1. The effect of pyro-convective fires on the global troposphere: comparison of TOMCAT modelled fields with observations from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) Sarah A Monks, University of Leeds This project has used observations which were collected during the ICARTT campaign (International Consortium for Atmospheric Research on Transport and Transformation) in summer 2004 to compare with the 3D global chemical transport model, TOMCAT. The main objective was to asses whether TOMCAT was capable of reproducing the observed trace species. During the campaign, there were extensive forest fires in Alaska. Biomass burning emits a wide variety of reactive trace gases into the atmosphere including CO 2, hydrocarbons and NO X. To improve the model performance, estimated surface fluxes from the forest fires in Alaska were included in the model. These additional emissions improved the model’s ability to reproduce observed trace gases. Simulated tracer fields showed large enhancements of CO, C 2 H 6, C 3 H 8 and O 3 in plumes being advected across the Atlantic. The location of the plume of CO being advected across Canada and the Atlantic matched observations from the MOPITT satellite instrument. However, the model underestimated the total column of CO by 20% over the Atlantic. The model was also unable to retain very high concentrations of CO, C2H6, C3H8 which were measured by the DC8 and BAe-146 aircraft in plumes on the 18th and 20th July 2004, respectively. The model results were shown to be very sensitive to the chosen Alaskan forest fire emission region which resulted in the displacement of the observed plumes on the 18th and 20th July in the model. The initial emission rate and simulated pyro-convection partially caused an underestimate in tracers over the Atlantic. Increasing the emission rate and reducing the mixing increased simulated CO concentrations by 8% in the plume travelling across the Atlantic. However, the model was still underestimating CO by 13%. A comparison of a 5.6°x5.6° resolution to a 2.8°x2.8° resolution showed that the higher resolution was much more capable at retaining the high concentrations observed in the thin forest fire plumes. On some days, overly strong parameterised convection led to unrealistic uniform concentrations in the troposphere over the Atlantic. Convection was also shown to be too strong on the 23 rd of July over Europe. Hydrocarbon ratios were also used to examine the model’s treatment of mixing and chemistry. Both a Lagrangian and Eulerian treatment of advection were compared to observations. The Eulerian model was shown to be much more effective at reproducing the observed relationship between mixing and chemistry. In this study, it has been shown that a high resolution, Eulerian model is better at simulating pollution events, such as pyro-convective fires. Abstract 1. Introduction Over recent years it has become accepted that emissions from one region can be easily transported to another. In the summer of 2004, several scientific groups from Europe and North America took part in the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). The main objective of this campaign was to gain a better understanding of regional air quality and intercontinental transport of ozone (O 3 ) precursors across the Atlantic. O 3 is a secondary pollutant formed from methane (CH 4 ), carbon monoxide (CO) and hydrocarbons in the presence of NO X. O 3 is of particular interest because its abundance and distribution greatly influences the oxidising capacity of our environment. It is also a greenhouse gas and can be harmful to living organisms. Flights conducted during the campaign sampled many air masses travelling from North America across the Atlantic towards Europe. This new data provides the opportunity for the chemical and transport processes that act upon air masses travelling across the Atlantic to be investigated in a previously data sparse region. An important use of observations is to validate models. This tests whether a model is capable of dealing with the complexities of chemistry and mixing processes in the atmosphere. Once a model has been validated, it can be used, for example, to test hypotheses or determine global distributions of trace species. 