Scientific literature makes it clear that small particles suspended in the air, especially those in the 1um to 2.5um diameter range, can have harmful effects on cardiovascular and respiratory health, especially when individuals are exposed at high levels or for long periods of time (Kim et al). The U.S. Environmental Protection Agency provides a basic overview of these health effects. These particles known as “PM2.5” are formed via two general mechanisms: they are either directly emitted into the atmosphere from sources such as coal power plants, diesel engines, or fugitive dust, or they are formed via reactions in the atmosphere. These atmospheric reactions are known as ‘secondary aerosol formation’ reactions since they don’t directly emit PM2.5 but instead react later on to form small particles once in the atmosphere. The substances reacting in secondary aerosol formation to make harmful PM2.5 are largely from vehicle emissions and include molecules with nitrogen and oxygen atoms called nitrous oxides, unstable organic molecules referred to as volatile organic compounds, and sulfate molecules.
In order to determine the effects of secondary aerosol formation, our research team studied PM2.5 levels at Coastal and Inland highways. California’s diurnal wind pattern causes vehicle emissions to blow from west to east, leading our team to hypothesize that San Diego highways in winter months have significantly higher PM 2.5 levels than coastal highways due to secondary particulate formation from coastward traffic’s NOx emissions. 
An Atmotube Pro Portable Outdoor and Indoor Air Quality Monitor was used to record PM2.5 levels as well as levels of larger particulate matter in the 10 micrometer diameter range (PM10) and general atmospheric characteristics such as temperature and humidity. Our team drove on the coastal I-5 and inland I-15 highway routes shown below. Data was recorded with both windows up and down to determine if this would have any affect on particulate matter levels, and the sensor was hung from the driver’s wrist, feeding data continuously every 30 seconds to a smartphone device. The prior path of air masses measured was analyzed via the National Oceanic and Atmospheric Administration’s backward air trajectory models and is displayed below, indicating a southwesterly wind direction suitable to testing our hypothesis.
Our team found higher PM2.5 levels at coastal highways when driving with windows up compared to inland locations with windows up. When driving with windows down, no statistically significant difference was observed between locations. This contradicts our hypothesis that inland San Diego highways in winter months have significantly higher PM 2.5 levels compared to coastal highways. Instead, secondary aerosol formation from coastward vehicle emissions is not causing an increase in harmful-to-breathe particulate matter at the inland highways.
One possible explanation for the decreasing and similar PM2.5 levels at the inland location is particle evaporation, a process by which tiny water molecules evaporate out of the PM2.5 range. Evaporation is increased under lower humidity conditions. These were observed at the inland location, illustrated by the blue bars in the graph below (left is inland, right is coastal). This correlates with humidity data in NOAA ‘s air trajectory models and supports the possibility that as air moves inland it loses PM2.5 due to evaporation of water particles suspended in the air.
Although limits on the duration of recording led to significant deviation in data values, driving with the windows up reduced PM2.5 and PM10 levels at both inland and coastal highways. Further experimentation is needed to support this, including an analysis of airflow differences over the sensor. However it does appear that when driving on highways, keeping the windows up may decrease a passenger’s exposure to harmful particulate matter. The mechanism by which coastal vehicle emissions do not significantly increase inland PM2.5 levels may be liquid particle evaporation, but more research is needed to determine the cause of this phenomenon.
Works Cited
Kim, Ki-Hyun, et al. A Review on the Human Health Impact of Airborne Particulate Matter. 24 Oct. 2014.