Chemical Instrumentation Test Research Articles

NOx and NOy over the northwestern North Atlantic: Measurements and measurement accuracy

Measurements of NOx (NO+NO2) and NOy (total reactive nitrogen oxides) during February‐April 1996 at the northern tip of Newfoundland are used to determine levels in the local marine boundary layer (MBL) and assess the adequacy of current understanding of the processes controlling NOx levels over the northern North Atlantic, as expressed through previously reported simulations using the Geophysical Fluid Dynamics Laboratory (GFDL) Global Chemical Transport Model (GCTM). Median mixing ratios of NOx and NOy in the local MBL were 24 parts per trillion by volume (pptv) and 200 pptv, respectively. These levels are 35–64% above background levels measured in the remote MBL in summer and fall during the Chemical Instrumentation Test and Evaluation (CITE 2), North Atlantic Regional Experiment (NARE‐93), and Pacific Exploratory Mission‐West (PEM‐West) A measurement campaigns and are similar to or somewhat higher than anthropogenically influenced levels observed in winter‐spring during the PEM‐West B campaign. The magnitude of median NOx and NOy, levels in the local MBL is not due to events with high reactive nitrogen oxides levels. Instead, these relatively high median levels are likely the result of dispersion of anthropogenic emissions over a large region. A detailed comparison with results from the GFDL GCTM indicates that measured March and April average NOx levels are significantly lower than simulated levels over the north central North Atlantic. The frequency and magnitude of modeled and observed elevated‐NOx events were similar, indicating that the conditions responsible for relatively direct long‐range transport events were similar. This indicates that interannual variability probably did not cause the discrepancy in monthly average NOx values. However, simulated elevated NOx events are much longer than are observed. This difference appears to be at least partially responsible for the higher average NOx values simulated by the model. These results indicate that model‐based estimates of this region's contributions to the global ozone budget may be too high. Accuracy of the NOx measurements is estimated to be 6%, while conservative analysis of conversion efficiencies indicates a negative bias of ≲18% in the determination of gas‐phase NOy compounds.

Influence of plumes from biomass burning on atmospheric chemistry over the equatorial and tropical South Atlantic during CITE 3

During all eight flights conducted over the equatorial and tropical South Atlantic (27°–35°W, 2°N–11°S; September 9–22, 1989) in the course of the Chemical Instrumentation Test and Evaluation (CITE 3) experiment, we observed haze layers with elevated concentrations of aerosols, O3, CO, and other trace gases related to biomass burning emissions. They occurred at altitudes between 1000 and 5200 m and were usually only some 100–300 m thick. These layers extended horizontally over several 100 km and were marked by the presence of visible brownish haze. These layers strongly influenced the chemical characteristics of the atmosphere over this remote oceanic region. Air mass trajectories indicate that these layers originate in the biomass burning regions of Africa and South America and typically have aged at least 10 days since the time of emission. In the haze layers, O3and CO concentrations up to 90 and 210 ppb were observed, respectively. The two species were highly correlated. The ratio ΔO3/ΔCO (Δ, concentrations in plume minus background concentrations) is typically in the range 0.2–0.7, much higher than the ratios in the less aged plumes investigated previously in Amazonia. In most cases, aerosol (0.12–3 μm diameter) number concentrations were also elevated by up to 400 cm−3in the layers; aerosol enrichments were also strongly correlated with elevated CO levels. Clear correlations between CO and NOxenrichments were not apparent due to the age of the plumes, in which most NOxwould have already reacted away within 1–2 days. Only in some of the plumes could clear correlations between NOyand CO be identified; the absence of a general correlation between NOyand CO may be due to instrumental limitations and to variable sinks for NOy. The average enrichment of ΔNOy/ΔCO was quite high, consistent with the efficient production of ozone observed in the plumes. The chemical characteristics of the haze layers, together with remote sensing information and trajectory calculations, suggest that fire emissions (in Africa and/or South America) are the primary source of the haze layer components.

