Instruments and procedures

I was reading through the paper A review of atmospheric aerosol measurements and found it quite useful, especially since aerosol collection is the core of the SPARTAN project.  It was authored by Peter H. McMurry, a specialist in both theoretical and experimental aerosol science (my kind of guy!). Even better is that he focuses on small aerosols, i.e. PM2.5 and below.

SPARTAN is not about aerosol nucleation theory, which is of course an active field of research but too far off topic from our direct interests: below a few hundred nanometers particles cease to scatter much light or have much mass, hence do not change nephelometer light scattering readouts. Actually I should qualify that: there is one pesky light-scattering gas: NO₂. Nitrogen dioxide is the reason polluted cities look polluted. That brownish colour in the sky is NO₂. At more than a few parts per billion, visible wavelengths get noticeably scattered.

NO2 cross section from here. Note the overlap with the visible spectrum, i.e. above 420 nm, which creates a brownish colour  

Back to instrumentation measurements. Although Dr. McMurry is principally interested in the theory of sulphuric acid nucleation (his definition of small is smaller than ours), his review is extensive. Some highlights:

Size selection methods

Diffusion of particles down the length of a tube is a convenient way to size-sort parties. But only particles below 100 nm can be practically size-sorted by diffusion. Particles are sorted into histogram-like bins from 10-100 nm. Since differences in diffusion are proportional to the square root of the mass then larger particles do not separate well. Sorting heavier aerosols is akin to the problem of sorting 235UF₆ and 238UF₆, i.e. requiring many times more path length. For heavier particles, i.e. 0.1-2.5 um, one would use a multi-stage impactor.

Interesting thing about impactors is that wet particles 'bounce' less than dry ones, so that >75% RH is a good way to capture these particles. Downside is that means we'll need to be extra careful about size correcting these hydroscopic particles.

Ammonium nitrate losses on filters

When it comes to capturing ammonium nitrate, which is volatile, it turns out impactor plates are better than Teflon filters:

Evaporative losses of particulate nitrates have been investigated in laboratory and field experiments with filters and impactors. The laboratory studies involved parallel sampling of ammonium nitrate particles with a Berner impactor and a Teflon filter. Both samplers were followed by nylon filters to collect evaporated nitric acid. Losses from the impactor were 3-7% at 35C and 18% relative humidity, and losses from the filter were 81-95% under the same conditions. This result (that evaporative losses from the filter exceeded those from the impactor) is consistent with theoretical predictions

In other words Teflon is not very good for capturing nitrates. Huge losses, which means total aerosol mass is under predicted, chemical speciation is mis represented and so on.

Here is another great review, but slightly older.

I want to play with one of these H-DTMA instruments (H-DTMA stands for hygroscopic tandem differential mobility analyzer).

Also reading this paper linking hygroscopic aerosol particle growth with organic composition, which is right up my alley. Here's a handy tabulation of aerosol species' properties: