The Pollution-Climate Connection How climate change could affect pollution episodes in the United States: a model study Loretta J. Mickley, Harvard University

Number of summer days with 8-hour ozone > 84 ppbv, average for northeast U.S. sites Probability of ozone exceedance vs. daily max. temperature Lin et al , hottest on record We know that day-to-day meteorology affects the severity and duration of pollution episodes. New England Why does probability of ozone episode increase with increasing temperature? Faster chemical reactions, increased biogenic emissions, and stagnation. days

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, and the answer matters. ozone, aerosol Top of boundary layer strong mixing { Soup of pollution precursors

How to make pollution: FiresBiosphereHuman activity Ozone (O 3 ) H2OH2O Hydroxyl (OH) deposition Nitrogen oxides CO, Hydrocarbons + Need sunlight, water vapor, and a mix of manmade or natural “ingredients.” winds rainout (important for aerosols) Our approach: focus on changes in winds and rainout.

Pilot Project: Implement “tracers of anthropogenic pollution” into GISS General Circulation Model Goddard Institute for Space Studies GCM: 9 layers, 4 o x5 o horizontal grid, CO 2 + other greenhouse gases increased yearly from 2000 to Carbon Monoxide: COt source: present-day manmade emissions sink: CO + present-day OH fields Black Carbon: BCt source: present-day manmade emissions sink: rainout Sensitive to climate change 1950 spin-up (ocean adjusts) 2000 increasing A1 greenhouse gas 2050 Timeline spin up { +2 o C Temp change Circulation also sensitive to climate change July global mean temperature

Our approach: Look at daily mean concentrations averaged over specific regions for two 8-year intervals ( ) and ( ). midwest California southeast northeast Cumulative probability plot shows the percentage of points below a certain concentration. Note: concentrations are low compared to observations since only source is direct manmade emissions. Histogram of COt concentrations averaged over Northeast for summers (July-Aug)

Frequency distributions for surface COt and BCt show significantly higher extremes in 2050s compared to present-day. July - August 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 In all regions, daily COt and BCt concentrations correlate (R 2 ~ 0.6 – 0.8) so changes are likely due to circulation.

How does depth of boundary layer change with changing climate? Triangles indicate days of high pollution. Extreme pollution events associated with lower boundary layer heights. Higher BL heights in future go in opposite direction to what is needed to explain air quality differences Northeast daily maximum boundary layer height.

Evolution of a typical pollution event. This happens repeatedly during summertime. 100 x  g/m 3 BCt and wind fields for 6 consecutive days in summer. weak winds cold front from Canada cyclone (low pressure system)

Is pollution more persistent in future? How often do cold fronts come through to sweep away pollution? Persistence of pollution episodes increases by % over Midwest. Mean frequency of cold fronts pushing into Midwest decreases by ~20% in future climate. 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. Agee, 1991 annual number of surface cyclones and anticylones for North America and nearby ocean McCabe et al., 2001 Standardized departure of cyclone frequency over Northern Hemisphere. cyclones anticyclones N Other model studies of future climate have found similar declines relative to the present-day

Two mechanisms for the meridional transport of energy on a round, wet world. warm tropics 1. Mid-latitude cyclones push warm air poleward ahead of front, push cold air equatorward behind front. cold poles 2. Eddy transport of latent heat carries energy to higher latitudes. cold front

Reduced temperature gradient and more efficient eddy transport of energy poleward Fewer cyclones generated More persistent pollution events Reasons for decline in cyclone generation over midlatitudes. 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 mid- troposphere in future atmosphere. TT

Reduced cloud cover 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. How do you translate our results into “ozone alert days”? Hotter maximum temperatures High maximum temperatures and reduced cloud cover suggest increased ozone production, amplifying effect of stagnation. Triangles indicate days of highest BCt concentrations.

AIRS CO Column July 18, 2004GEOS-CHEM CO Column July 18, 2004 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. Wallace McMillan (UMBC)Solene Turquety (Harvard) fires U.S. pollutionAsian pollution model

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 Will provide a detailed, global view of ozone, CO, and HNO 3 First day of data! Tropospheric ozone column on September 20, 2004 pollution biomass burning pollution

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. 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. 2050s 2000s