Production and Measurement of Organic Particulate Matter in a Flow Tube Reactor

Flow tube reactors are built to mimic the production processes of atmospheric organic particulate matter, and are used to study the mechanism, processes, and characterization of the particulate matter. The advantage of using a flow tube reactor is it enables the rapid synthesis of aerosol particles across a wide range of particle number and mass concentrations. For the set up presented here the flow tube reactor is equipped with a moveable injector which can sample organic particulate matter at different time points inside the flow reactor.

The particulate matter exiting the flow tube is analyzed by various kinds of online and offline techniques including a scanning mobility particle sizer and an aerosol particle mass spectrometer. And will also sample onto particulate filters. The flow tube is a suitable reactor platform for performing post analysis experiments and fast online, and offline analysis of the produced particulate matter.

Atmospheric particles have been part of the effects of climate, human health and visibility. The production mechanism for organic particulate matters, however still remain insufficiently characterized and aren't astute. One approach to solve this issue is to use a flow tube reactor to perform laboratory studies that help us understand the formation and the reaction mechanism for organic particulate matter.

The flow tube reactor consists of three parts. The first part of the flow tube experiment is the injection of organic precursor. The injection system consists of three elements.

A syringe pump, a glass syringe, and a three lag glass bulb. The organic solution is continuously injected using the syring pump into the glass bulb, and then it vaporizes. Then the vapor is swabbed into the flow tube where reactions take place to produce a particle population.

The second part of the flow tube reactor consists of the flow tube itself and also a movable sampler. The movable sampler can control the residence time of the particles inside the flow tube from three seconds to 42 seconds, therefore help us study the corrosive mechanism for this organic particles and also help us switch the gross mechanism between coagulation and condensation grows. The last part of the flow tube reactor system is the instruments that analyze organic particles.

We have the scanning mobility particle sizer and the aerosol particle mass analyzer to study the number mass concentration and also study the shape of the particles coming out from the flow tube. The protocols for performing the flow tube experiment are shown below. Gas phase injection of the flow tube reactor.

Depending on the purpose of the experiments, a wide range of volatile organic compounds can be used as the organic precursor for the experiment. Alpha pinene is used here as an example for the procedure of injecting the organic precursor into the flow tube reactor. Use a micro-pipette to withdraw one milliliter of alpha pinene and then transfer the liquid to a fifty milliliter volumetric flask.

Use two butanol to fill the volumetric flask to fifty milliliter thereby diluting the alpha pinene by a ratio of one to 49. Shake the volumetric flask to mix the solvent and the solute thoroughly. Use a five milliliter syringe to withdraw the alpha pinene solution.

Rinse the syringe three times with the solution and then fill the whole syringe. Remove any bubbles in the syringe. Connect the syringe to a sharp needle and then move the syringe onto a syringe injector.

Insert the needle tip into round bottom flask to vaporize the solution. Preheat the vaporizer flask to 135 plus or minus one degrees Celsius by adjusting the power of the heating tape. Set the mass flow controller rate to 5 standard liters per minute.

The purpose is to introduce a gentle flow of 5 standard liters per minute purified air to vaporize and carry away alpha pinene injected from the syringe. Turn on the syringe injector and adjust the ejection rate to a value set by the user. Passive flow of air at four standard liters per minute through an ozone generator.

Turn on the ozone generator. Control the ozone concentration to appropriate values by adjusting the length of the glass tube shielding the UV lamp inside the generator. Turn on the ozone concentration monitor.

Perform the experiments after the ozone concentration stabilizes. Particle production of the flow tube reactor. Unscrew the cap at the end of the flow tube reactor in order to adjust the position of the movable sampler tubing inside the flow tube reactor.

Change different positions of the movable sampler tubing subsequently to achieve different residence times. Position the movable sampler at the beginning of the flow tube reactor to obtain the shortest residence time. Position the movable sampler at the end of the flow tube reactor to obtain the longest residence time.

