North Atlantic Regional Experiment Research Articles

A case study of stratosphere‐troposphere exchange during the 1996 North Atlantic Regional Experiment

Passive tracers are employed in a relatively high spatial and temporal resolution three‐dimensional transport model to analyze a stratosphere‐troposphere exchange (STE) event over the eastern United States and western North Atlantic Ocean. The model is validated against measurements taken on board the National Oceanic and Atmospheric Administration WP‐3D Orion aircraft during the North Atlantic Regional Experiment study in the spring of 1996. Overall, the model reproduces the measurements well during the early part of the flight where there is indication of a small stratospheric intrusion. However, the very strong signatures of STE and mixing contained in the measurements later in the flight are not captured. Use of a finer horizontal resolution (20 km as opposed to 60 km) brings the model results closer to the aircraft measurements and yields higher values (50% at 7–8 km altitude) of ozone, O3, with a deeper penetration into the troposphere (20% at 80–120 ppbv levels).

Photolytic impact of a stratocumulus cloud layer upon the chemistry of an offshore advected plume of pollutants during the NARE 1993 intensive experiment: a numerical study

Layers of pollutants are frequently exported from the North American continent over the North Atlantic Ocean. Several intensive experiments were dedicated to study this long-range transport, which frequently occurs over cloud bands. Therefore, the photolytic impact of a persistent cloud layer upon an advected plume of pollutants is an important question and is addressed in the framework of the North Atlantic Regional Experiment (NARE) 1993 experiment, at a distance of 300 km from the continent. The comprehensive RAMS model coupled on-line with a gaseous and an aqueous chemical module is used to follow the chemistry of the photochemically aged plume. A thin cloud layer involves radicals (OH and HO 2) and hydrogen peroxide production in the polluted air advected over it and increases the oxidizing capacity of atmosphere. Additional sensitivity tests to study the radiative impact of the accompanying aerosol layer upon the photolysis rates are conducted. It is shown that aerosols moderate this radiative impact due to the cloud layer.

Export of NOy from the North American boundary layer: Reconciling aircraft observations and global model budgets

Fossil fuel combustion accounts for >50% of the global atmospheric emission of NOx, but this source is concentrated in the polluted continental boundary layer (CBL) and only a small fraction is exported as NOy (NOx and its oxidation products) to the global troposphere. Better quantification of this export efficiency is needed because of its implications for global tropospheric ozone. A recent Lagrangian analysis of the NOy‐CO correlations observed from the North Atlantic Regional Experiment in September 1997 (NARE'97) aircraft campaign downwind of eastern North America (September 1997) indicated a NOy export efficiency of <10%, with <10% of the exported NOy present as NOx. In contrast, previous three‐dimensional (3‐D) model Eulerian budget analyses for the North American boundary layer indicated NOy export efficiencies of 25–30%, with 30–35% of the exported NOy present as NOx. We investigated this apparent discrepancy by simulating the NARE'97 aircraft observations with a global 3‐D model of tropospheric chemistry (GEOS‐CHEM) and using the model to calculate the NOy export efficiency both through a Lagrangian analysis of the NOy‐CO correlations along the aircraft flight tracks and through an Eulerian budget analysis for the North American boundary layer. The model reproduces the variability and NOy‐CO correlations observed in the aircraft data and also at the Harvard Forest surface site in the northeastern United States. We show that the previous Lagrangian analyses of the NOy export efficiency during NARE'97 were probably biased low because of underestimation of the CO background. Correcting for this bias, we find a NOy export efficiency of 17 ± 7% in the model and 15 ± 11% in the observations. A similar NOy export efficiency (20%) in the model is obtained from the Eulerian budget analysis, demonstrating that the Lagrangian and Eulerian approaches are in fact consistent. Export efficiencies of NOy in previous 3‐D model Eulerian budget analyses were probably too high because of insufficient scavenging out of the CBL. Model results indicate that only 6% of the exported NOy is present as NOx along the aircraft flight tracks, in agreement with the observations, but that 40% of the NOy export flux is present as NOx, in agreement with the previous 3‐D model analyses. This result reflects the fast oxidation of NOx between the point of exit from the CBL and the point of sampling by the aircraft. The eventual ozone production in the global troposphere due to exported NOx and peroxyacetylnitrate (PAN), with equal contributions from each, is comparable in magnitude to the direct export of ozone pollution from the North American boundary layer.

