Tropospheric Ozone Production About The Spring Equinox Research Articles

An ozone depletion event in the sub-arctic surface layer over Hudson Bay, Canada

During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) program, aircraft flights during April 7–11, 2000 revealed a large area air mass capped below ∼500 m altitude over Hudson Bay, Canada in which ozone was reduced from normal levels of 30–40 ppbv to as low as 0.5 ppbv. From some of the in-situ aircraft measurements, back-trajectory calculations, the tropospheric column of BrO derived from GOME satellite measurements, and results from a regional model, we conclude that the event did not originate from triggering of reactive halogen release in the sub-Arctic region of Hudson Bay but resulted from such an event occurring at higher latitudes over the islands of the northern Canada Archipelago and nearby Arctic Ocean with subsequent transport over a distance of 1,000–1,500 km to Hudson Bay. BrOx remained active during this transport despite considerable changes in the conditions of the underlying surface suggesting that chemical recycling during transport dominated any local halogen input from the surface. If all of the tropospheric column density of BrO is distributed uniformly within the surface layer, then the mixing ratio of BrO derived from the satellite measurements is at least a factor of 2–3 larger than derived indirectly from in situ aircraft measurements of the NO/NO2 ratio.

Halogen‐driven low‐altitude O3 and hydrocarbon losses in spring at northern high latitudes

Halogen‐driven ozone and hydrocarbon losses in springtime Arctic boundary layer are investigated using a regional chemical transport model. Surface observations of ozone at Alert and Barrow and aircraft observations of ozone and hydrocarbons during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment from February to May in 2000 are analyzed. We prescribe halogen radical distributions on the basis of GOME BrO observations. Tropospheric GOME BrO column shows an apparent anticorrelation with surface temperature over high‐BrO regions. The enhancements of tropospheric BrO columns coincide with movements of cold polar air masses. While GOME BrO measurements reach the maximum in March, simulated near‐surface ozone loss peaks in April because of the increasing daylight hours and hence the time for chemical processing. At its peak, the area of simulated near‐surface ozone depletions (O3 < 20 ppbv) covers >50% of the northern high latitudes. Analysis of surface measurements at Alert and Barrow points to the importance of long‐range transport of ozone‐poor air from high‐BrO regions. We find that specifying a BrO layer thickness of 300 m results in the best overall agreement between observed and simulated ozone. The apparent halogen‐driven ozone loss up to 1 km was reproduced in the model because of vertical transport of ozone‐poor air from low altitudes. When the empirical Cl/Br ratios derived from previous observations are used, the model can reproduce the observed halogen loss of light alkanes and acetylene. The Cl/Br ratios from a recent box model study using an accepted chemical mechanism are, however, much higher than the empirical results. We show that the hydrocarbon loss is not as sensitive to the prescribed thickness of the halogen layer as the ozone loss, therefore representing a more robust measure for evaluating satellite BrO column measurements.

Evidence for the role of ion‐induced particle formation during an atmospheric nucleation event observed in Tropospheric Ozone Production about the Spring Equinox (TOPSE)

Various in situ measurements show evidence of new particle formation over a wide range of latitudes in the middle to upper troposphere and lower stratosphere. However, the exact mechanism of new particle formation is still uncertain. Using a combination of satellite‐derived brightness temperature, air parcel backward trajectory information, in situ measurements of aerosol and precursor gases and an aerosol microphysical model driven by parameterized ion‐induced nucleation, we investigate the mechanism responsible for, and factors leading to, new particle formation in the middle to upper troposphere during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment. The number concentrations of modeled ultrafine particles of diameters 3 to 4 nm (N3–4nm) and 3 to 8 nm (N3–8nm) are consistent with in situ measurements indicating that new particle formation likely occurred by ion‐induced nucleation a day prior to the time of measurement. A reduction in preexisting aerosol surface area in a region of cloud outflow probably triggered particle nucleation. These studies indicate that under typical middle to upper troposphere conditions, the ion mechanism is likely an important source of ultrafine particles, and these newly formed particles can grow to act as cloud condensation nuclei.