The impact of U.S. continental outflow on ozone and aerosol distributions over the western Atlantic

Aircraft measurements of selected trace gas species, aerosols, and meteorological parameters were performed in the lower troposphere off the U.S. east coast during August and September 1989 as part of the NASA Global Tropospheric Experiment (GTE) Chemical Instrumentation Test and Evaluation (CITE 3) expedition. In this paper, we examine these data to assess the impact of continental outflow on western Atlantic O3 and small aerosol budgets. Results show that mixed layer (ML) O3 concentrations and small aerosol number densities (Np) were enhanced by factors of 3 and 6, respectively, within air masses of predominantly continental origin compared with clean maritime background air. These enhancements exhibited a marked altitude dependence, declining rapidly above the ML to the point where only slight to moderate differences in O3 and Np, respectively, were notable above 2.4 km. Within continentally influenced ML's, both O3 and Np were correlated with CO, exhibiting linear regression slopes averaging 0.4 ppbv(O3)/ppbv(CO) for O3 and 7.7 (particles cm−3)/ppbv(CO) for Np and indicating a primarily anthropogenic origin for the observed enhancement of these species. Comparisons between profiles in continental and background maritime air masses suggest that photochemical production below 1.4‐km altitude adds over 10% to western Atlantic tropospheric column O3 abundance in continental outflow regimes. For aerosols, eastward advection of low‐level continental air contributes an average net flux of 2.8 metric tons of submicron (accumulation mode) particles per kilometer of shoreline per day to the western Atlantic troposphere.

An intercomparison of aircraft instrumentation for tropospheric measurements of sulfur dioxide

As part of the NASA Tropospheric Chemistry Program, a series of field intercomparisons have been conducted to evaluate the state‐of‐the art for measuring key tropospheric species. One of the objectives of the third intercomparison campaign in this series, Chemical Instrumentation Test and Evaluation 3 (CITE 3), was to evaluate instrumentation for making reliable tropospheric aircraft measurements of sulfur dioxide, dimethyl sulfide, hydrogen sulfide, carbon disulfide, and carbonyl sulfide. This paper reports the results of the intercomparisons of five sulfur dioxide measurement methods ranging from filter techniques, in which samples collected in flight are returned to the laboratory for analyses (chemiluminescent or ion chromatographic), to near real‐time, in‐flight measurements via gas chromatographic, mass spectrometric, and chemiluminescent techniques. All techniques showed some tendency to track sizeable changes in ambient SO2 such as those associated with altitude changes. For SO2 mixing ratios in the range of 200 pptv to a few ppbv, agreement among the techniques varies from about 30% to several orders of magnitude, depending upon the pair of measurements intercompared. For SO2 mixing ratios <200 pptv, measurements from the techniques are uncorrelated. In general, observed differences in the measurement of standards do not account for the flight results. The CITE 3 results do not unambiguously identify one or more of the measurement techniques as providing valid or invalid SO2 measurements, but identify the range of “potential” uncertainly in SO2 measurements reported by currently available instrumentation and as measured under realistic aircraft environments.

Atmospheric measurements of carbonyl sulfide, dimethyl sulfide,and carbon disulfide using the electron capture sulfur detector

Measurements of atmospheric dimethyl sulfide (DMS), carbonyl sulfide (COS), and carbon disulfide (CS2) were conducted over the Atlantic Ocean on board the NASA Electra aircraft during the Chemical Instrumentation Test and Evaluation (CITE 3) project using the electron capture sulfur detector (ECD‐S). The system employed cryogenic preconcentration of air samples, gas chromatographic separation, catalytic fluorination, and electron capture detection. Samples collected for DMS analysis were scrubbed of oxidants with NaOH impregnated glass fiber filters prior to preconcentration. The detection limits (DL) of the system for COS, DMS, and CS2 were 5, 5, and 2 ppt, respectively. COS concentrations ranged from 404 to 603 ppt with a mean of 489 ppt for measurements over the North Atlantic Ocean (31°N to 41°N), and from 395 to 437 ppt with a mean of 419 ppt for measurements over the Tropical Atlantic Ocean (11°S to 2°N). DMS concentrations in the lower marine boundary layer, below 600‐m altitude, ranged from below DL to 150 ppt from flights over the North Atlantic, and from 9 to 104 ppt over the Tropical Atlantic. CS2 concentrations ranged from below DL to 29 ppt over the North Atlantic. Almost all CS2 measurements over the Tropical Atlantic were below DL.

