The Tropospheric Ozone Production about the Spring Equinox (TOPSE) Experiment:  Highlights and Preliminary Results

 

Background to the experiment

 

One of the major research emphases in atmospheric chemistry continues to be the study of the photochemical and dynamic processes that determine the rates of formation and loss of oxidants throughout the atmosphere.  Oxidation processes shape the chemical composition of the atmosphere through transformations that act on the large variety of trace gases emitted from natural and anthropogenic sources.  Since these processes affect the distribution and trends of radiatively important trace gases in the atmosphere, there is a potential feedback between chemistry and climate that has implications for future global change. 

 

Tropospheric ozone plays a central role in the oxidative chemistry of the troposphere, has an important impact on the radiative balance of the atmosphere, and is known to have detrimental effects on human health and agricultural crop production. For these reasons, understanding the processes that control the origin, trends, distribution, and effects of tropospheric ozone have been high priority themes to atmospheric chemistry research over the last few decades. 

 

The processes that control the concentrations and distribution of tropospheric ozone are a complex interplay of chemistry and dynamics.  The sources, transformations, and interactions of radical and reservoir species in the HOx, NOx, and organic carbon families, as well as ozone itself, play a role in determining local rates of ozone production and loss.  In addition to strictly chemical and photochemical processes, ozone distributions are affected at least as significantly by atmospheric dynamics.  Ozone distributions are affected by direct advective transport of ozone and reactive precursors through the troposphere, by stratosphere/troposphere exchange, and through convective processes which rapidly transport ozone and precursors to different environments of radiation, temperature, humidity, aerosol loading, etc.

 

A feature of tropospheric ozone that is thought to have significant contributions from both chemical and dynamic origins is the widely observed springtime maximum of ozone in the Northern Hemisphere mid latitudes.  The origin of this maximum has been attributed to increased input of ozone from the stratosphere to the troposphere, but it has also been suggested that tropospheric photochemical processes may also play a significant role in producing the maximum.

 

Experimental Design

 

The TOPSE investigation combined model studies and simulations with a set of chemical and photochemical measurements taken over the critical winter-spring transition in the northern mid-to-high latitude troposphere. The overall goal of the experiment was to investigate the chemical and dynamic evolution of tropospheric chemical composition over mid- to high-latitude continental North America during the winter/spring transition; a particular emphasis was placed on the springtime ozone maximum in the troposphere. As the experiment progressed additional effort was used to investigate dramatic ozone depletion events found over large regions of the Arctic surface layer.

 

 

Figure 1. Interior of NCAR/NSF C-130 aircraft showing TOPSE chemical instrumentations and exterior at airfield at Churchill, Manitoba, Canada (Feb, 2000).

 

The experiment took place from February to May, 2000 in a series of 7 round trip missions from Colorado to northern latitudes. Using the NCAR C-130 flying laboratory (Figure 1), the TOPSE experiment covered a latitude range from 40 N to near the North

Pole, and from 100 ft to 25,000 ft. in altitude. The experimental design used in-situ and remote sensing observations to evaluate the rapid onset and early stages of active photochemistry in the Northern Hemisphere. These observations provided a unique characterization of the temporal and spatial distribution of ozone and ozone precursors over continental North America, and, more importantly, a better understanding of the primary photochemical and dynamic processes that control the budgets of radicals and reservoirs in the free troposphere. Measurements of ozone chemistry in the Arctic boundary layer provided important new data to document ozone loss processes near the surface.  Furthermore, through appropriate comparisons of measurements and model predictions, the TOPSE experiment is providing important tests and constraints of photochemical models that will advance the development of global and regional simulations of tropospheric ozone.   Investigators who contribute to experimental and modeling research activities in TOPSE are listed in Table 1.

 

 

Table 1. List of investigators and collaborators in the TOPSE Program.

