James W. Elkins

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

[1]xa0Sixteen 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.

Tropospheric SF6: Observed latitudinal distribution and trends, derived emissions and interhemispheric exchange time

Sulfur hexafluoride (SF6), an anthropogentically produced compound that is a potent greenhouse gas, has been measured in a number of NOAA GMDL air sampling programs. These include high resolution latitudinal profiles over the Atlantic and Pacific oceans, weekly flask samples from seven remote, globally distributed sites, hourly in situ measurements in rural North Carolina, and a series of archived air samples from Niwot Ridge, Colorado. The observed increase in atmospheric mixing ratio is consistent with an overall quadratic growth rate, at 6.9±0.2% yr−1 (0.24±0.01 ppt yr−1) for early 1996. From these data we derive an early 1996 emission rate of 5.9±0.2 Gg SF6 yr−1 and an interhemispheric exchange time of 1.3±0.1 years.

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.

Chlorine chemistry on polar stratospheric cloud particles in the Arctic winter

Simultaneous in situ measurements of hydrochloric acid (HCl) and chlorine monoxide (ClO) in the Arctic winter vortex showed large HCl losses, of up to 1 part per billion by volume (ppbv), which were correlated with high ClO levels of up to 1.4 ppbv. Air parcel trajectory analysis identified that this conversion of inorganic chlorine occurred at air temperatures of less than 196 � 4 kelvin. High ClO was always accompanied by loss of HCI mixing ratios equal to �(ClO + 2Cl2O2). These data indicate that the heterogeneous reaction HCl + ClONO2 → Cl2 + HNO3 on particles of polar stratospheric clouds establishes the chlorine partitioning, which, contrary to earlier notions, begins with an excess of ClONO2, not HCl.

Chemical loss of ozone in the arctic polar vortex in the winter of 1991-1992.

In situ measurements of chlorine monoxide, bromine monoxide, and ozone are extrapolated globally, with the use of meteorological tracers, to infer the loss rates for ozone in the Arctic lower stratosphere during the Airborne Arctic Stratospheric Expedition II (AASE II) in the winter of 1991-1992. The analysis indicates removal of 15 to 20 percent of ambient ozone because of elevated concentrations of chlorine monoxide and bromine monoxide. Observations during AASE II define rates of removal of chlorine monoxide attributable to reaction with nitrogen dioxide (produced by photolysis of nitric acid) and to production of hydrochloric acid. Ozone loss ceased in March as concentrations of chlorine monoxide declined. Ozone losses could approach 50 percent if regeneration of nitrogen dioxide were inhibited by irreversible removal of nitrogen oxides (denitrification), as presently observed in the Antarctic, or without denitrification if inorganic chlorine concentrations were to double.

Global tropospheric distribution and calibration scale of HCFC-22

Measurements of atmospheric chlorodifluoro-methane (HCFC-22), based upon a new calibration scale developed in this laboratory, suggest a global tropospheric mean that is ∼28% lower than determined previously from surface-based measurements. A global mean of 101.8 (±1.2, 1 s.d.) ppt and interhemispheric difference of 13 (±1) ppt were determined for HCFC-22 in 1992 from air collected in flasks from seven remote sites located between 82° N and 90° S. These results are consistent with mixing ratios predicted from recent emission estimates and a lifetime for HCFC-22 of 13.6 (+1.9, −1.5) yr. Based upon the analysis of flasks and archived air samples from mid-1987 through 1992, a mean growth rate for HCFC-22 of 7.3 (±0.3)% yr−1 was estimated for this period.

Chlorine budget and partitioning during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE)

[1]xa0The amount of chlorine in the stratosphere has a direct influence on the magnitude of chlorine-catalyzed ozone loss. A comprehensive suite of organic source gases of chlorine in the stratosphere was measured during the NASA Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) campaign in the arctic winter of 2000. Measurements included chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halon 1211, solvents, methyl chloride, N2O, and CH4. Inorganic chlorine contributions from each compound were calculated using the organic chlorine measurements, mean age of air, tropospheric trends, and a method to account for mixing in the stratosphere. Total organic chlorine measured at tropospheric levels of N2O was on the order of 3500 ppt. Total calculated inorganic chlorine at a N2O mixing ratio of 50 ppb (corresponding to a mean age of 5.5 years) was on the order of 3400 ppt. CFCs were the largest contributors to total organic chlorine (55–70%) over the measured N2O range (50–315 ppb), followed by CH3Cl (15%), solvents (5–20%), and HCFCs (5–25%). CH3Cl contribution was consistently about 15% across the organic chlorine range. Contributions to total calculated inorganic chlorine at 50 ppb N2O were 58% from CFCs, 24% from solvents, 16% from CH3Cl, and 2% from HCFCs. Updates to fractional chlorine release values for each compound relative to CFC 11 were calculated from the SOLVE measurements. An average value of 0.58 was calculated for the fractional chlorine release of CFC 11 over the 3–4 year mean age range, which was lower than the previous value of 0.80. The fractional chlorine release values for HCFCs 141b and 142b relative to CFC 11 were significantly lower than previous calculations.

Severe chemical ozone loss inside the Arctic Polar Vortex during winter 1999–2000 Inferred from in situ airborne measurements

Lower stratospheric in situ observations are used to quantify both the accumulated ozone loss and the ozone chemical loss rates in the Arctic polar vortex during the 1999–2000 winter. Multiple long-lived trace gas correlations are used to identify parcels in the inner Arctic vortex whose chemical loss rates are unaffected by extra-vortex intrusions. Ozone-tracer correlations are then used to calculate ozone chemical loss rates. During the late winter the ozone chemical loss rate is found to be −46±6 (1σ) ppbv/day. By mid-March 2000, the accumulated ozone chemical loss is 58±4% in the lower stratosphere near 450 K potential temperature (∼19 km altitude).

The distribution of hydrogen, nitrogen, and chlorine radicals in the lower stratosphere: Implications for changes in O3 due to emission of NOy from supersonic aircraft

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained in the lower stratosphere during SPADE are compared to results from a photochemical model that assimilates measurements of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N_2O_5 agree well with measured concentrations of NO and ClO, but concentrations of HO_2 and OH are underestimated by 10 to 25%, concentrations of NO_2 are overestimated by 10 to 30%, and concentrations of HCl are overestimated by a factor of 2. Discrepancies for [OH] and [HO_2] are reduced if we allow for higher yields of O(^1D) from O_3 photolysis and for heterogeneous production of HNO_2. The data suggest more efficient catalytic removal of O_3 by hydrogen and halogen radicals relative to nitrogen oxide radicals than predicted by models using recommended rates and cross sections. Increases in [O_3] in the lower stratosphere may be larger in response to inputs of NO_y from supersonic aircraft than estimated by current assessment models.

The diurnal variation of hydrogen, nitrogen, and chlorine radicals: Implications for the heterogeneous production of HNO2

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower stratosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N205 on sulfate aerosols agree with measured concentrations of NO, NO2, and C10 throughout the day, but fail to account for high concentrations of OH and HO2 observed near sunrise and sunset. The morning burst of (OH) and (HO2) coincides with the rise of (NO) from photolysis of NO 2, suggesting a new source of HOx that photolyzes in the near UV (350 to 400 nm) spectral region. A model that allows for the heterogeneous production of HNO2 results in an excellent simulation of the diurnal variations of (OH) and (HO2).