2. TOMCAT 3D Chemical Transport Model 3. Model Simulations 8. Results: Ethane and Propane (continued) TOMCAT: Global three-dimensional (3-D) chemical transport model (CTM) Off-line CTM: the large-scale circulation of the model is forced by winds from meteorological analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) 31 vertical levels (1000 hPa – 10 hPa) Parameterised convection and vertical diffusion Chemistry scheme: 41 chemical species, 118 chemical reactions Full Chemistry Simulations : A full chemistry run without the extra Alaskan forest fire emissions with a resolution of 5.6° x 5.6° (this will be known as the T21 control run). A full chemistry run with the extra Alaskan forest fire emissions with a resolution of 5.6° x 5.6° (this will be known as the T21 added emissions run). A full chemistry run without the extra Alaskan forest fire emissions with a resolution of 2.8° x 2.8° (this will be known as the T42 control run). A full chemistry run with the extra Alaskan forest fire emissions with a resolution of 2.8° x 2.8° (this will be known as the T42 added emissions). A sensitivity simulation with changes implemented to the initial conditions of Alaskan forest fires A sensitivity simulation without parameterised convection and vertical diffusion to test the strength of modelled convection and vertical diffusion. Idealised Tracer simulations ( 3 day, 7 day and 35 day tracers) : 40-day idealised tracer simulation: Decay at idealised rate according to lifetime. Emitted 1:1 at a constant rate from the North- east coast of America. 40-day idealised tracer simulation without parameterised convection and vertical diffusion Aims: Identify the impact of the fires in the model Can a global chemical transport model reproduce observed trace species Test TOMCAT’s treatment of mixing and chemistry Additional Forest Fire Emissions in initial full chemistry simulations: Additional surface fluxes emitted from Alaska for T21 and T42 added emission runs are shown in table 1 Emission region: 202.5 - 219.37E and 60.95 – 66.5N Mixing up to ~250hPa to simulate pyro-convection Table 1: Additional Alaskan forest fire emissions 5. Observations Observations used to compare to TOMCAT: Aircraft measurements of CO, O 3, C 2 H 6, C 3 H 8 from the DC8, BAe-146 and Falcon collected during the ICARTT campaign. CO observations from the satellite instrument, MOPITT. CO Total Column: ( figure 2) Large increase of CO near Alaska. Plume of CO from the fires is being advected across Canada and the Atlantic. T42 initial simulations: 33% increase in CO in the plume travelling across the Atlantic when forest fires are included. Plume location matches the plume observed by MOPITT. Plume over the Atlantic is initially underestimated by 20%. References 11. Summary 7. Results: Convection 6. Results: CO Trace Species Mass flux in initial runs (Tg) Mass flux in sensitivity run (Tg) CO3046.8 NO X 0.50.78 C2H6C2H6 0.270.42 C3H8C3H8 0.080.12 Additional forest fire emissions in sensitivity simulation: Increased surface fluxes to represent increased emission rate (table 1) Emission region extended to: 202.5 – 239E and 61 – 67N Mixing reduced to ~350 hPa CO Vertical Profiles : (figure 3) 15 th July: DC8: Model matches observations well. TOMCAT estimates that the fires have perturbed background CO by approximately 15 ppbv. 18 th July: DC8: Clear enhancement in observed CO at 400 hPa. The magnitude of this peak is not captured by TOMCAT. There is an increase of ~35 ppbv between the control run and the added emission run (for both resolutions), suggesting that this plume originated from the Alaskan fires. 19 th July: BAe-146: The gradient of the simulated profiles are too steep and do not match the observations well below 700 hPa due to overly strong convection. 20 th July: BAe-146: Clear enhancement in observed CO between 600-400 hPa. This is the same plume sampled by the DC8 on the 18 th. The magnitude is not captured by TOMCAT. 