Description
As the busiest land border crossing in the world, an estimated 1.8 million vehicles traveling northbound each year enter into California through the San Ysidro Land Port of Entry (LPOE). Currently, no local, state or federal agency monitors air quality in the community of San Ysidro. Exposure to traffic related air pollution has been associated with adverse health effects including contributions to respiratory and cardiovascular morbidity and mortality. This study characterized fine particulate matter (<2.5 _m) mass concentration and submicron particulate matter (<1.0 _m) surface area alveolar deposition in two sites, the border community of San Ysidro and in the Tijuana River National Estuarine Research Reserve (TRNRR). Measurements conducted from the roof tops of these two locations, were analyzed for differences between pollution concentrations at the two sites as well as for pollutant concentration with respect to meteorological parameters and vehicle delay times at the LPOE. Fine particulate matter mass concentration was measured utilizing two Airmetrics MiniVol Technical Air Samplers and submicron surface area deposition concentration was measured by one Nanoparticle Surface Area Monitor, during November 2-19, 2010. Results from the study indicate San Ysidro to have elevated concentrations of both PM_._ mass and submicron surface area deposition. Overall, PM_._ mass median concentrations for San Ysidro and TRNRR were 16±3.5 _g/m_ and 14±2.8 _g/m_, respectively. Median submicron surface area alveolar deposition concentrations were 36.2±47.3 _m_/cm_ and 27.4±25.9 _m_/cm_ for the San Ysidro site and Tijuana River Estuarine, respectively. Nighttime median submicron surface area alveolar deposition was observed to be higher in the San Ysidro site as compared to the Tijuana River Estuarine site, 67 _m_/cm_ vs. 42 _m_/cm_, respectively (p < 0.0005). In comparison, indoor surface area concentration has been observed to be on average 40-45 _m_/cm_. Using these results, a Generalized Estimating Equation (GEE) model was developed to predict submicron surface area alveolar deposition given meteorological conditions, time of day and vehicle delay times at the LPOE. Based on the model highest submicron surface area deposition for both sites occurred during the overnight hours (12:00 am - 6:00 pm) along with calm wind conditions (p < .0001). In addition, winds from the LPOE traveling towards the San Ysidro site also resulted in elevated surface area concentration (p < .0001). Border vehicle delay times (R_ = 0.245, p < .0001) along with relative humidity (R_ = 0.339, p < .0001) were shown to have a significant association with elevated surface area concentration. Calm winds overall demonstrated to have a relationship with increased levels of pollution at both sites (R_ = 0.661, p < .0001). Border winds also resulted in increased levels of pollution in both sites (R_ = 0.193, p < .0001). This study was the first to characterize submicron surface area alveolar deposition with respect to meteorological parameters and vehicle delay times at the LPOE. The GEE model developed can be utilized to predict the potential impact of idling vehicles at the LPOE in these two sites, although more sampling days over the calendar year are needed to refine model.
