Deposition Velocity Calculator

1) What deposition velocity represents

Deposition velocity (v_d) is an effective transfer speed that links air concentration to surface removal. It is commonly used in air-quality modeling to convert near-surface concentrations into dry deposition fluxes. Typical v_d values span orders of magnitude, from about 0.01 cm/s for weakly reactive gases over smooth surfaces to several cm/s for highly reactive species or efficient particle capture on rough canopies.

2) Flux–concentration approach in practice

When deposition flux (F) and near-surface concentration (C) are measured over the same averaging period, the ratio v_d = F/C provides a direct estimate. Field studies often report aerosol fluxes in µg/m²/s or mg/m²/day and concentrations in µg/m³. Converting both into consistent SI units ensures v_d is obtained in m/s and then displayed as cm/s or m/day for easier interpretation.

3) Resistance model and typical ranges

Many applications use a resistance framework: v_d = 1/(R_a+R_b+R_c). Aerodynamic resistance R_a can vary roughly from 10–200 s/m depending on wind and stability. Quasi-laminar resistance R_b is often 10–100 s/m, while surface resistance R_c can range from tens of s/m (strong uptake) to thousands of s/m (weak uptake). Lower total resistance yields higher v_d.

4) Drivers: surface type and atmospheric stability

Rough surfaces (forests, urban fabric) enhance turbulence and reduce R_a, increasing deposition. Smooth surfaces (water, snow) often yield lower v_d. Unstable daytime conditions typically increase turbulent transport, while stable nighttime stratification can suppress it. For particles, size matters: coarse particles deposit faster via impaction and settling, while ultrafine particles are influenced by diffusion and surface characteristics.

5) Unit checks and quality control

A quick diagnostic is to compare v_d against expected magnitudes. For many gases over vegetation, values around 0.1–1 cm/s are plausible, while very high values (greater than 10 cm/s) often indicate inconsistent averaging, unit errors, or concentration not representative of the flux footprint. This calculator shows intermediate conversions to help audits.

6) Common use cases

Deposition velocity supports emission control assessments, ecosystem loading calculations, and model evaluation. For example, if C = 30 µg/m³ and v_d = 0.5 cm/s, the implied flux is F ≈ 150 µg/m²/s. Over one day, that corresponds to about 13 g/m²/day, illustrating how small changes in v_d can strongly affect deposition totals.

7) Limitations and interpretation

v_d aggregates many processes into one parameter and should be interpreted as condition-specific. Wet deposition, resuspension, chemical transformation, and canopy storage can complicate direct flux–concentration ratios. When using the resistance method, input resistances must match the same reference height and land-surface assumptions.

8) Recommended workflow

Start with the method that matches your measurements. Use the flux–concentration mode for observed fluxes, and the resistance mode for modeling scenarios. Keep units consistent, review the steps, then export CSV or PDF for reporting. If results look unusual, test sensitivity by adjusting resistances or checking concentration units before finalizing conclusions.

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FAQs

1) What is a reasonable v_d value to start with?

For many gases over vegetation, 0.1–1 cm/s is a common starting range. For particles, values can be higher for coarse sizes and lower for ultrafine sizes.

2) Why does the resistance method use s/m?

Each resistance represents a time delay per meter of transport. Adding resistances gives a total s/m, and taking 1/R converts that combined delay into an effective speed in m/s.

3) Can I compute flux if I only know v_d and concentration?

Yes. Use the “Flux from v_d and concentration” mode. It applies F = v_d·C with unit conversion and reports flux in your selected unit.

4) What causes unrealistically high v_d results?

Common causes include mixing units (day vs second), mismatched averaging periods between flux and concentration, or a concentration measured away from the deposition footprint.

5) Does this calculator include gravitational settling?

No. It treats v_d as an effective parameter from your inputs. If settling is important, incorporate it in the flux or in the resistances you provide.

6) Which concentration height should I use?

Use the near-surface concentration representative of the same footprint as the deposition process, often at a reference height used by your monitoring or model configuration.

7) How should I report results for audits?

Include method, units, and averaging period. Export the result table as CSV or PDF, and document any assumptions about surface type, stability, and resistance parameter choices.