Name                                Institution                    Activity

D. Blake/N. Blake                 UC Irvine                    NMHC/Halocarbons

E. Browell/W.Grant               NASA-Langley          Ozone/Aerosol LIDAR

R. Cohen/J. Thornton            UC Berkeley              NO2 (LIF)

J. Dibb                                    UNH                            Be-7/Be-10/Pb-210

B. Heikes/J. Snow                URI                              Speciated Peroxides

R. Talbot                                 UNH                            Soluble Acids/Aerosols

R. Weber                                GIT                              Ultrafine Aerosol

J. Moody                                U.Va                           Traj./remote sensing

J. Merrill                                  URI                              Meterology/O3-sondes

D. Allen/K. Pickering            UMd                            Mesoscale/ 3-D

Y.Wang                                   Rutgers Univ.             Modeling

D. Jacob/M. Evans               Harvard                      3-D Modeling

B. Ridley                                 NCAR/ACD               NOx,y; in situ O3, Co-PI

F. Flocke/A. Weinheimer     NCAR/ACD               PAN/PPN

A. Fried                                  NCAR/ACD               Formaldehyde/Peroxide

L. Mauldin/F. Eisele              NCAR/ACD               OH, H2SO4, other (CIMS)

M. Zondlo                               NCAR/ACD               HNO3 (CIMS)

C. Cantrell                              NCAR/ACD               HO2/RO2; Mission Co-PI

M. Coffey/J. Hannigan          NCAR/ACD               CO, N2O

R. Shetter                               NCAR/ACD               Photolysis Rates

E. Atlas                                   NCAR/ACD               Mission Scientist, Co-PI

S. Madronich                         NCAR/ACD               Radiation/Process models

A. Klonecki/P. Hess              NCAR/ACD/ASP      Mesoscale Model and Forecast

L. Emmons/X. Tie/D. Kinnison                                NCAR/ACD   Global Modeling

 

Collaborative Research

J. Bottenheim                        Environ. Canada       Polar Sunrise 2000

J. D. Fast                               Pac. NW Lab             Strat. Folds

S. Penkett/K. Law/C. Reeves                                  UEA    MAXOX/UTLS Ozone

P. Shepson                            Purdue                        Polar Sunrise 2000

E. Schuepbach                      U. Berne                     Jungfraujoch Ozone

J. Dibb et al.                          UNH                            Summit Greenland Obs.

P. Newman                            NASA GSFC             Forecasting/Strat. Ozone

A. Richter/J. Burrows            U. Bremen                  GOME BrO

G. Brasseur                           MPI/NCAR                 Global Modeling

 

 

Highlights and Preliminary Results

           

The analysis and interpretation of the extremely large and rich data set from the TOPSE missions is in its very early stage.  Some of the significant observations and inferences are given in the text below, but it should be appreciated that the full scientific story of the TOPSE experiment will be based on more complete and detailed analyses than have been done to date.

 

Ozone and aerosols

           

Large-scale changes in lower tropospheric ozone, aerosols and in the Arctic were observed for the first time over the entire winter-to-spring transition during TOPSE.  Several significant features of the ozone distribution were noted from the measurements: 1) a broad scale ozone increase in the mid troposphere, including plumes with elevated ozone produced by photochemical processes in the troposphere; 2) stratospheric intrusions of containing high levels of ozone (mostly at the highest altitudes sampled by the aircraft); and 3) extensive areas in the boundary layer where ozone was depleted to near zero levels.   One example showing all of these features is shown in Figure 2 below.

 

During TOPSE, ozone in the Arctic lower troposphere was found to increase at a rate greater than 5 ppbv/mo from early February to mid-May, rising to over 60 ppbv by mid-May (tropospheric ozone from 60-85o N, 2-5 km increased 5 ppbv/mo according to the UV Differential Absorption Lidar (DIAL) system data and 6 ppbv/mo using the in situ ozone data.).

