North American Visibility rdyswth Seasonal Bext Horizontal Visibility: Determined by the Extinction Coefficient, Bext • Bext can be estimated from surface visual range data and WebCams • The dry BEXT (say RH = 50%) can also be estimated from the weighed sum of the mass concentrations of aerosol types: Bext = S (a1*Dust + a2*Smoke + a3*Haze + a4*Salt + a5*Soot) The weights a1…a5 are the mass extinction efficiencies for each species. • The chemical species concentrations are obtained from filter samples followed by chemical analysis Aerosol Types and Vertical Layering • At any given geographic location, the aerosol is composed of multiple types, e.g. dust, smoke and haze • The aerosol types most frequently reside in different layers • As a consequence, horizontal visibility is influenced by the aerosol in the surface layer while the vertical and slant visibility is determined by the layers in the aerosol column containing multiple aerosol types and layers. WMO Global Surface Meteorological Network Visibility over North America: A Global Perspective Surface Visibility AVHRR ShipObs POLDER SeaWiFS, TOMS and Surface Extinction Surface reflectance derived from the SeaWiFS satellite data for May 14-17 1998. The spectral reflectance data were rendered as a "true color" digital image by combining the blue (0.412 m), green (0.550 m), and red (0.670 m) channels. The TOMS absorbing aerosol index (green, levels 12 and 30) and the visibility-derived extinction coefficients are superimposed as green contours (red, levels 0.2. and 0.4 km-1). 3D SeaWiFS May 14, 1998 Average Excess TOMS Index for Mar., Apr., May 1998 Excess TOMS absorbing aerosol index averaged for March, April, May 1998 compared to 1999. The insert depicts the 1998 smoke impact from a global perspective. Fire Locations Surface Ozone Concentration Superposition of daily maximum ozone and aerosol extinction maps derived from surface visibility. Visibility Module: Calculates visibility from aerosol concentration and humidity data SeaWiFS Surface Reflectance on Clear and Smoky Days Spectral reflectance data derived from the SeaWiFS sensor on May 15, 1998; b) Excess aerosol backscattering over water. Synopsis • During a ten-day period, May 7-17, 1998, smoke from fires in Central America drifted northward into the USA and Canada. • The smoke caused exceedances of the PM standard, health alerts, and impairment of air traffic, as well as major reductions of visual range, and red sunsets. • It was a major air pollution event covered by the research community as well as by the national media. Background • Throughout the spring of 1998, thousands of fires in Central America have been burning as it happens every spring but the 1998 fires are said to be about twice as intense as the normal year. • Unlike earlier years, the research community has followed with keen interest the 1998 Central American fires by a variety of UV, visible and infrared remote sensors from satellites. • This is summary of the Web-based data as augmented by surface-based PM10 monitoring data by state agencies • This preliminary and incomplete but timely summary is intended for air quality managers and researchers interested in pursuing further detailed analysis of this unusual event. Forest Fires over Central America Throughout the spring of 1998, thousands of fires in Central America have been burning with twice the intensity of normal springtime fires. Location of fires (red dots) on May 15, 1998, based on Defense Meteorological NOAA’s Operational Significant Satellite Program (DMSP) satellite data Event Imagery (OSEI) Smoke from the Central American Fires Based on SeaWiFS and other satellite imagery, thick smoke has been lingering over southern Mexico, Guatemala and Honduras and adjacent oceans throughout the spring season. Smoke passes over Eastern North America GOES 8 Visible Imagery May 12 May 14 May 15 May 16 TOMS Aerosol Index Preliminary Surface Haze-Ozone Map Comparison • Surface haze 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 the surrounding areas US Visibility Trend Maps, 1980 - 1995 Click on the images to view larger versions In the Eastern US, throughout the 1980-95 period, the 75th percentile BEXT exceeded 0.15 or had an average visibility of less than 10 miles. Most notable are the hazy regions on both sides of the Appalachian Mountains where the BEXT exceeds 0.2 1/km. Since the early 1980s the BEXT decreased 10-15% with the largest decreases in the Southern and Central regions. Light Extinction Trends of the 75th and 90th Percentiles Trend Statistics Eastern US Northeastern US Southeastern US 75th %-ile 90th %-ile 75th %-ile 90th %-ile 75th %-ile 90th %-ile BEXT (1/km) 0.015 0.042 0.014 0.042 0.024 0.053 ('80 - '95) Stnd Error (1/km) 0.0095 0.018 0.014 0.023 0.011 0.023 /Error 1.58 2.29 1 1.83 2.14 2.32 Trend Confidence 85% 95% 65% 90% 95% 95% limit (t distribution) Over the Eastern US, the 75th percentile BEXT decreased about ~8 percent over the 15 years. The largest decreases occurred in the Southeast where the BEXT decreased 12% compared to 8% in the Northeast.