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Linking climate change and air quality: Results from a pilot study. Harmful components of smog: • ozone • aerosol (a.k.a. Particulate matter, PM) Collaborators: Daniel Jacob, Shiliang Wu, Brendan Field GISS: David Rind Caltech: John Seinfeld, Hong Liao U. Tennessee: Joshua Fu, Zuopan Li Argonne: David Streets Tropospheric ozone columns seen from GOME satellite, June 7-8, 1997 We know that day-to-day weather 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. . O3, PM Higher surface temperatures could also mean a deeper boundary layer, diluting concentrations at the surface. O3, PM 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 Many meteorological variables affecting pollution are sensitive to climate change. 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 What will people do? rainout (important for aerosols) deposition Fires Biosphere Human activity Many meteorological variables affecting pollution are sensitive to climate change. GCAP Project: Global climate and air pollution study GISS general circulation model, changing GHGs 1950 Spin-up 2000 2025 2050 archive temperatures, humidity, winds, etc precursor emissions GEOS-CHEM global chemistry model 2075 2100 MM5 mesoscale model archive chemistry CMAQ regional chemistry model Why so many models? How do these models work? All the models cut up the atmosphere into gridboxes and describe exchange of air mass, and water vapor, chemical species between boxes. More boxes = more CPU time. Feedbacks between chemistry and climate require “on-line chemistry” or iteration between models. Uncertainties: cloud feedbacks, land cover effects, ocean circulation, abrupt climate change, CO2 uptake. Pilot project: We focus on future changes in just winds (circulation) and rainout. H2O Hydroxyl (OH) winds Ozone (O3) + Nitrogen oxides CO, Hydrocarbons rainout (important for aerosols) deposition Fires Biosphere Human activity Pilot Project: Implement “tracers of anthropogenic pollution” into simpler version of 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 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 two U.S. regions in July-August show increased severity of pollution episodes. 2050 2000 Daily COt and BCt concentrations correlate (R2 ~ 0.6 – 0.8) so much of the difference is likely due to changes in circulation. How does depth of boundary layer change with changing climate? Maximum boundary layer heights. 2045-2052 Triangles indicate days of high pollution. 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 We found 20% decrease in summertime cyclone frequency in future climate. 100 x mg/m3 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 in the model. 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 + Increased transport of latent heat 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. 2050s 2000s Reduced cloud cover High maximum temperatures and reduced cloud cover suggest increased ozone production, amplifying effect of stagnation. 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 “In an ideal world, scientists would learn in graduate school how to tell ordinary people about their world.” Cornelia Dean, NYT reporter and editor. 2000s