FIGURE 1-2 National average emission categories for carbon monoxide (CO), sulfur dioxide (SO₂), nitric oxide and nitrogen dioxide (NO_x), volatile organic compounds (VOCs), particulate matter less than 10 micrometers in diameter (PM₁₀), and particulate matter less than 2.5 micrometers in diameter (PM_2.5) for 2001. For primary PM₁₀ and PM_2.5, a significant fraction of the PM fugitive dust emissions return to the ground after a few minutes. Biogenic emissions (such as VOC emissions from vegetation and NO_x emissions from soil microbial processes) are not included. SOURCE: Adapted from EPA 2003a.