F. E. Grahek

Distributions of NO, NOx, NOy, and O3 to 12 km altitude during the summer monsoon season over New Mexico

During late July and August 1989, 12 flights of the National Center for Atmospheric Research Sabreliner jet aircraft were made over New Mexico when the region was dominated by either synoptic high pressure or moist “monsoon” flow. In the latter case, sampling was made within and about deep convective clouds which were sometimes electrically active. A summary of the measurements of the species listed in the title and their ratios are given. These distributions include signatures from deep convection, lightning production of odd nitrogen, aircraft exhaust emissions, and possible stratospheric input. The averages and range of these distributions are considered to be more representative of typical summer conditions over the region compared to flights that are often restricted more to fair weather situations. Coherence between the O3 and the NOy observations is compared to results from other ground-based and aircraft programs and possible contributing factors are discussed. Because the measurements were made with then newly developed instrumentation, its capabilities and shortcomings are summarized.

Partitioning and budget of NO y species during the Mauna Loa Observatory Photochemistry Experiment

During the Mauna Loa Observatory Photochemistry Experiment (MLOPEX), measurements were made of total odd nitrogen (NOy) and the known individual daytime odd-nitrogen species. The individual species measured were NO, NO2, HNO3, paniculate NO3−, peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), methyl nitrate, and >C3 alkyl nitrates. The most abundant component of NOy was nitric acid; its median contribution to NOy in free tropospheric samples was 43%. The large fraction of HNO3 is consistent with the long transport times and photochemical processing of air masses reaching the mid-Pacific site as well as possible stratospheric input of NOy. The median contribution of NOx to NOy in the free troposphere near 3.4 km was ≈14%. PAN and other measured organic nitrates contributed < 7% to NOy. The median sum of the individually measured species was 102% of NOy in upslope periods which consist of a mixture of island-modified marine boundary layer and free tropospheric air. This total was 75% of NOy during downslope periods representative of the free troposphere. This shortfall in the odd-nitrogen budget in the free troposphere corresponds to 72 pptv of reactive nitrogen, which is over 2 times median NOx. The NOy shortfall and the composition of NOy appeared to have a regular variation in the free troposphere during the experiment which was related to air mass origin, recycling of odd nitrogen, and loss processes during transport. The presence of an odd-nitrogen deficit in the remote free troposphere suggests that our understanding of the NOy system is incomplete. Unidentified odd-nitrogen species, such as organic nitrates, may be present, but sampling limitations and analytical uncertainties in NOy and individual (NOy)i measurements still restrict our ability to accurately define an NOy budget, especially in remote regions.

On the production of active nitrogen by thunderstorms over New Mexico

In July and August of 1989 the National Center for Atmospheric Research (NCAR) Sabreliner jet aircraft was used to probe electrically active and inactive convective storms over west central New Mexico to examine the production of odd nitrogen in the middle and upper troposphere by thunderstorms. In the anvil outflow or cloud top region of active and nonactive storms, the majority of flights showed that O 3 was reduced relative to the extracloud air owing to transport of ozone-poor air from lower altitudes. A similar result was found for active nitrogen (NO x ) and total odd nitrogen (NO y ) in nonelectrically active storms, but the reduction in NO y was also enhanced by removal of soluble constituents during convective transport. Examples of efficient removal from the gas phase are described. There was no evidence of O 3 production by lightning discharges. Indeed, O 3 was a good tracer over the lifetime (∼1 hour) of the storms. During the active-to-mature stage of air mass thunderstorms, large enhancements in active nitrogen were observed in the anvil altitude region (9-11.8 km) and, in one case, in the midlevel outflow (near 7 km) of a dissipating thunderstorm. Two thunderstorms allow good estimates of the NO x production by lightning within or transport to the upper altitude region (8-11.8 km). Thunderstorms of August 12 and August 19 yield amounts in the range of 253-296 kg(N) and 263-305 kg(N), respectively. If, as an exercise, these amounts are extrapolated to the global scale on the basis of the number of cloud-to-ground and intracloud lightning flashes counted or estimated for each storm and a global flash frequency of 100 s -1 the result is 2.4-2.7 and 2.0-2.2 Tg(N)/yr. Alternatively, an estimate for the two storms made on the basis of the average number of thunderstorms that occur per day globally (44,000) yields amounts in the range of 4.1-4.7 and 4.2-4.9 Tg(N)/yr, respectively. These estimates only apply to the production or transport of lightning generated NO x in or to the altitude region between 8 km and the top of the thunderstorm anvil (∼11.8 km in these studies). Since in some large-scale models, lightning-generated NO x is equally distributed by mass into each tropospheric layer, our estimates are roughly equivalent to those model runs that use a global source strength of about twice our estimate for the upper altitude region. In several flights where the region below the base of thunderstorms was examined, no large enhancements in odd nitrogen which could be clearly attributed to lightning were observed. Apparently, the aircraft was not in the right place at the right time. Thus no estimate of the NO x production by lightning that remains below ∼8 km could be made.