A photostationary state analysis of the NO2‐NO system based on airborne observations from the subtropical/tropical North and South Atlantic

The Chemical Instrumentation Test and Evaluation 3 (CITE 3) NO‐NO2 database has provided a unique opportunity to examine important aspects of tropospheric photochemistry as related to the rapid cycling between NO and NO2. Our results suggest that when quantitative testing of this photochemical system is based on airborne field data, extra precautions may need to be taken in the analysis. This was particularly true in the CITE 3 data analysis where different regional environments produced quite different results when evaluating the photochemical test ratio (NO2)Expt/(NO2)Calc, designated here as RE/RC. The quantity (NO2)Calc was evaluated using the following photostationary state expression: [NO2]Calc = (k1[O3] + k4[HO2] + k5[CH3O2] + k6[RO2]) [NO]Expt/J2. The four most prominent regional environmental data sets identified in this analysis were those labeled here as free‐tropospheric northern hemisphere (FTNH), free‐tropospheric tropical northern hemisphere (FTTNH), free‐tropospheric southern hemisphere (FTSH), and tropical‐marine boundary layer (plume) (TMBL(P)). The respective RE/RC mean and median values for these four data subsets were 1.74, 1.69; 3.00, 2.79; 1.01, 0.97; and 0.99, 0.94. Of the four data subsets listed, the two that were statistically the most robust were FTNH and FTSH; for these the respective RE/RC mean and standard deviation of the mean values were 1.74 ± 0.07 and 1.01, ± 0.04. The FTSH observations were in good agreement with theory, whereas those from the FTNH data set were in significant disagreement. An examination of the critical photochemical parameters O3, UV(zenith), NO, NO2, and non‐methane hydrocarbons (NMHCs) for these two databases indicated that the most likely source of the RE/RC bias in the FTNH results was the presence of a systematic error in the observational data rather than a shortcoming in our understanding of fundamental photochemical processes. Although neither a chemical nor meteorological analyses of these data identified this error with complete certainty, they did point to the three most likely possibilities: (1) an NO2 interference from a yet unidentified NOy species; (2) the presence of unmeasured hydrocarbons, the integrated reactivity of which would be equivalent to ∼2.7 parts per billion by volume (ppbv) of toluene; or (3) some combination of points (1) and (2). Details concerning hypotheses (1) and (2) as well as possible ways to minimize these problems in future airborne missions are discussed.

Measurements of atmospheric carbonyl sulfide during the NASA Chemical Instrumentation Test and Evaluation project: Implications for the global COS budget

Atmospheric COS concentrations were measured by three analytical systems during the Chemical Instrumentation Test and Evaluation (CITE 3) project. The three systems all used cryogenic sample preconcentration and gas chromatographic (GC) separation but differed in the method of detection. The FPD system used a flame photometric detector, the MS system used a mass selective detector, and the ECD‐S system used a fluorinating catalyst followed by an electron capture detector. With the FPD system, we found a mean COS concentration of 510 ppt over the North Atlantic and 442 ppt over the Tropical Atlantic. With the ECD‐S system, we found a mean COS concentration of 489 ppt over the North Atlantic and 419 ppt over the Tropical Atlantic. All three systems registered a latitudinal gradient in atmospheric COS of between 1.6 and 2.0 ppt per degree of latitude, with increasing COS concentrations northward which was similar to the gradient measured by Bingemer et al. (1990). It is difficult to reconcile the measured latitudinal concentration gradient with present theories of the global COS budget since the largest sink of COS is thought to be a flux to land plants, most of which are in the northern hemisphere.

