Wandering dust, the seed of rain – the international journey of an aerosol particle

Saudi Arabia; a storm in May. The column of air, water, and ice towers over the sand dunes. As the hail falls, it melts to form rain, cooling the air. As the rain continues to fall, most of it evaporates in the drier air nearer the ground, cooling the air still further. And as the cool dense air sinks, it spreads out rapidly as it approaches the surface. A few kilometres away, a Nubian ibex, a goat sporting impressive horns, feels the gust of air. Sand is blown along the surface of the desert. Some grains are lifted up, airborne for a few metres, before crashing back to the surface, knocking yet more sand and dust into the air. As the process continues, fuelled by the gusts of cool air, a wall of dust begins to form. The “haboob” dust storm makes its way across the arid landscape, coating plants, roads, and buildings in a layer of dust. As the haboob closes in on the Gulf of Oman, most of the larger grains of sand are deposited. But some of the finer dust particles, the size of bacteria, have been lifted up to a height of five kilometres and are blown across the Arabian Sea to Pakistan and northern India.

In northern India, a family sits around a wood-burning stove. Inside their home, the air is heavy with black carbon soot. Much of this smoke escapes into the air outside, adding to the black carbon aerosol released from a nearby coal-fired power station. The power station adds yet another chemical to the noxious airborne soup: sulphur dioxide. Over the next few days, much of this sulphur dioxide gas is oxidized to sulphuric acid, forming small liquid particles of sulphate aerosol. Some of the sulphate coats the particles of black carbon and the dust that has been blown across the Arabian Sea.

When the sun rises, the family observes that it is yet another hazy day, a common sight during the pre-monsoon season. The milky mixture of aerosol reduces the amount of sunlight reaching the ground. Much of the sunlight is scattered back to space by the sulphate and dust. However, some of the sunlight is absorbed by the dust and black carbon, warming the atmosphere, inhibiting the formation of clouds. But the inhibition of cloud formation is only temporary: the monsoon storms are on their way, and the atmospheric heating may even hasten their arrival.

Over the course of the next few days, energy builds up gradually in the warm moist air near the ground. Finally, one sunny afternoon, a parcel of air gains enough energy to rise upwards. As it rises and cools, water condenses on many of the sulphate particles, diluting the sulphate to form cloud droplets. As the water condenses, heat is released and the air parcel rises even further. After a few minutes, the top of the cloud is so high that the air temperature is below 0°C. But the cloud droplets do not freeze immediately. The water molecules do not have a surface to freeze onto. Even when the “supercooled” droplets have risen high enough to cool below -15°C, most of the droplets remain unfrozen. But some of the cloud droplets have grown on dust that has been coated by sulphate. And some of these dust particles act as the seed, or “ice nucleus”, on which ice can grow. As the air rises and cools even further, the ice crystals grow rapidly at the expense of the liquid droplets. Some of the ice crystals collide with other ice crystals and liquid droplets, accelerating their growth. When they grow large enough, their downward terminal velocity becomes faster than the upward movement of the air, and they fall through the cloud, collecting yet more water on the way, growing to form hailstones. By the time the hail reaches the ground, most of it has melted to form rain.

The family recognizes the sound of thunder. They feel a gust of cool air. The monsoon rains have finally come.

[Benjamin Grandey, June 2015]

Radiative effects of interannually varying vs. interannually invariant aerosol emissions from fires

Grandey, B. S., H.-H. Lee and C. Wang, Radiative effects of interannually varying vs. interannually invariant aerosol emissions from fires, Atmospheric Chemistry and Physics, doi:10.5194/acp-16-14495-2016


A news release, written by Mark Dwortzan, can be read via MIT News and
the Joint Program on the Science and Policy of Global Change news page.


Wildfires emit organic carbon aerosols, small particles suspended in the atmosphere. These aerosols may cool the climate system via interactions with sunlight and clouds. We have used a global climate model to investigate the cooling effects of these aerosols. We find that ignoring interannual variability of the emissions may lead to an overestimation of the cooling effect of the aerosols emitted by fires.


Global mean net radiative flux perturbation (RFP) associated with aerosols emitted by wildfires. F1997 to F2006 are ten simulations using fire aerosol emissions for individual years from 1997 to 2006. {Fyyyy} represents the mean of this ten-member ensemble. FMEAN, a simulation that uses fire aerosol emissions averaged across 1997-2006, overestimates the strength of the cooling effect by 23%. [Adapted from Fig. 3 of Grandey et al. (2016).]

Transient Climate Impacts for Scenarios of Aerosol Emissions from Asia: A Story of Coal versus Gas

Grandey, B. S., H. Cheng and C. Wang, Transient climate impacts for scenarios of aerosol emissions from Asia: a story of coal versus gas, Journal of Climate, doi:10.1175/JCLI-D-15-0555.1