23 rd July: Falcon flight: a Lagrangian match to the plume measured by the DC8 on the 18 th and BAe-146 on the 20 th. An enhancement in the observed profile is visible at 600- 500 hPa. Extensive vertical mixing in TOMCAT has resulted in a nearly uniform concentration throughout the troposphere Figure 3: Vertical profiles of CO observed by the DC8 on 15 th and 18 th, observed by the BAe-146 on the 19 th and 20 th and observed by the Falcon on the 23 rd compared to simulated profiles. The higher resolution run (T42) is more capable of maintaining the high concentrations of CO in the plumes over the Atlantic than the lower resolution (T21). Extending the emission region moves the plume over the Atlantic closer to the flight track: reason for the underestimate seen in the profiles on the 20 th July shown in figure 2. Changing the emission rate and reducing the mixing over the fires increases CO in the plume from 230 ppbv to 245 ppbv. The emission region displaced the plume observed on the 18 th by the DC8, causing the discrepancy between the observed and simulated vertical profiles. With changes implemented to the fire emissions in Alaska in the sensitivity simulation there is still no peak concentrations of CO of the same magnitude as the maximum observed (460 ppbv). Figure 4: Simulated CO concentration (ppbv) on the 20 th July at 500 hPa from T21 added emission run (top), T42 added emission run (middle) and the sensitivity simulation (bottom) The flight track of the BAe-146 is shown in the mid-Atlantic by the black and white dashed lines. At 350 hPa: Comparisons to MOPITT at 350 hPa showed that the mixing scheme, used to simulate pyro- convection, was too strong initially and was mixing the trace species too high. This will reduce the concentrations of CO at lower altitudes causing an underestimate of trace species. Reducing the pyro-convection in the model from 250 to 350 hPa increased concentrations. Figure 5: Vertical profiles of ethane and propane observed by the DC8 on 15 th and 18 th July 2004 and observed by the BAe-146 on the 19 th and 20 th July compared to simulated profiles. TOMCAT routinely underestimates the observed profiles indicating an underestimate in forest fire emissions. (figure 5) 15 th July: DC8. The profiles for the DC8 on the 15th are well matched. T42 reproduces the plume which was encountered at 300 hPa. This is believed to be from industrial emissions. 18 th July: DC8. Forest fire plume at 400 hPa as seen in the CO profile. The peak in propane is underestimated by a factor of 2.6 whereas ethane is only underestimated by a factor of 1.2. This plume is mostly comprised of forest fire emissions which indicates that the 0.08 Tg of C 3 H 8 included in TOMCAT is not a sufficient estimate. 19 th July: BAe-146. The TOMCAT simulated profiles match the observations quite poorly with the unrealistic gradients due to extensive mixing in the troposphere. 20th July: BAe-146. Enhanced propane and ethane concentrations at 500 - 400 hPa. The model the profiles show that ethane is enhanced by 400 pptv and propane by 40 pptv. These enhancements are not large enough to match the magnitude of observed ethane and propane. The plume, which is visible in the T42 TOMCAT CO tracer fields at 8°N of the BAe-146 flight region at 500 hPa, can also be seen in C 2 H 6 and C 3 H 8. In this plume the peak concentrations simulated by the model are up to 1800 pptv of ethane and 170 pptv of propane. These concentrations are still lower than the measured concentrations seen in the vertical profile. This is attributed to the overly strong mixing used to simulate pyro- convection, underestimate in forest fire emissions and the constant emission rate. Displaced plumes due to chosen emission region in initial T42 added emission runs. Figure 6: Simulated propane and ethane tracer fields from TOMCAT T42 added emission run on the 20 th July 2004 at 500 hPa. 9. Results: Ozone Vertical Profiles Generally, the model does a good job at reproducing the profiles. 18 th July: At 400 hPa increase from 50 ppbv to 57 ppbv (1%) when the forest fires are included. 20 th July: At 500 hPa increase from 50 ppbv to 58 ppbv when forest fires included. However, the tracer fields also show the displaced plumes which are a better match to the observed O 3 at these altitudes. 