The community of San Ysidro is located just north of the international U.S.-Mexico border with Tijuana and adjacent to the San Ysidro Port of Entry, the busiest land border crossing in the western hemisphere. The mainly Latino and low-income community is heavily burdened by pollution from three freeways crossing the community, and by border transport and idling vehicles waiting to cross the border. To examine spatial and temporal variation in concentrations of particulate matter air pollution in the community, we measured concentrations within and adjacent to San Ysidro, CA over a one-year period (March 2017- March 2018). Four ‘snapshot’ campaigns were conducted to record fine particulate matter (PM2.5) and black carbon (BC) concentrations at four sampling sites (two schools, a cultural center, and a coastal reference site). In our campaigns, the urban portion of San Ysidro had higher PM2.5 concentrations than the coastal site for all snapshots except Snapshot 3 (November-December 2017). PM2.5 varied over the year with the highest median site value measured during Snapshot 3 (46.7 μg/m3) and the lowest recorded in Snapshot 2 (July-August 2017, 5.5 μg/m3). Large peaks were observed throughout most campaigns, usually occurring late at night to the early morning. Nighttime PM2.5 levels were higher than the daytime for all snapshots except Snapshot 2 (July-August 2017). Black carbon represented a significant portion of the PM2.5 during early March 2018 (Snapshot 4), with site percentages as high as 19.6%; over three times the highest percentages found in other snapshots. The coastal reference site had the lowest %BC for each snapshot (2.9-9.6%). These data indicate that current government monitors do not completely capture the pollution patterns in San Ysidro, and that community-based pollution monitoring can help fill in data gaps. The sources of air pollution throughout San Ysidro are out of the control and regulation of the community. This inequity between polluters and those enduring the pollution leads to an environmental justice issue. Solutions that may reduce the burden on community members include policies to decrease idling times at the POE crossing, improve vehicle maintenance, and mitigate exposures through indoor filtration, diesel exhaust scrubbers, and urban green spaces.