 

Aerosols were found in numerous tropospheric layers throughout the TOPSE mission, but the thickness and horizontal extent of the layers increased as the season progressed.  These layers also had photochemically enhanced ozone in them, and the ozone levels in the layers also increased with the advance of the season. In situ measurements (by the UHH group) of fine aerosol sulfate mixing ratios were similar to previous values reported for the springtime Arctic troposphere at ground level. Mixing ratios of 250 – 600 pptv were widespread throughout the Arctic at altitudes up to ~7 km. In addition, the influence of Arctic haze on aerosol sulfate was not confined to high latitudes, contrary to expectations from previous measurements.  The correlation between fine aerosol sulfate and condensation nuclei (CN) was variable in the TOPSE region; the correlation ranged from very high in some air parcels to no correlation in others.  Both of these data sets show complex geographic and vertical distributions that will require detailed analyses to understand them.

 

Measurements of large-scale ozone and aerosol layers with the UV DIAL system provided a unique opportunity to document the pervasive nature and important role of transport from urban/industrial regions into the Arctic as a leading mechanism whereby ozone levels are increased during the spring.  While all northern hemisphere continents are likely source regions for the pollution, the specific places of origin will have to be determined from back trajectory calculations. 

 

Stratospheric intrusions/tropospheric folds (SI/TF) were found to play a relatively minor role in affecting tropospheric ozone levels during the mission.  While SI/TF were observed on 14 of 32 flight days, they were less frequent than urban/industrial plumes and seemed to transport less ozone irreversibly into the troposphere than was produced photochemically in the plumes.  (The months with the most observed events were March and April.)

 

 

Figure 2. TOPSE Flight 41.  This image of ozone from the surface to 10.5 km obtained on a flight from Thule to Winnipeg on May 22, 2000 shows different impacts on tropospheric ozone. Note a stratospheric intrusion just before 1500 and a pollution plume between 1630 and 1800.  An ozone depleted layer is measured in the lower 2 km (1400 – 1500) over Baffin Bay.

 

 

The first large-scale airborne observations of very low ozone (<5-10 ppbv) regions just above the surface (0 to 500-2500 m) in the Arctic were made in March through May over Hudson Bay, Baffin Bay, Davis Strait, and over the Arctic Ocean.  A virtual surface ozone hole was observed for the first time over much of Hudson Bay, and over the Arctic Ocean.  Surface ozone loss was observed to occur over a larger region of the arctic than had been previously recognized.

 

Ozone loss in these layers is thought to occur from destruction by natural halogen compounds that become photochemically reactive during polar sunrise. It is believed that the mechanisms leading to the destruction of the ozone involves interplay between gas, aerosol and snow pack chemistry centering on bromine and chlorine. Indeed, measurements of enhanced levels of gaseous and particulate Br by the UNH group and of NMHC by the UCI research team demonstrate the presence of reactive halogen processes associated with the surface ozone depletion. Efforts to understand this process will be enhanced by the comprehensive suite of chemical and radiation measurements during TOPSE.

 

Major differences between the composition of the boundary layer during Low Ozone Events (LOEs) and the normal boundary layer were found during TOPSE. The total reactive oxidized nitrogen (NOy) concentrations are elevated during LOEs due to increases in the PAN (peroxyacetyl nitrate) and PPN (peroxyproprionyl nitrate) concentrations. The NOx concentration seems similar in both normal and low ozone air masses.  Hydrocarbons are significantly depleted during LOEs with the higher carbon alkanes being relatively more depleted. Ethyne is especially depleted, and this indicates the influence of reactive bromine. The NO/NO2 ratio is significantly perturbed from a simple steady state involving photolysis of NO2 and oxidation of NO by O3. H2O2, CH3OOH and CH2O concentrations vary little between these cases

 

To understand this process, TOPSE investigators at Harvard are employing a model that considers the gas and aerosol phase chemistry important in the arctic and includes fluxes from both the ice pack and the free troposphere. The model reproduces some of these features such as the rapid loss of ozone and the decrease in the hydrocarbon concentrations. Increases in NOy due to the change of the speciation towards increased concentrations of PANs is also calculated by the model. The model also has a perturbed NO/NO2 ratio and this is due to the existence of BrO and large radical concentrations. The model fails to reproduce some features. The NOx concentration in the model reduces dramatically as production of BrNO3 and PANs removes NOx. This is not seen in the measurements. The model also calculates less H2O2 and CH2O and more CH3OOH during the LOE than in the normal boundary layer, again a features not observed in the measurements.  

