6.0 Air Quality Standards and Concluding Remarks
6.1 National and International Air Quality Standards
The scientific understanding of the sources, transformations, and effects of air pollutants provides the foundation for establishing regulatory standards. These standards are designed to protect public health and welfare by setting limits on the allowable concentrations of key pollutants in the ambient air. Below are the U.S. National Ambient Air Quality Standards (NAAQS) and a comparison of standards from around the world.
Table: U.S. National Ambient Air Quality Standards (NAAQS)
| Pollutant | Primary Standard | Averaging Times | Secondary Standard |
| Carbon Monoxide | 9 ppm | 8-hour¹ | None |
| 35 ppm | 1-hour¹ | None | |
| Lead | 1.5 µg/m³ | Quarterly average | Same as primary |
| Nitrogen Dioxide | 0.053 ppm | Annual (arith. mean) | Same as primary |
| Particulate Matter (PM₁₀) | 50 µg/m³ | Annual² (arith. mean) | Same as primary |
| 150 µg/m³ | 24-hour¹ | Same as primary | |
| Particulate Matter (PM₂.₅) | 15 µg/m³ | Annual³ (arith. mean) | Same as primary |
| 65 µg/m³ | 24-hour⁴ | Same as primary | |
| Ozone | 0.08 ppm | 8-hour⁵ | Same as primary |
| 0.12 ppm | 1-hour⁶ | Same as primary | |
| Sulfur Oxides | 0.03 ppm | Annual (arith. mean) | — |
| 0.14 ppm | 24-hour¹ | — | |
| — | 3-hour¹ | 0.5 ppm | |
| ¹Not to be exceeded more than once per year. | |||
| ²To attain this standard, the expected annual arithmetic mean PM₁₀ concentration must not exceed 50 µg/m³. | |||
| ³To attain this standard, the 3-year average of the annual arithmetic mean PM₂.₅ concentrations must not exceed 15 µg/m³. | |||
| ⁴To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations must not exceed 65 µg/m³. | |||
| ⁵To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average must not exceed 0.08 ppm. | |||
| ⁶The 1-hour standard is attained when the expected number of days per year with maximum hourly averages above 0.12 ppm is ≤ 1. |
Table: Recommended Ambient Air-Quality Limits for Selected Gases Throughout the World
| Country/Organization | CO (ppm) | SO₂ (ppm) | O₃ (ppm) | NO₂ (ppm) | PM₁₀ (µg/m³) |
| WHO | 26 (1 hr), 8.7 (8 hr) | 0.048 (24 hr), 0.019 (annual) | 0.061 (8 hr) | 0.105 (1 hr), 0.021 (annual) | |
| EU | 8.7 (8 hr) | 0.132 (1 hr, ≤24x), 0.047 (24 hr, ≤3x) | 0.061 (8 hr, ≤25x/yr) | 0.105 (1 hr, ≤18x), 0.021 (annual) | 50 (24 hr, ≤35x), 40 (annual) |
| UK | 10 (8 hr) | 0.132 (1 hr, ≤24x), 0.047 (24 hr, ≤3x) | 0.050 (8 hr) | 0.105 (1 hr, ≤18x), 0.021 (annual) | 50 (24 hr, ≤35x), 40 (annual) |
| Russia | 4.4 (24 hr) | 0.02 (24 hr) | 0.045 (24 hr) | ||
| Australia | 9 (8 hr) | 0.20 (1 hr), 0.08 (24 hr), 0.02 (annual) | 0.10 (1 hr), 0.08 (4 hr) | 0.12 (1 hr), 0.03 (annual) | 50 (24 hr, ≤5x) |
| New Zealand | 9 (8 hr, ≤9x) | 0.132 (1 hr, ≤9x) | 0.08 (1 hr) | 0.105 (1 hr, ≤9x) | 50 (24 hr, ≤5x) |
| China | 9 (1 hr), 3.5 (24 hr) | 0.19 (1 hr), 0.06 (24 hr), 0.02 (annual) | 0.10 (1 hr) | 0.13 (1 hr), 0.06 (24 hr), 0.04 (annual) | 150 (24 hr), 100 (annual) |
| Japan | 20 (8 hr), 10 (24 hr) | 0.10 (1 hr), 0.04 (annual) | 0.06 (1 hr) | 0.04–0.06 (24 hr) | 200 (1 hr), 100 (24 hr) |
| Canada | 0.065 (8 hr) | 30 (PM₂.₅ 24 hr) | |||
| Mexico | 11 (8 hr) | 0.13 (24 hr), 0.03 (annual) | 0.11 (1 hr) | 0.21 (1 hr) | 150 (24 hr), 50 (annual) |
| Brazil | 35 (1 hr), 9 (8 hr) | 0.14 (24 hr), 0.03 (annual) | 0.08 (1 hr) | 0.17 (1 hr), 0.05 (annual) | 150 (24 hr), 50 (annual) |
| Note: Numbers in parentheses represent the averaging time period and, where noted, the number of exceedances allowed (x). |
6.2 Synthesis and Final Remarks
As we have seen, the chemistry of our atmosphere is extraordinarily complex and deeply interconnected. Throughout these lectures, we have journeyed from the urban scale of photochemical smog, to the regional scale of acid deposition and secondary aerosols, and finally to the global scale of stratospheric ozone depletion. This progression demonstrates how fundamental chemical principles and transport processes manifest in profoundly different ways across these scales. The same hydroxyl radical that initiates ozone formation in a polluted city also cleanses the atmosphere of sulfur dioxide and methane. The nitrogen oxides that fuel smog are also precursors to the nitric acid that contributes to acid rain.
Our current understanding of these intricate processes is the result of decades of diligent and cumulative research by countless individuals across many scientific disciplines. This persistent scientific effort is essential. It allows us to diagnose the causes of environmental degradation, develop models to predict future changes, and design effective, science-based solutions to protect the fragile atmospheric systems that sustain life on our planet.