1. Aerosol Pattern over Southern North America Tropospheric Aerosols: Science and Decisions in an International Community A NARSTO Technical Symposium on Aerosol Science Queretero, Mexico October 23-26, 2000 Janja D.Husar and Rudolf B. Husar Center for Air Pollution Impact and Trend Analysis (CAPITA) Washington University, St. Louis, MO

2. Outline Background: –North America is part of the global aerosol system. –The aerosol pattern and trends over the US and southern Canada are reasonably well documented. –The aerosol levels and characteristics over the southern part of North America are not well documented. Goal: Explore the Regional Aerosol Pattern Over North America, esp. Central America Approach: –Use surface visibility –Multiple satellite data –Combine visibility and satellite data to obtain aerosol pattern Results: –Seasonal aerosol maps and charts –Long-term trends –Smoke event

3. Regional Aerosol Detection by Surface Visibility and Satellites At any given geographic location, the aerosol is composed of multiple types, e.g. dust, smoke and haze. Horizontal visibility is influenced by the aerosol in the surface layer while the vertical optical depth is determined by the aerosol column concentration. The data from multiple sensors yield complementary information on aerosols : Light extinction (visibility), Scattering (AVHRR, SeaWiFS) aerosol absorption (TOMS) The data from multiple sensors yield only semi-quantitative aerosol pattern

4. Oceanic Aerosol – AVHRR satellite sensor During the winter and fall season the optical depth (AOD) is low surrounding N. America In the spring, elevated AOD is observed adjacent to Mexico In the summer an aerosol plume is evident near the Eastern US

5. POLDER Aerosol Polarization Index (fine particles with polarized scattering) During spring, the polarization index is high over Central America and the adjacent oceans. In the late fall, the polarization index is low throughout North America.

6. Surface Visibility over North America The surface extinction coefficient, Bext, over North America is relatively low compared to other continents. The Bext contours are based on 7500 surface visibility stations.

7. Surface Visibility over North America Over Central America, Bext is highest in the spring season. Over the Eastern US, Bext is highest in the summer. The size of the yellow rectangle is proportional to Bext at each station.

8. Summer Visibility Trends, 1980 – 1995 Since the 1980s the Bext has decreased throughout the US. Over the Eastern US, the decline during 1980-195 was 10-15% The highest regional Bext values cover the Eastern US

9. Seasonal Pattern and Long-Term Trend of Bext The Bext has a sharp spring peak throughout C. America. The noon Bext values are not influenced by humidity. The spring peak occurs regularly every year However, the peak spring Bext varies from one year to another.

10. Bext Pattern in 1998 Data from multiple stations show that Bext in 1998 was 2- 4 times higher than in previous years During 1998, the daily pattern of Bext varied significantly from one station to another.

11. Fire Locations (ATSR-ESA) TOMS Aerosol Index The excess AI in 1998 compared to 1997 shows the pattern of smoke due to the Central American fires. The fires and the smoke occur mainly at lower elevations. In 1998 the number of fire locations was much higher then in 1997

12. Smoke from the Central American Fires Based on SeaWiFS and other satellite imagery, thick smoke was seen over southern Mexico, Guatemala and Honduras and adjacent oceans throughout the 1998 spring season.SeaWiFS

13. 3D SeaWiFS May 14, 1998

14. Smoke Transport over North America (SeaWiFS, TOMS and Bext) On May 15 the smoke plume was swiftly transported northward along the Mississippi Valley. By May 16 the smoke plume reached Hudson Bay in Canada. The smoke was also observed at the the surface visibility stations.

15. Surface Bext-Ozone Comparison Surface Bext maps show the north and eastward transport of smoke aerosol Regionally, the smoke does not appear to add ozone to the existing values Rather, ozone in the smoky airmass tends to be lower than over the surrounding areas

16. Approximate PM Concentration in Central American Cities Urban centers in Central America are PM ‘hot spots’. PM10 levels are in the range of the EPA-WHO standard. High TSP levels indicate that much of the urban PM is composed of coarse particles.

17. Summary and Conclusions The aerosol levels in North America are relatively low compared to other continents. Over the Eastern US the highest Bext occurs during the summer season and has been declining by 10-15% since 1980. During spring, the AOT, TOMS and POLDER index and Bext are all high over Central America and the adjacent oceans. The spring peak over Central America occurs regularly every year but varies in magnitude from one year to another. Biomass smoke from forest and agricultural fires is the main contributor to the springtime aerosol peak over the lower-lying areas of Central America. In 1998 the fires and the smoke were 2-4 times higher then in previous years. In May 1998, the Central American smoke plume covered much of eastern North America from Mexico to Hudson Bay. Urban centers in Central America are PM ‘hot spots’ dominated by coarse particles.