Mobile personal air monitors that give individuals feedback as to the quality of the air they are breathing in different locations are new and potentially important tools to improve public health. CitiSense, developed at the University of California at San Diego, is a novel personal air quality monitor based on a cell phone than can sample ambient air and display pollution levels to the user. The device currently measures pollutants associated with traffic exhaust, carbon monoxide (CO) and nitrogen dioxide (NO_), as well as ozone (O_). Exposure to traffic related air pollutants has been associated with respiratory and cardiovascular impairment as well as increases in asthma rates and cancer. The aim of the present study was to examine how well traffic pollutants measured by CitiSense with inexpensive monitors, CO and NO_, correlated with other established pollutants associated with traffic, ultra fine particulates (UFP) and black carbon (BC), that were measured with higher cost monitors not as suitable for personal use. Samples were taken in the city of San Ysidro, California for two to three times per week between December 11th, 2012 and March 15th, 2013, for a total of 18 sampling days. At each site samples taken included NO_, CO, O_, UFPs, BC as well as temperature and humidity readings. UFP levels were sampled using a condensation particle counter (TSI model 3007) and BC with a microaethalometer (MicroAeth model AE51). Simultaneous border wait time and wind speed and direction data was collected. Sampling locations included the San Ysidro Port of Entry (POE), regarded as the busiest internal land border crossing in the United States as well as a location directly adjacent to the POE, 6 elementary schools, 1 middle school, and 1 school office in the community of San Ysidro. The Tijuana River Estuary Visitor Center in the city of Imperial Beach, California was chosen as a control location. Elevated levels of UFP were found at the POE (mean 42,811 pt/cc) compared to the control location (mean 8,707 pt/cc) Black carbon levels were also seen to be elevated at the POE compared to the control location (mean 4,152 ng/m_ compared to a mean of 1,324 ng/m_ respectively. Poor correlations, r<0.3, were observed between the CitiSense readings and the UFP and BC readings over all sampling days. However, when examined within one day at a time, correlations were often markedly improved, suggesting that baseline drift and calibration are important factors for the CitiSense devices. The results presented in this study will help further the development of the CitiSense device for field deployment. Furthermore, the results of this study illustrate the high levels of UFP and BC in the city of San Ysidro in relation to the POE and provide further evidence that the POE is a source of pollution for this community.