Evaluation of inlets used for the airborne measurement of formaldehyde

The performance of three aircraft inlets used for sampling gas‐phase formaldehyde (CH2O) was evaluated. These 1.5 m long inlets were operated with the National Center for Atmospheric Research Tunable Diode Laser Absorption Spectrometer (TDLAS) at flow rates between 7 and 9 standard liters per minute. Laboratory tests were performed on the 1997 North Atlantic Regional Experiment (NARE 97) TDLAS inlet, involving a wide range of sample temperatures (−40° to 25°C), pressures (250–625 torr), relative humidities (<1 to 85%), and CH2O concentrations (0–25 ppbv). Standard additions on ambient air were performed in the field with another inlet. Sampling artifacts were not observed in either case at CH2O levels less than about 10 ppbv to within the measurement precision (25–120 parts per trillion by volume (1 min, 1σ)) and/or accuracy of standard generation (±6%). Desorption associated with the Herriott Cell was measured under highly polluted conditions, and was largely corrected for by subtracting a frequently acquired instrument background. Inlet shielding and heating minimized error due to liquid water collection. Common inlet materials such as PFA Teflon and silica‐coated steel efficiently transmitted CH2O.

Export of NOy from the North American boundary layer during 1996 and 1997 North Atlantic Regional Experiments

This paper studies the removal timescales of the NOy originating from surface emissions of nitrogen oxides (NOx) and studies what fraction of the emissions reaches the free troposphere. These are important questions because NOx, which limits ozone production in the free troposphere, can be recycled from NOy after the primary NOx has been exhausted. We present an analysis of aircraft measurement data of odd nitrogen (NOy) and carbon monoxide (CO) taken during the North Atlantic Regional Experiment (NARE) in spring 1996 and fall 1997. Because CO has a relatively long chemical lifetime in the atmosphere and because it has similar emission patterns to NOx, CO concentrations can serve as a reference against which NOy removal can be studied. Correlating NOy and CO for different heights, we show that for the NARE 1997 (and NARE 1996) measurement campaigns, 78–91% (33–76%) of the surface NOy emissions were lost within the boundary layer or during transport from it, and only 3% (5%) of the emissions reached altitudes above 3 km. This accounted for ∼35% (∼41%) of the total NOy present above 3 km, the remainder coming from in situ emissions. In order to establish the timescales over which NOy is removed from the atmosphere, we determine the time elapsed between emission and measurement using a Lagrangian tracer transport model. We obtain approximate, effective mean e‐folding lifetimes of NOy of 1.7 (1.8) days for the two intensives. On average, only 4% (9%) of the NOy remains in the atmosphere after 4–5 days. After 5–6 days, NOy from anthropogenic surface sources has been further depleted and accounts for <50% of the observed NOy. The rest of the observed NOy must come from in situ emissions, not from anthropogenic surface emissions.

Airborne CH2O measurements over the North Atlantic during the 1997 NARE campaign: Instrument comparisons and distributions

Airborne CH2O measurements were acquired by tunable diode laser absorption spectroscopy (TDLAS) and coil/2,4‐dinitrophenylhydrazine (CDNPH) techniques over remote regions of the North Atlantic Ocean from the surface to 8 km during the North Atlantic Regional Experiment (NARE‐97) in September of 1997. There were eight aircraft flights when both instruments were simultaneously operating, producing 665 overlapping time intervals for comparisons. A number of approaches were used in the comparisons, and indicated that on average both instruments measured identical ambient CH2O concentrations to within 0.1 ppbv, and more typically within 0.08 ppbv, over the 0 to 0.8 ppbv‐concentration range. However, significant differences, larger than the combined 2σ total uncertainty estimates, were observed in 29% of the full time‐coincident data set. The two instruments produced very similar altitude trends. Under clean background conditions in the 35° to 55°N latitude band, the median TDLAS and CDNPH CH2O concentrations were 0.399 and 0.410 ppbv for 0–2 km, 0.250 and 0.355 ppbv for 2–4 km, and 0.217 and 0.280 ppbv for 4–8 km, respectively. Elevated CH2O concentrations were observed in this study at both high altitudes (4–8 km) and in the marine boundary layer by both instruments. Thus vertical transport of CH2O and/or its precursors may provide a greater contribution to upper tropospheric HOx than previously thought. The results of this study, which are based upon instruments employing entirely different measurement principles, calibration, and sampling approaches, not only reinforce this conclusion but also provide a high‐quality database necessary to further explore CH2O measurement‐model relationships in the clean background atmosphere.

