Lecture 2: Trace gases and aerosols

Question 1

Aerosols

Answer 1

= Particles suspended in the atmosphere
▪ The particles can be either solid or liquid
▪ Contributes to cloud formation and air pollution, such as smog
▪ Two types of aerosols:
➢ Natural (wildfire smoke, volcanic emissions, desert and soil dust, etc.)
➢ Anthropogenic (industrial emissions, biomass burning etc.)

Question 2

Trace gases

Answer 2

= gases present in the atmosphere in very low concentration
(typically < 0.1 %)
▪ ≃ gases other than nitrogen, oxygen and argon (> 99.9 % of the atmosphere)
▪ Main gases monitored with RS: O3, HCHO, NO2, SO2, CO
▪ Two types of trace gases:
➢ Natural (biogenic processes, oceanic emissions, volcanic emissions, etc.)
➢ Anthropogenic (fossil fuel combustion, mining, biomass burning, industrial activity, etc.)

Question 3

Remote sensing of trace gasses

Answer 3

Trace gases and aerosols are commonly detected thanks to their absorption and thermal emission properties

Trace gases
▪ Selection of band(s) at absorbed wavelength
and at not absorbed wavelength
▪ Difficult for some gases (overlapping signature
characteristics)
▪ Characteristics of absorption spectra also depend on P/T conditions (hence, on altitude)
▪ In practice, complex algorithms used to infer quantities (density, partial pressure, column amount)

The column burden of gases measured with satellite RS is expressed in different units,
depending on the gas
➢ For NO2 and HCHO = 1015 molecule/cm2
➢ For ozone and SO2 = Dobson Unit (DU)
1 DU = 2.69 x 1016 molecules / cm2
(= 0.01 mm of ozone at standard P/T (i.e., 105 Pa / 273.15 K) )

Question 4

Planck’s Law

Answer 4

Planck’s Law
▪ Blackbody (perfect emitter, perfect absorber)
▪ Planck’s Law describes how the intensity of emitted radiation depends on wavelength and temperature.
▪ Higher temperatures → more radiation emitted overall.
▪ The peak wavelength shifts to shorter values as temperature increases.
This relation allows us to:
▪ Use the shape of the emission curve to identify temperature (via brightness temperature).
▪ Detect gases and aerosols through their absorption features in the emitted spectrum.

▪ Monochromatic evolution of the intensity of
radiation as a function of temperature
with:
Bλ (T)= (2hc^2/λ^5)/(e^(hc/(λkT)) −1)
h = Planck’s constant (6.626 x 10-34 J s)
k = Bolzmann’s constant (1.381 x 10-23 J/K)
▪ Physical dimensions of intensity (power per unit area per unit solid angle) per unit wavelength
→ W m-2 µm-1 sr-1

Question 5

Brightness temperature

Answer 5

Planck’s law lets you estimate of the temperature of an object, based on the amount of energy that it radiates. This is the brightness temperature.

Question 6

Brightness Temperature Difference (BTD)

Answer 6

Compare BT at two thermal infrared bands: 10.8 µm (BT₁₁) and 12.0 µm (BT₁₂).
Normal condition (no ash):
For water/ice clouds: BT₁₁ > BT₁₂ (12 µm absorbed more).
With volcanic ash:
Ash absorbs more at 10.8 µm, so BT₁₁ < BT₁₂.
⇒ BTD = BT₁₁ – BT₁₂ < 0 → indicates ash cloud presence.
Why it works:
Ground and regular clouds emit similarly at both wavelengths.
Ash alters this pattern due to selective absorption, allowing detection.

Underestimations (make BTDash positive)
▪ Moisture rich environment and ash water
content
▪ Cold environment and cold volcanic
clouds → ice formation
▪ High zenith angle
Overestimations (make BTDcloud negative)
▪ Mineral dust clouds
▪ Desert conditions with dry atmosphere
▪ Night-time thermal relaxation
▪ High-altitude meteorological cloud
convection
▪ Misalignment between bands at 11 and 12 µm

Question 7

Alternative method for volcanic ash detection:
RSTash (Robust Satellite Technique)

Answer 7

RSTash (Robust Satellite Technique):
→ Uses a time series of satellite images to understand normal BTD behavior.
→ Detects anomalies that deviate from the daily pattern → likely ash.

Question 8

Alternative method for volcanic ash detection:
3- or 4-band methods

Answer 8

3- or 4-band methods:
→ Combine multiple thermal bands to better separate clouds vs. ash.
→ Use simple equations or thresholds to enhance ash signal.

Question 9

Alternative method for volcanic ash detection:
Dynamic BTD thresholds

Answer 9

Dynamic BTD thresholds:
→ Adjust the BTD cutoff based on local water vapor content.
→ Makes ash detection more adaptive and reliable.

Question 10

GOES-17

Answer 10

▪ Geostationary satellite imagery (NOAA, U.S.A.)
▪ Spatial resolution (km): 0.5 (red), 1 (blue, NIR), 2 (NIR, SWIR, TIR)
▪ Temporal resolution: 10 minutes
▪ EO instrument = ABI (Advanced Baseline Instrument)

Question 11

Sentinel-5 Precursor (S5P)

Answer 11

▪ Sun synchronous satellite imagery (Copernicus Programme, ESA)
▪ Mission objective = atmospheric RS, continuity of previous ENVISAT mission
▪ Instrument = TROPOMI (Tropospheric Monitoring Instrument)
▪ Spatial resolution (km): 21x28 (UV1), 7x7 (UV, VIS, NIR), 7x1.8 (NIR2)
▪ Temporal resolution: 1 day

Question 12

Google Earth Engine

Answer 12

= cloud platform for Earth Science data and analysis
▪ Catalogue of various satellite image collections and geospatial datasets (mostly rasters)
▪ Data analysis capabilities (online platform [JavaScript] or Python API)
▪ Processing performed on GEE servers (users only need an internet connection and a web browser)
▪ Free for academic and research use (commercial licences available)