What are aerosols?
Aerosols are small and complex chemical mixture of liquid and particles suspended in the air. In practice, referring to aerosols include then both particles and its surrounding medium. Due to their tiny dimensions, they are usually invisible to the human eye. However they efficiently interact with solar radiation and affect strongly its distribution throughout the atmosphere (Dubovik et al., 2019). Their presence not only perturbs our climate system, total atmospheric energy budget, atmospheric visibility, human health & safety, but also interferes with satellite observations of atmospheric trace gases. Furthermore, scattering and absorption by aerosols impact the actinic flux, and consequently modifies the photolysis rates of important processes in the atmosphere (Palancar et al., 2013).
They differ from gases as they are bigger than molecules. There are several classifications of atmospheric aerosols but the most widely used is according to their size. They range from the smallest superfine mode, with diameters of a few nanometers (nm), to large coarse mode particles, with diameters to more than 100 micrometers (m) 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 um (Seinfeld, 1986). In polluted conditions, they are often denoted as particulate matter (PM): e.g. PM10 referring to the dry mass of particles with a diameter less than 10 um.
How are aerosols produced?
Aerosol sources combine both natural and anthropogenic processes, and are of two types:
- a direct emission resulting from dispersion of material at the Earth surface (e.g. sea spray aerosol from sea surface or waves, dust from desert outbreak – See an example of dust transport next to the Death Valley park (USA) – , biomass burning aerosol, volcanic ash resulting from eruptions, primary organic aerosol, industrial debris).
- or indirectly from precursor trace gas emission leading to the formation of secondary aerosols (e.g. sulfates, nitrates, ammonium salts, secondary organic aerosol), or condensation or coagulation processes.
There are still some gaps in our understanding and modelling of aerosol sinks (IPCC: The Core Writing Team Pachauri and Meyer, 2014). After their release, they undergo various physical and chemical processes modifying then their size and optical properties. They are generally removed from the air by dry or wet depositions, depending on their size and Earth’s surface characteristics, at the surface (Kerkweg et al., 2006; IPCC: The Core Writing Team Pachauri and Meyer, 2014). Wet deposition occurs by in-cloud and below-cloud scavenging. The remaining particles go through dry deposition processes such as turbulent diffusion, gravitational sedimentation, impact with obstacles and/or Brownian diffusion.
The global total aerosol mass is dominated by natural processes at the surface, in particular sea spray aerosol and desert dust. However, anthropogenic emissions (e.g. industries, vehicles, agriculture, wildfires etc…) of both primary particles and precursor gases greatly increases the total aerosol load and can locally outweigh the natural aerosols (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). Yet accounting for the effects of aerosol particles is very difficult since they represent one of the most complex atmospheric constituents. For example, it has widely been recognized that the lingering uncertainty in the knowledge of aerosol properties drives the global climate change estimation uncertainly (Dubovik et al., 2019). These 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