Description
The San Ysidro Port of Entry (POE) is the world's busiest international land border crossing. Each day, an average of 50,000 vehicles travel northbound through the port, entering the community of San Ysidro, California. Traffic related air pollution is associated with adverse effects to pulmonary, cardiovascular, and reproductive systems. This study characterizes traffic related air pollutants at several rooftop locations in San Ysidro and nearby Imperial Beach, including two elementary schools within two miles (3.2 km) of the POE. Relationships between pollution concentrations, wind patterns, and border crossing traffic delays were also investigated. During February-March, April-June and November 2010, the pollutants particulate matter (PM) with aerodynamic diameter equal to or less than 2.5 micrometers (µm; PM2.5), ultrafine particles (UFP), carbon monoxide (CO), and black carbon (BC) were monitored in continuous five minute intervals, utilizing active flow personal dataRAMs with cyclones (pDR 1200), TSI condensation particle counters (CPC 3007 and P-TRAK), HOBO CO three channel data loggers and aethalometers (model AE42). In general, pollution levels were elevated at near border measurement sites, including elementary schools, and UFP and BC concentrations were comparable to those associated with adverse health effects in previous research. Nighttime median UFP, BC and PM2.5 concentrations were significantly higher than daytime concentrations at the near-border site. For CO, median concentrations were significantly higher during the day. Median daytime UFP (8,929 particles/cm_), BC (632 ng/m_), and CO (2.7 ppm) concentrations were highest at the measurement site closest to the border crossing (p < .05). PM2.5, unlike the other pollutants, appeared to have a lower concentration gradient relative to the crossing, suggesting that this pollutant is more regional. The highest mean daytime BC (4,614 ng/m_) and UFP (33,657 particles/cm_) concentrations at the near-border site were associated with calm wind conditions (p < .001). Winds blowing from the direction of the POE toward the store measurement site were also associated with elevated mean daytime concentrations relative to concentrations measured when the winds was blowing from northwesterly directions: 2,694 versus 634 ng/m_ for BC (p < .001), and 22,896 versus 11,535 particles/cm_ for UFP (p < .05). Positive correlations were found between wait times (an indicator of idling cars) at the POE and daytime median BC concentration during November, the period in which calm wind conditions predominated, at the store near the border (rho = 0.320, p < .01) and at an elementary school 1.7 mi (2.7 km) from the crossing (rho = 0.584, p < .001). Filtering for only calm wind conditions revealed an even stronger correlation (rho = 0.447, p < 0.05) between BC and northbound border vehicle wait times at the near-border site. This study is the first to characterize the relationships between traffic related air pollutants, wind direction, and border crossing delays in San Ysidro, and could be used to guide public health and transportation policy in this border community.
