Research Highlights

Particulate matters (PM) suspended in the atmosphere can scatter/absorb solar radiation, act as cloud condensation nuclei, through which ways to impact the climate directly or indirectly. PM2.5 refers to those with a diameter smaller than 2.5 micro meter, which can profoundly damage human health since they can penetrate efficiently into deep stages of the lung. During most of PM's lifetime in the atmosphere, they interact with each other as well as the gas molecules/radicals surrounding them. Thus the properties (e.g., chemical composition, size, and phase state etc.) of these suspended particles are always changing. We are interested in how these properties are evolving so that we can predict and evaluate them by models (just like the weather prediction). To do this, we need both instruments and models.

Caltech PhotoOxidation flow Tube reactor (CPOT)

Flow tube reactor is a complementary reactor to the smog chamber reactor, both of which have been widely used in the laboratory to study atmospheric chemistry. The CPOT is designed to run experiments at steady state under a specific conditions that can perform a much higher exposure (reagent * time) than a chamber reactor. We have carefully characterized this system: the residence time distribution, the OH exposure as a function of relative humidity and ozone, and simulation of the photochemistry with an axial dispersion plug flow reactor model (AD-PFR).

You can find the two characterization papers here (1, 2).

We are going to run heterogeneous oxidation of secondary organic aerosol with CPOT. More papers are on the way.

Vapor-Wall Mass Transfer

Compared with the real atmosphere, all the experiments in the lab have to concern about the boundaries because the air parcel is not as free as those ambient ones. The interaction between vapor molecules and the Teflon-walled environmental chamber can be described as a multi-step dissolving process. This project proposes a unified model to reconcile the observed two significantly different dissolving timescales.

You can find more details from this paper.

Oxidation at Air-Water Interface

This project essentially builds a model that can be used to simulate multi-phase (gas-interface-aqueous phases) mass transport and reaction. The experimental technique is called field induced droplet ionization (FIDI) that focuses on in-situ air-water interfacial reaction . The model framework follows the PRA definition.

You can find more details about this model from this paper. The model is written in MATLAB, and tested by R2016b. If you are interested in the source code, please contact me.

Particle-Phase Component Detection

Detection of particle phase component is always very challenging since the particle is a condensed phase. Online aerosol mass spectrometer usually employs hard ionization (EI, LDI etc.) and provides information of fragments. To keep the most original form of the molecules in the particle phase, soft ionization (ESI, CI etc.) has been developed. The problem then lies on the pretreatment of particles. For chemical ionization (CI), one popular method to pretreat particles is thermal desorption (e.g., Filter Inlet for Gases and AEROsols, FIGAERO). I developed a system that is essentially the same idea as FIGAERO to our CIMS (quadrupole) and combined with UPLC-tofMS to study the volatility of aerosol. The manuscript on this is submitted.

Particle Size Measurement

Differential mobility analyzer (DMA, the classifier) and condensation particle counter (CPC, the detector) are usually combined to measure particle size distribution. The classifier has a size-dependent transmission efficiency (so called "transfer function") and the detector have a delayed response time. The deconvolution of this system to get the correct particle size distribution is called inversion. I derived an analytical solution of the transfer function in scanning mode and developed an empirical method to simplify the inversion with my colleagues. You can find the details on diffusional transfer function for details here. The other manuscripts are in preparation.