Scavenging Coefficient Calculator

Formula used

• Time-decay (first-order removal): C(t) = C₀ · e^-Λt, so Λ = (1/t) · ln(C₀/Cₜ).
• Scavenging ratio model (well-mixed layer): Λ = (R · W) / H, where R is precipitation rate, W is scavenging ratio, and H is mixing height.

How to use this calculator

1. Pick a method based on your available observations or model inputs.
2. Enter values with consistent units, then choose any needed unit options.
3. Click Calculate. Your scavenging coefficient appears above the form.
4. Use Download CSV for spreadsheets, or print to save as PDF.
5. Compare scenarios by changing rainfall, mixing height, or concentrations.

Example data table

Wet deposition and washout context

Rain and cloud processes remove gases and aerosols through wet deposition. Many air‑quality models approximate this as first‑order removal, where concentration falls exponentially during precipitation. This calculator turns common observations into a scavenging coefficient (Λ), summarizing how effectively rainfall cleans the air.

What the scavenging coefficient represents

Λ is a removal rate per unit time: larger Λ means faster washout. Under constant Λ, the remaining fraction after time t is exp(−Λt). Real storms are not perfectly steady, so Λ is best interpreted as an event‑average value for comparison across cases.

Units and typical magnitudes

Λ is reported in 1/s and 1/hr. Strong washout can reach tenths per hour, changing concentrations within hours, while drizzle can be much smaller. Values depend on pollutant solubility, particle size, temperature, and drop size spectra. A convenient interpretation is half‑life = ln(2)/Λ, the time for concentration to halve under the same conditions.

Deriving Λ from concentration time series

With two measurements, use the time‑decay method. Enter C₀ at the start, Cₜ after a known duration, and the time unit. The calculator applies Λ = (1/t)·ln(C₀/Cₜ). This is useful for field campaigns, chamber tests, or validating model output during storms.

Estimating Λ from precipitation inputs

Without concentration data, the scavenging ratio method uses Λ = (R·W)/H. R is precipitation rate, W is the scavenging ratio (rain concentration divided by air concentration), and H is an effective mixing height for a well‑mixed layer. Rainfall is converted to m/s internally.

Sensitivity to rainfall and mixing height

Λ scales linearly with rainfall in the ratio model: doubling R doubles Λ if W and H stay fixed. Λ decreases as mixing height grows because pollutant mass is distributed through a deeper layer. These relationships help compare convective bursts versus stratiform rain.

Quality checks and uncertainty

For time‑decay, confirm Cₜ < C₀ and consider other drivers such as emissions, transport, or chemistry. For the ratio method, W varies by species, solubility, and particle size, often by orders of magnitude. Treat Λ as a practical indicator, not a universal constant.

Applications in research and operations

Scavenging coefficients support storm cleansing comparisons, parameter tuning in dispersion models, and interpretation of monitoring drops during rainfall. Exporting to CSV enables event libraries, while PDF printing documents assumptions and results for reports, quality reviews, and stakeholder communication. It also helps estimate recovery time after short emission spikes during rainy periods.

FAQs

1) What is the scavenging coefficient used for?

It summarizes how rapidly rainfall removes a pollutant from air. Larger values mean faster wet removal and shorter persistence during a storm, making it useful for comparing events and testing model assumptions.

2) Which method should I choose in this tool?

Use time‑decay if you have two concentration measurements and a duration. Use the scavenging ratio method when you know rainfall rate, scavenging ratio, and an effective mixing height for a well‑mixed layer.

3) Why must Cₜ be lower than C₀ in the decay method?

The exponential model assumes net removal during the interval. If Cₜ is higher, emissions, transport, or chemistry likely increased concentrations, so Λ derived from ln(C₀/Cₜ) would not represent wet scavenging.

4) What units should my concentrations use?

Any consistent unit works because the decay method uses a ratio C₀/Cₜ. For the ratio method, W is dimensionless by definition, rainfall is entered with selectable units, and mixing height is in meters.

5) How do I interpret the half-life output?

Half‑life is the time required for concentration to drop by 50% under the same Λ. Short half‑life indicates strong storm cleansing; long half‑life suggests weak removal or light precipitation.

6) Does Λ stay constant throughout a storm?

Not always. Rain intensity, droplet sizes, and boundary‑layer mixing can change during events. Treat Λ as an average over your chosen time window, and avoid mixing periods with very different storm regimes.

7) Can I use this for gases and particles?

Yes, but parameter meaning differs. Gases with high solubility and fine particles often scavenge more efficiently than coarse or hydrophobic species. Choose W and H values that match your pollutant type and study context.