The effects of chemical properties on time scales for experiments involving organic condensation onto particulate matter.

摘要

Research in organic aerosol growth using a TDMA is well-established. In laboratory TDMA setups, however, it may be challenging to select an appropriate organic with which to conduct TDMA experiments. When choosing an organic for TDMA work, emphasis is often placed on which organic family a compound comes from, or its presence in the atmosphere, or the historical use of the compound. As equilibrium is the only state in which results from one experiment are comparable to results from another experiment, it is important to know what timescales are theoretically expected in order for the lab setup to be designed accordingly. Many TDMA setups have residence times on the order of a few seconds. Our model suggests that, for most organic vapors, a significantly longer amount of time is necessary to achieve equilibrium.; A MATLAB model has been developed based on fundamental principles for heterogeneous condensation of organics onto transition-regime particles. Beginning with a single inert particle surrounded by a super-saturated organic vapor, condensation of the organic vapor onto the particle is modeled by the diffusion flux of the vapor to the particle. While this idealized model does not consider wall losses in an experimental system, it allows us to systematically examine the effect of each individual chemical property of an organic (e.g., molecular weight) via a series of sensitivity analyses. We can also assess the importance of various experimental parameters in the lab setup, such as the particle concentration or the temperature used to achieve supersaturation. We are thus able to tease out the individual effect of each organic characteristic or experimental property on the timescales necessary to reach equilibrium.; Candidate organics for thin coatings are largely determined by low saturation vapor pressures. Results also show that the time necessary for condensation of organic vapor onto a particle typically ranges from tens of seconds to minutes. This suggests that TDMA data utilizing equilibration times of a few seconds may not be at equilibrium. With this model, guidance can be provided to assist experimenters in the selection of appropriate organics for their particular TDMA particle growth experiments so that equilibrium is reached.