The Stratospheric/Tropospheric Measurements (S/TM) group (Elliot Atlas, Frank Flocke, Sue Schauffler, Stephen Donnelly, Verity Stroud, and Kristen Johnson) investigates the sources, budgets, distribution and variations of atmospheric trace gases, with particular emphasis on those species related to the ozone formation and destruction processes in both the troposphere and the stratosphere. An integral part of the program is to evaluate and develop state-of-the-art sampling and analytical facilities for trace gas measurement from different environments.
During this past year, S/TM participated in the TRAnsport and Chemical Evolution over the Pacific (TRACE-P) field campaign in collaboration with Donald Blake (University of California, Irvine). In a collaboration with William Sturges (University of East Anglia, United Kingdom) and Jakob Schwander (University of Berne, Switzerland), the group also analyzed firn air samples collected in Greenland, as well as samples collected along the Trans-Siberia Railway by Eva Oberlander (Max Planck Institute, Germany). In addition to the analytical work, ST/M continued with the evaluation of measurements from several previous field campaigns that were designed to study trace gas distributions and chemistry in the Arctic troposphere and lower stratosphere during SAGE III Ozone Loss and Validation Experiment (SOLVE) and the Tropospheric Ozone Production about the Spring Equinox (TOPSE) field campaign, and in a polluted urban region during the Texas Air Quality Study (TexAQS) 2000.
Transport and Chemistry Evolution over the Pacific (TRACE-P)
The NASA TRACE-P experiment was conducted February to April, 2001. The campaign focused on the influence of Asian pollution outflow on the tropospheric chemistry over the central and western Pacific Ocean. The campaign included 20 flights, covering the Western Pacific from about 10N to 40N latitude. Two aircraft were deployed, the NASA DC-8 and the NASA P-3. S/TM contributed two projects to the TRACE-P mission. These were trace gas analyses from whole air sample and in-situ analysis of PANs.
Whole air samples were collected by Blake’s group. All samples were analyzed at the University of California, Irvine, and a subset was analyzed at NCAR for a variety of gases, including alkyl nitrates, halocarbons, and hydrochlorofluorocabons (HCFCs). Flocke and Andrew Weinheimer (ACD-AON) deployed an in-situ instrument on the P-3 that measured PAN, PPN, PiBN, APAN and MPAN.
Flights of the P-3 aircraft originated from Wallops Island, Virginia, went through NASA Dryden, California, Hawaii, Wake Island, Guam, and Midway. Major bases were Hong Kong, China, and Yokota Air Base, Japan. Pollution plumes were sampled over the western Pacific. Particularly intense pollution was sampled over the Yellow Sea, the South China Sea, the Sea of Japan, and the Pacific Ocean off the East Coast of Japan. PAN/PPN and PiBN mixing ratios suggest anthropogenic pollution as the main contributor. Data is currently being finalized for inclusion in the data archive in December. Preliminary analysis of PAN altitude profiles show an increase with altitude in Central Pacific and a decrease with altitude in Western Pacific due to pollution outflow. Mixing ratios at 6-8 km are comparable in the two regions. Comparisons with CO and long-lived NMHC are currently being evaluated to estimate the NOx source from PAN decomposition during subsidence. Ozone/PAN relationships in pollution plumes are being analyzed as well.
The measurements from the whole air sampler are being used to characterize trace gas signatures from different source regions in Eastern Asia and further downstream. Initial analyses indicate that there may be significant differences in trace gas signatures from emissions from Japan compared to those from mainland China and other regions. For example, high levels of vinyl chloride and selected chloroalkanes in samples near Japan reflect emissions from specific chemical production processes. Also, high levels of HCFCs were routinely observed in the lower altitudes near Yakota Air Base (Tokyo region). High levels of sulfur species, e.g. carbonyl sulfide, relative to carbon monoxide in the TRACE-P study region may be related to widespread coal burning, though other sources may also contribute. Alkyl nitrates were found to correlate well with emissions of parent hydrocarbons, and nitrate/hydrocarbon ratios may be used to examine air mass evolution in the TRACE-P region. Interestingly, methyl nitrate distributions showed both a marine signature (as observed in previous studies of the equatorial Pacific Ocean), and an anthropogenic source. The marine signature appeared in the upper troposphere, apparently as a result of convective transport from tropical regions to higher latitudes, though more detailed studies are underway to confirm this hypothesis. Studies are just beginning to examine the trace gas distributions in relation to specific air mass origins, meteorological factors, and all the related trace gas and aerosol measurements obtained during the study.
