Have you ever wondered what you breathe while taking a walk or the beach or driving on a freeway? The answer is particulate matter (PM). By definition, PM is an aggregate of all hazardous particles- solid, liquid, or gas- suspended in the atmosphere. This subject is of the utmost importance to the public due to the implication on human health caused by PM 2.5 and PM 10. The distinction between different PMs is their particle size and their source. PM 2.5 particles are about 1um to 2.5um diameter range. Primary PM 2.5 originate from vehicles, power plants, fireplaces, agricultural burning whereas secondary PM 2.5 is created by atmospheric reactions of nitric oxides (NOx), volatile organic compounds (VOCs), and sulfates (SO2) from vehicle emissions, coal burning, and indoor cooking among other sources (Baird 121 and Barnard 1). The high levels of PM 2.5 correspond to serious cardiovascular and respiratory health issues (Kim 2014, Wilson 2005). On the other hand PM 10 particles are about 10um in diameter. Sources of PM 10 are usually primary, directly forming PM via mechanical processes that create larger particles of sea salt, pollen, and brake dust (Natural Sources of Particulate Matter 2018). According to the U.S. Environmental Protection Agency (EPA), San Diego ranks 5th in terms of PM pollution. Thus, by providing a solution to limit inhalation of PM, lives can be saved.
The two goals of this experiment were to determine if location has an impact on PM levels and whether or not the wind blowing from the beach contributed to particulate matter growth or evaporation. Secondary PM 2.5 is produced via multiple compounds reacting in the atmosphere over a period of time. These reactions are not well established in the realm of environmental chemistry. Thus, we wanted to test the impact of wind on either particulate matter growth or evaporation. Our hypothesis was that inland highways, during winter months will have higher levels of PM 2.5 than coastal highways. We speculate that since secondary PM formation from the coast would take precedence over the process of evaporation, especially during the winter in San Diego.
In order to test our hypothesis that wind contributes to volatile organic compounds (VOCs) and nitric oxides (NOXs) being evaporated from particulate matter. PM 2.5 and PM 10 were measured at different locations in San Diego. As shown in the map below (figure 1) , data was collected on a coastal highway (Route 5), inland highway (Route 15), and at the beach (La Jolla Shores). The beach levels were recorded to act as a background measurement. Data was collected on the same day at all three locations to limit effects of other variables (wind trajectory, sunlight, temperature). A portable Atmotube sensor connected to a smartphone was utilized to gather data. In order to minimize error, the sensor was calibrated. This refers to comparing the readings of the Atmotube with measurements obtained from a known device.To analyze the data, we generated a Hysplit model back trajectory, which is a trajectory that analyzes and graphs wind patterns at different locations.
Figure 1- Measuring PM 2.5 and PM 10 at various locations in San Diego
Data was collected at first on an inland freeway along route 15. Next, we gathered PM data on a coastal freeway along route 5, going toward La Jolla. Lastly, beach measurements were obtained at La Jolla Shores.
Figure 2- Hysplit model wind trajectories at various locations
The leftmost wind trajectory is mapped at the inland location at which PM measurements were collected. In the middle, the hysplit model represents wind direction at the coastal highway. The rightmost model shows the wind direction at the beach.
The horizontal axis denotes the three different locations at which PM 2.5 and PM 10 data was gathered. The beach, the coastal highway, as well as the inland highway. Meanwhile the vertical axis represents measurements of PM 2.5 and PM 10 in ug/m3.
Table 1- Analyzing variance in PM 2.5 and PM 10 data collected at various locations
The wind trajectories plotted by the hysplit model confirms that the wind traveled coast ward towards the inland location (Figure 2). Based on data collected, we observed that there was a significantly higher mean of PM 2.5 (7.59+/- 1.59 ug/m3) as well as PM 10 (10.7 +/- 2.39 ug/m3) at the coastal highway as shown in Figure 3. Due to the higher standard deviation for the PM 10 values, we determined that there is no significant difference between the PM 10 values at both locations. In simple terms, this means that we are not certain what caused the PM 10 levels to elevate on a coastal highway.
The data refuted our hypothesis. A higher level of PM 2.5 was found at the coastal location compared to inland location meaning that the secondary PM formation reactions occurring in the atmosphere are not as dominant as the process of evaporation. Thus, we can conclude that during wintertime in San Diego, there is significantly higher PM 2.5 found on a coastal highway as compared to an inland highway. It is important to note that these statistics may change based on the location of measurement as well as other factors such as humidity, temperature, speed, traffic or not.
Although our experiment answered our main question- what is the root cause of changes in PM 2.5 and PM 10 at various locations, some questions still remain unanswered. For instance, would data vary during summer months? How would Santa Ana winds (winds that blow from inland to coast ward in San Diego) impact secondary PM 2.5 formation? More experiments need to be conducted on a larger scale in order to determine whether or not PM varies by locations across the globe in a similar trend, and if so, what measures should be taken in California in order to prevent detrimental health effects across the state.
Baird, Colin, and Michael Cann. Environmental Chemistry. New York, NY: W.H. Freeman and Company, 2012. Print.
Barnard, William, and William Hodan. “Evaluating the Contribution of PM2.5 Precursor Gases and Re-Entrained Road Emissions to Mobile Source PM2.5 Particulate Matter Emissions.” ww3.Epa.gov, MACTEC Under Contract to the Federal Highway Administration, www3.epa.gov/ttn/chief/conference/ei13/mobile/hodan.pdf.
Kim, Ki-Hyun, et al. A Review on the Human Health Impact of Airborne Particulate Matter. 24 Oct. 2014.
Natural Sources of Particulate Matter: Stats NZ. Natural Sources of Particulate Matter | Stats NZ. 2018.