US aerosols: observation from space, interactions with climate

1. US aerosols: observation from space, interactions with climate Daniel J. Jacob with Easan E. Drury, Loretta J. Mickley, Eric M. Leibensperger, Amos Tai and funding from NASA, EPRI, EPA

2. IMPORTANCE OF ATMOSPHERIC AEROSOLS Public health Chemistry Visibility Cloud formation Ocean fertilization Climate forcing

3. number AEROSOL CHARACTERISTICS area Typical size distribution (Seinfeld and Pandis, 1998) PM2.5 (EPA std.) volume Chemical composition of PM2.5 (NARSTO, 2004) sulfate (coal combustion) nitrate (fossil fuel combustion) ammonium (agriculture) black carbon (combustion) organic carbon (combustion, vegetation) soil other

4. a revolution over the past decade SATELLITE OBSERVATIONS OF TROPOSPHERIC COMPOSITION: Integrated observing system The NASA “A-Train” Satellites Models aircraft, ships, sondes, lidars Surface sites • Principal tropospheric species measured from space: • Ozone , NO2, formaldehyde, BrO, glyoxal • CO, CO2, methane • Aerosols, SO2

5. EARTH HOW TO OBSERVE AEROSOLS FROM SPACE? Solar occultation (SAGE, POAM…) Active system (CALIPSO…) Solar back-scatter (MODIS, MISR…) laser pulse Surface Surface Pros: high S/N, vertical profiling Cons: sparse sampling, cloud interference, low horizontal resolution Pro: vertical profiling Con: sparse sampling, low S/N Pro: horiz. resolution Con: daytime only, no vertical resolution

6. Aerosol observation from space by solar backscatter Relatively easy to do qualitatively for thick plumes over ocean… California fire plumes Pollution off U.S. east coast Dust off West Africa …but difficult quantitatively! Fundamental quantity is aerosol optical depth (AOD) Il () Measured top-of-atmosphere reflectance = f (AOD, aerosol properties, surface reflectance, air scattering, gas absorption, Sun-satellite geometry) aerosol scattering, absorption Il (0)=Il()exp[-AOD]

7. Aerosol optical depths (AODs) measured from space Jan 2001 – Oct 2002 operational data MODIS (c004) return time 2x/day; nadir view known positive bias over land 550 nm AODs MISR 9-day return time; multi-angle view better but much sparser van Donkelaar et al. [2006]

8. MODIS AEROSOL RETRIEVAL OVER LAND • Operational retrieval: • Use top-of-atmosphere (TOA) reflectance • at 2.13 mm (transparent atmosphere) to derive surface reflectance • Assume fixed 0.47/2.13 and 0.65/2.13 surface reflectance ratios to derive atmospheric reflectances at 0.47 and 0.65 mm by subtraction • Assume generic aerosol optical properties to convert atmospheric reflectance to AOD • Our improved retrieval: • Derive local values of 0.47/2.13 and 0.65/2.13 surface reflectance ratios from statistics of low-aerosol scenes • Use local aerosol column information from the GEOS-Chem chemical transport model to convert atmospheric reflectance to AOD TOA reflectance 0.47 mm 0.65 mm 2.13 mm SURFACE Drury et al. (JGR in press)

9. GEOS-Chem CHEMICAL TRANSPORT MODEL (geos-chem.org) • Global model of atmospheric composition driven by NASA/GEOS assimilated meteorological data with 0.5ox0.625o (~50 km) resolution • Simulates coupled oxidant-aerosol chemistry for • sulfate-nitrate ammonium • organic aerosol • black carbon aerosol • dust (4 size classes) • sea salt (2 size classes) • on 2o x2.5o grid • Size distributions and optical properties for different aerosol types are specified

10. TESTING THE MODIS AEROSOL RETRIEVALUSING ICARTT AIRCRAFT DATA OVER US (Jul-Aug 2004) fit AODs synthetic TOA reflectance = f(AOD,…) MODIS satellite instrument: TOA reflectance NASA, NOAA, DOE aircraft: speciated mass concentrations, microphysical & optical properties GEOS-Chem model evaluate MODIS local surface reflectance and ratio NASA DC-8 EPA AQS/IMPROVE surface networks: mass concentrations NASA AERONET surface network: AODs EASTERN U.S. Drury et al. [JGR in press]

11. ORGANIC AEROSOL IN ICARTT Water-soluble organic carbon (WSOC) measured on NOAA P-3 IMPROVE measurements of organic carbon Fu et al. (AE, 2009) Two mechanisms for formation of secondary organic aerosol (SOA): • Standard reversible SOA (Pankow/Seinfeld): • Dicarbonyl SOA (Liggio/Fu):

12. AEROSOL OPTICAL PROPERTIES IN ICARTT Single-scattering albedo = fraction of aerosol extinction due to scattering standard model assumption (GADs) improved fit (this work) AERONET Drury et al., JGR in press

