Direct measurements of CCN chemistry during the CalWater Experiment
GC ROBERTS, SD Noblitt, JM Creamean, DB Collins, JL Collett, CS Henry, SV Hering, KA Prather
Scripps Inst. Oceanogr.; Meteo France
Abstract Number: 303
Working Group: Recent Campaigns in the North American West Coast
Last modified: April 4, 2011
The CalWater 2011 Experiment took place in winter in the foothills of the Sierra Nevada Mountains in Central California to assess the sources of cloud active aerosol and their interaction with the hydrological cycle in California. We coupled the capabilities of demonstrated miniaturized instrumentation – cloud condensation nuclei (CCN), water condensation nuclei (WCN) and microchip capillary electrophoresis (MCE) – to provide direct chemical measurements of cloud active aerosols. Ion concentrations of CCN droplets attribute the anthropogenic, marine and secondary organic contributions to cloud-active aerosols. The MCE system was also coupled to a WCN device to determine the mass fraction of soluble ions that serve as cloud-active nuclei. In addition to direct measurements of the CCN and aerosol chemistry (electrophoresis and single particle analysis), continuous CCN spectra, aerosol size distributions and concentrations were also measured.
CCN concentrations were low (< 100 cm$^(-3) at 0.4% S$_c) when high pressure systems prevented boundary layer development and intrusion of the Central Valley pollution to the site - suggesting relatively low sources of CCN from the surrounding forest during this period. Storm fronts and changes in atmospheric boundary layer brought aerosol and anions associated with Central Valley pollution to the site with concentrations reaching several thousand particles cm$^(-3). CCN spectra show large variations depending on the aerosol sources and often exhibit bi-modal distributions with minima at 0.4% S$_c. MCE results show that nitrates and sulfates comprise most of the fraction of the aerosol anion mass (PM$_1). Nitrate and sulfate concentrations were often low (< 0.2 ug m$^(-3)) due to the frequent rains in February and March; however, regional sources did generate peaks of several ug m$^(-3) of nitrate during the passage of storm fronts. Chloride was also an important component during precipitation events indicating contributions of marine sources as well.