 

Ozone Precursors

 

The chemistry of ozone is driven by a series of photochemical reactions involving nitrogen oxides, carbon monoxide, and volatile organic carbon compounds (hydrocarbons and other compounds).   TOPSE has provided a new and detailed view of the distribution and evolution of these critical ozone precursors that is necessary to fully understand the chemical evolution.  For example, the family of reactive nitrogen oxides includes compounds that interchange between active (NO/NO2) and reservoir (e.g., HNO3, PAN) species.  A major unexpected finding was the predominance of PAN as the most abundant NOy species in TOPSE region.  PAN (and related) compounds account for 50 – 90% of the total oxidized nitrogen reservoir in the winter/spring atmosphere.  Further, the correlation of PAN with ozone during the spring build-up of ozone tends to confirm that in-situ photochemical production of ozone is the major process contributing to the spring maximum in the mid-troposphere. 

 

In addition to the direct measurement of PAN, TOPSE investigators from UC-Berkeley have used a newly developed instrument to make high time resolution observations of the sum of PAN, PPN, N2O5, HNO4 and other NOy compounds of the same bond energy. Intercomparison of these measurements with those of PAN and PPN (NCAR/ACD) show that the new approach is reasonably accurate and precise. Details await a more thorough study. The combination of these two appraoches to PANs will provide constraints on the abundance of HNO4 and of N2O5, two compounds that play an important role in the nitrogen oxide budget but have been left as free parameters in most models because of a lack of observational evidence constraining their abundance.

 

Investigators at UNH have examined the preliminary NOy budget for each of the 42 TOPSE flights.  In general, the agreement between measured NOy and NOy determined from the sum of individual species measurements was quite good (NOy sum = 1.005 NOy meas. + 32).  The latitudinal distribution of the ratio of individual species to measured NOy showed trends not captured well by the initial model simulations.  The data show the following trends:  NOx/NOy was ~ 5% at high latitudes increasing to 15% at 40 N; HNO3/NOy was ~  10% at high latitudes increasing to 40% at 40 N; PAN/NOy was ~ 80% at high latitudes decreasing to 50% at 40 N; and alkyl nitrates/NOy ~ 2% at all latitudes.  The large fraction existing as PAN was unexpected but consistent with its long lifetime at the cold temperatures of the Arctic troposphere.

 

Measurements of non-methane hydrocarbons also provided insight into the evolution of photochemical oxidation from winter to spring.  Consistent with other measurements, the levels of hydrocarbons decreased from their wintertime maximum as increase in oxidation rates proceeded into spring. Details of the rates of hydrocarbon loss will be used to make evaluate integrated radical abundances over the seasonal transition. Also, the NMHC will be valuable as airmass tracers to diagnose source characteristics and long-range transport as more detailed diagnostic studies proceed.

 

Stratospheric/Troposphere Exchange (STE)

 

While some of the TOPSE measurements suggest a minor role for stratospheric ozone in producing the spring maximum, other observations indicated a more significant role for STE. Investigators from UNH identified ~ 45 individual samplings of stratospherically influenced air parcels where O3 and HNO3 were both significantly elevated during the TOPSE missions.  The 7Be aerosol data also indicated a stratospheric influence in nearly all of these cases, supporting our inference that the enhanced O3 and HNO3 were not of anthropogenic origin.  These events occurred over the entire suite of flight profiles during TOPSE, with no bias toward any sampled geographic region.  Overall 57% of the flights (i.e., 24 of 42) were observed to have stratospheric influence in the sampled air parcels.  The correlation between HNO3 and O3 in the stratospherically influenced air parcels showed a slope of 3.5 x 10-3 pptv/ppbv  (r2 = 0.75), which is very similar to values previously reported for stratospheric air.  These results suggest potentially significant inputs of O3 and NOy to the middle troposphere in the middle-to-high latitudes from the stratosphere in late winter and springtime.  However, more detailed meteorological and computer model analyses is required to quantify this input.