Comparisons of box model calculations and measurements of formaldehyde from the 1997 North Atlantic Regional Experiment

[1] Formaldehyde (CH2O) measurements from two independent instruments are compared with photochemical box model calculations. The measurements were made on the National Oceanic and Atmospheric Administration P-3 aircraft as part of the 1997 North Atlantic Regional Experiment (NARE 97). The data set considered here consists of air masses sampled between 0 and 8 km over the North Atlantic Ocean which do not show recent influence from emissions or transport. These air masses therefore should be in photochemical steady state with respect to CH2O when constrained by the other P-3 measurements, and methane oxidation was expected to be the predominant source of CH2O in these air masses. For this data set both instruments measured identical CH2O concentrations to within 40 parts per trillion by volume (pptv) on average over the 0–800 pptv range, although differences larger than the combined 2σ total uncertainty estimates were observed between the two instruments in 11% of the data. Both instruments produced higher CH2O concentrations than the model in more than 90% of this data set, with a median measured-modeled [CH2O] difference of 0.13 or 0.18 ppbv (depending on the instrument), or about a factor of 2. Such large differences cannot be accounted for by varying model input parameters within their respective uncertainty ranges. After examining the possible reasons for the model-measurement discrepancy, we conclude that there are probably one or more additional unknown sources of CH2O in the North Atlantic troposphere.

Alkyl nitrate measurements during STERAO 1996 and NARE 1997: Intercomparison and survey of results

Alkyl nitrates were measured over the northeastern plains of Colorado during the Stratospheric Tropospheric Exchange: Radiation, Aerosols and Ozone (STERAO) 1996 campaign and over the North Atlantic during the North Atlantic Regional Experiment (NARE) 1997. Alkyl nitrate measurements were performed by two different laboratories enabling an intercomparison of analytical techniques. A weighted bivariate linear regression analysis of comparable samples (National Oceanic and Atmospheric Administration versus National Center for Atmospheric Research) for the four alkyl nitrates measured by both techniques (2‐propyl nitrate, 1‐propyl nitrate, 2‐butyl nitrate, and 3‐pentyl nitrate) suggests there is neither a significant calibration difference nor a systematic offset. The precision of the measurements is indicated by the scatter in the correlations. Good precision is observed for the 2‐butyl nitrate measurements (square of the correlation coefficient, R2 = 0.90). However, relatively small values of R2 and relatively large uncertainties in the determination of the slopes and intercepts (2σ confidence intervals) for 2‐propyl nitrate, 1‐propyl nitrate, and 3‐pentyl nitrate suggest significant imprecision when compared to ambient mixing ratios for at least one of the measurement techniques. Vertical profiles during NARE 1997 show high levels of alkyl nitrates in well‐defined photochemical pollution layers of continental origin. A comparison of alkyl nitrate to hydrocarbon ratios with a sequential reaction scheme model suggests our understanding of C4–C5 alkyl nitrate photochemistry is adequate to reproduce the observations. Air masses with larger ratios of alkyl nitrate to hydrocarbons were observed during NARE 1997 than STERAO 1996, consistent with the NARE 1997 measurements being in the more remote North Atlantic. These results suggest that information related to the photochemical age of an air mass can be derived from alkyl nitrate to hydrocarbon ratios.

Trace gas signatures of the airstreams within North Atlantic cyclones: Case studies from the North Atlantic Regional Experiment (NARE ’97) aircraft intensive

This study reveals how airstreams within midlatitude cyclones draw and export trace gases from the polluted continental boundary layer, the midtroposphere, and the stratosphere of North America to the troposphere of the North Atlantic Ocean. The North Atlantic Regional Experiment (NARE) produced aircraft‐based trace gas measurements from eight midlatitude cyclones during the autumn of 1997. Meteorological and back trajectory analysis identified the various component airstreams of several cyclones, including a cold conveyor belt, two warm conveyor belts, a dry airstream, a previously undefined post cold front air‐stream, and a streamer fragment that originated in a dry airstream off the coast of California. O3, CO, and NOy mixing ratio distributions and relationships were determined for each airstream. Airstream chemical composition was related to the origin and transport history of the associated air mass. The lowest O3 values were associated with airstreams originating in Canada or the Atlantic Ocean marine boundary layer; the highest O3 values were associated with airstreams with a recent stratospheric component. The highest CO values were associated with lower tropospheric outflow from New England and a warm conveyor belt that advected boundary layer CO from the southeast United States to the mid and upper troposphere. The highest NOy values were also the result of lower troposphere polluted outflow from New England. Most NOy was removed from the airstreams that transported polluted boundary layer air to the free troposphere. A steep and positive O3/NOy slope was found for all airstreams in the free troposphere regardless of air mass origin.