Late‐spring increase of trans‐Pacific pollution transport in the upper troposphere

The observations during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment show large enhancements of NOx, PAN, O3, CO, CFCs, and Halon‐1211 in the upper troposphere over North America in late spring. Analysis of these observations and model results indicate that the enhancements are most likely driven by a surge of trans‐Pacific pollutant transport in late spring. The rapid seasonal transition is particularly striking for upper tropospheric NOx, resulting in large increases in photochemical oxidation and O3 production during the period. The transition is later in season than that of low‐altitude trans‐Pacific transport, which peaks in March and April. The current generation of global chemical transport models clearly underestimates this long‐range transport of pollutants, implying an underestimation in the model‐projected impact on regional air quality over North America (through subsidence).

Measurements of the sum of HO<sub>2</sub>NO<sub>2</sub> and CH<sub>3</sub>O<sub>2</sub>NO<sub>2</sub> in the remote troposphere

Abstract. The chemistry of peroxynitric acid (HO2NO2) and methyl peroxynitrate (CH3O2NO2)is predicted to be particularly important in the upper troposphere where temperatures are frequently low enough that these compounds do not rapidly decompose. At temperatures below 240K, we calculate that about 20% of NOy in the mid- and high-latitude upper troposphere is HO2NO2. Under these conditions, the reaction of OH with HO2NO2 is estimated to account for as much as one third of the permanent loss of hydrogen radicals. During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign, we used thermal dissociation laser-induced fluorescence (TD-LIF) to measure the sum of peroxynitrates (PNs HO2NO2+CH3O2NO2+PAN+PPN+...) aboard the NCAR C-130 research aircraft. We infer the sum of HO2NO2 and CH3O2NO2 as the difference between PN measurements and gas chromatographic measurements of the two major peroxy acyl nitrates, peroxy acetyl nitrate (PAN) and peroxy propionyl nitrate (PPN). Comparison with NOy and other nitrogen oxide measurements confirms the importance of HO2NO2 and CH3O2NO2 to the reactive nitrogen budget and shows that current thinking about the chemistry of these species is approximately correct. During the spring high latitude conditions sampled during the TOPSE experiment, the model predictions of the contribution of (HO2NO2+CH3O2NO2) to NOy are highly temperature dependent: on average 30% of NOy at 230K, 15% of NOy at 240K, and 5% of NOy above 250K. The temperature dependence of the inferred concentrations corroborates the contribution of overtone photolysis to the photochemistry of peroxynitric acid. A model that includes IR photolysis (J=1x10-5s-1) agreed with the observed sum of HO2NO2+CH3O2NO2 to better than 35% below 240K where the concentration of these species is largest.

Photochemistry in the Arctic Free Troposphere: Ozone Budget and Its Dependence on Nitrogen Oxides and the Production Rate of Free Radicals

Local ozone production and loss rates for the arctic free troposphere (58–85° N, 1–6 km, February–May) during the TroposphericOzone Production about the Spring Equinox (TOPSE) campaign were calculated using a constrained photochemical box model. Estimates were made to assess the importance of local photochemical ozone production relative to transport in accounting for the springtime maximum in arctic free tropospheric ozone. Ozone production and loss rates from our diel steady-state box model constrained by median observations were first compared to two point box models, one run to instantaneous steady-state and the other run to diel steady-state. A consistent picture of local ozone photochemistry was derived by all three box models suggesting that differences between the approaches were not critical. Our model-derived ozone production rates increased by a factor of 28 in the 1–3 km layer and a factor of 7 in the 3–6 kmlayer between February and May. The arctic ozone budget required net import of ozone into the arctic free troposphere throughout the campaign; however, the transport term exceeded the photochemical production only in the lower free troposphere (1–3 km) between February and March. Gross ozone production rates were calculated to increase linearly with NOx mixing ratiosup to ∼300 pptv in February and for NOx mixing ratios up to ∼500 pptv in May. These NOx limits are an order of magnitude higher thanmedian NOx levels observed, illustrating the strong dependence ofgross ozone production rates on NOx mixing ratios for the majority of theobservations. The threshold NOx mixing ratio needed for netpositive ozone production was also calculated to increase from NOx∼ 10pptv in February to ∼25 pptv in May, suggesting that the NOx levels needed to sustain net ozone production are lower in winter than spring. This lower NOx threshold explains how wintertime photochemical ozone production can impact the build-up of ozone over winter and early spring. There is also an altitude dependence as the threshold NOx neededto produce net ozone shifts to higher values at lower altitudes. This partly explains the calculation of net ozone destruction for the 1–3 km layerand net ozone production for the 3–6 km layer throughout the campaign.

