Tropospheric Chemistry

Atmospheric Odd Nitrogen

The Atmospheric Odd Nitrogen Group’s (Brian Ridley, Andrew Weinheimer, Denise Montzka, David Knapp, and Frank Grahek) expertise is in measurements and analysis of NO, NO₂, total reactive nitrogen (NO_y), O₃, and PAN and related homologues.  Much of the effort in 2001 was focused upon data analysis and interpretation of results from field programs conducted between 1998 and 2000: the Polar Sunrise Experiment conducted on the ground at Alert (82.5^oN) in spring 1998; the NASA PEM-Tropics B aircraft mission conducted in the spring of 1999; the NASA/NOAA Atmospheric Chemistry of Combustion Emissions Near the Tropopause (ACCENT) WB-57 aircraft experiment conducted from near Houston in the fall of 1999; the ACD-led Tropospheric Ozone Production about the Spring Equinox (TOPSE) C-130 aircraft experiment conducted over latitudes of 40-85^oN from February through May 2000; and the NOAA P3 or NCAR Electra aircraft missions based near Nashville in the summer of 1999 (Southern Oxidant Study [SOS ’99]) and near Houston in the summer of 2000 (TexAQS2000).  In addition Andrew Weinheimer with Frank Flocke (Stratosphere/Troposphere Measurements [STM] group) participated in the recent NASA GTE TRACE-P aircraft project conducted this past spring by making measurements of PAN and related species on the NASA P3 aircraft.  A summary of results from the fieldwork utilizing the PAN instrumentation is given in the STM section.

One of the more interesting products of Arctic and Antarctic surface studies over the past few years has been the discovery that the sunlit snow surface can be a source of reactive species including NO_y constituents.  This finding has been a result of studies made in Michigan, Alert (Canada), Summit (Greenland) and, more recently, at the South Pole during the Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) program (See the Photochemical Oxidation and Products Section). Our measurements made at Alert in 1998 contributed to this finding as was summarized in last year’s report.  With John Orlando (Kinetics Group), a manuscript was completed which describes measurements and modeling of the effect of these emissions from the snow surface during a surface ozone depletion event that occurred during our 1998 study (Ridley and Orlando, J. Atmos. Chem., submitted).  The paper examines the chemistry and effects of the NO_x emissions under conditions of normal (~30-40 ppbv) to very small ozone mixing ratios (~0.5 ppbv). 

With Elliot Atlas (STM Group) and Flocke, a considerable effort was made on examining some of the measurements acquired during the TOPSE program.  One effort focused on examining the observations made while flying at 30 m above the snow/ice surface in the Arctic to probe the location, frequency, composition, and extent of surface ozone depletion events that are known to occur from previous studies at Alert, Barrow Alaska, Ny Ålesund and elsewhere in the Arctic in the spring after polar sunrise.  A number of these ozone depletion events were found during TOPSE.  In particular, for the first time in the sub-Arctic, an ozone depletion event extending over a large fraction of Hudson Bay was found.  The event which persisted for at least four days was examined in detail with the Edward Browell’s lidar instrument group (NASA Langley) and with the in situ instruments on board the aircraft.  Other ozone depletion events were found over Baffin Bay and on every low-altitude flight over the Arctic Ocean.  Surprisingly, analysis of the observations and transport patterns revealed that surface depletion events that occurred to the extreme south of the Arctic Ocean did not form in situ.  Instead, it is reasonably clear that the chemical depletion occurred over the Arctic Ocean, and the depleted air masses were transported intact over large distances (of order 1000 km) to lower latitudes.  Thus, the conditions or properties of the Arctic Ocean surface that actually trigger the surface ozone destruction remain unidentified, although the chemistry causing the depletion seems to be well understood.  The analysis and results from TOPSE were presented at the Spring AGU meeting, and a manuscript was recently submitted (Ridley, et al., J. Geophys. Res., submitted).

Analysis of the partitioning of reactive nitrogen species during the spring season from the TOPSE data was also made.  A major and chemically-important finding was that PAN was the major reactive nitrogen constituent in tropospheric air above the higher latitude surface regions throughout the winter/spring period.  PAN/NO_y ratios were typically 75% or larger in tropospheric air masses.  The suite of measurements made on the aircraft also allowed an examination of the budget of NO_y.  To first order, a reasonable balance between the measurement of NO_y and the sum of individually measured constituents (NO, NO₂, PAN, PPN, HNO₃, alkyl nitrates, and inferred HNO₄) was determined.  Preliminary results were presented in a poster session at the “Workshop on Nitrogen Oxides in the Lower Stratosphere and Upper Troposphere” held at the University of Heidelberg, Germany, 19-22 March 2001.  (Ridley served as chair of one of the workshop sessions.)  A somewhat different presentation was also made at the Spring AGU meeting by Flocke.  Further analysis of uncertainties in the budget analysis is yet to be completed prior to submitting a manuscript.  These analyses are ongoing with collaborative input from Robert Talbot and Jack Dibb (University of New Hampshire) and Ronald Cohen (University of California, Berkeley).