Total reactive oxidized nitrogen (NOy) in the remote Pacific troposphere and its correlation with O3 and CO: Mauna Loa Observatory Photochemistry Experiment 1988

As part of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX) total reactive oxidized nitrogen (NOy) was measured during May and early June of 1988 at the Mauna Loa Observatory, the NOAA-Geophysical Monitoring for Climatic Change Baseline Monitoring Station, located at 3.4-km elevation on the island of Hawaii. Gold catalytic surface conversion of individual reactive oxidized nitrogen species to NO and subsequent quantification of the NO by NO/O3 chemiluminescence was used to measure the NOy mixing ratio. The NOy abundance at the site was governed by the local downslope/upslope wind systems as well as synoptic-scale transport. With some exceptions, downslope wind brought air representative of the free troposphere, while upslope winds transported air from below the trade wind inversion to the site. The upslope air masses could be a mix of marine boundary layer air and free tropospheric air modified by anthropogenic and natural emissions from island sources. It was possible to identify free tropospheric air in the downslope flow through meteorological and chemical tracers. Reflecting the remote location, low NOy mixing ratios with median values of 262 and 239 pptv were found in free tropospheric and upslope air masses, respectively. The median NOy levels in free tropospheric air are consistent with airborne NOy measurements made during NASAs Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (CITE 2) program over the northeastern Pacific Ocean at corresponding altitudes. The median NOy values in upslope flow are significantly higher than those measured in the remote marine boundary layer during CITE 2, reflecting probably the influence of island source and/or mixing of free tropospheric air with boundary layer air. The low correlation found between NOy and tracers of anthropogenic sources, such as carbon monoxide, tetrachloroethylene, and n-propyl nitrate, in free tropospheric air samples is consistent with a stratospheric or upper tropospheric source for NOy. Simultaneous particulate nitrate (NO3−) measurements suggest that at times not all aerosol NO3− was quantitatively converted to NO by the Au-surf ace converter technique. These episodes were usually found during upslope flow and were characterized by high sodium concentrations, suggesting that possibly the sodium nitrate contained in these aerosols was not converted efficiently by the Au converter.

Observations of peroxyacetyl nitrate, peroxypropionyl nitrate, methyl nitrate and ozone during the Mauna Loa Observatory photochemistry experiment

Measurements of the title species were made during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX) conducted between May 1 and June 4, 1988, at the Geophysical Monitoring for Climatic Change (GMCC) station at 3.4-km elevation on the Island of Hawaii. Diurnal changes in the organic nitrates primarily resulted from the transition between downslope flow (usually free tropospheric air) and upslope flow (marine boundary layer or a mix of marine boundary layer and free tropospheric air, both influenced by island sources of precursors) characteristic of the site. Longer term trends in the mixing ratios reflected changes in air mass origins from mid-latitudes to more tropical latitudes. The average mixing ratios in free tropospheric samples were peroxyacetyl nitrate (PAN, 17 pptv), peroxypropionyl nitrate (PPN, 0.3 pptv), methyl nitrate (MN, 4 pptv), and O3 (43 ppbv). The organic nitrates (PAN, PPN, MN) represent minor components of the total odd nitrogen budget at the site. In free tropospheric samples, PAN, PPN, and MN constituted average percentages of 7%, <1%, and 2% of total odd nitrogen. In more tropical air masses, MN could constitute as much as 10% of total odd nitrogen. A photochemical model is used to investigate the sensitivity of free tropospheric PAN to local precursor concentrations. The observed mixing ratios of PAN are also contrasted with measurements made at continental surface sites and during aircraft programs.

