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Effect of climate change on air pollution episodes in the United States: a model study Loretta J. Mickley, Daniel J. Jacob, Brendan D. Field Harvard University David Rind Goddard Institute for Space Studies We know that day-to-day meteorology affects the severity and duration of pollution episodes. New England days Number of summer days with 8-hour ozone > 84 ppbv, average for northeast U.S. sites 1988, hottest on record Probability of ozone exceedance vs. daily max. temperature Lin et al. 2001 Why does probability of ozone episode increase with increasing temperature? Faster chemical reactions, increased biogenic emissions, and stagnation. How will a changing climate affect pollution? Answer: we don’t know. Rising temperatures could mean faster chemical reactions. . . Higher surface temperatures could also mean a deeper boundary layer, diluting concentrations at the surface. The picture is complicated. Top of boundary layer Soup of pollution precursors { ozone, aerosol strong mixing How to make pollution: Need sunlight, water vapor, and a mix of anthropogenic or natural “ingredients.” H2O Hydroxyl (OH) winds Ozone (O3) + Nitrogen oxides CO, Hydrocarbons rainout (important for aerosols) deposition Fires Biosphere Human activity Our approach: focus on changes in winds and rainout. Previous studies have focused mainly on chemical response to temperature change (e.g. Aw and Kleeman, 2003) Increase in surface ozone and aerosol due to 5K temperature change DO3 all other met variables --e.g. circulation, boundary layer height-- the same Ozone increase 10-15% due to faster reaction rates. DAerosol Aerosol decreases 10-15% due to increased volatilization of ammonia. What have long-lived tracer studies shown about changes in transport? DSF6 surface Rind et al., 2001 31-layer GISS GCM, several longlived tracers, 2xCO2 Increased convection leads to: less surface SF6 and DSF6 500 mb more SF6 aloft. Holzer and Boer, 2001 coupled global model, 2xCO2 Less vigorous flow, increased plume concentrations How to make pollution: Need sunlight, water vapor, and a mix of anthropogenic or natural “ingredients.” H2O Hydroxyl (OH) winds Ozone (O3) + Nitrogen oxides CO, Hydrocarbons rainout (important for aerosols) deposition Fires Biosphere Human activity Our approach: focus on changes in winds and rainout. Pilot Project: Implement “tracers of anthropogenic pollution” into GISS General Circulation Model Timeline 1950 spin-up (ocean adjusts) 2000 increasing A1 greenhouse gas 2050 Goddard Institute for Space Studies GCM: 9 layers, 4ox5o horizontal grid, CO2 + other greenhouse gases increased yearly from 2000 to 2050. July global mean temperature Carbon Monoxide: COt source: present-day anthro emissions sink: CO + present-day OH fields 2045-2052 +2o C Temp change spin up Sensitive to climate change Circulation also sensitive to climate change { Black Carbon: BCt source: present-day anthro emissions sink: rainout 19952002 Anthropogenic emissions: • What changes: Well-mixed greenhouse gas concentrations over time Climate response to greenhouse gas trends, including rainout of black carbon tracer CO emissions (molecules /s) • What remains the same: Emissions of CO and black carbon tracers Sink of CO (monthly mean, present-day OH) BC emissions (kg/s) Timeline 1950 spin-up (ocean adjusts) 2000 increasing A1 greenhouse gas 2050 Large-scale meteorological changes from 2000 to 2050, Jul-Aug mean D Surface temp D Surface pressure D Precipitation Increased surface temperature co-located with decreased surface pressure in mid-U.S. Decreased precipitation in Southeast. (20-30% less in some gridboxes) How much do July-August mean COt mixing ratios change at the surface? present-day 1995-2002 future 2045-2052 Answer: not much Not much change either for BCt. Note: only source of COt is direct emission Difference 2050-2000 Our approach: Look at daily mean concentrations averaged over specific regions for two 8-year intervals (1995-2002) and (2045-2052). Histogram of COt concentrations averaged over Northeast for 1995-2002 summers (July-Aug) midwest California northeast southeast Cumulative probability plot shows the percentage of points below a certain concentration. Frequency distributions for surface COt and BCt show significantly higher extremes in 2050s compared to present-day. July - August 2045-2052 1995-2002 Changes at the extremes are due solely to changes in circulation and rainfall. Frequency distributions for three U.S. regions in July-August show increased severity of pollution episodes. 