Intercontinental transport of pollution manifested in the variability and seasonal trend of springtime O3 at northern middle and high latitudes

Observations (0–8 km) from the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment are analyzed to examine air masses contributing to the observed variability of springtime O3 and its seasonal increase at 40°–85°N over North America. Factor analysis using the positive matrix factorization and principal component analysis methods is applied to the data set with 14 chemical tracers (O3, NOy, PAN, CO, CH4, C2H2, C3H8, CH3Cl, CH3Br, C2Cl4, CFC‐11, HCFC‐141B, Halon‐1211, and 7Be) and one dynamic tracer (potential temperature). Our analysis results are biased by the measurements at 5–8 km (70% of the data) due to the availability of 7Be measurements. The identified tracer characteristics for seven factors are generally consistent with the geographical origins derived from their 10 day back trajectories. Stratospherically influenced air accounts for 14 ppbv (35–40%) of the observed O3 variability for data with O3 concentrations <100 ppbv at middle and high latitudes. It accounts for about 2.5 ppbv/month (40%) of the seasonal O3 trend at midlatitudes but for only 0.8 ppbv/month (<20%) at high latitudes, likely reflecting more vigorous midlatitude dynamical systems in spring. At midlatitudes, reactive nitrogen‐rich air masses transported through Asia are much more significant (11 ppbv in variability and 3.5 ppbv/month in trend) than other tropospheric contributors. At high latitudes the O3 variability is significantly influenced by air masses transported from lower latitudes (11 ppbv), which are poor in reactive nitrogen. The O3 trend, in contrast, is largely defined by air masses rich in reactive nitrogen transported through Asia and Europe across the Pacific or the Arctic (3 ppbv/month). The influence from the stratospheric source is more apparent at 6–8 km, while the effect of O3 production and transport within the troposphere is more apparent at lower altitudes. The overall effect of tropospheric photochemical production, through long‐range transport, on the observed O3 variability and its seasonal trend is more important at high latitudes relative to more photochemically active midlatitudes.

Unexplained enhancements of CH3Br in the Arctic and sub-Arctic lower troposphere during TOPSE spring 2000

[1] Elevated concentrations of methyl bromide (CH3Br) were observed in the Arctic atmospheric boundary layer (BL) during periods of widespread BL ozone (O3) depletion episodes (ODEs: O3 mixing ratios < 20 × 10−9 or parts per billion by volume, ppbv) particularly during major ODEs (MODES: O3 < 4 ppbv). No other organic gases measured during TOPSE (Tropospheric Ozone Production about the Spring Equinox) exhibited anti-correlations with O3 during these ODEs. Methyl bromide has both natural and anthropogenic sources and contributes ∼ half of the bromine (Br) to the stratosphere, where it can catalytically destroy O3. Several known CH3Br sources are evaluated, but the current knowledge cannot explain the observed enhancements. If the mechanism is direct gas-phase photochemical production, a significant portion of the unknown CH3Br source may be found.

Budget of tropospheric ozone during TOPSE from two chemical transport models

The tropospheric ozone budget during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign has been studied using two chemical transport models (CTMs): HANK and the Model of Ozone and Related chemical Tracers, version 2 (MOZART‐2). The two models have similar chemical schemes but use different meteorological fields, with HANK using MM5 (Pennsylvania State University, National Center for Atmospheric Research Mesoscale Modeling System) and MOZART‐2 driven by European Centre for Medium‐Range Weather Forecasts (ECMWF) fields. Both models simulate ozone in good agreement with the observations but underestimate NOx. The models indicate that in the troposphere, averaged over the northern middle and high latitudes, chemical production of ozone drives the increase of ozone seen in the spring. Both ozone gross chemical production and loss increase greatly over the spring months. The in situ production is much larger than the net stratospheric input, and the deposition and horizontal fluxes are relatively small in comparison to chemical destruction. The net production depends sensitively on the concentrations of H2O, HO2 and NO, which differ slightly in the two models. Both models underestimate the chemical production calculated in a steady state model using TOPSE measurements, but the chemical loss rates agree well. Measures of the stratospheric influence on tropospheric ozone in relation to in situ ozone production are discussed. Two different estimates of the stratospheric fraction of O3 in the Northern Hemisphere troposphere indicate it decreases from 30–50% in February to 15–30% in June. A sensitivity study of the effect of a perturbation in the vertical flux on tropospheric ozone indicates the contribution from the stratosphere is approximately 15%.