House the flow tube reactor in a temperature controlled double walled, water jacketed, stainless steel box. Perform a leak check and a water level check prior to each set of experiments. Set the temperature of the thermostat in the water circulator to 20 degrees Celsius.

Turn on the temperature recording software in the main computer and set the data sampling time to 10 seconds. Record the temperature measured from the temperature sensor when turning on the record button. Turn on the pressure monitor software and set the sampling interval to 10 seconds.

Set the sampling length to 36, 000 points. Characterization of produced particle population of the flow tube reactor. Connect the outlet of the flow tube reactor to a scanning mobility particle sizer by electrostatic resistant tubing.

Start the software that records the number diameter distribution. Create a new file and each parameter to appropriate values. Record the number diameter distributions of the particles exiting the flow tube reactor by clicking on Okay button.

Connect the two inlets of a water bubbler to two mask flow controllers so as to adjust the humidity of the sheathe air in the flow tube. Adjust the flow rate of the two inlets from zero to 10 standard liters per minute so as to change the relative humidity of the sheathe air from less than 5%to greater than 95%Connect the outlet of the water bubbler to the sheathe air inlet of the Nafion tube. Connect the outlet of the flow tube reactor to the main sampling inlet of the Nafion tube.

Connect a relative humidity sensor to the outlet of the Nafion tube. To measure the relative humidity of the sampling air. Connect the outlet of the relative humidity control setup to the inlet of a differential mobility analyzer.

Connect the outlet of the differential mobility analyzer to the inlet of the APM instrument by electrostatic resistant tubing. Connect the outlet of the APM to a condensation particle counter. Turn on the APM instrument and the APM control box by pressing the respective power buttons.

Click Remote button on the APM control box so that the instrument can be operated from the software interface in the computer. Turn on the APM control software. Load a preset scanning file by clicking the File and Load buttons as shown in the video.

Click on the Start button of the APM control software so that the APM instrument starts to collect data. Clean a silicon substrate by a cycle of methanol water and again methanol to remove any contaminates. Dry the substrate using a gentle flow of nitrogen.

Place the clean substrate onto the electrode of the nanometer aerosol sampler. Secure the edge of the substrate with tape to keep it stable during the collection. Turn on the nanometer aerosol sampler.

Set the voltage to minus 9.9 kilovolts. Set the flow rates to 1.8 liters per minute. Afterwards, remove the silicon substrate loaded with collected particles from the nanometer aerosol sampler.

Perform further analysis of particles on the substrate such as morphology by scanning electron, microscope or surface analysis. Representative results. There is a range of number and mass concentrations of organic particulate matter that can be produced depending on the selected alpha pinene and ozone concentrations.

As shown in this table, these conditions produced 4.4 plus or minus 6 to 6.3 plus or minus 7 times 10 to the five particles per centimeter cubed and mass concentrations of 10 to the one to 10 to the four microgram per meter cubed respectively. The evolution of the dynamic characteristics of the particle population can be studied inside the flow tube reactor. This figure shows the number diameter distributions of the aerosol particle population for this experiment.

The total number concentration and the mode diameter of the particles increased with the residence time. The particle mass and mobility diameters were used to calculate the dynamic shape factor kai across particle subpopulation. This figure shows the dynamic shape factors of the particles exiting the flow tube at various mobility diameters and humidity levels.

As the RH was increased, kai decreased for all three populations reaching a final value of 1.02 plus or minus 01 at 35%relative humidity and corresponding with an uncertainty to spherical particles. The flow tube reactor described above is a great tool for studies of physical or chemical properties and evolution of organic particles. However, the relative short residence time and the high precursor concentration limits its ability study the organic particles formed under close ambient conditions.

We have shown that the flow tube can synthesis particles across a very wide range of mass concentration and number concentrations and is very suitable to distinguish a particle gross from coagulation to condensation. The flow tube is also suitable for collecting organic particles under a relative high mass.