Operational overview of the NASA GTE/CITE 3 airborne instrument intercomparisons for sulfur dioxide, hydrogen sulfide, carbonyl sulfide, dimethyl sulfide, and carbon disulfide

This paper reports the overall experimental design and gives a brief overview of results from the third airborne Chemical Instrumentation Test and Evaluation (CITE 3) mission conducted as part of the National Aeronautics and Space Administration's Global Tropospheric Experiment. The primary objective of CITE 3 was to evaluate the capability of instrumentation for airborne measurements of ambient concentrations of SO2, H2S, CS2, dimethyl sulfide, and carbonyl sulfide. Ancillary measurements augmented the intercomparison data in order to address the secondary objective of CITE 3 which was to address specific issues related to the budget and photochemistry of tropospheric sulfur species. The CITE 3 mission was conducted on NASA's Wallops Flight Center Electra aircraft and included a ground‐based intercomparison of sulfur standards and intercomparison/sulfur science flights conducted from the NASA Wallops Flight Facility, Wallops Island, Virginia, followed by flights from Natal, Brazil. Including the transit flights, CITE 3 included 16 flights encompassing approximately 96 flight hours.

CITE 3 meteorological highlights

Meteorological highlights from the third NASA Global Tropospheric Experiment Chemical Instrumentation Test and Evaluation (GTE/CITE 3) are presented. During August and September 1989, research flights were conducted from Wallops Island, Virginia, and Natal, Brazil, and included airborne sampling of air masses over adjacent regions of the Atlantic Ocean. Isentropic backward trajectory calculations, wind vector/streamline fields, rawinsonde data, and GOES and METEOSAT satellite imagery are utilized to examine the meteorological conditions for each flight and to determine the transport paths of the sampled air masses. Some aspects of the chemical signatures of the sampled air are also discussed. During the series of flights based at Wallops Island, Virginia, the flow into the experiment area was governed primarily by the position of the North Atlantic subtropical anticyclone. The large‐scale tropospheric circulation switched from primarily a marine flow during flights 1–4, to a predominantly offshore mid‐latitude continental flow during flights 5–10. During these later flights, the regional influences of large eastern U.S. cities along with vertical mixing by typical summertime convective activity strongly influenced the chemical characteristics of the sampled air. During the series of flights based at Natal, Brazil, the dominant synoptic feature was the South Atlantic subtropical anticyclone which generally transported air across the tropical Atlantic toward eastern Brazil. Pronounced subsidence and a well‐defined trade wind inversion often characterized the lower and middle troposphere over the Natal region. Some high‐altitude recirculation of air from South America was observed, as was cross‐equatorial transport which had come from northern Africa. Biomass burning plumes were observed on segments of all of the flights, the source region being the central and southern savannah regions of Africa.

Measurements of dimethyl sulfide and SO2 during GTE/CITE 3

As part of NASA's Global Tropospheric Experiment Chemical Instrumentation Test and Evaluation (GTE/CITE 3) Sulfur Gas Intercomparison, we conducted measurements of dimethyl sulfide (DMS) and SO2 using two techniques well suited for sampling from an aircraft due to their simplicity of design. DMS was collected by preconcentration on gold wire preceded by a KOH‐impregnated filter oxidant scrubber, and analyzed by gas chromatography with flame photometric detection. SO2 was collected on K2CO3/glycerol‐impregnated filters and analyzed by ion chromatography. In blind tests, both techniques produced excellent agreement with National Institutes of Standards and Technology (NIST) standards. For field measurements, the DMS technique produced excellent correlation with the mean of the six different techniques intercompared. For SO2, the five techniques intercompared were rather poorly correlated, but correlations between the three techniques which passed NIST standards tests were somewhat better. Our SO2 filter measurements exhibited rather large uncertainties due to higher than normal variability of the filter blanks, which we believe was caused by extended storage in the field. In measurements conducted off the coast of Natal, Brazil, a diurnal afternoon minimum in DMS concentrations accompanied by a corresponding maximum in SO2 concentrations was observed. However, due to rather large uncertainties in the SO2 measurements, any conclusions about the SO2 trend must be considered tentative.