Peroxy radical observations using chemical ionization mass spectrometry during TOPSE

Peroxy radicals (HO2 + RO2) were measured by chemical conversion‐chemical ionization mass spectroscopy in the TOPSE (Tropospheric Ozone Production about the Spring Equinox) campaign that took place February through May 2000. Instrumentation for these measurements was deployed on the NCAR/NSF C‐130 aircraft that flew at latitudes from 40 to 85°N, and altitudes from the surface to 7.5 km over the North American continent. The measurements demonstrate the evolution of photochemical activity as time progresses through the study period due to increases in free radical source rates. The increase in average peroxy radical concentration moves northward as the maximum solar elevation and length of sunlit days increase. HOxROx (HO2 + RO2) concentrations are distributed lognormally with means of 11.5 and 7.8 pptv for the middle‐latitude band (MLB) and high‐latitude band (HLB), respectively. The observations agree well on average with steady state derived concentrations; measurement‐model concentration ratios are 1.04 (MLB) and 0.94 (HLB). Concentrations within a given latitude band and altitude region sometimes appear to increase with NOx concentrations, but this correlation nearly disappears at low and moderate NOx levels when the data are parsed by radical production rate; lower radical levels are observed at the highest NOx levels measured (near 1 ppbv). These data are compared with results from other recent observations utilizing a variety of platforms.

Measurements of OH, H2SO4, and MSA during Tropospheric Ozone Production About the Spring Equinox (TOPSE)

Data from OH, H2SO4, and MSA measurements performed during Tropospheric Ozone Production About the Spring Equinox (TOPSE) are presented. Model simulations of OH results made at solar zenith angles of 60° or less showed a tendency of the model to overestimate OH concentrations. This overestimation was a factor of 1.7 at the lower latitudes (&lt;57°N) of the study. At higher latitudes (&gt;57°N) the model tended toward agreement and ultimately an underestimation of OH concentrations by a factor of 0.7. Comparisons of measurements and model showed the model underestimates OH concentrations at solar zenith angles greater than 70°. The results of the present study are discussed in the context of previous studies. Possible model discrepancies are discussed. Measurements of H2SO4 showed the highest concentrations at the lowest altitudes (&lt;2000 m) and the lowest latitudes of the study. Larger H2SO4 concentrations observed at higher latitudes were accompanied at times by particle nucleation as indicated by the presence of UCN (ultra‐fine condensation nuclei) with diameters of 3–4 nm. Concentrations of MSA were generally low with typical values of &lt;2 × 105 molecule cm−3. High concentrations (&gt;107 molecule cm−3) were found in layers and were accompanied by other compounds such as NOx and NOy indicating MSA may also have an industrial source.

Seasonal changes in the transport of pollutants into the Arctic troposphere‐model study

In association with Tropospheric Ozone Production about the Spring Equinox (TOPSE) measurement campaign a regional episodic chemical transport model is used to study the seasonal mechanisms of transport of pollutants from their Northern Hemisphere emission regions into the remainder of the troposphere. The model simulates the strong seasonal cycle for CO and hydrocarbons that agrees well with TOPSE measurements. In this study we use the isentropic perspective to analyze transport during different seasons and from different emission regions. Simulations with diagnostic tracers are conducted to quantify (1) the contribution of different emission regions to tracer distribution and (2) the importance of cross‐isentropic transport. In the high latitudes during winter, in agreement with previous studies, the European and Siberian emission sources are the largest contributors to the diagnostic tracer distributions in the lower troposphere due to large‐scale circulation patterns, low temperatures at the source and presence of a cold stable boundary layer that facilitates diabatic cooling. North American emissions are located further south, often south of the polar front, and are on average emitted at higher potential temperatures. Owing to their predominant transport over the relatively warm Atlantic, they experience strong diabatic heating due to the general instability in the air column and heavy precipitation in the storm track. During the summer months the pollution from all emission regions is more likely to be diabatically transported to higher potential temperatures and diluted. In addition, direct transport of pollutants into the Arctic is less frequent during summer due to the differences in the large‐scale circulation patterns.

