hemisphere is dominated by H2 emissions from biomass burning.

Airborne gas chromatograph for in situ measurements of long-lived species in the upper troposphere and lower stratosphere

A new instrument, the Airborne Chromatograph for Atmospheric Trace Species IV (ACATS-IV), for measuring long-lived species in the upper troposphere and lower stratosphere is described. Using an advanced approach to gas chromatography and electron capture detection, the instrument can detect low levels of CFC-11 (CCl 3 F), CFC-12 (CCl 2 F 2 ), CFC-113 (CCl 2 F-CClF 2 ), methyl chloroform (CH 3 CCl 3 ), carbon tetrachloride (CCl 4 ), nitrous oxide N 2 O), sulfur hexafluoride (SF 6 ), Halon-1211 (CBrClF 2 ), hydrogen (H 2 ), and methane (CH 4 ) acquired in ambient samples every 180 or 360 s. The instrument operates fully-automated onboard the NASA ER-2 high-altitude aircraft on flights lasting up to 8 hours or more in duration. Recent measurements include 24 successful flights covering a broad latitude range (70°S-61°N) during the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/ MAESA) campaign in 1994.

A new look at methane and nonmethane hydrocarbon emissions from oil and natural gas operations in the Colorado Denver‐Julesburg Basin

Emissions of methane (CH4) from oil and natural gas (O&G) operations in the most densely drilled area of the Denver-Julesburg Basin in Weld County located in northeastern Colorado are estimated for 2 days in May 2012 using aircraft-based CH4 observations and planetary boundary layer height and ground-based wind profile measurements. Total top-down CH4 emission estimates are 25.8 ± 8.4 and 26.2 ± 10.7 t CH4/h for the 29 and 31 May flights, respectively. Using inventory data, we estimate the total emissions of CH4 from non-O&G gas-related sources at 7.1 ± 1.7 and 6.3 ± 1.0 t CH4/h for these 2 days. The difference in emissions is attributed to O&G sources in the study region, and their total emission is on average 19.3 ± 6.9 t/h, close to 3 times higher than an hourly emission estimate based on Environmental Protection Agencys Greenhouse Gas Reporting Program data for 2012. We derive top-down emissions estimates for propane, n-butane, i-pentane, n-pentane, and benzene from our total top-down CH4 emission estimate and the relative hydrocarbon abundances in aircraft-based discrete air samples. Emissions for these five nonmethane hydrocarbons alone total 25.4 ± 8.2 t/h. Assuming that these emissions are solely originating from O&G-related activities in the study region, our results show that the state inventory for total volatile organic compounds emitted by O&G activities is at least a factor of 2 too low for May 2012. Our top-down emission estimate of benzene emissions from O&G operations is 173 ± 64 kg/h, or 7 times larger than in the state inventory.

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

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.

Changes in CH4 and CO growth rates after the eruption of Mt. Pinatubo and their link with changes in tropical tropospheric UV flux

Trace gas measurements from air samples collected weekly at a globally distributed network of sampling sites revealed sharp increases in the growth rates of CH4 and CO in the tropics and high southern latitudes immediately following the eruption of Mt. Pinatubo on June 15, 1991. The eruption emitted ∼20 Mt SO 2 into the lower stratosphere. Calculations made with a radiative transfer model show that UV actinic flux in the wavelength region 290-330 nm was attenuated by ∼12% immediately after the eruption due to direct absorption by SO 2 , and that it was perturbed for up to I year after the eruption due to scattering by sulfate aerosols. We suggest that the decreased UV flux decreased the steady-state [OH] and led to the observed anomalously large growth rates for CH 4 and CO during late-1991 and early-1992.

Inter‐annual variability of summertime CO concentrations in the Northern Hemisphere explained by boreal forest fires in North America and Russia

Background measurements of Carbon Monoxide in the extra-tropical Northern Hemisphere during the 1990s showed no clear trends, but significant inter-annual variations. In this study, the measured summertime averaged CO concentrations north of 30° N were correlated with area burned by forest fires in North America and Russia. According to a linear regression analysis, 14% of the CO variability in the extra-tropical Northern Hemisphere can be explained by boreal forest area burnt in North America, 53% by area burnt in Russia, and 63% by the combination of both. There are strong indications that the officially reported Russian fire areas are significantly underestimated. By statistically correcting these areas, we can show that, on the average, boreal burning provides a summertime CO source term comparable with all anthropogenic sources in the United States and Europe.