In collaboration with Andrew Neuman, Ru-Shan Gao, Peter Popp, and David Fahey (NOAA Aeronomy Laboratory) analysis of our NO_y and NO and their HNO₃ measurements from the ACCENT program progressed with submission of one manuscript led by the NOAA group (Neuman, et al., Atmos. Environ., submitted).  It summarizes the changes in partitioning of NO_y into HNO₃ and other constituents in the upper troposphere and across the tropopause.  Another goal of the missions was to examine the composition of the exhaust from an Athena rocket (solid rocket motor, ammonium perchlorate/aluminum propellant) as part of the necessity for understanding the effects of such emissions on the troposphere and especially on stratospheric ozone.  A surprise of the measurements compared to previous model scenarios was that a large fraction of the NO_x emissions was transformed to HNO₃, even with an elapsed time before sampling at 16 km altitude of only 4 minutes.  The results suggest a rapid conversion of emitted reactive nitrogen to HNO₃ on the surface of alumina particles that have abundant surface area and are a principal component of the exhaust emissions.  The measurements also allow a good estimate of the reactive nitrogen emission index for the Athena rocket which can be applied to other launch vehicles that employ this type of engine.  These results will be presented in several talks at the Fall AGU meeting.

In the laboratory, a number of experiments were conducted with the NO_x,y and PAN instruments to settle some of the fine details of the output of the calibration system for each instrument.  Weinheimer and Flocke made a number of improvements to their PAN instrument to increase the sampling frequency and to expand the number of constituents measured.  In addition, several new modules were constructed and tested for the intercomparison of NO calibration standards used on both instruments, and for re-conditioning the gold tube converters on the NO_y instrument.  David Knapp started the design and construction of a new and more current data acquisition and control computer for the aircraft instrument.  A start was also made on redesigning the sampling inlet and other modules for the high altitude WB-57 NO_xy instrument.  The changes will allow simultaneous forward and aft sampling of NO_y constituents in order to probe the general reactive nitrogen content of ice particles or aerosol particles for an upcoming (summer 2002) NASA WB-57 aircraft study of cirrus and other clouds in the upper troposphere to lower stratosphere altitude region.

PAN Instrumentation and Field Studies

Over the past year, in addition to analysis of the TOPSE program data (see Atmospheric Odd Nitrogen section), substantial analysis of results from participation in the NOAA TexAQS 2000 aircraft program was made by Weinheimer and Flocke (STM Group). This project focused on air quality/ozone photochemistry in the greater Houston Area. Measurements were contributed of several PAN-type compounds aboard the NCAR Electra: PAN, APAN, PPN, PiBN, and MPAN (Weinheimer et al., J. Geophys. Res., accepted). These organic nitrates are co-produced with ozone. Their mixing ratios, and how they correlate with each other and with ozone, have implications for identification of the type of hydrocarbon precursors involved in 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 has a source from biogenic hydrocarbons such as isoprene. Thus it can be concluded that biogenic hydrocarbons do not contribute significantly to the high ozone events that occur frequently in summer in the Houston area. Additionally, based on PAN-PPN, ozone-PAN, and ozone-PPN correlations in the greater Houston area, the mix of hydrocarbons contributing to ozone formation does not appear atypical in an overall sense (though the case of APAN is unique). Moreover, the same result appears to be true in the very high ozone events (O3 ~ 200 ppbv), 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 the coming year. In at least some cases, PAN samples corresponding to very high ozone values (~200 ppbv) show PAN/ozone ratios and PAN/PPN ratios that are consistent with data obtained from other locales (based on data obtained in other aircraft projects: Southern Oxidant Stydt (SOS) '99 (Nashville); TOPSE (mid-high latitudes); TRACE-P (Asian outflow regions). Thus it appears that the photochemical sources of much of the high ozone in Houston are similar to those occurring in other polluted regions in the US and Asia. There are cases where PAN/PPN ratios are anomalous, but these occur generally when ozone is not extremely high.

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 from refineries/industry concentrated in the Houston area, is mainly responsible for 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 does not yield any of the PAN-type compounds that were 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. 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 were observed. Thus there is a dilemma. There is a strong case for the importance of ethene and propene, yet this appears to be inconsistent 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, by consideration of the effects of mixing and loss processes on the correlations of the PAN-type compounds among themselves and with ozone, and through collaboration with Michael Trainer (NOAA Aeronomy Laboratory) who is modeling the Houston studies.

More recently, the group participated in the NASA TRACE-P experiment (Feb.-April, 2001) by flying instrumentation for measurements of PAN, PPN, PiBN, and MPAN (not detected) on 20 flights of the P-3 aircraft. The study focused on the influence of outflow of Asian on the tropospheric chemistry over the central and western Pacific Ocean. Flights originated from Wallops, went through Dryden, 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, and 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 point to anthropogenic pollution the being main contributor. Data is currently being finalized for archival in December. In a collaboration with Yutaka Kondo (Nagoya University, Japan) and his group during the missions to compare calibration standards, preliminary analysis of the flight data reveals that PAN showed an increase with altitude in the Central Pacific more distant from source regions, but showed a strong increase with decreasing altitude in the Western Pacific due to pollution outflow at lower altitudes. At 6-8 km PAN mixing ratios were comparable in the two regions. Analyses to estimate the source of NOx to the more remote Pacific from PAN decomposition and to determine ozone/PAN relationships in pollution plumes, analyses similar to that described above for TexAQS, will be done in the coming year.