Blog by Xiaoxi Liu, CIRES researcher
The formation of this particulate matter is largely driven by organic vapor condensation. How promptly particles exchange vapors with the gas-phase influences secondary organic aerosol (SOA) formation and is an important parameter in models that simulate the formation process. The mass accommodation coefficient (α) describes the probability that a molecule will pass through the gas-particle interface and be taken up by the particle. The value of α determines if partitioning is kinetically limited and impacts the gas/particle equilibration timescale in the atmosphere. Previously, α typically was only inferred from indirect methods, resulting in a very wide range of values.
Our work, for the first time, directly measures in both the gas and particle phases the dynamic, isothermal partitioning of chemically-speciated organic compounds in a state-of-the-art Teflon chamber. We have found near-unity α values during gas/particle partitioning for representative types of aerosol in the atmosphere, including aerosol of different phases (solid, liquid, and glassy) and polarities. Being near the value of one, this means there is a very high chance vapor will be taken up by a particle. The large α values imply insignificant gas-to-particle mass transfer limitations in the lower atmosphere. By providing a measurement of α for several low- and semi-volatile organics, as well as a highly accurate and precise method for its measurement, our results will enable faster advances in this field.
Figure 1. Schematic of experimental procedures, instrumentation, and partitioning processes considered in this work. Note that the dependence of the system on a is embedded in the condensational sink term (CS).
I worked closely with CIRES fellows Jose Jimenez and Paul Ziemann on this project—and it builds on a couple things: 1) our previous work on gas-wall partitioning that must be understood in order to study G/P partitioning in chambers. And 2) A previous study (Krechmer et al., 2017) was a proof of concept for the methods, but their results only applied to liquid dioctyl sebacate (DOS) seeds, which is not representative of atmospheric particles, and limited to gas-phase measurements only. Given that α could be different for different particle substrates, there was a need to quantify α for atmospherically-relevant particles with different phases and properties using experimental techniques that are capable of fast, direct measurements.
A number of advances in speciation of compounds relevant to G/P partitioning for SOA have been achieved by new developments in instrumentation for laboratory and field studies. We harnessed a new tool, the extractive electrospray ionization (EESI)-mass spectrometry instrument, and applied it successfully to measure particle-phase organics in our chemical system.
This paper was possible because we combined new methods with cutting-edge instrumentation—it drove our science. We achieved better, more accurate results by combining the methods with instrumentation capabilities, results one could not achieve with either one on its own.
But working with this instrumentation was, of course, not without its own set of challenges. This is a new field, so we are stretching the capabilities of instruments in both time resolution and detection limit. We had to take care to execute an experimental design to isolate the partitioning process from chamber wall loss and chemical reaction.
Read the paper: https://www.nature.com/articles/s42004-019-0200-x