Production and Measurement of Organic Particulate Matter in the Harvard Environmental Chamber

The overall goal of this project is to produce organic particular matter in the environmental chamber and characterize its chemical and physical properties. The Harvard Environmental Chamber was built to study the formation of organic particulate matter and the reaction of gas phase components at close to ambient conditions. This is the tenth year anniversary of chamber operations.

The unique part of this chamber is it is operated as a completely mixed flow reactor providing the chance to do steady state operation across days. To ensure the steady state operation, the chamber uses a feedback system to control important chamber parameters and keep them stable. We can conduct various online and offline measurements that require days to finish because the concentrations of the gas and particle phase species remain stable indefinitely.

The Harvard Environmental Chamber, consist of three parts. The first part is the seed areas of generations and the volatile organic compounds generation system. The second part is the environmental chamber itself, and the third part is the instrument analyzing this system.

Using seed particles, in the reactions in the Harvard Environmental Chamber, is a critical technique, which allows a stable generation, of particle phase organic materials. We specifically choose inorganic salts as seed particles and in reactions they will be coated with organic materials. We later do data analysis on the organic materials, and since we choose inorganic salts, the interference from seed particles can be minimized.

One of the instruments that we use here in our lab, is a high resolution time of flight Aerosol Mass Spectrometer, or for short AMS. It's an instrument mercelized by Aerodyne Research, and nowadays its widely used both in laboratory and field studies, including air craft deployments. The AMS provides real time measurements of non-refractory particle chemical composition, and it can also be operated in a mode that provides information on particle size by measuring the particle time of flight.

So, by combining information on size and information of chemical composition from the Mass Spectra, we can get mass ionomer distributions for the measured ions. They key measured environmental parameters include ozone, NO and NO2, relative humidity, temperature, and the differential pressure between the bag and the chamber. Set the physical parameters of the environmental chamber by the feedback system.

Set the differential pressure to four Pascal or 30 mini Torr. Turn on the ozone generator to generate ozone flow by passing the dry air through an ultraviolet lamp. Set the flow rate to 0.1 standard liter per minute.

Set the relative humidity of the bag to the designated values. Set the temperature of the chamber to 25 plus or minus 0.1 degrees Celsius. Connect inlets of instruments to the environmental chamber.

Start the self developed software by clicking the Start button. Check the real time data displayed on the self developed software that integrates the feedback control. Turn on all the instruments and wait for them to warm up completely.

Dissolve ammonium sulfate in high purity water in 100 milliliter volumetric glass to prepare an ammonium sulfate solution. Use an atomizer to produce ammonium sulfate particles at a flow rate of three standard liters per minute. Pass the aerosol flow through a diffusion drier to bring the relative humidity down to 10%Pass the aerosol flow through a bipolar charger and a differential mobility analyzer to size select the particles and prepare a quasi-monodispursed distribution by electric mobility.

Use a syringe to withdraw one milliliter of isoprene solution. Rinse the syringe three times with the solution prior to final withdrawal. Place the syringe into a syringe injector.

Insert the needle tip through a rubber seal into a round bottomed flask. Preheat the flask to 90 plus or minus 1 degree Celsius by heating tape. Turn on the syringe injection and set it to an appropriate value.

The gas phase concentration of the precursor is adjusted for different experiment by controlling the syringe injection rate. For long experiments, refresh the syringe as needed. Introduce a flow of two standard liters per minute of purified air to vaporize and carry away isoprene injected in the round bottomed flask.

The flow of the air is large enough that the sessile droplet at the tip of the syringe is vaporized instead of dripping into the flask. The combination of isoprene and UV light leads to the production of secondary organic material. Start the aerosol measurement software and create a new file by clicking on Create a New File.

Each parameter is set as shown. Record the number diameter distributions of particles exiting the bag by clicking on the Okay button. Measure aerosol flow by using a high resolution time of flight aerosol mass spectrometer.

Start the data acquisition software by pressing the Acquire button on the lower left of the panel. High resolution mass spectra of the organic PM are recorded during the time course of the experiments. The total organic mass concentration is also obtained.

Open the sampling value of a PTFE Teflon tube inside the bag. The sampled flow is guide to a proton transfer reaction time of flight mass spectrometer. Parameter settings of the ion source of the proton transfer reaction time of flight mass spectrometer are shown in this video.

Start the data acquisition by accessing the drop down menu, Acquisition, in the top deck of your software and then pressing Start. Record the time series of each ion through this software. Stop the injection of the gas phase precursors and the aerosol seed particles.

For several days, continuously inject pure air at 40 liters per minute into the bag. Turn on all ultraviolet lights. Set the ozone concentration to 600 parts per billion and set the temperature to 40 degrees Celsius.

In this way, an aggressive oxidation environment is maintained for several days to scrub the bag. The data acquired from aerosol mass spectrometer are recorded and processed. The experimental conditions are 490 PPB of isoprene with UV lights turning on to provide OH radical as oxidant.

The mass concentration of secondary organic materials was increasing at the beginning of the experiment and after about four hours it reached a steady state. The plot suggests the environmental chamber is able to produce SOM from the gasses precursors. The evolution of gas phase organic compounds inside the chamber can be studied using the PTRTOFMS.

An example experiment on isoprene photo oxidation was conducted with approximately 16 PPB of isoprene being put into the chamber continuously. The figure shows the time series of the C4H6O+ion one of isoprene's major oxidation products measured by the PTRTOFMS. At the beginning of the experiment, there was no UV light inside the chamber.

At about eight minutes, the UV light was turned on and there was a clear trend of the rising C4H8O+ion. After about 50 minutes, the reaction reaches steady state. Laboratory chamber studies are really important in the field of aerosol science or more broadly, atmospheric sciences and that's because they allow us to simulate and investigate, in a controlled way, the complex chemical and physical phenomena that happen in the atmosphere.

Chambered studies have greatly helped in developing our understanding of formation and evolution of secondary organic aerosols for example, SOA, which are a dominant component of the particulate matter on a global basis. So, data coming from these chamber studies, looking at questions related to SOA, have been used to guide development of chemical mechanisms and also have been used in parameterizations for SOA formation and evolution in models.