The Tropospheric Ozone Production about the Spring Equinox (TOPSE) Experiment: Introduction

The Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment was conducted during February–March, 2000 to study the evolution of tropospheric chemistry at mid to high latitudes over North America. The experiment used airborne in situ and remote sensing measurements of trace gases, radiation, and aerosols, combined with model simulations, to describe the major processes that control evolution of ozone and oxidants during the winter‐spring transition. This paper introduces the major scientific questions of the TOPSE experiment, describes the conduct of the mission, and introduces the scientific results discussed in a series of companion papers in this special section. Among the significant findings of the experiment was that tropospheric ozone increases from winter to spring were dominated by in situ photochemical production in the troposphere, rather than by transport from the stratosphere. Also unique observations of widespread surface ozone depletion events, and of seasonal evolution of trace gases and aerosols as a function of latitude and altitude, were obtained over the course of the mission.

Ozone, aerosol, potential vorticity, and trace gas trends observed at high‐latitudes over North America from February to May 2000

Ozone (O3) and aerosol scattering ratio profiles were obtained from airborne lidar measurements on thirty‐eight flights over seven deployments covering the latitudes of 40°–85°N between 4 February and 23 May 2000 as part of the Tropospheric Ozone Production about the Spring Equinox (TOPSE) field experiment. Each deployment started from Broomfield, Colorado, with bases in Churchill, Canada, and on most deployments, Thule Air Base, Greenland. Nadir and zenith lidar O3 measurements were combined with in situ O3 measurements to produce vertically continuous O3 profiles from near the surface to above the tropopause. Potential vorticity (PV) distributions along the flight track were obtained from several different meteorological analyses. Ozone, aerosol, and PV distributions were used together to identify the presence of pollution plumes and stratospheric intrusions. Ozone was found to increase in the middle free troposphere (4–6 km) at high latitudes (60°–85°N) by an average of 4.6 ppbv/mo (parts per billion by volume per month) from about 54 ppbv in early February to over 72 ppbv in mid‐May. The average aerosol scattering ratios at 1064 nm in the same region increased rapidly at an average rate of 0.36/mo from about 0.38 to over 1.7. Ozone and aerosol scattering were highly correlated over the entire field experiment, and PV and beryllium (7Be) showed no significant positive trend over the same period. The primary cause of the observed O3 increase in the mid troposphere at high latitudes was determined to be the photochemical production of O3 in pollution plumes with less than 20% of the increase from stratospherically‐derived O3.

Effect of sulfate aerosol on tropospheric NOx and ozone budgets: Model simulations and TOPSE evidence

The distributions of NOx and O3 are analyzed during TOPSE (Tropospheric Ozone Production about the Spring Equinox). In this study these data are compared with the calculations of a global chemical/transport model (Model for OZone And Related chemical Tracers (MOZART)). Specifically, the effect that hydrolysis of N2O5 on sulfate aerosols has on tropospheric NOx and O3 budgets is studied. The results show that without this heterogeneous reaction, the model significantly overestimates NOx concentrations at high latitudes of the Northern Hemisphere (NH) in winter and spring in comparison to the observations during TOPSE; with this reaction, modeled NOx concentrations are close to the measured values. This comparison provides evidence that the hydrolysis of N2O5 on sulfate aerosol plays an important role in controlling the tropospheric NOx and O3 budgets. The calculated reduction of NOx attributed to this reaction is 80 to 90% in winter at high latitudes over North America. Because of the reduction of NOx, O3 concentrations are also decreased. The maximum O3 reduction occurs in spring although the maximum NOx reduction occurs in winter when photochemical O3 production is relatively low. The uncertainties related to uptake coefficient and aerosol loading in the model is analyzed. The analysis indicates that the changes in NOx due to these uncertainties are much smaller than the impact of hydrolysis of N2O5 on sulfate aerosol. The effect that hydrolysis of N2O5 on global NOx and O3 budgets are also assessed by the model. The results suggest that in the Northern Hemisphere, the average NOx budget decreases 50% due to this reaction in winter and 5% in summer. The average O3 budget is reduced by 8% in winter and 6% in summer. In the Southern Hemisphere (SH), the sulfate aerosol loading is significantly smaller than in the Northern Hemisphere. As a result, sulfate aerosol has little impact on NOx and O3 budgets of the Southern Hemisphere.