Firn Air Analysis
Firn air samples were collected at the NGRIP site at core depths from the surface to near 78 m by Schwander. Initial analyses of trace gases in the firn samples indicate that air dated to approximately 1960 or slightly older was sampled. More accurate dating will be determined based on modeled diffusion analyses in collaboration with Sturges. Samples from NGRIP show similar trace gas trends to those observed in another Northern Hemisphere firn core (Devon Island, Canada) analyzed in our laboratory. CFCs and HCFCs reflect the expected changes from known emissions and long-term monitoring networks and archives. NMHC and selected halocarbon solvents appear to have gone through a maximum in concentration approximately 20 years ago, and concentrations in the firn air show a slow decline that likely reflects emission controls and decreased emissions. Interestingly, as in the Devon Island core, anomalous increases in some trace gases (notably methyl nitrate and methyl halides) were observed with increasing depth in the core. The causes of this increase have not been determined.
Texas Air Quality Study (TexAQS) 2000
The TexAQS 2000 field campaign, Aug/Sept. 2000, focused on obtaining an improved understanding of the processes that control the formation and distribution of fine particles and ozone in the Houston and southeast Texas areas. In particular, the study examined the relative importance of various local sources as well as local versus transported emissions on local and regional ozone chemistry. TexAQS 2000 addressed these issues through a series of coordinated measurements involving instrumented aircraft and a ground-based network of chemistry and meteorological measurements. The S/TM whole air sampler designed and built to fly on the NOAA WP-3D during the 1999 Southern Oxidant Study (SOS) campaign in Tennessee was modified and flown on the NCAR Electra. The PAN, PPN, MPAN instrument was also deployed on board the NCAR Electra.
The whole air sampler was utilized during TexAQS 2000 to investigate specific plumes from industrial and power plant sources, and to characterize the trace gas composition in these plumes and in the surrounding area. A large variety of hydrocarbons were quantified in the samples, in addition to halocarbons and organic nitrates. Samples were also examined with full-scan GC-MS instrumentation to identify other compounds that were not included in the list of target compounds analyzed. The measurements revealed large amounts of reactive hydrocarbons (especially ethene and propene) in the plumes from specific industrial sources. High levels of alkanes were also seen, though these compounds contributed less to the photochemical reactivity of the plume compared to the alkenes and their carbonyl oxidation products. The high levels of alkenes in specific plumes represented the highest mixing ratios of any trace gas measured from the whole air sampler. However, on a mean or median scale, the light alkanes and carbonyls were the most abundant gases measured.
Several PAN-type compounds were measured: PAN, APAN, PPN, PiBN, and MPAN. These organic nitrates are co-produced with ozone. Their mixing ratios, and how they correlate with each other and with ozone, have implications for the hydrocarbon precursors to ozone formation. For example, high ozone during the TexAQS project was found to be co-located with high levels of anthropogenic PANs (PAN, PPN, APAN, PiBN). On the other hand, the very high ozone was not co-located with high MPAN, which is biogenic. Thus it can be concluded that biogenic hydrocarbons such as isoprene do not contribute significantly to high ozone events in Houston. Additionally, based on PAN-PPN, ozone-PAN, and ozone-PPN correlations in the Greater Houston area, the mix of hydrocarbons contributing to ozone formation in Houston does not appear atypical in an overall sense (though the case of APAN is unique). Moreover, the same appears to be true of the very high ozone events as well, at least to the extent that these have been examined on a case-by-case basis. Further work along these lines will be pursued. In at least some cases, PAN samples corresponding to high ozone values (~200 ppbv) show PAN/ozone ratios and PAN/PPN ratios that are consistent with data from other locales (based on data obtained in these projects: SOS, TOPSE, TRACE-P). There are cases where PAN/PPN ratios are anomalous, but these occur generally when ozone is not extreme.
