Land use change impact on aerosols and clouds in southwest Australia

Approximately 13 million hectares of native vegetation in southwest Australia has been replaced by non-native, rain-fed agricultural species over the last several decades. Prior studies link land use change to reduction in convective cloudiness and precipitation in this region. However, recent aircraft observations also show that land use change and associated effects on ground water hydrology impacts the atmospheric CCN distribution, with distinct differences found between adjacent areas of native vegetation and agriculture. Over the agricultural areas, substantially higher concentrations of very small aerosols were observed during the morning hours. Smaller concentrations of larger aerosols were found over the native vegetation area in the morning. Even though the smaller particles over the agricultural area grow and the size distribution is shifted to higher particle size ranges in the afternoon, they are smaller and more numerous compared to those on the native vegetation area. Aircraft observations also show differences in cloud particle size distribution consistent with differences in aerosols, with larger droplet sizes existing in higher concentrations over the native vegetation region. The source of the small particles in the agricultural area was traced to salt lakes. Shallow rooted vegetation in the agricultural area, causing the rise of the water table and altering the lake geochemistry is a potential reason for the enhanced aerosol emissions over the lakes in the agricultural area. The impact of differing atmospheric CCN on cloud precipitation is being explored using numerical modeling and results from this analysis will also be presented.