Abstract
Abstract. Carbonaceous aerosols impact climate directly by scattering and absorbing radiation, and hence play a major, although highly uncertain, role in global radiative forcing. Commonly, ambient carbonaceous aerosols are internally mixed with secondary species such as nitrate, sulfate, and ammonium, which influences their optical properties, hygroscopicity, and atmospheric lifetime, thus impacting climate forcing. Aircraft-aerosol time-of-flight mass spectrometry (A-ATOFMS), which measures single-particle mixing state, was used to determine the fraction of organic and soot aerosols that are internally mixed and the variability of their mixing state in California during the Carbonaceous Aerosols and Radiative Effects Study (CARES) and the Research at the Nexus of Air Quality and Climate Change (CalNex) field campaigns in the late spring and early summer of 2010. Nearly 88% of all A-ATOFMS measured particles (100–1000 nm in diameter) were internally mixed with secondary species, with 96% and 75% of particles internally mixed with nitrate and/or sulfate in southern and northern California, respectively. Even though atmospheric particle composition in both regions was primarily influenced by urban sources, the mixing state was found to vary greatly, with nitrate and soot being the dominant species in southern California, and sulfate and organic carbon in northern California. Furthermore, mixing state varied temporally in northern California, with soot becoming the prevalent particle type towards the end of the study as regional pollution levels increased. The results from these studies demonstrate that the majority of ambient carbonaceous particles in California are internally mixed and are heavily influenced by secondary species that are most prevalent in the particular region. Based on these findings, considerations of regionally dominant sources and secondary species, as well as temporal variations of aerosol physical and optical properties, will be required to obtain more accurate predictions of the climate impacts of aerosol in California.
Highlights
Carbonaceous aerosols, comprised of soot and/or organic carbon (OC), affect climate directly through scattering and absorbing radiation and indirectly by influencing cloud formation, albedo, and lifetime (Ackerman et al, 2000; Poschl, 2005; Ramanathan et al, 2007; Rosenfeld and Givati, 2006)
A large area of northern California was characterized during the Carbonaceous Aerosols and Radiative Effects Study (CARES), a study that sought to follow the evolution of soot as particles are transported from fresh urban sources in Sacramento into the more remote Sierra Nevada foothills
This study focused on the surrounding urban Sacramento area and Sierra Nevada foothills, results from this campaign are interchangeably referred to as northern California
Summary
Carbonaceous aerosols, comprised of soot and/or organic carbon (OC), affect climate directly through scattering and absorbing radiation and indirectly by influencing cloud formation, albedo, and lifetime (Ackerman et al, 2000; Poschl, 2005; Ramanathan et al, 2007; Rosenfeld and Givati, 2006). The potentially large effect on radiative forcing calculations due to mixing state necessitates empirical measurements to determine the extent of soot and non-absorbing species, such as OC, present in the atmosphere as internal or external mixtures. A large area of northern California was characterized during the Carbonaceous Aerosols and Radiative Effects Study (CARES), a study that sought to follow the evolution of soot as particles are transported from fresh urban sources in Sacramento into the more remote Sierra Nevada foothills (campaign.arm.gov/cares/). These two studies conducted consecutively in May and June 2010 provide an assessment of particle mixing state throughout most of California
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