Effect of pollution from Central American fires on cloud-to-ground lightning in May 1998

In the spring of 1998, numerous fires from seasonal biomass burning in Cent ral America, mainly in Mexico and Guatemala, produced aerosol particles tha t were advected into the central plains of the United States. The effects o f the fires continued from approximately April 9 through June 11. The most intense smoke concentration was on May 8 as seen from a NASA Shuttle missio n. Characteristics of cloud-to-ground (CG) lightning flashes recorded by th e National Lightning Detection Network were examined for May 1998 by subtra cting the lightning characteristics for May 1995-1997, and 1999 This produc es "difference value" maps with approximately 100 km resolution for polarit y, peak currents, and multiplicity. Several significant differences are rev ealed. The percentage of positive flashes increased by a factor of two. Med ian first stroke peak currents have been calculated for both positive and n egative flashes. For negative flashes, the median peak current decreased du ring the fire period. For positive flashes, the median peak current increas ed by over 20 kA in Texas, Oklahoma, Kansas, Arkansas and Nebraska. Mean mu ltiplicity values, or the number of strokes per flash, were calculated. Pos itive flash multiplicity did not change. Mean multiplicity values for negat ive flashes, however, changed dramatically. For a region including Texas, O klahoma, Kansas, and Louisiana multiplicity values for negative flashes dec reased from 2.8 to 1.0-1.4 strokes per flash. The data analyzed were a corr ected set from which we had deleted positive flashes with peak currents les s than 10 kA, thus removing most of the intracloud contamination. Further s tudies may reveal the relation between the effect of pollution and aerosol size on the characteristics of CG flashes.