IMPROVE Extinction Equation
The IMPROVE (Interagency Monitoring of Protected Visual Environments) equation estimates atmospheric extinction coefficients from chemical composition, widely used in visibility research.
Revised IMPROVE (2007)
Complete Equation
\[b_{ext} = b_{sp} + b_{ap} + b_{sg} + b_{ag}\]
Particle Scattering (\(b_{sp}\))
\[b_{sp} = 2.2 \times f_S(RH) \times [Small\ (NH_4)_2SO_4] + 4.8 \times f_L(RH) \times [Large\ (NH_4)_2SO_4]$$
$$+ 2.4 \times f_S(RH) \times [Small\ NH_4NO_3] + 5.1 \times f_L(RH) \times [Large\ NH_4NO_3]$$
$$+ 2.8 \times [Small\ OM] + 6.1 \times [Large\ OM]$$
$$+ 1 \times [Soil] + 1.7 \times f_{SS}(RH) \times [SS]\]
Particle Absorption (\(b_{ap}\))
\[b_{ap} = 10 \times [EC]\]
Size Classification
- Small (fine mode): When \([X] < 20\ \mu g/m^3\), \([Small] = [X]\)
- Large (coarse mode): \([Large] = [X] - [Small]\)
Hygroscopic Growth Factor f(RH)
| RH (%) | \(f_S(RH)\) | \(f_L(RH)\) | \(f_{SS}(RH)\) |
|---|---|---|---|
| 30 | 1.0 | 1.0 | 1.0 |
| 50 | 1.3 | 1.4 | 1.5 |
| 70 | 2.0 | 2.4 | 2.8 |
| 80 | 3.0 | 3.8 | 4.5 |
| 90 | 5.5 | 7.5 | 9.0 |
Modified IMPROVE
Simplified version using a single f(RH):
\[b_{ext} = 3 \times f(RH) \times [(NH_4)_2SO_4 + NH_4NO_3]$$
$$+ 4 \times [OM] + 10 \times [EC] + 1 \times [Soil] + 1.7 \times f(RH) \times [SS]\]
Mass Extinction Efficiency (MEE)
| Component | Dry MEE (m2/g) | Description |
|---|---|---|
| (NH4)2SO4 | 2.2-4.8 | Highly hygroscopic |
| NH4NO3 | 2.4-5.1 | Highly hygroscopic |
| OM | 2.8-6.1 | Depends on organic type |
| EC | 10 | Strong absorption |
| Soil | 1 | Low efficiency |
| SS | 1.7 | Highly hygroscopic |
Gas Extinction
Rayleigh Scattering
$\(b_{sg} = 11.4\ Mm^{-1}\)$ (at sea level standard conditions)
NO2 Absorption
\[b_{ag} = 0.33 \times [NO_2]\ (ppb)\]
In AeroViz
Usage lives in the guide: improve for the call and return keys, and Optical Closure for a worked example.
References
- Pitchford, M., et al. (2007). Revised Algorithm for Estimating Light Extinction from IMPROVE Particle Speciation Data. JAPCA, 57(11), 1326-1336.
- Malm, W. C., & Hand, J. L. (2007). An examination of the physical and optical properties of aerosols collected in the IMPROVE program. Atmos. Environ., 41(16), 3407-3427.