| Summary: | The mass composition of Particulate Matter (PM) with an aerodynamic diameter of 2.5 microns (PM<sub>2.5</sub>) in San Joaquin Valley (SJV) is dominated by ammonium nitrate (NH<sub>4</sub>NO<sub>3</sub>), a secondary pollutant. The goal of this research was the investigation of the relationship between emissions, meteorology and PM<sub>2.5</sub> concentrations in Fresno for the winter season. It was found that location of sites near emission sources such as freeways compared with residential sites strongly affected measured PM<sub>2.5</sub> concentrations. It was found that although long-term trends showed declines in both emissions and PM<sub>2.5</sub> concentrations, there was substantial variability between the years in the PM<sub>2.5</sub>−emissions relationship. Much of the yearly variation in the relationship between emissions and PM<sub>2.5</sub> concentrations can be attributed to yearly variations in weather, such as atmospheric stability, precipitation frequency and average wind speed. There are moderate correlations between PM<sub>2.5</sub> concentrations and temperature differences between nearby surface stations at varying elevations which explains some of the daily and seasonal variation in PM<sub>2.5</sub>. Occurrence of precipitation was related to low PM <sub>2.5</sub>, although the higher wind speeds and lower atmospheric stability associated with precipitation likely explain some of the low PM<sub>2.5</sub> as well as washout of PM.
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