Vertical distributions of sulfur species simulated by large scale atmospheric models in COSAM: Comparison with observations

A comparison of large-scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on wet deposition measurements and sulfate aerosol concentrations in source regions at the surface. They found that different models simulated the observed sulfate surface concentrations mostly within a factor of two, but that the simulated column burdens and vertical profiles were very different amongst different models. In the COSAM exercise, one aspect is the comparison of sulfate aerosol and precursor gases above the surface. Vertical profiles of SO2, SO2−4, oxidants and cloud properties were measured by aircraft during the North Atlantic Regional Experiment (NARE) experiment in August/September 1993 off the coast of Nova Scotia and during the Second Eulerian Model Evaluation Field Study (EMEFSII), in central Ontario in March/April 1990. While no single model stands out as being best or worst, the general tendency is that those models simulating the full oxidant chemistry tend to agree best with observations although differences in transport and treatment of clouds are important as well.

Effects of cloud microphysical parameters on the global energy budget

Cloud microphysical parameters, such as cloud droplet size (r), droplet number concentration (Nd) and liquid water content (LWC), play an important role in the energy budgets of clouds, and thus the climate of the earth's atmosphere. The observations were collected during the North Atlantic Regional Experiment (NARE) near Yarmouth, Nova Scotia between 5 August and 8 September 1993. Measurements collected by the Twin Otter of the National Research Council (NRC) of Canada, and averaged over approximately 60 km (15 min) at constant altitudes, were used to estimate mean, standard deviation (σ) median, 20% and 80% percentiles. Frequency occurrence and autocorrelation coefficients are obtained for 15 cases. For each case, two-day back-trajectories are used to understand better how the source of aerosols plays a role in affecting cloud microphysical parameters. The results suggest that frequency occurrence values of microphysical data at 20% and 80% can be important for climate studies of extreme temperature values and the energy budget of the atmosphere.

Mixing of anthropogenic pollution with stratospheric ozone: A case study from the North Atlantic wintertime troposphere

As part of the North Atlantic Regional Experiment (NARE), instrumentation for the measurement of O3 and CO was included on research flights conducted by the National Oceanic and Atmospheric Administration WP‐3D Orion aircraft from St. John's, Newfoundland, Canada, and Keflavik, Iceland, from February 2 to 25, 1999. These flights sampled the lower troposphere over the western North Atlantic Ocean. One significant feature observed during these flights was the close proximity of air masses with contrasting source signatures: high levels of anthropogenic pollution immediately adjacent to elevated O3 of stratospheric origin. Here we present a case study showing the most pronounced example of this proximity, which was associated with a frontal passage across North America and out into the North Atlantic region. Trajectory analyses and satellite imagery are used to investigate the transport mechanisms that create the interleaving of air masses from the different sources. One important chemical feature was noted: in air masses with differing amounts of anthropogenic pollution admixed, O3 was negatively correlated with CO, which indicates that emissions from surface anthropogenic sources had reduced O3 in this wintertime period, even in air masses transported into the free troposphere.

Airborne intercomparison of vacuum ultraviolet fluorescence and tunable diode laser absorption measurements of tropospheric carbon monoxide

During the fall 1997 North Atlantic Regional Experiment (NARE 97), two separate intercomparisons of aircraft‐based carbon monoxide measurement instrumentation were conducted. On September 2, CO measurements were simultaneously made aboard the National Oceanic and Atmospheric Administration (NOAA) WP‐3 by vacuum ultraviolet (VUV) fluorescence and by tunable diode laser absorption spectroscopy (TDLAS). On September 18, an intercomparison flight was conducted between two separate instruments, both employing the VUV fluorescence method, on the NOAA WP‐3 and the U.K. Meteorological Office C‐130 Hercules. The results indicate that both of the VUV fluorescence instruments and the TDLAS system are capable of measuring ambient CO accurately and precisely with no apparent interferences in 5 s. The accuracy of the measurements, based upon three independent calibration systems, is indicated by the agreement to within 11% with systematic offsets of less than 1 ppbv. In addition, one of the groups participated in the Measurement of Air Pollution From Satellite (MAPS) intercomparison [Novelli et al., 1998] with a different measurement technique but very similar calibration system, and agreed with the accepted analysis to within 5%. The precision of the measurements is indicated by the variability of the ratio of simultaneous measurements from the separate instruments. This variability is consistent with the estimated precisions of 1.5 ppbv and 2.2 ppbv for the 5 s average results of the C‐130 and the WP‐3 instruments, respectively, and indicates a precision of approximately 3.6% for the TDLAS instrument. The excellent agreement of the instruments in both intercomparisons demonstrates that significant interferences in the measurements are absent in air masses that ranged from 7 km in the midtroposphere to boundary layer conditions including subtropical marine air and continental outflow with embedded urban plumes. The intercomparison of the two VUV instruments that differed widely in their design indicates that the VUV fluorescence technique for CO measurements is not particularly sensitive to the details of its implementation. These intercomparisons help to establish the reliability of ambient CO measurements by the VUV fluorescence technique.

