Example data table
These sample inputs show how flux scales linearly with concentration and velocity.
| Scenario | Concentration (µg/m³) | Vd (cm/s) | Flux (µg/m²/s) | Flux (mg/m²/day) |
|---|---|---|---|---|
| Urban PM | 45 | 0.6 | 0.270 | 23.328 |
| Industrial plume | 120 | 1.2 | 1.440 | 124.416 |
| Background | 8 | 0.3 | 0.024 | 2.074 |
Formula used
Dry deposition flux expresses how fast material transfers from air to a surface. The core relationship is:
- F = Veff × C
- F is deposition flux (mass / area / time)
- C is air concentration (mass / volume)
- Veff is effective deposition velocity (length / time)
If you include gravitational settling, the calculator uses: Veff = Vd + Vg.
In resistance mode, the deposition velocity is computed with: Vd = 1 / (Ra + Rb + Rc), where each resistance has units of s/m.
How to use this calculator
- Choose a mode: direct Vd, or the resistance model.
- Enter air concentration and select its unit.
- Provide Vd (or Ra, Rb, Rc), using realistic values.
- Optionally enable settling and enter Vg for particles.
- Add area and time to estimate deposited mass.
- Press Calculate and download CSV or PDF if needed.
For compliance work, document the data source and assumptions.
Practical notes
- Linearity: If C doubles, flux doubles when velocities are fixed.
- Surface dependence: Vd varies by land cover, moisture, and roughness.
- Particle effects: Larger particles often have higher Vg.
- Uncertainty: Field Vd estimates can vary by orders of magnitude.
Dry deposition flux in applied environmental physics
1) Why dry deposition matters
Dry deposition is a major removal pathway for gases and particles from the atmosphere. It influences ground-level exposure, ecosystem loading, corrosion risk, and compliance assessments. Flux links an air measurement to a surface impact rate, supporting studies from urban air quality to agricultural foliar uptake.
2) What the calculator computes
The calculator applies F = Veff × C to estimate flux in µg/m²/s and converts results to convenient reporting units such as mg/m²/day. It can also integrate flux over a user-defined surface area and duration to estimate deposited mass, useful for roof dust loading or crop deposition.
3) Typical deposition velocity ranges
Deposition velocity varies widely by pollutant and surface. Fine particles can have Vd on the order of 0.1–1.0 cm/s over rough urban surfaces, while smoother or wet surfaces may differ. Reactive gases can exhibit higher effective uptake when surface resistance is low. Always justify Vd with site and species context.
4) Concentration data quality
Concentration uncertainty propagates linearly into flux. Hourly averages can reduce random noise but may mask short peaks. Report the averaging time, sampler method, and unit basis. This tool converts inputs like ng/m³ and mg/m³ to µg/m³ so mixed datasets remain comparable in one workflow.
5) Settling for larger particles
For coarse particles, gravitational settling can dominate. Enabling settling adds Vg to Vd, increasing Veff. As a quick check, a Vg of 0.2 cm/s added to Vd of 0.6 cm/s increases flux by about 33% at the same concentration. Use particle-size-informed values.
6) Using resistances to estimate Vd
When direct Vd is unavailable, the resistance pathway estimates Vd from Ra + Rb + Rc. Lower resistances imply faster transfer. As an example, Ra=50 s/m, Rb=20 s/m, Rc=100 s/m yields Vd ≈ 0.00588 m/s (≈0.588 cm/s). This is often suitable for screening-level analyses.
7) Unit conversions for reporting
Many reports prefer daily mass per area. The calculator converts µg/m²/s to mg/m²/day using 86,400 s/day and 1,000 µg/mg. This ensures consistent rollups when comparing scenarios, seasons, or surface types. Export tools help preserve a traceable calculation record.
8) Interpreting results and uncertainty
Flux is sensitive to micrometeorology, surface wetness, roughness, and chemical reactivity. Treat single values as estimates, and consider ranges or sensitivity runs for Vd and Rc. If results drive decisions, document assumptions and use field-calibrated parameters where possible.
FAQs
1) What does dry deposition flux represent?
It is the rate at which material transfers from air to a surface, expressed as mass per area per time. It converts an air concentration into a surface loading rate for impact and exposure analysis.
2) When should I use settling velocity?
Use settling when coarse particles or droplets are relevant and gravitational fall-out is non‑negligible. For fine aerosols and gases, settling is often small compared with turbulent deposition.
3) How do I choose a realistic Vd value?
Select Vd from literature or site studies that match your pollutant, surface type, and stability conditions. Use a range when uncertain, because Vd can vary by orders of magnitude across surfaces.
4) What are Ra, Rb, and Rc in resistance mode?
Ra is aerodynamic transport above the surface, Rb is the near-surface quasi‑laminar layer, and Rc is surface uptake resistance. Together they limit transfer and determine Vd through their sum.
5) Why does flux change linearly with concentration?
The formula multiplies concentration by velocity. If velocities are fixed, doubling concentration doubles the flux. This makes measurement quality and averaging choices very important for reliable flux estimates.
6) Can I estimate total deposited mass?
Yes. Provide surface area and exposure time. The tool computes mass as Flux × Area × Time, then reports it in grams and milligrams for straightforward comparisons across sites or periods.
7) Which output unit should I report?
Use µg/m²/s for process studies and mg/m²/day for compliance and daily loading summaries. Keep units consistent across datasets, and include the averaging time and parameter sources in reports.