In-situ measurements of the mixing state and optical properties of soot with implications for radiative forcing estimates

Abstract

Our ability to predict how global temperatures will change in the future is currently limited by the large uncertainties associated with aerosols. Soot aerosols represent a major research focus as they influence climate by absorbing incoming solar radiation resulting in a highly uncertain warming effect. The uncertainty stems from the fact that the actual amount soot warms our atmosphere strongly depends on the manner and degree in which it is mixed with other species, a property referred to as mixing state. In global models and inferences from atmospheric heating measurements, soot radiative forcing estimates currently differ by a factor of 6, ranging between 0.2-1.2 W/m 2 , making soot second only to CO 2 in terms of global warming potential. This article reports coupled in situ measurements of the size-resolved mixing state, optical properties, and aging timescales for soot particles. Fresh fractal soot particles dominate the measured absorption during peak traffic periods (6 -9 AM local time). Immediately after sunrise, soot particles begin to age by developing a coating of secondary species including sulfate, ammonium, organics, nitrate, and water. Based on these direct measurements, the core-shell arrangement results in a maximum absorption enhancement of 1.6؋ over fresh soot. These atmospheric observations help explain the larger values for soot forcing measured by others and will be used to obtain closure in optical property measurements to reduce one of the largest remaining uncertainties in climate change.aerosol ͉ atmosphere ͉ climate ͉ composition

Characteristics of individual particles in the atmosphere of Guangzhou by single particle mass spectrometry

“…The physical and chemical properties (including density, shape, hygroscopicity, and refractive index) of these particle types are distinctly different from each other, influenced by various factors such as meteorological conditions and emission sources (e.g., Moffet and Prather, 2009;Zelenyuk et al, 2008). Therefore, it is expected that these particle types behave differently in the atmosphere and probably contribute differently to the environmental and climatic effect.…”

Variation of secondary coatings associated with elemental carbon by single particle analysis

“…Many studies have demonstrated that optical properties of EC are sensitive to their physical (e.g., size and morphology) and chemical (e.g., composition, mixing state) properties (Moffet and Prather, 2009). While freshly emitted EC may contain limited coating, atmospheric aging such as coagulation with other particles, condensation of vapors, and in-cloud processing could cause EC to internally mix with other chemical species (e.g., Moffet and Prather, 2009).…”

“…While freshly emitted EC may contain limited coating, atmospheric aging such as coagulation with other particles, condensation of vapors, and in-cloud processing could cause EC to internally mix with other chemical species (e.g., Moffet and Prather, 2009). Cappa et al (2012) and Lan et al (2013) had observed a limited enhancement due to the mixing state of ambient EC, however, other studies confirmed the enhancement effect of internally mixing under various conditions (Wei et al, 2013;Zhang et al, 2008).…”

“…Single particle mass spectrometry (SPMS) can provides the details of chemical composition measurements for single particles, including EC (e.g., Cahill et al, 2012;Pratt and Prather, 2012 and references therein;Zauscher et al, 2013). For example, Moffet and Prather (2009) observed a rapid coating process of organic carbon (OC) and sulfate on the EC core and assessed the related absorption enhancement in the polluted atmosphere of the Mexico city. Healy et al (2012) found that the mixing state of EC-containing particles showed dependency on vacuum aerodynamic diameter (d va ), with smaller particles (d va 0.4 mm) mainly externally mixed and larger particles (d va > 0.4 mm) mainly internally mixed, influenced by sources and transport.…”

Mixing state of biomass burning particles by single particle aerosol mass spectrometer in the urban area of PRD, China

“…It has been proved to be a powerful tool for analyzing the mixing state and formation mechanisms of aerosols in single particle levels (Gaston et al, 2010;Angelino et al, 2001;Moffet et al, 2004;Freney et al, 2006;Denkenberger et al, 2007;Moffet and Prather, 2009;Wang et al, 2009a;Yang et al, 2009). These accomplishments are extremely helpful to understand the impact of aerosols on the global climate.…”