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.

Design and initial characterization of an inlet for gas‐phase NOy measurements from aircraft

An understanding of gas‐phase HNO3 transmission through an inlet is necessary to evaluate the quality of NOy measurements from an aircraft platform. A simple, inexpensive, low‐volume Teflon inlet is described and its suitability as an aircraft inlet for gas‐phase NOy is assessed. Aerosol transmission is not characterized, but inlet design and orientation probably discriminates against the majority of aerosol by mass. Laboratory data, in‐flight HNO3 standard addition calibrations, and ambient NOy measurements from the 1997 North Atlantic Regional Experiment aircraft mission are used to characterize inlet transmission efficiencies and time constants. Laboratory tests show high transmission efficiencies for HNO3 which are relatively independent of ambient temperature and humidity. In‐flight standard addition calibrations were carried out at ambient temperatures ranging from −20° to +8°C and relative humidities from 3% to 71%. These data suggest that nearly all the sampled air contacts an inlet surface, with 90% of added HNO3 being transmitted in ∼1.5 s. Ambient data are presented to demonstrate negligible hysteresis in 1‐Hz NOy measurements, relative to variability observed in ozone data, from an air mass where HNO3 is expected to be a large fraction of the total NOy. Power spectra of ambient NOy (at temperatures from −35° to +35°C and relative humidities from 3% to 100%) and ozone measurements suggest an effective NOy instrument time constant of ∼2 s.

Comparing Different Cloud Schemes of a Single Column Model by Using Mesoscale Forcing and Nudging Technique

Abstract Different cloud schemes are compared using the single column model (SCM) version of the general circulation model of the Canadian Centre for Climate Modelling and Analysis. Emphasis is placed on the differences between a statistical cloud scheme and an explicit one, two approaches commonly used in GCMs. The microphysical processes are identical in both schemes so that the differences can be attributed to cloud formation and dissipation only. Two case studies are chosen, one for a day during the European Cloud and Radiation Experiment (EUCREX) and one for a day during the North Atlantic Regional Experiment (NARE). During the EUCREX case study the SCM is forced by advection from the mesoscale model GESIMA (Geesthacht Simulation Model of the Atmosphere). The comparison of ice water content as a function of height shows that the SCM cannot reproduce the observed nearly linear decrease with height as well as GESIMA does above 8.5 km. If temperature, specific humidity, and cloud ice advection are used ...

Photochemical production and loss rates of ozone at Sable Island, Nova Scotia during the North Atlantic Regional Experiment (NARE) 1993 summer intensive

Three weeks of summertime surface‐based chemical and meteorological observations at Sable Island, Nova Scotia during the North Atlantic Regional Experiment (NARE) 1993 summer intensive are used to study instantaneous photochemical production and loss rates of ozone by means of a numerical photochemical model. Results are most sensitive to the averaging scheme of data used to constrain the model and the ambient variability of the measurements. Model simulations driven by a time series of 5 min averaged data, most representative of the chemistry at the site, yield an average net photochemical ozone production of 3.6 ppbv/d. Estimates of net ozone production designed to filter out local sources, by using 1000–1400 LT median values of observations to drive the model and by excluding short‐lived hydrocarbons, give values ranging from 1 to 4 ppbv/d. These positive values of net ozone production within the marine boundary layer over Sable Island demonstrate the impact of polluted continental plumes on the background photochemistry of the region during the intensive. The dominant ambient variables controlling photochemical production and loss rates of ozone at the site during the measurement campaign appear to be levels of nitrogen oxides, ozone, nonmethane hydrocarbons, and solar intensity determined by cloud cover. The model partitioning of nitrogen oxides agrees for the most part with measurements, lending credence to calculated photochemical production and loss rates of ozone as well as inferred levels of peroxy radicals not measured at the site. Discrepancies, however, often occur during episodes of intermittent cloud cover, fog, and rain, suggesting the influence of cloud processes on air masses reaching the site.

