The mixing state of carbonaceous aerosol particles in northern and southern California measured during CARES and CalNex 2010

摘要

Carbonaceous aerosols impact climate directly by scattering and absorbingradiation, and hence play a major, although highly uncertain, role in globalradiative forcing. Commonly, ambient carbonaceous aerosols are internallymixed with secondary species such as nitrate, sulfate, and ammonium, whichinfluences their optical properties, hygroscopicity, and atmosphericlifetime, thus impacting climate forcing. Aircraft-aerosol time-of-flightmass spectrometry (A-ATOFMS), which measures single-particle mixing state,was used to determine the fraction of organic and soot aerosols that areinternally mixed and the variability of their mixing state in Californiaduring the Carbonaceous Aerosols and Radiative Effects Study (CARES) and theResearch at the Nexus of Air Quality and Climate Change (CalNex) fieldcampaigns in the late spring and early summer of 2010. Nearly 88% of allA-ATOFMS measured particles (100–1000 nm in diameter) were internally mixedwith secondary species, with 96% and 75% of particles internally mixedwith nitrate and/or sulfate in southern and northern California,respectively. Even though atmospheric particle composition in both regionswas primarily influenced by urban sources, the mixing state was found to varygreatly, with nitrate and soot being the dominant species in southernCalifornia, and sulfate and organic carbon in northern California.Furthermore, mixing state varied temporally in northern California, with sootbecoming the prevalent particle type towards the end of the study as regionalpollution levels increased. The results from these studies demonstrate thatthe majority of ambient carbonaceous particles in California are internallymixed and are heavily influenced by secondary species that are most prevalentin the particular region. Based on these findings, considerations ofregionally dominant sources and secondary species, as well as temporalvariations of aerosol physical and optical properties, will be required toobtain more accurate predictions of the climate impacts of aerosol inCalifornia.