What are aerosols?
Aerosols are small and complex chemical mixture of liquid and solid matter suspended in the atmosphere: i.e. particles and droplets, depending on their aggregation state. In practice, referring to aerosols include then both particles and its surrounding medium.
They play an important components of Earth’s climate system. There are several classifications of atmospheric aerosols but the most widely used is according to their size. They range in size from the smallest superfine mode, with diameters of a few nanometers (nm), to large coarse mode particles, with diameters to more than 100 micrometers (μm) or more. Between the superfine and the coarse mode particles are the fine mode particles, with diameters ranging from 0.1 mm to a few μm (Seinfeld and Pandis 1998).
Because of their large differences regarding physical properties and aggregation states, aerosols cannot be confused with cloud droplets.
How are aerosols produced?
Sources of aerosols combine both natural processes and anthropogenic activities. They combine two main processes:
- a direct emission resulting from dispersion of material at the Earth’s surface (e.g. sea spray aerosol, dust, biomass burning aerosol, volcanic ash, primary organic aerosol, industrial debris);
- or an indirect emission from atmospheric chemical reactions due for example by precursor trace gases (e.g. sulphates, nitrates, ammonium salts, secondary organic aerosol), or condensation or coagulation processes.
The total aerosol mass is dominated by natural processes at the surface, in particular sea spray aerosol and desert dust. However, anthropogenic emission of both primary particles and precursor gases greatly increases the total aerosol load (Andreae and Rosenfeld 2008).
Why shall we observe atmospheric aerosols?
Earth-atmosphere climate, air quality issues, accurate trace gas observations from satellites…
The reasons are numerous:
- Aerosols directly impact the radiation budget of the Earth-atmosphere system through the scattering and absorption of solar and terrestrial radiation (Feingold et al., 1999). High concentrations of fine particles lead to reduced clouds droplet size, enhanced cloud reflectance (Twomey et al., 1984), and reduced precipitation (Rosenfeld, 2000; Ramanathan et al., 2001; Rosenfeld et al., 2002). Therefore, large uncertainties of aerosol optical properties limit our climate predictive capabilities (IPCC: Solomon et al., 2007). In spite of more robust climate predictions in the last years, radiative forcing (RF) induced by aerosols still contributes to the largest uncertainty to the total RF estimate (IPCC: The Core Writing Team Pachauri and Meyer, 2014). The vertical distribution and relative location are determining factors of aerosol radiative forcing in the long-wave spectral range (Dufresne et al., 2002; Kaufman et al., 2002).
- Aerosols play a significant role in air quality, in particular near the surface. Due to the rapid growth of both population and economic activity, such as in Asian region, increase in fossil fuel emissions gives rise to concerns about fine particles formation and dispersion. Aerosols include a variety of hazardous organic and inorganic substances, reduce visibility, lead to reductions in crop productivity and strongly affect health of inhabitants in urban regions (Chameides et al., 1999; Prospero, 15 1999; Eck et al., 2005).
- In the absence of clouds, vertical distribution of aerosols, combined with their optical properties strongly affect our ability of accurately deriving trace gas concentrations as derived from air quality satellite spectral measurements. Negative biases on the Ozone Monitoring Instrument (OMI) tropospheric NO2 – Nitrogen dioxide columns, between 26% and 50 %, are found in urban and very polluted areas in cases of high aerosol pollution and particles located at elevated altitude (Shaiganfar et al., 2011; Ma et al., 2013; Kanaya et al., 2014). HCHO for GOME-2 and SCIAMACHY shows about 20-50% sensitivity to aerosols, depending if they are located within or above the boundary layer (Barkley et al., 2012; Hewson et al., 2015). Dust aerosols (large particles, with strong absorption in UV) can double the retrieved SO2 (Krotkov et al., 2008). This impacts the ability of sensors like OMI to monitor Planetary Boundary Layer (PBL) SO2 with a sensitivity to local anthropogenic sources (Lee et al., 2009). Therefore, aerosol parameters (or retrievals) are a pre-requisite before retrieving trace gas vertical column densities.
In its last report (Clear the air for children, October 2016), UNICEF has emphasized these striking numbers: globally, 1), 2 billion children live in areas where outdoor air pollution exceeds international limits, 2) 300 million children live in areas where outdoor air pollution exceeds 6 times international limits.
The Americas and Europe are also concerned: 120-130 (20) million children live in areas where outdoor exceeds (2 times) international limits (cf. UNICEF).
A typical satellite aerosol map?
Aerosol Optical Thickness, or AOT, is one of the most common and important parameter retrieved from satellite measurements: it describes the extinction of the sunlight due to particles present in the atmosphere. This parameter is spectrally dependent (i.e. function of wavelength), and can be considered, at a first approximation, as a proxy of aerosol concentrations.
In the map above, very strong aerosol pollution can be visualized over East Asia, Central Africa and India. The main reasons are a combination of industries, coal burning power plants, biomass burning activities and car traffic. Although much lower, Western countries also face aerosol pollution episodes during days or weeks.
Note the large plume over East Russia and Siberia: probably the consequence of some wildfires caused by very warm temperatures and dry forests…
Some reference satellite aerosol missions products?
Nowadays, the most likely famous product is the aerosol optical depth (AOD) from the two identical satellite aerosol sensors MODIS on-board the NASA Terra (early morning, 10:30) and Aqua (early afternoon, 13:30) platforms. MODIS AODs are retrieved at 550 nm by the Dark-Target (Levy et al., 2013) and the Deep-Blue (Hsu et al., 2013) algorithms. The widely used 10 km resolution MODIS aerosol product provides valuable information on aerosol distribution in space and time, and has been widely used to characterize aerosol dynamics and distribution, simulate climate change, and assess population PM exposure (Levy et al., 2010, 2013).
Others may be mentioned:
- NASA current mission VIIRS, on board the NASA-NOAA S-NPP, early afternoon (13:30), AOD.
- South Korea current GOCI mission, on-board COMS, hourly measurements eight times per day from 09:00 to 16:00 Korean LT, sampling area of 2500 x 2500 km2 centered at [130 E, 36 N] in East Asia, AOD.
- French CNES CALIOP current mission, on-board NASA CALIPSO, early afternoon (13:30), aerosol backscattering, extinction and vertical profile (e.g. altitude or height)
- Dutch-Finnish current OMI mission, on-board NASA EOS-Aura, early afternoon (13:30), AOD, Aerosol Aborsbing Index (AAI)
- POLDER current mission, early afternoon (13:30), AOD
- NASA AVHRR current mission (more than 30 years!), on-board on NASA TIROS-N, NOAA-6, NOAA 15, and then, Metop A early morning (06:00, 09:30, and 10:00), AOD
- AATSR past mission, on-board ESA ENVISAT, early morning (10:00), AOD, Angstrom coefficient, mixing ratio of dominant aerosol classes
- MERIS past misson, on-board ESA ENVISAT, early morning (10:00), AOD, Angstrom coefficient
- WebPage of an example of aerosol transport: Dust storm next to the Death Valley park (USA) – An example of aerosol transport! here