Spatial and temporal variability of nonmethane hydrocarbon mixing ratios and their relation to photochemical lifetime

The relationship between temporal and spatial variability of C2‐C8 nonmethane hydrocarbon mixing ratios and their HO lifetimes (τ) is presented for samples collected during the 1993 North Atlantic Regional Experiment (NARE) and from other urban and remote sites. The C2‐C4 alkanes, acetylene and benzene typically define a trend of the form slnx = Aτ−b where slnx is the standard deviation of the ln of the mixing ratio. The relationship extended over a wider range of hydrocarbons in winter. The exponent b ranged in value from 0.28±0.023 for winter urban data where C2‐C8 hydrocarbons defined a strongly correlated trend, to 0.56±0.15 for C2‐C4 hydrocarbons at a coastal site in Nova Scotia during NARE. The trends are significantly different from that given by the Junge relationship [Junge, 1974]. Data from the Azores do not display such a trend and were likely influenced by local emissions. Variance trends are a useful analytical tool for examining the validity of hydrocarbon measurements.

Photochemical box modeling of long‐range transport from North America to Tenerife during the North Atlantic Regional Experiment (NARE) 1993

A photochemical trajectory model was used to reproduce the concentrations of several trace gases measured at Izaña/Tenerife during a selected episode of August 1993 with long‐range transport from the North American continent across the Atlantic Ocean. The set of initial conditions necessary to match the observations is analyzed and compared with available data in the literature. Initial and final concentrations of several hydrocarbons are in reasonable agreement with other measurements made at similar latitudes, and the CO/NOy and NMHC/NOy ratios are within the range of those observed from airborne measurements over the eastern United States and Canada. Important exceptions are the light olefines ethene and propene which may originate from oceanic emissions. The simulated concentration of ozone at Izaña is very similar to the initial concentration used, and there is no indication for photochemical net ozone production during long‐range transport. The abundance of NOx at the end of the simulation is largely due to decomposition of peroxyacetyl nitrate during the descent of the air before reaching Izaña. The results differ significantly from those of a steady state calculation which was made for comparison, and the height evolution of the trajectory was found to be a critical parameter. A parameter variation study revealed that significant net ozone production only occurs for very low initial ozone concentrations and/or NOy levels that are well above those observed during the period at Izaña at night. This study also emphasizes the important role of water vapor for the tropospheric ozone budget and the role ozone itself plays in its photochemical balance.

Measurement of total nonmethane organic carbon (Cy): Development and application at Chebogue Point, Nova Scotia, during the 1993 North Atlantic Regional Experiment campaign

A technique was developed for the measurement of total nonmethane organic carbon (Cy) in the gas phase based on cryogenic collection, preseparation of CO, CH4 and CO2, conversion of all carbon to CH4, and measurement by flame ionization detector. Collection and recovery efficiencies were found to be essentially quantitative for C2‐C7 hydrocarbons and better than 95 and 93% for methanol and formaldehyde, respectively. CO2 prevented the measurement of the C2 hydrocarbons in atmospheric samples, and system response to ambient water vapor limited the precision of the measurement at very low Cy. The measurement system had an estimated uncertainty of 10% at ambient mixing ratios of 100 ppb C, and a detection limit of between 5 and 7 ppb C. Cy was measured in Boulder, Colorado, during June of 1993 and at Chebogue Point, Nova Scotia, during the North Atlantic Regional Experiment intensive campaign, August to September 1993. Cy values ranged from several hundred ppb C to 11 ppb C in Boulder, and from 177 to below 5 ppb C at Chebogue Point, with medians of 55 and 11 ppb C, respectively. Cy was found to correlate with both CO and O3 at higher levels of those two pollutants. The sum of Cy and the C2 hydrocarbons was virtually always equal to or greater than the sum of the hydrocarbons. Several exceptions to this occurred when high HC values were observed in cannister samples but not Cy measurements. These instances were attributed to short‐term perturbations from local sources. There was a net difference between total nonmethane organic carbon and the sum of hydrocarbons and carbonyls of 3.5±7.8 ppb C overall. This difference appears to be broadly correlated with anthropogenic pollution. There does not appear to be a significantly large reservoir of reactive carbon unaccounted for by hydrocarbon or carbonyl measurements.