Tunable diode laser measurements of formaldehyde during the TOPSE 2000 study: Distributions, trends, and model comparisons

Airborne measurements of formaldehyde (CH2O) were acquired employing tunable diode laser absorption spectroscopy (TDLAS) during the 2000 Tropospheric Ozone Production About the Spring Equinox (TOPSE) study. This study consisted of seven deployments spanning the time period from 4 February to 23 May 2000 and covered a wide latitudinal band from 40°N to 85°N. The median measured CH2O concentrations, with a few exceptions, did not show any clear temporal trends from February to May in each of five altitude and three latitude bins examined. Detailed measurement–model comparisons were carried out using a variety of approaches employing two different steady state models. Because recent emissions of CH2O and/or its precursors often result in model underpredictions, background conditions were identified using a number of chemical tracers. For background conditions at temperatures warmer than −45°C, the measurement–model agreement on average ranged between −13% and +5% (measurement–model/measurement), which corresponded to mean and median (measurement–model) differences of 3 ± 69 and −6 parts per trillion by volume (pptv), respectively. At very low temperatures starting at around −45°C, significant and persistent (measurement–model) differences were observed from February to early April from southern Canada to the Arctic Ocean in the 6–8 km altitude range. In such cases, measured CH2O was as much as 392 pptv higher than modeled, and the median difference was 132 pptv (83%). Low light conditions as well as cold temperatures may be important in this effect. A number of possible mechanisms involving the reaction of CH3O2 with HO2 to produce CH2O directly were investigated, but in each case the discrepancy was only minimally reduced. Other possibilities were also considered but in each case there was no compelling evidence to support any of the hypotheses. Whatever the cause, the elevated CH2O concentrations significantly impact upper tropospheric HOx levels at high latitudes (&gt;57°N) in the February–April time frame.

Winter‐spring evolution and variability of HOx reservoir species, hydrogen peroxide, and methyl hydroperoxide, in the northern middle to high latitudes

The Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment examined the evolution of tropospheric chemical compositions from February to May 2000 over North America, 40 to 85 N. Hydrogen peroxide (H2O2) and methyl hydroperoxide (CH3OOH) were investigated using instrumental observations aboard the NCAR C‐130 research aircraft. Primary TOPSE results indicate both photochemistry and atmospheric dynamics are critical factors controlling the variability of peroxides in this region. From February to May, H2O2 and CH3OOH mixing ratios increased with the greatest relative changes at mid‐altitudes. H2O2 ranged from below the detection limit (BDL = 25 pptv) to 380 pptv in winter and from BDL to 1330 pptv during spring. Winter measurements of CH3OOH were from BDL (35 pptv) to 740 pptv with higher levels of BDL to 1400 pptv measured during spring. Peroxides also decreased with latitude at all altitudes. These findings are consistent with those expected from photochemical theory. Evidence also supports a source of CH3OOH to the Arctic from the transport of subtropical air masses. Air mass back trajectories and GOES‐derived specific humidity products indicate transport of moist tropical air to the study region coincides with elevated levels of CH3OOH up to 940 pptv. The concurrence of this transport regime with episodic elevated CH3OOH events suggests a source of HOx to the Arctic. However, evidence from this study shows CH3OOH does not greatly contribute to total HOx production which is dominated primarily by reactions of O(1D) and H2O at low latitudes and CH2O at high latitudes.

Steady state free radical budgets and ozone photochemistry during TOPSE

A steady state model, constrained by a number of measured quantities, was used to derive peroxy radical levels for the conditions of the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign. The analysis is made using data collected aboard the NCAR/NSF C‐130 aircraft from February through May 2000 at latitudes from 40° to 85°N, and at altitudes from the surface to 7.6 km. HO2 + RO2 radical concentrations were measured during the experiment, which are compared with model results over the domain of the study showing good agreement on the average. Average measurement/model ratios are 1.04 (σ = 0.73) and 0.96 (σ = 0.52) for the MLB and HLB, respectively. Budgets of total peroxy radical levels as well as of individual free radical members were constructed, which reveal interesting differences compared to studies at lower latitudes. The midlatitude part of the study region is a significant net source of ozone, while the high latitudes constitute a small net sink leading to the hypothesis that transport from the middle latitudes can explain the observed increase in ozone in the high latitudes. Radical reservoir species concentrations are modeled and compared with the observations. For most conditions, the model does a good job of reproducing the formaldehyde observations, but the peroxide observations are significantly less than steady state for this study. Photostationary state (PSS) derived total peroxy radical levels and NO/NO2 ratios are compared with the measurements and the model; PSS‐derived results are higher than observations or the steady state model at low NO concentrations.

