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  1. We have used a 3D Chemistry Climate Model (CCM) STOCHEM to study the changes in the distribution of methane CH4, ozone O3 and OH radical resulting from N. American NOx emissions. Continuous one month emission pulses resulted in: (1) CH4 deficits that lead to climate cooling (negative radiative forcing) and decay with e-folding times of 10-15 years; (2) Excess O3 mixing ratio (climate warming) in the short-term that decay over several months to produce deficits; and (3) Long-term O3 deficits (negative radiative forcing) which decay away similar to CH4 deficits. Time-integrated net radiative forcing over a 100 year horizon from negative CH4 andlong-term O3 contributions, and positive short-term O3 leave a small negative residual. However, over a 20 year time horizon the integrated net radiative forcing from winter NOx emission pulses show both negative (at low latitudes) and positive (at high latitudes) values. This signifies the effect of either climate cooling or warming depending on the location of the NOx emissions over 20 year period. CH4 RF CH4 RF Climate-Chemistry Model STOCHEM • STOCHEM is a global Lagrangian tropospheric chemistry- transport model (Collins et al., 1997). • 50,000 air parcels carry mixing ratios of 70 chemical species with 174 chemical reactions, are mapped after each 3 hour advection time-step to an Eulerian grid of 5o x 5o x 9 vertical levels. • Meteorological input is from the UK Met Office numerical weather prediction models with resolution of 1.25o long and 0.83o lat on 12 vertical levels. • Advection uses a fourth order Runge-Kutta method with bilinear and cubic interpolations for horizontal and vertical, respectively. • Chemical scheme uses a backward Euler solver using 5 minutes time step. • Wide range of emission fields are used, included are man-made (Dententer et al., 2005), Aircraft (IPCC, 1999), International Shipping (Olivier and Berdowski, 2001), 500 Tg y-1 of isoprene. • Dry and wet removal follow resistance and scavenging coefficient approaches, respectively. O3 OH CH4 Winter Summer O3 RFLong O3 RFLong O3 OH CH4 O3 RFshort O3 RFshort Newfoundland Minnesota Kansas Florida Mexico 3 -600 -360 -270 -180 -90 0 ppt -250 -100 -10 0 10 100 250 x10-6 ppt -100 -40 -0.01 0 0.01 20 60 100 ppt Honduras Net RF Net RF NO + HO2= OH + NO2 OH NO2 + hv = NO + O O3 O + O2 + M = O3 + M OH + CH4 = CH3 + H2O CH4 Fig. 1.Monthly mean changes in surface CH4 (column 1), O3 (column 2) and OH (column 3) ppt from Idaho-Utah (approximately the black rectangles) NOx emissions pulses. The upper and lower panels show January and July pulses, respectively. Immediately after the NOx emission densities had been perturbed we observed a decrease in O3 and OH at the locations of the emission but build up at downwind of nearby areas which decayed away rapidly during the following month or two. The increased OH ultimately led to a decrease in CH4 concentrations. Below are the main reactions responsible for the changes in O3, OH and CH4. Fig. 3. 100 year time-integrated global CH4 (1st row), O3 long-term (2nd row), O3 short-term (3rd row) and Total net (4th row) radiative forcing (mWm-2 years) responses to NOx emissions pulses emitted at all the 21 locations both in winter (left column) and summer (right column) seasons. CH4 and O3 long-term show negative forcing (cooling) whereas O3 short-term gives a positive forcing (warming) leaving a net cooling. Summer pulses in all the 21 locations result in higher net radiative forcing in terms of magnitude compared to winter pulses. The magnitude also increases towards the south with just a slight increase to the east. Fig. 5.A time-integrated radiative forcing similar to fig. 4 above but over 20 year time horizon. On like fig. 4, the net radiative forcing from winter pulses show weak positives at high latitudes and weak negatives at low latitudes. o Experiments The transient behaviour of CH4 and O3 in response to emission pulses of NOx were investigated by starting the model from an initial set of trace gas mixing ratios on 1st October 1997 and using analysed wind fields to run through to 1st January 1998. At that point, three model runs were initiated. The first model run, the base case, continued on from the annual cycle without change until 30th June 1999. In the second and third model runs, the transient cases, NOx emission pulses of 0.1 Tg N were added in each of 21 10o