The Dark side of VOCs

The emissions of VOCs into the environment can be scary no matter what circumstances are occurring in the world.

Introduction

During the COVID-19 pandemic, many businesses and industries have had to shut down. But not everything has been for the worst. With the shutdown of manufacturing industries, the production of VOCs and therefore their reactions with other atmospheric gases have been severely reduced so much so that the air quality has gotten detectably better.

During Covid-19 Times

With the pandemic still going on, tons of things in the world have changed. In manufacturing, the production rate and transportation have severely changed. But that isn't necessarily a bad thing. The environment or more specifically the atmosphere has seen a significant improvement. In a study taking data in Nanjing, China on air quality, they found that there was a 47% decrease in the concentration of VOCs in the air. So instead of 50.9 parts per billion (ppb) it was 26.9 ppb. This plainly means that with manufacturing slowing down, so has the emission of VOCs into the atmosphere as well. Vehicle emissions were also significantly reduced. In a similar study as before, they found that vehicle emissions of VOCs were reduced by about 75%. Basically, less people were driving to their jobs, to events, or to other private establishments.

This reduction of pollutants has its benefits but it also causes some of its own problems. In another study taken in China, the emission of NOx has also been reduced by about 48%. It's good that NOx pollutants are being reduced, but the production of ozone actually increased by about 92%. When there are low concentrations of NOx but higher concentrations of VOCs, ozone is able to be produced more frequently. That could lead to some more issues in the future.

In General, the emission rates of things like VOCs, NOx, and other atmospheric pollutants have been significantly reduced and therefore their environmental effects. Researchers do need to keep monitoring the atmosphere for any changes in VOCs, NOx, and ozone levels to maintain decent air quality.

What are VOCs?

VOC stands for volatile organic compounds. These compounds have a high vapor pressure and low water solubility. Meaning VOCs don't dissolve in water well and evaporate quickly. VOCs also are generally made during manufacturing, but can be emitted by a variety of things. Some examples are paints, cleaning products, pesticides, and fuels.

Why are VOCs created?

VOCs can be both a byproduct and be an ingredient for a product. In manufacturing, VOCs can be used as industrial solvents or fuel oxygenates. In commercially available products, VOCs can be used in paints, Isoprene for example can be used in other ways. Isoprene can be used as synthetic rubber, a material in pesticides, and used to make other chemicals. Other than being made in a manufacturing process, some VOCs are naturally emitted from finished products or living organisms. Isoprene is a chemical that can be emitted by just about every living being.

Figure 1.1: 3D model of isoprene

Figure 1.2: 3D model of Nitrogen Dioxide

How do VOCs impact the environment?

VOCs by themselves do not impact the environment a lot. VOCs cause issues though when they react with nitrous oxides (NO/NO2). The same example of a VOC is the chemical isoprene. Isoprene is an organic compound that can be produced in a variety of ways. Most living things will release isoprene, but the majority of emissions come from agriculture and manufacturing. This organic compound can be used to make synthetic rubber and other chemicals. Again, however, the main issue is how it can react when in the presence of nitric oxides.

Reaction

When isoprene is in the presence of high levels of nitric oxides, it can become oxidized (electrons taken from another substance) and turn into hydroperoxides. Hydroperoxides are molecules that have a weak oxygen bond and can oxidize other chemicals very easily. 2 (NO2 +02 → NO +O3) For example, this new compound can react with nitrogen monoxide (NO) to produce nitrogen dioxide (NO2). Nitrogen dioxide molecules are able to decompose under sunlight. That reaction is what results in ozone, which is considered as a pollutant.

Figure 1.3: Reactions with isoprene to form ozone and other things

Source: National Library of Medicine - National Institutes of Health

Quantitative Analysis

Concentrations have been found to be 2-5 times as dense within homes compared to the outside atmosphere, regardless of living conditions within the city. The net reaction for isoprene and nitrogen oxides is:

\(RH + 4O_2 → R'HCO + 2O_3 + H_2O \)

When the resulting compound, Isoprene with an HCO radical (R'HCO) is multiplied by 5, the rest of the function must be increased with it, resulting in:

\(5RH + 2ØO_2 → 5R'HCO + 1ØO_3 + 5H_2O \)

In other words, for each isoprene in the outside atmosphere, there are 5 in many households, caused by household products. This results in a higher concentration of R'HCO, which is the main VOC that is produced by isoprene, and is a health hazard to human beings.

How Do We Know?

One way that scientists can observe and understand the chemical properties and reactions of VOCs is that they can use Gas Chromatography (GC). GC can have a variety of different types of detectors. The primary ones used for VOCs analysis are flame ionization, electron capture, and mass spectrometry.

Flame lonization

The Flame lonization Detector is one of the best detection methods, mainly because of its ability to measure hydrocarbons. This is done by using a carrier gas, in which the sample being tested is ionized from a hydrogen-air flame. That hydrogen-air is then collected using a polarized (opposite) voltage. The current produced is what is measured and used to quantify the amount of the sample that is being burned.

Figure 1.4: Diagram on the inside of a flame ionization detector

Source: lab-training.com

Electron Capture

The Electron Capture method utilizes beta particles, which are electrons. This detector contains radioactive nickel-63, which is what emits these beta particles. These particles collide with a carrier gas within the cell of the detector. This allows for stable free electrons. When an electronegative molecule, molecules that want to gain electrons, enters the cell of the detector. This molecule almost immediately combines with the free electrons to fulfill the octet rule (Fills in its most outer shell of electrons). The slight reduction is what gets measured by the detector. In other words, the detector can tell how much of the sample there was being tested, by the use of electrons combining with the sample.

Figure 1.5: Diagram on the inside of the Electron Capture Detector

Source: SlideServe

Mass Spectrometry

Mass Spectrometry is also a possibility to qualitatively analyze VOCs. To do this, the sample being tested is ionized by an electron beam. This will typically fragment the sample and produce a mass spectrum that matches with the sample's chemical structure. Moreover, the ionization allows for the sample to be broken down to ions. Those ions get pulled through slits toward a magnetic field then analyzed for the differences based on the mass to charge. The sample can then be identified by simply comparing the results to a reference.

Figure 1.6: Diagram on the inside of the Mass Spectrometry to a computer to compile the data

Source: Kumar, D., et al. Bio-oil and biodiesel as biofuels derived from microalgal oil and their characterization by using instrumental techniques. InAlgae and environmental sustainability. 2015, pp 87-95.

Periodic Table

Here as you can see is the periodic table of elements. As most of you all know, the elements that scientists are aware of get organized here. This particular way of organization allows scientists to predict and compare other elements by their properties. In context of this exhibit, this periodic table highlights the major elements discussed during the main duration of this exhibit and a little bit on the Troublesome Aluminum exhibit. This provides a little bit of supplementary information on those elements. More specifically, it shows their atomic mass, weight, their atomic number, how many protons the element has, and many other things.