Measurements of nitric oxide and nitrogen dioxide during the Mauna Loa Observatory Photochemistry Experiment

NO and NO2 were simultaneously measured by photolytic conversion / chemiluminescence techniques during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX). The field site, located at an elevation of 3.4 km on the north side of the Mauna Loa Volcano, was subject to two airflow regimes which typically corresponded to upslope (marine boundary layer plus island sources) conditions during the day and downslope (middle free tropospheric) conditions at night to mid-morning. Median values of NOx (NOx = NO + NO2) were 37 and 31 pptv during upslope and downslope conditions, respectively, with the downslope measurements consistent with previous measurements made from aircraft in the middle free troposphere over the North Pacific. Although the difference in median NOx mixing ratios in the upslope and downslope regimes is small, the influence of island sources of NOx is apparent. Indeed, the median upslope values were approximately 2.5 times greater than measurements made previously in the remote marine boundary layer. The data have been examined according to downslope / free tropospheric and upslope air flow regimes for relationships between NOx and the various species that were measured simultaneously (e.g., peroxyacetyl nitrate (PAN), HNO3, NO3, NOy, O3, CO, and hydrocarbons). While positive correlations between NOx and O3 and PAN were typically observed in free tropospheric air, these correlations were considerably weaker than those observed during previous campaigns. This is likely primarily due to the lower sampling altitude during the MLOPEX study. NOx and dew point temperature were weakly anticorrelated in free tropospheric air masses. Linear correlations between NOx and the peroxides, formaldehyde, alkyl nitrates, and hydrocarbons were also weak in the free tropospheric air masses at the MLO. NOx/NOy was typically on the order of 0.1–0.2 in free tropospheric flow. Considerably higher values of NOx/NOy, were occasionally observed under upslope conditions. The NOx/NOy and HNO3/NOx values obtained under downslope conditions were similar to those previously obtained during aircraft measurements in the middle free troposphere over the northeast Pacific. On the whole, the downslope air masses sampled appear to be characteristic of well-aged, marine free tropospheric air, and this conclusion is supported by 10-day trajectory analyses.

Uptake of NOy on wave-cloud ice particles

In a flight through a wave cloud during SUCCESS on 2 May 1996, simultaneous forward- and aft-facing NOy inlets were used to infer the amount of condensed-phase NOy present on ice particles that were up to a few minutes old. Condensed-phase amounts were 25–75 pptv, or 10–20% of gas-phase NOy. Given the rapid HNO3 uptake on ice observed in the laboratory, a model calculation implies that virtually all of the gas-phase HNO3 will be depleted in the first 1–2 minutes after the appearance of ice. Thus the NOy observations are consistent with the laboratory results only if the ambient HNO3/NOy ratio is 10–20%.

Aircraft measurements made during the spring maximum of ozone over Hawaii: Peroxides, CO, O3, NOy, condensation nuclei, selected hydrocarbons, halocarbons, and alkyl nitrates between 0.5 and 9 km altitude

Between April 22 and May 11, 1992, ten flights of the University of Wyoming King Air were made during the maximum in tropospheric ozone over the central North Pacific Ocean in conjunction with the spring intensive of the second Mauna Loa Observatory Photochemistry Experiment. During the first week of flights, an episode of remarkably large total reactive nitrogen, NO y (∼2 ppbv) persisted in the 5-9 km altitude region for 3-4 days. Backtrajectory calculations combined with the trace gas and aerosol measurements confirm that its source was due primarily to export from northern latitude continental surface regions. The total amount of odd nitrogen transported over Hawaii during this event was estimated to be 1-2% of the annual emissions from subsonic aircraft or from stratospheric input. Throughout the measurement program layers of elevated O 3 , NO y , condensation nuclei (CN), and other species were frequently found between the onset of the marine boundary layer temperature inversion and 4-5 km altitude. Structure and strong gradients within these layers contribute to the daily variations seen at the 3.4 km elevation of the Mauna Loa Observatory during the nighttime downslope flow. The dryness of these low-altitude layers and the calculated air mass trajectories indicate that export from northern latitudes occurred mainly with subsidence to the Hawaii region rather than from transit just above the boundary layer inversion. There was no evidence of recent stratospheric input to the altitude region sampled below 9 km. However, the observations cannot distinguish whether O 3 input from the stratosphere occurred earlier in the air mass histories at higher latitudes. Fine vertical scale anticorrelations between CN and O 3 or NO y were also often observed particularly in the last week of the program when NO y mixing ratios were more typical of the remote troposphere. These features are attributed to new particle formation near the tops of cloud convection episodes and they illustrate the importance of such processes in contributing to the detailed layering and dilution of some chemical species in the free troposphere during this time of year. Mean and median profiles for many of the title species are given for high and low-to-moderate NO y categories.

