This study examines the behavior of particulate matter in El Paso Texas during the North American Monsoon. A laser particle counter was utilized to record size segregated surface measurements of particulate number densities within the accumulation and coarse modes (0.3 mm – 10.0 mm). Changes in particle number size distributions as a function of relative humidity for case studies during the 2005 – 2007 monsoon seasons have been analyzed. It is intended to investigate the evolution of aerosol number size distributions in the presence of increasing relative humidity typical of the monsoon system. Fractional losses and increases for accumulation (0.3mm < r0 < 3.0 mm) and coarse (r0 >3.0 mm) modes have been calculated. Hygroscopic growth factors of aerosols as a function of relative humidity were calculated according to Hänel, 1976.