Advection of sulfur dioxide over the western Atlantic Ocean during CITE 3

During the NASA Chemical Instrumentation Test and Evaluation 3 sulfur intercomparison over the western Atlantic Ocean, five techniques for the determination of sulfur dioxide were evaluated. The response times of the techniques varied from 3 to 30 min. Based on the ensemble of measurements reported, it was clear that advection of SO2 from the North American continent occurred in the boundary layer (altitude <1 km) with only one exception. The vertical distribution of SO2 above the boundary layer for the northern and southern Atlantic Ocean was remarkably similar during this experiment.

An intercomparison of airborne PAN measurements

As part of the NASA Tropospheric Chemistry Program a series of field intercomparisons have been initiated to evaluate state‐of‐the‐art capability for measuring key tropospheric species. These intercomparisons, designated as Chemical Instrumentation Test and Evaluation (CITE), are conducted as part of NASA's Global Tropospheric Experiment. The objectives of the second series of instrument tests, CITE 2, were to evaluate instrumentation for measuring NO2, HNO3, and peroxyacetyl nitrate (PAN) and to determine for various tropospheric environments the relative abundances and partitioning among the major nitrogen species. This paper summarizes the results from the PAN instrument intercomparisons. Other results are addressed in companion papers. Both PAN instruments use the same detection principle of electron capture gas chromatography of a cryogenically enriched sample of ambient air. NASA Ames Research Center and the National Center for Atmospheric Research were responsible for the respective instruments. The intercomparisons included three exchanges of standards and 13 intercomparison flights in which a variety of types of air masses were sampled. Nine flights were based from Ames Research Center, California, and the remaining four were ferry flights between Ames and Wallops Flight Center, Virginia (aircraft home base). Flight altitudes ranged from 150 to 5000 m above ground level. All flights but one were during daylight hours. PAN mixing ratios during the flight intercomparison periods were generally <300 parts per trillion by volume (pptv), and about 40% of the results for mixing ratios were <100 pptv. At mixing ratios of <100 pptv the two instruments agreed on the average to about 17 pptv with a 95% confidence interval of ±9 pptv. Instrument agreement at PAN levels of 100–300 pptv was of the order of 25% with a confidence interval of ±6%. These levels of agreement are within expected limits based on the stated accuracy and precision of the two instruments. However, it is noted that for some of the individual intercomparison data periods, agreement was outside expected levels.

A tunable diode laser system for aircraft measurements of trace gases

The advantages of tunable diode laser absorption spectrometry (TDLAS) for measuring trace atmospheric gases are universality, positive identification, good sensitivity, and rapid response time. A TDLAS system is described which is capable of making measurements on board an aircraft. The instrument was employed during the Chemical Instrumentation Test and Evaluation (CITE) 2 program to measure NO2, on 11 flights and HNO3, on five flights. Detection limits were determined to be 25 parts per trillion by volume [pptv] for NO2 and 75 pptv for HNO3 with response times of 3 min. This was sufficient to permit measurements of NO2 in the free troposphere and HNO3, in the continental boundary layer a significant fraction of the time.

An intercomparison of airborne nitric oxide measurements: A second opportunity

Results from an airborne intercomparison of techniques to measure tropospheric levels of NO are discussed. The intercomparison was part of the National Aeronautics and Space Administration's Global Tropospheric Experiment and was conducted during the summer of 1986. Instruments intercompared included a two‐photon laser‐induced fluorescence system and two NO/O3 chemiluminescence detectors. These three instruments also participated in the NO intercomparisons conducted during the Chemical Instrumentation Test and Evaluation (CITE) 1 mission in 1983–1984, and therefore the current results are a revisit to the question of how well NO can be measured from an aircraft in remote tropospheric environments. The intercomparisons were weighed to mixing ratios less than 20 pptv. Results from CITE 2 were similar to those of CITE 1. The most noteworthy observation common to both campaigns is that at these low mixing ratios, agreement among the three instruments is almost always within about 15 or 20 pptv for sampling periods of 1–6 min. Analyses of multipoint data sets suggest that on the average the difference between data from any pair of the instruments at the low mixing ratios is about 5–7 pptv. Since this agreement was obtained among instruments using fundamentally different detection principles (chemiluminescent and laser) and the data suggested that all provided equally valid measurements of NO, the observed level of agreement perhaps represents the uncertainity in the current measurement of low mixing ratio ambient NO. The major question which must be addressed by the scientific community is the influence of, for example, a 5‐pptv uncertainty in NO at 10 pptv in photochemistry studies, nitrogen budget studies, and so on, in the “clean” remote troposphere.