18th July: the plume has up to a 31% increase in ozone, with concentrations of 60-78 ppbv which matches the magnitude of the concentrations measured at this altitude shown in the vertical profile. Figure 7: Vertical profiles of O 3 (lines are the same as figure 2). Figure 8: O 3 tracer fields from T42 added emission run. 4. Alaskan Forest Fire Emissions Compare resolutions Compare Eulerian and Lagrangian treatments of advection. Turning off the convection and vertical diffusion in the model (blue dashed line) : Greatly improves the gradients of some of the profiles over the Atlantic and Europe. Highlights that the Alaskan fire emissions are underestimated in the model. Shows the importance of including a convection scheme in a model. Over the US, where there are a lot of surface emissions, convection and vertical diffusion are needed to prevent a build up of pollution in the boundary layer. Sensitivity simulation: 44% increase in CO in the plume over the Atlantic, but still underestimates observed CO by 13% in this region. c) b) a) Figure 2: CO total column in molecules/cm 2 averaged over 19 th -21 st July 2004 taken from a)T42 control run, b) T42 added emission run, c) T42 sensitivity run and c) MOPITT. d) 10. Results: Mixing and Chemistry Model can reproduce CO and O 3 vertical profiles on days when the aircraft did not target fire plumes. The location of the plume over the Atlantic matched the plume observed by MOPITT. However, the magnitude was underestimated by 20% in the initial T42 added emission run and 13 % in the forest fire sensitivity run. On the 18 th and 20 th July, the displacement of the sampled plumes in the model resulted in the poor vertical profiles. Changing the emission region moved the plume sampled on the 20 th closer to the flight region. The maximum concentrations of CO in the plumes on the 18 th and 20 th July were not reproduced in the model. This would need a higher resolution which was not possible in this study for time and computer limitations. Out of the two resolutions used, the 2.8°x2.8° was more capable at retaining the high concentrations of the forest fire plumes. Ethane and propane were regularly underestimated indicating that the surface emissions in the model were not sufficient. Propane was underestimated more than ethane in forest fire plumes, suggesting that the 0.08 Tg of propane emitted from the fires was too low. Comparisons to MOPITT identified that CO was underestimated near the east coast of North America. More work needs to be done to know if this is due to the Alaskan forest fire emissions or anthropogenic emissions in the east coast of North America. Increasing the emission rate and reducing the pyro-convection increased CO concentrations by 15 ppbv in the forest fire plume on the 20 th July. Parameterised convection and vertical diffusion was too strong on some days and was resulting in uniform concentrations of trace species throughout the troposphere on some days. The Eulerian advection scheme was able to reproducing the coupling of chemistry and mixing that was observed. A high resolution Eulerian model is suitable for studying long-range tropospheric pollution events such as the Alaskan forest fires. Figure 1: ICARTT Flight tracks interpolated to 12 UTC. 8. Results: Ethane and Propane Observations show hydrocarbons have similar sources and losses. Relationship reproduced by the model. Less scatter because there is only one source and tracers are emitted 1:1. Ratio versus ratio relationship shows observations lie between the lines expected from mixing with negligible background only and from decay by OH chemistry only. (Agrees with results of Parrish et al. (1992) and McKeen and Liu (1993)). Both mixing and chemistry are important in the evolution of hydrocarbon ratio relationships where there is a background of hydrocarbons. The Eulerian tracers lie in between the mixing and chemistry only lines, showing that the affects of mixing and chemistry on the ratios are reproduced in the Eulerian model. The Lagrangian tracers lie exactly on the chemistry only line (as expected because mixing is not considered). Parrish, D.D., C.J. Hahn, E.J. Williams, R.B. Norton, and F.C. Fehsenfeld, Indications of photochemical histories of Pacific air masses from measurements of atmospheric trace species at Point Arena, California, J. Geophys. Res., 97, 15,883 – 15,901, 1992. McKeen, S.A., and S.C. Liu, Hydrocarbon ratios and photochemical history of air masses, Geophys. Res. Lett., 20, 2363 – 2366, 1993. 20th July: Ozone increases by 47 % in the displaced plume with concentrations of up to 78 ppbv. Acknowledgements: Thanks to Prof. Martyn Chipperfield and Dr. Steve Arnold for all their help and ideas. CSAR and HPCX are also thanked for the use of their computer facilities. Chemistry only Mixing only