x 10o latitude-longitude regions across North America for the months of January and July 1998. They were then reset to the base case value and allowed to continued until 30th June 1999. The impacts of NOx on the CH4 and O3 were followed by taking the differences between the base and transient cases. Time-integrated radiative forcing calculations for CH4 and O3 perturbations over 20 and 100 year time periods (Derwent et al., 2001; Wild et al., 2001) as explain in Stevenson et al. (2004), was employed to estimate the overall climate forcing from the NOx emissions. Time-integrated CH4 deficits in ppbv years are converted into radiative forcings using 0.37mWm-2 ppb-1 (Schimel et al., 1996). O3 is converted to radiative forcings using an off-line radiation code described in Stevenson et al. (2004). CH4 RF CH4 RF Minnesota Mexico 3 Florida Newfoundland Mexico 3 Newfoundland Conclusions:It is evident from our studies that time-integrated CH4,O3 and OH responses from one month-long NOx emission pulses in North America show both spatial and seasonal variations. The responses reveal a clear latitudinal gradient from north to south. The south show the highest in magnitude compared to the north by a factor of two. The west-east gradient show a slight increase, although it is not systematically clear and hence needs more work. Summer responses outweigh that of winter by almost the same factor. The studies also reveal a much larger variability of the net time-integrated global radiative forcing over 20 and 100 year period with location and season. However, it is also evident that the 100 year period net radiative forcings for surface NOx sources leave a negative values (climate cooling) irrespective of location or season of the emission. Whereas for 20 year period emission in winter results in climate cooling at low latitudes and warming at high latitudes. Winter Summer O3 RFLong O3 RFLong Kansas Honduras Kansas Mexico 3 Kansas O3 RFshort O3 RFshort References:Collins et al., 1997. Tropospheric ozone in a global-scale three-dimensional Lagrangian model and its response to NOx emission controls. Journal of Atmospheric Chemistry 26, 223-274. Dentener, et al., 2005. The impact of air pollutant and methane emission controls on tropospheric ozone and radiative forcing: CTM calculations for the period 1990-2030. ACP 5, 1731-1755. Derwent, et al., 2001. Transient behaviour of tropospheric ozone precursors in a global 3-D CTM and their indirect greenhouse effects. Climatic Change 49, 463-487. Edwards, J.M., Slingo, A., 1996. Studies with a flexible new radiation code. I. Choosing a configuration for a large-scale model, QJRMS 122, 689-719. IPCC, 1999. IPCC special report on aviation and the global atmosphere. Cambridge University Press, New York. Olivier, J.G.J., Berdowski, J.J.M., 2001. Global emissions sources and sinks. In: The Climate System. Eds. J.J.M. Berdowski, R. Guicherit and B.J. Heij. Swets& Zeitlinger Publishers, Lisse, Netherlands. Schimel et al., 1996. Radiative forcing of climate change, In: Climate change 1995: The scientific basis. Cambridge University Press, New York. Stevenson et al., 2004. Radiative forcing from aircraft NOx emissions: Mechanisms and seasonal dependence, J. Geophys. Res., 109, D17307, doi:10.1029/2004JD004759. Net RF Net RF Fig. 2.Time development of CH4, O3 and OH global anomalies due to NOx pulses from all the 21 locations. The red and blue lines show the locations with the extreme response from the pulses in the month of emission. Rows 1 and 2 are pulses emitted in January and July, respectively. For CH4 is Minnesota and Mexico, O3 is Florida and Newfoundland, and for OH is Mexico and Newfoundland. The emission pulses produces deficits (negative) in CH4 that peaked in the 2-3 months and steadily decay away. The O3 response show a short-term excess (positive) that peaked during the 1-2 months after the pulses and decay away after several months to produce deficits which also decay in step with the CH4 deficits. Acknowledgements:This research was supported by the U.K. Natural Environment Research Council (NERC) fellowship. Time-integrated Radiative Forcing from North American NOX Emissions: Climate Effect on 20 and 100 year time scales. R. Damoah and D. Stevenson Institute for Atmospheric and Environmental Science, The University of Edinburgh, UK R. Derwent Rdscientific, Newbury, Berkshire, UK ABSTRACT

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