Impact of Asian emissions on the remote North Pacific atmosphere: Interpretation of CO data from Shemya, Guam, Midway and Mauna Loa

In this study we look at the concentration of CO at four remote stations in the North Pacific to evaluate the impact of Asian industrial emissions on the remote atmosphere. Using a locally weighted smoothing technique to identify individual data outliers from the seasonal cycle, we have identified 22–92 outliers or “events” (greater than 5 ppbv above the seasonal cycle) at each site for the 3–6 year data records. Using isentropic back trajectories, we identify a possible source region for each event and present a distribution of the trajectory types. For the events at Midway, Mauna Loa, Guam, and Shemya, we are able to identify a source region for the elevated CO in 82, 72, 65, and 50% of the events, respectively. At Mauna Loa and Midway a majority of the events occur during spring and are usually associated with transport from Asia. These events bring the highest CO mixing ratios observed at any time during the year to these sites, with CO enhancements up to 46 ppb. At Guam, easterly trade winds are the norm, but occasionally synoptic events bring Asian emissions to the island, generally during late summer and fall, from either East Asia or Southeast Asia (e.g., Indonesia). These events bring with them the largest CO enhancements of any of the four sites considered in this paper, up to 58 ppb. Finally, to examine the robustness of our conclusions, we redo our analysis using the more stringent definition that an event must be either 10 or 15 ppb above the seasonal cycle. Although this reduces the number of events identified at each site, it does not significantly change the fraction of events which can be attributed to a known source.

The development and evaluation of a gravimetric reference scale for measurements of atmospheric carbon monoxide

We have prepared a set of 17 carbon monoxide (CO) reference mixtures for use in the calibration of measurements of atmospheric concentrations of this gas. The mixing ratios of these standards range from 25 to 1003 ppb (parts per billion by mole fraction) in zero natural air and are contained in 5.9-L, high-pressure aluminum cylinders. Carbon monoxide was measured using gas chromatography with a mercuric oxide detector. The concentration range of the standards is sufficient to cover that of the background troposphere and also that found in remote locations affected by anthropogenic activities. The low concentration standards were prepared by gravimetric methods using one of three high concentration standards as the parent. Two of the parents were prepared by gravimetric methods starting from high-purity (99.97%) CO to have concentrations of about 250 ppm (parts per million) CO. A total of 14 atmospheric level primary standards were prepared from these two parents. The third parent was a NIST SRM (National Institute of Standards and Technology, Standard Reference Material) having 9.7 ppm CO, from which three standards were prepared. Monitoring of CO levels in the primary standards, relative to natural air contained in 29.5-L high-pressure aluminum cylinders, suggests that the CO content of some primaries may be increasing at rates of between 1 and 2 ppb yr−1. The CO concentration scale defined by the gravimetric standards was used to calibrate a set of 10 secondary standards. The secondary standards are all contained in 29.5-L high-pressure aluminum cylinders and range in concentration from 35 to 200 ppb. Examination of the CO content in several of the oldest secondary standards indicates their CO concentrations have not changed relative to each other over the 2 years they have been studied. Comparison of the low concentration standards derived from the gravimetric parents to those prepared from the NIST SRM show no difference to within 1% between the two scales. We also compared our standards to commercially available, NIST-traceable, CO standards (approximately 0.5 and 1 ppm of CO in air). The concentrations assigned these standards by the manufacturer agreed to within 3% of concentrations we calculated referenced to our standard scale. In addition, we compared our concentration scale to a CO standard used at the Commonwealth Scientific and Industrial Research Organization (CSIRO), Australia. Intercomparison of a cylinder of natural air between our laboratory and CSIRO (which used a CO reference gas traceable to the standards of the Oregon Graduate Institute for Science and Technology, formerly the Oregon Graduate Center) indicated that the CO concentration determined for this air based upon our reference scale was approximately 25% greater than the concentration determined by CSIRO. Carbon monoxide concentrations determined in flask samples collected at Mauna Loa, Hawaii, referenced to this concentration scale, are compared to the earlier reports of CO levels at this location by Seiler et al. [1976] and Khalil and Rasmussen [1988].