Video Introduction


Fuel usage is an important driver of anthropogenic aerosol emissions. In Asia, it is possible that aerosol emissions may increase if business continues as usual, with economic growth driving an increase in coal burning. But it is also possible that emissions may decrease rapidly as a result of the widespread adoption of cleaner technologies or a shift toward noncoal fuels, such as natural gas. In this study, the transient climate impacts of two aerosol emissions scenarios are investigated: a representative concentration pathway 4.5 (RCP4.5) control, which projects a decrease in anthropogenic aerosol emissions, and a scenario with enhanced anthropogenic aerosol emissions from Asia. A coupled atmosphere–ocean configuration of the Community Earth System Model (CESM), including the Community Atmosphere Model, version 5 (CAM5), is used. Three sets of initial conditions are used to produce a three-member ensemble for each scenario. Enhanced Asian aerosol emissions are found to exert a large cooling effect across the Northern Hemisphere, partially offsetting greenhouse gas–induced warming. Aerosol-induced suppression of the East Asian and South Asian summer monsoon precipitation occurs. The enhanced Asian aerosol emissions also remotely impact precipitation in other parts of the world. Over Australia, austral summer monsoon precipitation is enhanced, an effect associated with a southward shift of the intertropical convergence zone, driven by the aerosol-induced cooling of the Northern Hemisphere. Over the Sahel, West African monsoon precipitation is suppressed, likely via a weakening of the West African westerly jet. These results indicate that fuel usage in Asia, through the consequent aerosol emissions and associated radiative effects, might significantly influence future climate both locally and globally.

Video: Climate Change and Climate Engineering

This is the video version of a CENSAM Public Lecture, exploring four questions:
1. What is happening to the climate?
2. How may climate change impact Singapore and elsewhere?
3. What might be the consequences of deliberate climate engineering?
4. How can we respond to the challenge of climate change?

If you have any comments or suggestions, please do drop me an email.

Irrigation impacts on rainfall in Sudan

Dr Im Eun-Soon (a colleague at SMART-CENSAM) and co-authors have recently published some interesting work looking at irrigation impacts on precipitation:

Alter, R. E., E.-S. Im, and E. A. B. Eltahir (2015), Rainfall consistently enhanced around the Gezira Scheme in East Africa due to irrigation, Nature Geoscience, doi:10.1038/ngeo2514

A short introduction can be read at MIT News.

A professional video introduction is available on Youtube:

Enhanced marine sulphur emissions offset global warming and impact rainfall

Grandey, B. S., and C. Wang (2015), Enhanced marine sulphur emissions offset global warming and impact rainfall, Scientific Reports, doi:10.1038/srep13055

Video Introduction

MIT News

A news release, written by Mark Dwortzan, can be read at http://globalchange.mit.edu/news-events/news/news_id/480 [URL added on 7-Sep-2015] or http://news.mit.edu/2015/fertilize-ocean-cool-planet-0908 [URL added on 9-Sep-2015].

Straits Times [23rd October 2015]

Audrey Tan has written a really well researched article in the Straits Times: http://www.straitstimes.com/singapore/shivers-over-growing-plankton-to-cool-earth [URL added on 23-Oct-2015]. I really like the Straits Times’ infographic.


Artificial fertilisation of the ocean has been proposed as a possible geoengineering method for removing carbon dioxide from the atmosphere. The associated increase in marine primary productivity may lead to an increase in emissions of dimethyl sulphide (DMS), the primary source of sulphate aerosol over remote ocean regions, potentially causing direct and cloud-related indirect aerosol effects on climate. This pathway from ocean fertilisation to aerosol induced cooling of the climate may provide a basis for solar radiation management (SRM) geoengineering. In this study, we investigate the transient climate impacts of two emissions scenarios: an RCP4.5 (Representative Concentration Pathway 4.5) control; and an idealised scenario, based on RCP4.5, in which DMS emissions are substantially enhanced over ocean areas. We use mini-ensembles of a coupled atmosphere-ocean configuration of CESM1(CAM5) (Community Earth System Model version 1, with the Community Atmosphere Model version 5). We find that the cooling effect associated with enhanced DMS emissions beneficially offsets greenhouse gas induced warming across most of the world. However, the rainfall response may adversely affect water resources, potentially impacting human livelihoods. These results demonstrate that changes in marine phytoplankton activity may lead to a mixture of positive and negative impacts on the climate.

Publication: Rainfall-aerosol relationships explained by wet scavenging and humidity

Grandey, B. S., A. Gururaj, P. Stier and T. M. Wagner (2014), Rainfall-aerosol relationships explained by wet scavenging and humidity, Geophysical Research Letters, doi:10.1002/2014GL060958

Key points

  • Negative rainfall-aerosol relationships arise due to wet scavenging (removal of aerosol by rainfall).
  • Satellites poorly sample aerosol populations depleted by wet scavenging.
  • Positive rainfall-aerosol relationships arise due to humidification effects. Increased relative humidity causes many aerosols to swell, and high relative humidity is required for rainfall to occur.
  • Key figure

    Maps of the differences in precipitation rate between clean and polluted conditions during June-July-August

    Maps of the differences in precipitation rate between clean and polluted conditions during June-July-August. (a) Difference in total precipitation rate between clean (total aerosol optical depth < 33rd percentile) and polluted (total aerosol optical depth > 67th percentile) conditions for the ECHAM5-HAM control simulation. (b) Similar to (a) but for convective precipitation rate instead of total precipitation rate. (c) Similar to (b) but for the NoConvScav simulation, in which scavenging of aerosols by convective precipitation has been turned off. (d) Similar to (c) but for dry aerosol optical depth instead of total aerosol optical depth. The annotated arrows show the conceptual steps linking each figure. Area-weighted means are provided at the side of each figure.