This PAN perspective, which is "business-as-usual" in many respects, appears to conflict with the proposal that a unique hydrocarbon mix, dominated by ethene and propene, is responsible for ozone formation in Houston in the high-ozone events. A dominance by ethene and propene should lead to unusual correlations of the PAN-type compounds with each other and with ozone, because ethene oxidation results in the formation of none of the PAN-type compounds measured, while the oxidation of propene leads to the formation of PAN but not PPN. The observations of large amounts of ethene and propene downwind of petrochemical industrial sources in the Houston area no doubt suggest that these compounds play a significant role as ozone precursors. Furthermore, this is corroborated by the high rate of ozone formation that is found in these cases, the high efficiency of ozone production that is inferred, and by the large amounts of formaldehyde that are observed. Thus we have a dilemma. There is a strong case for the importance of ethene and propene, yet this appears not to be consistent with the perspective portrayed by the PAN-type compounds. A resolution to this dilemma will be pursued by a careful examination of the data on a case-by-case basis, and by consideration of the effects of mixing and loss processes on the correlations of the PAN-type compounds among themselves and with ozone.
SAGE III Ozone Loss and Validation Experiment (SOLVE)SOLVE was a NASA-sponsored measurement campaign designed to examine the processes controlling ozone levels at mid to high latitudes in the northern hemisphere. Measurements were made in the Arctic high-latitude region in winter using the NASA DC-8 and ER-2 aircraft, as well as balloon platforms and ground-based instruments. The flights were based out of Kiruna, Sweden. The ST/M whole air sampler (WAS) flew aboard the ER-2, and, for this campaign, the redesigned instrument was located in the centerline drop tank.
An initial manuscript describing SOLVE results (Schauffler, et al., J. Geophys. Res., submitted) was focused on chlorine. The manuscript includes quantitative evaluation of correlations between organic and inorganic chlorine from each compound and N2O as well as between total organic and inorganic chlorine and N2O. The manuscript also includes calculations of the fractional chlorine release (FC) from each compound, which is the amount of chlorine released relative to the entry-mixing ratio for each sample. The entry-mixing ratio was determined by the age of the sample and the tropospheric trend, if any, of each compound. FC values for each compound relative to the FC value of CFC-11 are used in calculations of the amount of chlorine available for involvement in current and future stratospheric ozone loss. The FC calculations in the manuscript represent updates of previously published FC values as well as FC values for additional compounds not previously reported. Finally, the manuscript includes a unique method of examining stratospheric lifetimes. In this method, organic and inorganic mixing ratios of a given compound were plotted versus N2O, and correlation curves for each were generated. The N2O value where the correlation curves cross, i.e., the point where the correlation curves were equivalent, was then plotted versus the stratospheric lifetime of the given compound. The resulting curve generated using the N2O crossing points and stratospheric lifetimes of several compounds was then used to evaluate previously published stratospheric lifetimes for CH3CCl3, CH4, and HCFC 141b and for lifetimes of additional compounds not previously reported.
Work is continuing on analysis of stratospheric trends of HCFCs and HFCs by comparing SOLVE results to previous measurements in the stratosphere. The HCFCs and HFCs (hydrofluorocarbons) show rapid growth in the stratosphere and make significant contributions to the organic chlorine and fluorine budgets. For these compounds, the growth in the stratosphere is maintained at a significant fraction of their rate of growth in the troposphere. The mixing ratios and distributions of PFCs (perfluorocarbons) in the lower stratosphere are also being examined. PFC contribution to the stratospheric fluorine budget will increase as the CFCs decrease.