NATURAL AND TRANSBOUNDARY POLLUTION INFLUENCES ON AEROSOL CONCENTRATIONS AND VISIBILITY DEGRADATION IN THE UNITED STATES Rokjin J. Park, Daniel J. Jacob, Colette L. Heald, T. Duncan Fairlie, Robert M. Yantosca AWMA, October 29, 2004 with support from EPRI, NASA, EPA (ICAP)

OBJECTIVES and APPROACH Determine present-day natural PM concentrations in the U.S., compared to EPA “default” values. Determine the contributions from foreign anthropogenic emissions (Canada, Mexico, Asia) to visibility degradation in the U.S. Determine the pervasiveness of natural fires and dust events, particularly from outside the U.S, in decreasing natural visibility in the U.S. Start from best a priori estimates of natural and anthropogenic PM sources Conduct global oxidant- PM simulation with GEOS-CHEM chemical tracer model Evaluate model with aerosol data from IMPROVE, CASTNET, other networks Conduct sensitivity simulations Assessment of transboundary pollution influences Assessment of EPA “default estimated natural PM concentrations” Improved emission estimates

 g m -3 ] Ammonium Sulfate West East Ammonium Nitrate West East Elemental Carbon West East Organic Carbon Mass West East Baseline (2001) Background Natural Transboundary pollution Canada & Mexico Asia EPA natural defaults < < Annual regional means from GEOS-CHEM standard and sensitivity simulations AEROSOL CONCENTRATIONS IN THE U.S.: contributions from natural sources and transboundary pollution We find that EPA default natural concentrations are too low by factors of 2-3 except for OC in eastern U.S. – quantifying fire influences is critical Transboundary pollution influence from Asia is comparable in magnitude to that from Canada + Mexico [Park et al., 2003, 2004]

IMPLICATIONS FOR IMPLEMENTATION OF REGIONAL HAZE RULE Illustrative calculation for mean western U.S. conditions, assuming linear relationship between emissions and PM concentrations, and assuming zero trend in anthropogenic sources from foreign countries between now and 2064 Desired trend in visibility Required % decrease of U.S. anthropogenic emissions Phase 1 29% 44%44% Park et al. [2004]

VISIBILITY DEGRADATION STATISTICS IN THE UNITED STATES (2001): IMPROVE vs. GEOS-CHEM Visibility decrease (deciviews: dv = 10ln(b ext /10) ) from sulfate, nitrate, and carbonaceous aerosols R.J. Park (Harvard)

FREQUENCY DISTRIBUTION OF DAILY VISIBILITY DEGRADATION (2001): IMPROVE SITE OBSERVATIONS vs. MODEL IMPROVE (EAST) MEAN > 18STD > 5 MEAN > 18 STD < 5 IMPROVE vs. MODEL REGION 1 REGION 2 MEAN < 9STD < 3.5 REGION 4 IMPROVE (WEST) Successful simulation of low end of pdf increases confidence in ability of model to describe natural (and transboundary?) signatures R.J. Park (Harvard)

DUST STORMS PROVIDE VISIBLE EVIDENCE OF INTERCONTINENTAL TRANSPORT OF AEROSOLS Glen Canyon, AZ Clear dayApril 16, 2001: Asian dust! …and anthropogenic pollution is transported together with the dust Colette Heald et al. (Harvard) satellite data

ASIAN PM POLLUTION INFLUENCE OVER WESTERN U.S. - * - AERONET __GEOS-CHEM __ Asian SO 4 +NH 4 +Nit __ Asian Dust Spring 2001 What about in surface air? Observations at clean western U.S. sites indicate Asian sulfate influence up to 2.2  g m -3 (24-h average) [Jaffe et al., 2003] AERONET aerosol optical depth (AOD) measurements at Missoula, MT Colette Heald et al. (Harvard) AOD

GEOS-CHEM SIMULATION OF TRACE-P OBSERVATIONS BC underestimated by factor of 2 because emissions [Bond et al.] are too low Park et al. [2004b] P3B DATA over NW Pacific (30 – 45 o N, 120 – 140 o E) Scavenging from Asian outflow is 80-90% efficient for sulfate and BC, ~100% for nitrate Black carbon (BC)

ASIAN POLLUTION INFLUENCES ON SO 4 2- /NO 3 - /NH 4 + PM Annual means as determined from a GEOS-CHEM 2001 sensitivity simulation with Asian anthropogenic sources shut off SO 4 2- NO 3 - NH 4 +  g m -3 Mean Asian pollution enhancements for U.S. surface aerosols are greatest for sulfate (0.1 μg m -3 ) and less for other aerosols (scavenging during outflow)

SEASONAL VARIATION OF ASIAN POLLUTION INFLUENCES ON SO 4 2- /NO 3 - /NH 4 + PM IN THE UNITED STATES The transpacific transport of Asian pollution is largest during spring and minimum in summer. Monthly mean SO NO 3 - -NH 4 + enhancements due to Asian pollution

RELATIVE HUMIDITY WEIGHTING FACTORS, f(RH): IMPROVE vs. GEOS-CHEM (2001) Use ISORROPIA + MIE computation with 3-h SO NO 3 - -NH 4 +, RH Aerosol acidity results in higher f(RH) than recommended values especially at RH greater than 60%. [NH 4 + ] / ([SO 4 2- ]+[NO 3 - ]) WESTEAST

SEASONAL AND SPATIAL DISTRIBUTION OF BACKGROUD AEROSOL ACIDITY IN THE UNITED STATES Seasonal means as determined from a GEOS-CHEM 2001 sensitivity simulation with US anthropogenic sources shut off DJFMAM JJA SON [NH 4 + ] / (2[SO 4 2- ]+[NO 3 - ]) Background aerosols are not fully neutralized in the west except in summer implying higher background visibility endpoint?

WHERE ARE ASIAN EMISSIONS HEADED? It’s anybody’s guess Past trends and future projections of Chinese SO 2 emissions [Streets et al., 2002]