Cloud Parcel Modelling – Part 1: Temperature Change and Equation (b: Salt Vs BC effect)

  • How black carbon and salt influence dwv/dt via activation?
  • Why this balance controls cloud formation and droplet growth?

Let’s break them down clearly and concisely....

These get to the heart of aerosol-cloud interactions and their role in the cloud parcel model — especially in relation to the water vapor change term dwv/dt.

Let’s break them down clearly and concisely so you can explain them in your blog.

πŸ”₯ How Black Carbon and Salt Influence dwv/dt via Activation

πŸ“Œ dwv/dt is the rate of change of water vapor in the parcel.

This change comes mainly from condensation onto cloud droplets.

Now, how do aerosols affect this?

1. Salt (e.g., NaCl): Hygroscopic Growth Enhancer

  • Salt particles are highly hygroscopic, meaning they attract water vapor easily.

  • This lowers the critical supersaturation needed for them to activate into droplets.

  • So, more droplets activate earlier.

  • → More surface area for condensation.

  • → Faster depletion of water vapor.

  • ✅ Result: Larger (more negative) dwv/dt.

2. Black Carbon (BC): Complex, Often Inhibiting

  • Hydrophobic fresh BC (e.g., from fires or diesel) does not absorb water easily.

  • If not coated by sulfate or organics, it won’t activate → fewer droplets.

  • Fewer activated droplets → less surface area → slower condensation.

  • ✅ Result: Smaller (less negative) dwv/dt.

But:

  • Internally mixed BC (e.g., coated with organics or sulfates) can become partially hygroscopic, changing its behavior.

  • Also, BC can heat the parcel (semi-direct effect) → delays saturation and droplet activation.

πŸ’§ Why the Balance Controls Cloud Formation and Droplet Growth

Let’s go back to Equation (4):

dTdt=−gVcp−Lcpdwvdt\frac{dT}{dt} = -\frac{gV}{c_p} - \frac{L}{c_p} \frac{dw_v}{dt}

πŸ“Œ Here's the logic:

  1. More droplets activate → more surface area for condensation → faster decrease in water vapor (dwv/dt more negative).

  2. Faster condensation → more latent heat release → slows the cooling (dT/dt becomes less negative).

  3. Latent heating counters adiabatic cooling → helps maintain or even increase temperature.

  4. This temperature balance controls:

    • How quickly supersaturation builds up.

    • How many more droplets can activate.

    • How fast existing droplets grow.

So:
➡️ Aerosols (BC or salt) control droplet activation → which controls dwv/dt → which alters latent heating → which feeds back on temperature → which affects the cloud microphysics.

Additional role of black carbon...

Black carbon (BC) does have a unique dual role. While uncoated BC tends to suppress droplet activation, it also absorbs solar radiation, heats the surrounding air, and modifies the cloud parcel thermodynamics.

πŸ”₯ Additional Notes:

  • BC's heating effect is not included directly in dwv/dt, but it contributes to dT/dt via radiative absorption, especially at visible wavelengths.

  • This heating effect can:

    • Warm the parcel.

    • Suppress RH and delay cloud formation.

    • Lead to cloud thinning or dissipation (semi-direct effect).

✅ Conclusion:

Salt enhances condensation and latent heating, while black carbon can suppress condensation but add heat radiatively. This duality makes BC a complex and sometimes disruptive player in cloud microphysics.

🧠 Summary

Aerosol TypeActivation BehaviorEffect on dwv/dtEffect on dT/dt (Latent + Radiative)
Salt (NaCl)Easy activation (low critical S)Faster condensation (more negative)More latent heating → slows cooling (positive term)
Black Carbon (uncoated)Poor activation (hydrophobic)Slower condensation (less negative)Less latent heating from condensation
+ Radiative absorption (semi-direct)—Direct atmospheric heating (can delay saturation)

 

Referrence(s) or reading(s):

Nenes, A., Ghan, S., Abdul-Razzak, H., Chuang, P. Y., & Seinfeld, J. H. (2001). Kinetic limitations on cloud droplet formation and impact on cloud albedo. Journal of Geophysical Research: Atmospheres, 106(D6), 7629–7639. https://doi.org/10.1029/2000JD900091

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