2050 2000 In all regions, daily COt and BCt concentrations correlate (R2 ~ 0.6 – 0.8) so much of the difference is likely due to circulation. How does depth of boundary layer change with changing climate? Northeast daily maximum boundary layer height. Triangles indicate days of high pollution. 2045-2052 Extreme pollution events associated with lower boundary layer heights. 1995-2002 Higher BL heights in future go in opposite direction to what is needed to explain air quality differences. Evolution of a typical pollution event. This happens repeatedly during summertime. weak winds cyclone (low pressure system) BCt and wind fields for 6 consecutive days in summer. cold front from Canada 100 x mg/m3 Is pollution more persistent in future? How often do cold fronts come through to sweep away pollution? Mean frequency of cold fronts pushing into Midwest decreases by ~20% in future climate. Persistence of pollution episodes increases by 30-100% over Midwest. Cyclone number and cold front frequency decline in future, allowing pollutants to build up. A decrease in cyclone frequency over midlatitudes has also been observed in recent decades. 1000 cyclones Agee, 1991 500 100 1950 anticyclones 1980 annual number of surface cyclones and anticylones for North America and nearby ocean McCabe et al., 2001 30-60N Standardized departure of cyclone frequency over Northern Hemisphere. Model studies of future climate have found similar declines relative to the present-day. (e.g., Zhang and Wang, 1997; Carnell and Senior, 2001; Geng and Sugi, 2003) Two mechanisms for the meridional transport of energy on a round, wet world. 1. Mid-latitude cyclones push warm air poleward ahead of front, push cold air equatorward behind front. warm tropics cold poles cold front 2. Eddy transport of latent heat carries energy to higher latitudes. Reasons for decline in cyclone generation over midlatitudes. DT Change in zonally averaged temperature for July-August. Increase is greatest at high latitudes. Reason is ice-albedo feedback. Change in northward transport of latent heat by eddies in midtroposphere in future atmosphere. Reduced temperature gradient and more efficient eddy transport of energy poleward Fewer cyclones generated More persistent pollution events How do you translate our results into “ozone alert days”? Model predicts high-pollution days will occur about 66% more frequently in future due to changes in circulation over Northeast and Midwest. Best calculation includes full chemistry responding to all the meteorological changes. Hotter maximum temperatures Triangles indicate days of highest BCt concentrations. 2050 2000 Reduced cloud cover High maximum temperatures and reduced cloud cover suggest increased ozone production, amplifying effect of stagnation. GCAP: ongoing project to diagnose effect of changing climate on US air quality Global Climate and Air Pollution collaborators: David Streets, John Seinfeld, David Rind, Joshua Fu 23L GISS GCM, with changing GHGs 1950 2000 Spin-up of ocean 2025 2050 2075 2100 archived temperatures, humidity, winds, etc GEOS-CHEM Calculate chemistry, aerosol present-day & future precursor emissions CMAQ regional model Monitoring pollution and biomass burning over North America with satellites AIRS instrument onboard the AQUA satellite enables observation of complex and overlapping long-range transport. AIRS CO Column July 18, 2004 fires Asian pollution GEOS-CHEM CO Column July 18, 2004 model U.S. pollution Wallace McMillan (UMBC) Solene Turquety (Harvard) First day of ozone column data from TES TES = Tropospheric Emission Spectrometer Measures infrared radiances in both limb and nadir mode. Launched July 15, 2004 pollution Will provide a detailed, global view of ozone, CO, and HNO3 pollution First day of data! biomass burning Tropospheric ozone column on September 20, 2004 Summary Model predicts an increase in the severity and duration of pollution episodes over the Midwest and Northeast U.S. by 2050, even with constant emissions. Change in pollution tied to a decrease in the frequency of cold fronts arriving from Canada, which sweep away the pollution. 2050s Observed correlations between meteorological parameters and pollutant concentrations provide a tool for predicting trends in GCM simulations. A new era of satellite observations probing the troposphere can supply data to assess our model predictions. 2000s Extra slides Meteorology in NE associated with high CO and BC events includes average to high temp and pressures, low winds, and clear skies Low trop clouds 2050 2000 Surf temp Surf press Surface wind speed Boundary layer height Static stability Days of high CO Present-day 1995-2002 Future 2045-2052 July-August mean BC concentrations at the surface Change from present-day