Signatures of tropopause folding in satellite imagery

A satellite‐based conceptual picture of the stratospheric contribution to the spring ozone peak is presented through a survey of flight measurements and satellite imagery from the Tropospheric Ozone Production about the Spring Equinox (TOPSE) program. We address the stratospheric contribution through the process of tropopause folding across air mass boundaries (the arctic and polar tropopause break) in the polar and midlatitude upper troposphere. These boundaries and the boundaries of their related outgrowing streamers are easily identifiable as sharp gradients in the altered water vapor (AWV) imagery, a linear modification of the GOES water vapor channel that represents upper tropospheric specific humidity. The close relationship between altered brightness temperature and total ozone (from the Total Ozone Mapping Spectrometer (TOMS) and the GOES sounder) along this gradient confirms the utility of AWV imagery in detecting the tropopause break. Transects constructed from aircraft lidar and in situ observations of ozone illustrate the existence of tropopause folding along the AWV gradient. Ten out of ten available transects during the February to May study period show tropopause folding where indicated by the satellite imagery. This is strong evidence that folding is ubiquitous across the polar/stratospheric air mass boundary during the study period, and that the associated intrusions of stratospheric air into the troposphere can be located continuously using AWV imagery.

The seasonal evolution of NMHCs and light alkyl nitrates at middle to high northern latitudes during TOPSE

The Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment was designed to follow the role of photochemistry in the evolution of the springtime maximum of tropospheric ozone (O3) in the Northern Hemisphere (NH) at high latitudes. Determination of the composition and seasonal evolution of volatile organic carbon (VOC) species, which take part in and are good indicators for photochemical processes in the troposphere, was an important part of this study. We report measurements of a large number of C2–C10 nonmethane hydrocarbons (NMHCs), selected C1–C2 halocarbons, and C1–C4 alkyl nitrates. These gases were quantified in whole air samples collected aboard the National Center for Atmospheric Research (NCAR) C‐130 aircraft at altitudes between 30 m and 8 km. Seven TOPSE sampling trips were flown between early February and mid‐May 2000 covering the region from Colorado (40°N) to Churchill (in Manitoba, Canada), Thule (in northern Greenland), and as far north as 85°N. These measurements represent the most comprehensive spatial characterization of the North American Arctic to date and revealed strong latitudinal, vertical, and temporal NMHC gradients. In the midtroposphere north of Churchill (58°N), ΣNMHCs decreased by ∼6.2 ppbC between February and May (1.6 ppbC month−1) and the magnitude of this change diminished with altitude. Over the same period, midtropospheric O3 levels increased by ∼16 ppbv (4.2 ppbv month−1). Free tropospheric NMHC decreases were consistent with removal by hydroxyl (OH) radicals at an average mixing ratio for mid‐March to mid‐May of 4.1 × 105 mol cm−3. The alkyl nitrates, which are a reservoir species for tropospheric reactive odd nitrogen (NOY), revealed similar latitudinal, vertical, and temporal gradients to their parent NMHCs. Their total decreased by ∼4 pptv month−1, representing 10% or less of NOY. In conjunction with meteorological trajectory analysis, different trace gas signatures provided significant clues to the origins of individual polluted air masses. Several of these air masses were rapidly advected over the Pole from source regions in northeastern and western Europe as well as an air mass that originated over the southwestern United States/Baja California that contained unusually high levels of alkanes. In addition, episodes of low boundary layer (BL) O3 associated with low NMHC mixing ratios and trajectories from over the Arctic Ocean were frequently sampled toward the latter part of the experiment. The TOPSE data described here provide a unique picture of NH trace gas evolution from winter to summer that will be invaluable to models investigating the role that anthropogenic emissions play in high latitude O3 chemistry.