13. AEOSOL OPTICAL DEPTHS (0.47 mm), JUL-AUG 2004 c005 is current MODIS operational data; statistics are relative to AERONET data in circles bias=+2% r = 0.84 bias=-21% r = 0.82 Beyond improving on the operational products, our MODIS retrieval enables quantitative comparison to model results (consistent aerosol optical properties); indicates model underestimate in Southeast US, likely due to organic aerosol bias=-15% r = 0.87 Drury et al., JGR in press

14. EPA AQS surface network data MODIS PM2.5 (this work) Can we use AODs measured from space as proxy for PM2.5? Infer PM2.5 from AOD by • MODIS captures general observed patterns in PM2.5 • but is 50% higher than observed. Could reflect • Clear-sky bias • Time-of-day bias • Model error in vertical aerosol distribution Drury et al., JGR in press

15. RADIATIVE FORCING OF CLIMATE BY AEROSOLS Anthropogenic aerosols may have offset more than half of global greenhouse warming from 1750 to present IPCC (2007) But this aerosol radiative forcing is very inhomogeneous: what are the regional climate consequences? Aerosol direct radiative forcing Leibensperger et al., in prep.

16. CLIMATE IMPLICATIONS OF US AIR QUALITY POLICY Radiative forcing of US anthropogenic aerosols is small globally but important regionally Leibensperger et al., in prep. US sulfur emissions are decreasing rapidly: what are the regional climate impacts? today

17. CALCULATING THE CLIMATE RESPONSE FROM SHUTTING DOWN U.S. AEROSOL Use NASA/GISS general circulation model (GCM) GISS GCM Consider two scenarios: Control: aerosol optical depths fixed at 1990s levels. Sensitivity: U.S. aerosol optical depths set to zero (radiative forcing of about +2 W m-2 over US) Conduct ensemble of 3 simulations for each scenario. Mickley et al. (AE, submitted)

18. Removing aerosols over US causes 0.5-1o C annual mean warming in the East. No-US-aerosols case Temperature (oC) Control, with US aerosols Additional warming due to zeroing of aerosols over the US. 2010-2050 warming due to greenhouse gases Mickley et al. [AE, submitted] Additional effects include increased summer heatwaves (1-2 o C warming) and increased precipitation in the East

19. EFFECT OF CLIMATE CHANGE ON SURFACE AIR QUALITY Expected effect of 21st century climate change Ozone PM (aerosol) Stagnation Temperature Mixing depth Precipitation Cloud cover Relative humidity ? ? = ? = ? ? = Jacob and Winner, AE 2009

20. 2000-2050 change of 8-h daily max ozone in summer, keeping anthropogenic emissions constant ppb Northeast Midwest California Texas Southeast EFFECT OF FUTURE CLIMATE CHANGE ON US AIR QUALITY Models show consistent increase of ozone, mainly driven by temperature Results from six coupled GCM-CTM simulations Weaver et al. [BAMS, 2010] …but model results for aerosols show no such consistency, including in sign. How can we progress?

21. OBSERVED AEROSOL CORRELATION WITH METEOROLOGICAL VARIABLES Multilinear regression model fit to 1998-2008 deseasonalized EPA/AQS data for PM2.5 (total and speciated) mostly precipitation mostly temperature and stagnation R2 fit Tai et al. [AE, submitted]

22. PM2.5 CORRELATION WITH METEOROLOGICAL VARIABLES Tai et al. [AE , submitted]

23. TEMPERATURE COEFFICIENTS FOR SPECIATED PM2.5 Tai et al. [AE , submitted]

24. IMPORTANCE OF MID-LATITUDES CYCLONES IN AIR POLLUTION METEOROLOGY Cold fronts from mid-latitude cyclones are the principal ventilation process for U.S. Midwest/Northeast, western Europe, China Clean air sweeps behind cold front GCMs show decrease + N shift of cyclones from 21st-century climate change; already seen in 1950-2000 climatological data

25. CORRELATIONS AND TRENDSOF POLLUTION EPISODES AND CYCLONES IN NORTHEAST U.S. # pollution episode days (O3>80 ppb) and # cyclones tracking across SE Canada in summer 1980-2006 observations Cyclone track # cyclones # episodes • Strong correlation; cyclone frequency is predictor of pollution episode frequency • 1980-2006 decrease in cyclone frequency would imply a corresponding degradation of air quality if emissions had remained constant • Expected # of > 80 ppb days in Northeast dropped from 30 in 1980 to 10 in 2006, but would have dropped to zero by 2001 in absence of cyclone trend! Leibensperger et al. [ACP2008]

26. EFFECT OF INCREASED STAGNATION ON PM2.5 Difference in PM2.5 between stagnant days (wind < 8 m s-1, 500 hPa wind <13 m s-1, no precipitation) and non-stagnant days, 1998-2008 data PM2.5 is expected to be highly sensitive to Increasing stagnation in future climate Tai et al. [AE , submitted]