Operational overview of NASA GTE/CITE 2 airborne instrument intercomparisons: Nitrogen dioxide, nitric acid, and peroxyacetyl nitrate

This paper provides the rationale, objectives, approach, and a brief description of the instrumentation included in the second airborne Chemical Instrumentation Test and Evaluation (CITE 2) mission conducted by NASA. This mission, conducted on NASA's Electra aircraft, was based at Moffett Field, California, and intercompared data from instruments measuring NO2, HNO3, and PAN in the troposphere. Ancillary measurements augmented the intercpmparison data in order to address specific questions related to the budget and partitioning of odd nitrogen species in the troposphere. This study, conducted in August 1986, encountered marine and continental air with free tropospheric mixing ratios of NO2, HNO3, and PAN typically less than 120, 150, and 200 parts per trillion by volume, respectively. Other papers in this issue provide the detailed results of the intercomparison tests and also discuss issues related to the budget and distribution of odd nitrogen species in the troposphere.

Ozone precursors and ozone photochemistry over eastern North Pacific during the spring of 1984 based on the NASA GTE/CITE 1 airborne observations

Simultaneous, high‐resolution measurements of O3, NO, CO, dew point temperature, and UV flux obtained during the NASA Global Tropospheric Experiment Chemical Instrumentation Test and Evaluation (GTE/CITE 1) spring 1984 airborne field exercise over the eastern North Pacific Ocean are analyzed. Mid‐tropospheric CO, O3, and NO mixing ratios averaged about 120 parts per billion by volume (ppbv), 50 ppbv, and 10 parts per trillion by volume (pptv), respectively. Statistical analysis of the high‐resolution data indicates the existence of two ozone sources: one related to the downward transport of ozone‐rich air from the upper troposphere and stratosphere and the other to the transport of ozone‐rich air from the continents. Modeling calculations based on these average levels imply that from the surface to about 8 km, photochemical reactions probably supplied a net sink of ozone to the region overlying the eastern North Pacific Ocean during the sampling period. However, because the NO levels measured during the flights were frequently at or near the detection limit of the instruments and because our results are very sensitive to the absolute NO levels and their temporal variability, our conclusion must be considered provisional.

Fast‐response, high‐precision carbon monoxide sensor using a tunable diode laser absorption technique

A tunable diode laser instrument denoted as DACOM (Differential Absorption CO Measurement) has been developed to meet the fast‐response, high‐precision CO measurement needs of the GTE (Global Tropospheric Experiment) program. Under the GTE program, DACOM participated in the three field missions of CITE 1 (Chemical Instrumentation Test and Evaluation 1), a project involving the intercomparison of trace gas measurement techniques. DACOM performance, including analyses of measurement error sources, is discussed for the ground‐based mission at Wallops Island, Virginia (summer 1983), and two missions on the NASA CV‐990 (fall 1983 and spring 1984). Examples of fast‐response (≈1 s), high‐precision (±1 part per billion by volume, ±1.5% of reading) airborne data are included to illustrate the capability of this instrument.

Carbon monoxide measurements over the eastern Pacific during GTE/CITE 1

As part of the Global Tropospheric Experiment's Chemical Instrumentation Test and Evaluation (GTE/CITE 1) intercomparison, carbon monoxide (CO) measurements were made from the NASA CV‐990 aircraft during the fall of 1983 and again in the spring of 1984. The experimental measurements for CO obtained during those flight series over the eastern and mid‐Pacific are presented here. Data were acquired from 10° to 20°N latitude over the mid‐Pacific and from 30° to 37°N latitude over the eastern Pacific off the coast of California. A seasonal variation of approximately 34 parts per billion by volume was measured over the altitudes and latitudes sampled, and a small latitudinal variation was also noted. The data are discussed in terms of the meteorological context in which they were collected.