Detailed measurements of cloud drop activation and hygroscopicity in the desert Southwest of the United States

Cloud condensation nuclei (CCN) comprise a key component of ambient aerosol with critical influences on weather and climate. The importance of CCN concentration is often linked to radiative feedbacks associated with cloud albedo, which has important consequences for climate sensitivity; however, the impacts of CCN may also extend to cloud dynamics in convective environments and atmospheric electricity.

Size-resolved CCN, aerosol number concentration and sub-saturated hygroscopicity were measured from May to September 2014 at a rooftop laboratory in Tucson, Arizona on the campus of the University of Arizona. The measurements spanned the pre-monsoon and monsoon summer seasons. In addition, detailed measurements of aerosol composition were made (using an OC/EC analyzer, a particle-into-liquid sampler (PILS) and several multi-orifice uniform deposition impactors (MOUDI)). We investigate sub-seasonal and diurnal patterns in the data to identify the role of local and regional emissions, meteorology and photochemistry on aerosol hygroscopicity and cloud drop activation.

We place a particular focus on the North American Monsoon season, which begins near the end of June, to investigate the changes in aerosol properties during the moist monsoon compared to the preceding hot and dry months. We also compare data measured in Tucson with more temperate regions, where other similar studies have been completed, to identify key mechanisms that drive CCN variability in arid climates.