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San Diego occupies a distinctive coastal position in Southern California, nestled between the Pacific Ocean to the west and rugged mountain ranges to the east, creating a complex geographic context that profoundly shapes its air quality. The city's urban character blends dense coastal development with sprawling suburban communities extending inland toward semi-arid foothills, establishing a pronounced urban-rural gradient. San Diego's regional position as a major metropolitan hub near the U.S.-Mexico border subjects it to cross-border pollution transport, particularly from Tijuana's industrial activities and occasional agricultural burning in northern Baja California. The terrain features a narrow coastal plain rising sharply to the Peninsular Ranges, including the Laguna Mountains, which can trap pollutants when offshore winds weaken. Proximity to the Pacific Ocean generally provides marine breezes that disperse contaminants, but this cleansing effect diminishes inland where valleys like Mission Valley experience poorer ventilation. The city lacks heavy industrial belts comparable to Los Angeles but hosts port operations, military bases, and transportation corridors that contribute localized emissions. Surrounding landscapes include chaparral-covered hills susceptible to wildfires—a significant episodic pollution source—and agricultural zones in northern San Diego County that occasionally generate particulate matter. Elevation varies from sea level to over 1,200 feet in city limits, creating microclimates where temperature inversions can form in inland basins, temporarily worsening air quality. This geographic mosaic makes San Diego's pollution patterns highly variable, with coastal areas typically enjoying cleaner air while eastern suburbs face greater exposure to ozone and particulates, especially during Santa Ana wind events that bring desert dust and smoke.

San Diego's air quality follows a distinct seasonal rhythm shaped by its Mediterranean climate, with pollution peaks and troughs closely tied to meteorological patterns. Spring (March-May) brings generally good air quality as marine layers strengthen, dispersing pollutants, though occasional Santa Ana winds can elevate particulate levels from desert dust; this is an ideal season for outdoor activities. Summer (June-August) sees the highest ozone concentrations due to intense sunlight reacting with vehicle and port emissions, particularly in inland valleys where sea breezes weaken; sensitive groups should limit exertion on hot afternoons. The marine layer often creates morning clouds that burn off by midday, allowing pollution buildup—July and August are peak months for smog advisories. Autumn (September-November) begins with persistent ozone issues but transitions to cleaner conditions as temperatures drop and onshore flows resume; however, Santa Ana winds return, carrying smoke from regional wildfires that can create hazardous episodes, making October a month to monitor closely. Winter (December-February) offers the cleanest air overall, with frequent rain showers scrubbing pollutants and stable marine air dominating; temperature inversions are rare along the coast but can trap particulates in inland basins during calm periods. Throughout the year, sensitive individuals—especially those with respiratory conditions—should track daily forecasts, as San Diego's proximity to wildfire-prone areas and cross-border sources can trigger sudden deteriorations regardless of season.