 

Three-dimensional computer modeling and analysis

 

An essential part of the TOPSE project has been the interaction between measurements and chemical transport models used to expand the TOPSE data into a broader, hemispheric context.  This interaction has led to some significant findings even with models and data in their preliminary form.  Further modeling efforts are planned.  Thus far, two large scale models have been applied to the TOPSE experiment. – MOZART and HANK.  These models are used to assess the importance of the two processes that are responsible for the build-up of ozone in the mid-troposphere during winter and spring: transport from the stratosphere, where ozone mixing ratios are much higher; and production in the background troposphere from ozone precursors coming from both natural and anthropogenic sources.

 

Models that incorporate the most recent findings relating to atmospheric chemistry and transport are very useful in testing our understanding of processes taking place in the atmosphere, with the discrepancies between measured and modeled values often pointing to areas where more work needs to be done.

 

MOZART, Model for OZone and Related chemical Tracers, a global 3D chemical transport model was run in near-real-time using observed (ECMWF) winds, allowing for immediate preliminary data-model comparisons at the completion of each mission.  These comparisons provided an indication of the influences on the sampled air masses, such as the long-range transport of pollutants from Europe and Asia, or clean air from the Pacific. The TOPSE data set has also provided a unique set of measurements for evaluating the model at northern high latitudes in winter and spring, and shown that the model can generally reproduce long-lived pollutants such as CO and C2H6.  Comparisons with the data have also helped uncover some deficiencies in the model.  For example, the dry deposition of O3 over ice was too high in the model, leading to calculated values of O3 much lower than observed throughout the free troposphere at high latitudes.  The model also underestimates NOx while overestimating HNO3, leading us to investigate possible missing processes in the model.  Further analysis of the model results will provide additional understanding of the reactive nitrogen and ozone budgets during the winter-spring transition; identifying the influence of the stratosphere, long-range transport, and in situ chemistry on the observed distributions during TOPSE.  

 

Results from MOZART show that most of the ozone at high northern latitudes during winter is transported from the stratosphere.  The increase of ozone through the spring was caused by a combination of increased transport from the stratosphere in the upper troposphere and, in the lower troposphere, increased photochemical production from anthropogenic precursors.  These findings are in broad agreement with inferences based on measurements from the C-130 and from ozonesondes.

 

The HANK regional chemical transport model was designed to run for the conditions and domain covering the Arctic region. In a novel approach the model simulated the chemical composition of the troposphere in a "real-time" mode during the campaign, with the model dynamics being driven by the analyzed winds obtained from the weather forecast models from the National Center of Environmental Prediction.

 

A comparison of the tracer concentrations simulated by the HANK model against measured values shows that the model simulated levels of NOx, which is a necessary precursor for ozone production in the troposphere, are significantly lower than the measured ones. The sensitivity studies conducted so far indicate that the heterogeneous removal of NO3 and N2O5 on the surface of aerosols is the major sink for nitrogen oxides. It is possible, that the rate of this removal in the model, which reflects our current understanding of this process, is too high in the Arctic region.

 

The model was used to test earlier hypothesis relating to the transport of pollutants into the Arctic region. In agreement with these earlier studies (Arctic Haze), it was found that the lower troposphere over the Arctic is exposed to significant levels of anthropogenic pollutants in the winter and the major transport pathways are from Europe and Siberia. In the winter, this transport pathway is especially efficient with the pollutants being transported towards and over the north pole, and on one occasion, reaching as far south as the Hudson Bay. The model indicates that during the winter the pollution from the European and Siberian sources are often trapped in the cold continental boundary layer and can be transported over large distances into the Arctic. In the Spring, different meteorological circulation patterns and warming up of the boundary layer, lead to weaker transport into the Arctic and enhanced mixing of pollutants with the free troposphere.

 

Preliminary analysis of the model results indicates that the tropospheric values simulated by the model are very sensitive to the ozone mixing ratios in the lower stratosphere, and therefore to the cross-tropopause transport of ozone. The sensitivity of tropospheric ozone to the levels of stratospheric ozone actually observed in the TOPSE region will be examined in more detail through a variety of different model approaches.