Evolution of black carbon mixing-state in the Sacramento urban-biogenic admixed environment

 

Authors

Larry Kleinman — Brookhaven National Laboratory
Stephen R. Springston — Brookhaven National Laboratory
Arthur J Sedlacek — Brookhaven National Laboratory
Rahul Zaveri — Pacific Northwest National Laboratory
John E Shilling — Pacific Northwest National Laboratory
R. Subramanian — Carnegie Mellon University

Category

Field Campaigns

Description

Comparison of the incandescence-based mass equivalent diameter for BC particles to the light scattering-based estimates optical diameter enables the SP2 to probe the mixing state of BC. In the plot above, it can be seen that while coated 115 nm BC particles were negligible in the a.m., by the afternoon flight these particles gained a coating to yield optical diameter Dp ~ 225 nm. Put another way, between the morning and afternoon flights conducted on June 28, 115 nm diameter BC particles become encapsulated within a shell that is nominally 55 nm thick.
As part of the CARES campaign, the G-1 was deployed to investigate the temporal evolution of aerosols in a mixed urban-biogenic environment through morning and afternoon flights. Of specific interest to the present poster is the evolution of the black carbon (BC) mixing state. As will be discussed, the incandescence and scattering channels available on the SP2 can be combined to allow details of the BC mixing state to be studied. SP2 analysis of data collected on June 28 reveals that significant evolution in the BC mixing state took place, as evidenced by the growth of coated accumulation mode BC particles. In addition, probing the BC mixing state, it is also shown that the coated soot distribution for the afternoon flight can be reconstructed via an estimate of the coating thickness for a given soot core diameter along with a simplified condensation model. Using the G-1 AMS observation that nominally 90% of the non-refractory material was organic and the reconstructed coated core distribution, the expected light absorption enhancement can be calculated and compared to that measured in the field. Comparison of this model with that tabulated using the PSAP (Mm-1) and SP2 (ng/m3) reveals that the field measurement is nominally 2x larger. It is suggested that this difference is attributed to an organic aerosol bias in the PSAP measurement (Lack et al. 2008). In addition to the strictly airborne comparisons, comparisons were also carried out between the G-1 and ground sites. On June 15, SP2 calculated mass mean diameters (MMD) for the T0 (Sacramento) and T1 (Cool) ground sites were found to be 143 nm and 175 nm respectively. It is also reported that mixing-state analysis suggests that while both sites were dominated by thinly coated BC, there are likely more thickly coated, sub-100 nm MED BC cores at T1. (ASR poster: Subramanian et al. 2011) This mixing-state observation is consistent with that observed aboard the G-1 where analysis of the mixing layer suggests that BC is dominated by fresh emissions while the residual layer is comprised of an admixture of nascent and thickly coated soot aggregates, similar to that observed over the foothills (T1 site location). However, comparison of the G-1/ground MMDs is mixed. Whereas the G-1 mixing layer MMD of 147 nm is in excellent agreement with that reported for the T0 site (143 nm), the T1 MMD (175 nm) is nominally 15% higher than the measured 150 nm aboard G-1 while over the foothills area.