Aerosols

This morning one of the most important (and most delayed) satellite launches in ages took place. The mission was to launch the Glory satellite into a polar orbit, where three key instruments would have been looking at solar irradiance, aerosols and clouds. Unfortunately, one of the stages failed to separate and the satellite did not make orbit.

The irradiance measurements were to be an important continuation of the SORCE mission results, and are needed to stably continue the Total Solar Irradiance (TSI) timeseries. However the big new measurements were those associated with the Aerosol Polarimeter Sensor (APS). A similar instrument has flown in space twice before (the French-developed POLDER instrument), but unfortunately only for short periods. Its uniqueness lies in its ability to detect aerosols over bright surfaces (like land), and more importantly, to distinguish what kind of aerosols it is seeing. (Update: There is a third POLDER instrument, PARASOL, that is currently in orbit, see comments).

It may seem surprising, but despite many different attempts, almost all remote sensing of aerosols from space is only capable of detecting the total optical depth of all aerosols. MISR can provide some discrimination in special cases (picking out dust via a retrieval of non-spherical particles, or using the single scattering albedo to distinguish black carbon), but overall the estimates mix up sulphates, dust, black carbon, sea salt, nitrates and secondary organics. These originate from different processes, have different properties and different impacts on both radiation and clouds. Sea salt comes from sea spray over the oceans, dust from dry desert areas, black carbon from burning of forests and fossil fuels, sulphates derive from ocean plankton and burning coal, nitrates derive from fertiliser use, car exhausts and lightning, and secondary organics come from the stew of volatile organic compounds from industrial and natural sources alike. There are also pollen, and fat particles from outdoor cooking etc.

Because we can’t easily distinguish what’s what from space, we don’t have good global coverage of exactly how much of the aerosol is anthropogenic, and how much is natural. That uncertainty is a big player in the overall uncertainty in the human caused aerosol radiative forcing. Similarly, we have not been able to tell how much of the aerosol is capable of interacting with liquid or ice clouds (which depends on the different aerosols’ affinity for water), and that impacts our assessment of the aerosol indirect effect. These uncertainties are reflected in the model simulations of aerosol concentrations which all show similar total amounts, but have very different partitions among the different types.

The APS technology is a big step forward on these issues. It turns out that while the reflected SW from many different aerosols is similar, the polarisation of that reflected light depends quite strongly on what kind of aerosol it is. This varies depending on the angle at which the light is shining, So by scanning through the angles and measuring the polarisation, we can get a better constraint on the distribution of key aerosols. Scientists have already been working with aircraft mounted versions of the instrument, and this will continue.

The story of how this launch actually happened is very long and twisted, and needless to say, has taken far longer than anyone envisaged at the start (over a decade ago). With the failure to make orbit this morning, the wait will unfortunately go on.

This is of course a huge setback for the mission team (many of whom I know), and I can only imagine how frustrating this must be. The loss of OCO two years ago was due to a similar problem, though 3 launches since then have been successful (and the same system is being replicated as OCO-2). With the postponement of CLARREO in the proposed 2012 budget, there is a huge hole building in the US contribution to Earth and Sun observing systems.

Working from space is hard, expensive and risky. We cannot take it for granted, and yet we need that information more than ever.

In a new GRL paper, Svensmark et al., claim that liquid water content in low clouds is reduced after Forbush decreases (FD), and for the most influential FD events, the liquid water content in the oceanic atmosphere can diminish by as much as 7%. In particular, they argue that there is a substantial decline in liquid water clouds, apparently tracking a declining flux of galactic cosmic rays (GCR), reaching a minimum days after the drop in GCR levels. The implication would be that GCR can affect climate through modulating the low-level cloudiness. The analysis is based on various remote sensing products.

[Read more…] about Still not convincing

Guest post by Bart Verheggen, Department of Air Quality and Climate Change , Energy research Institute of the Netherlands (ECN)

In Part I, I discussed how aerosols nucleate and grow. In this post I’ll discuss how changes in nucleation and ionization might impact the net effects.

Cosmic rays

Galactic cosmic rays (GCR) are energetic particles originating from space entering Earth’s atmosphere. They are an important source of ionization in the atmosphere, besides terrestrial radioactivity from e.g. radon (naturally emitted by the Earth’s surface). Over the oceans and above 5 km altitude, GCR are the dominant source. Their intensity varies over the 11 year solar cycle, with a maximum near solar minimum. Carslaw et al. give a nice overview of potential relations between cosmic rays, clouds and climate. Over the first half of the 20th century solar irradiance has slightly increased, and cosmic rays have subsequently decreased. RC has had many previous posts on the purported links between GCR and climate, e.g. here, here and here.

[Read more…] about Aerosol effects and climate, Part II: the role of nucleation and cosmic rays