Abstract
Abstract. Black carbon (BC) is a short term climate forcer that directly warms the atmosphere, slows convection, and hinders quantification of the effect of greenhouse gases on climate change. The atmospheric lifetime of BC particles with respect to nucleation scavenging in clouds is controlled by their ability to serve as cloud condensation nuclei (CCN). To serve as CCN under typical conditions, hydrophobic BC particles must acquire hygroscopic coatings. However, the quantitative relationship between coatings and hygroscopic properties for ambient BC particles is not known nor is the time scale for hydrophobic-to-hydrophilic conversion. Here we introduce a method for measuring the hygroscopicity of externally and internally mixed BC particles by coupling a single particle soot photometer with a humidified tandem differential mobility analyzer. We test this technique using uncoated and coated laboratory generated model BC compounds and apply it to characterize the hygroscopicity distribution of ambient BC particles. From these data we derive that the observed number fraction of BC that is CCN active at 0.2 % supersaturation is generally low in an urban area near sources and that it varies with the trajectory of the airmass. We anticipate that our method can be combined with measures of air parcel physical and photochemical age to provide the first quantitative estimates for characterizing hydrophobic-to-hydrophilic conversion rates in the atmosphere.
Highlights
Black carbon (BC) particles absorb solar radiation, leading to heating and suppression of convection in the atmosphere
The purpose of this work is to describe the humidified tandem differential mobility analyzer (HTDMA)-SP2 setup, present tests with laboratory generated pure and mixed model BC particles, explain the data processing procedures, report on initial ambient data collected in a major urban center, and discuss how data products from this technique can be used in future targeted campaigns to improve our understanding of refractory black carbon (rBC) aging processes
We examined the relationships between several estimates of rBC coating thickness and mixing state for hydrophobic and hygroscopic rBC particles
Summary
Black carbon (BC) particles absorb solar radiation, leading to heating and suppression of convection in the atmosphere. Black carbon has an average lifetime of 1 ± 1 weeks (Rodhe et al, 1972) and contributed ∼0.9 W m−2 to the top of atmosphere forcing during 2005 (Ramanathan and Carmichael, 2008) Forcing estimates such as these rely on up-to-date emission inventories and accurate description of the wet removal process. Detailed microphysical models predicting τ indicate that the time scale varies between day and night, and that aging proceeds rapidly during daytime (Riemer et al, 2010). Verifying these differences experimentally is important because hydrophobic-tohydrophilic conversion time scales represent a major uncertainty in determining BC lifetime and predicting its distribution throughout the atmosphere
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