Measurements of NO x and PAN and estimates of O3 production over the seasons during Mauna Loa Observatory Photochemistry Experiment 2

Measurements of peroxyacetyl nitrate (PAN) and NOx and a variety of other constituents were made over approximately 1-month-long intensives in the autumn of 1991 and the winter, spring, and summer of 1992 during the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2). PAN and NOx in the free troposphere had maximum abundances in spring in concert with the well-known maximum in O3. The ratio of the spring to summer averages was a factor of 4.1 for PAN, a factor of 1.6 for O3, and only a factor of 1.4 for NOx. During most intensives, variations over periods of a few days to a week were often larger than the average seasonal amplitude. In free tropospheric air masses local to Hawaii, average PAN/NOx ratios were a maximum in winter through spring but in the range of 0.25–0.86 in all intensives. PAN decomposition is unlikely to be the major net source of NOx in local air masses in summer and fall. The low HNO3/NOx ratios determined during MLOPEX 1 were confirmed during MLOPEX 2. Intensive average ratios of 1.6–3.8 over the year are lower than some model predictions. Both the low ratio and the magnitude of NOx imply a shortcoming in our understanding of the transformations and sources of NOy constituents in the central Pacific, The 3- to 4-km altitude region near Hawaii was a net importer of O3, on average, over the year. The average net rate of production of O3 in free tropospheric air was near zero in winter, −0.4 to −0.8 ppbv/d in spring, −1.4 ppbv/d in summer, and −0.6 ppbv/d in autumn. Thus the spring maximum in O3 is not due to local photochemistry. We believe, as has been concluded from the long-term measurements of long-lived constituents by the Climate Monitoring and Diagnostics Laboratory, that the variation of ozone precursors over the year and on shorter timescales of a few days to a week is controlled predominantly by changes in long-range transport: more frequent sampling of higher-latitude and higher-altitude air masses in winter and spring versus more frequent sampling of well-aged air from lower altitudes and latitudes in summer and autumn.

Marine latitude/altitude OH distributions: Comparison of Pacific Ocean observations with models

Reported here are tropical/subtropical Pacific basin OH observational data presented in a latitude/altitude geographical grid. They cover two seasons of the year (spring and fall) that reflect the timing of NASAs PEM-Tropics A (1996) and B (1999) field programs. Two different OH sensors were used to collect these data, and each instrument was mounted on a different aircraft platform (i.e., NASAs P-3B and DC-8). Collectively, these chemical snapshots of the central Pacific have revealed several interesting trends. Only modest decreases (factors of 2 to 3) were found in the levels of OH with increasing altitude (0–12 km). Similarly, only modest variations were found (factors of 1.5 to 3.5) when the data were examined as a function of latitude (30°N to 30°S). Using simultaneously recorded data for CO, O3, H2O, NO, and NMHCs, comparisons with current models were also carried out. For three out of four data subsets, the results revealed a high level of correspondence. On average, the box model results agreed with the observations within a factor of 1.5. The comparison with the three-dimensional model results was found to be only slightly worse. Overall, these results suggest that current model mechanisms capture the major photochemical processes controlling OH quite well and thus provide a reasonably good representation of OH levels for tropical marine environments. They also indicate that the two OH sensors employed during the PEM-Tropics B study generally saw similar OH levels when sampling a similar tropical marine environment. However, a modest altitude bias appears to exist between these instruments. More rigorous instrument intercomparison activity would therefore seem to be justified. Further comparisons of model predictions with observations are also recommended for nontropical marine environments as well as those involving highly elevated levels of reactive non-methane hydrocarbons.