While air pollution in cities is detrimental to the health of those living in the city, it does contribute to global climate change which affects the entire world.
Earlier this year Ezra Wood, PhD, a professor in Drexel University’s College of Arts and Sciences and an expert in outdoor air pollution and atmospheric chemistry, co-authored a study published in Environmental Science and Technology about air pollution in Philadelphia.
The study focused on two important classes of pollutants:
The first is nitrogen oxides, “NOx,” comprising nitrogen dioxide (NO2) and nitric oxide (NO), which are created by combustion. These compounds also catalyze the formation of ozone, which is harmful to public health at ground level and an important component of photochemical smog. The second, methane, is the main component of natural gas and is a potent greenhouse gas.
Wood spoke with Drexel News Blog about the study and what it can teach us about indoor air quality during, and following, the COVID-19 pandemic.
Why is this study important?
To adequately reduce air pollution and mitigate the worst effects of global warming, as a society we need to reduce emissions of both of these compounds – NOx and methane. To enact an emission reduction program and also to accurately model the effects of current emissions, it is important to know how much is being emitted at both local and global scales.
Cities are hotspots for emissions of many compounds, including NOx and methane. In cities, NOx mainly comes from cars and trucks, and methane appears to mainly result from leaks in the natural gas infrastructure – especially in cities like Philadelphia, where the network of underground natural gas pipes is aging. Currently there are not “standard” methods for quantifying these emissions. For example, measuring the exhaust from individual cars and trucks and scaling up to the entire fleet is a time-consuming process and might not account for the presence of “super-emitters” – the dirtiest 5-10% of vehicles that can account for 50% or so of total emissions.
How did you measure these emissions?
In this study we demonstrated a fairly simple way of inferring useful information on these emissions – simply by measuring concentrations of carbon dioxide (CO2) alongside the concentrations of NOx and methane and determining the ratio of the urban excess NOx or CH4 (methane) concentrations relative to CO2.
In the winter in a city, when photosynthesis and respiration are minimal, CO2 concentrations are mainly from the global background plus a more local component from fossil fuel combustion. NOx emissions also result from this combustion. So, the NOx/CO2 ratio allowed us to calculate the fuel-based emission factor for the Philadelphia vehicle fleet, i.e. how many grams of NOx are emitted per liter of fuel (gasoline or diesel) consumed.
The ratio of methane to CO2, meanwhile, tells us the ratio of urban methane emissions to urban CO2 emissions. We learned that this CH4/CO2 ratio is quite a bit higher than was thought based on the official EPA emission inventories for these two compounds. While this sounds like bad news, on the flip side it means that efforts to reduce natural gas leaks may be more effective than previously thought in reducing overall greenhouse gas emissions from Philadelphia, and likely from other similar cities.
We did this all, mainly using measurements of outdoor air from Drexel’s campus!
What does this mean for residents of Philadelphia and beyond?
NOx pollution affects those closest to the emission source — in the city. The resulting ozone pollution is spread out on a more regional scale. The impact of global warming is, as the name suggests, global. But it disproportionately affects those with lower incomes who are more susceptible to flooding, heat waves, drought and other natural disasters.
What do you hope follows this research?
We’d like to see the incorporation of CO2 and CH4 measurements into the suite of measurements made at air quality monitoring stations. These greenhouse gases aren’t considered classical “pollutants” and are not currently measured in as many places. This is slowly changing — the Philadelphia Air Management laboratory has started to measured methane, recently in fact. This should provide a long-running record of these emissions for which we just provided a two-week snapshot one year.
Following up on that “more measurements” theme, society would also benefit from being empowered with the knowledge that would result from ubiquitous measurements of CO2 indoors as well – since they are a great indicator of ventilation. High indoor concentrations of CO2 from occupants’ breath can result if the ventilation is not adequate. Teachers should know what the CO2 concentration is in their classrooms and potential patrons of a restaurant or gym should know what the CO2 levels are inside.
Can you explain further about indoor air quality measurements?
The pandemic really hammered home how crucial indoor air quality is. In the beginning of the pandemic everyone was told to wash their hands and clean surfaces. We were also told to always stay at least six feet from others because it was thought that respiratory droplets, which were erroneously believed to be too big to travel more than a few feet, were responsible for most viral transmission. But aerosol scientists were telling the CDC and WHO to acknowledge the dominant role of respiratory aerosols, which travel much farther than six feet – very similar to cigarette smoke. The crucial intervention that was given too little attention was indoor ventilation. Masks, social distancing, washing your hands are all great but the science is crystal clear that without adequate ventilation, indoors can be a dangerous place. Think of it like being in a room with a smoker. Without good ventilation, even if you distance yourself from the smoker, you’re still going to end up inhaling second-hand smoke.
We can envision a future where every indoor location has a visible measurement of CO2 – like a thermometer telling us the temperature of the room. This is not going to happen overnight, but is slowly catching on, and in light of COVID-19, the demand is increasing for this level of knowledge of air quality in indoor spaces.
Media interested in speaking with Wood should contact Annie Korp, news manager, at 215-571-4244 or amk522@drexel.edu.
