Thursday, August 30, 2012

SPARTAN update

At the moment it seems that the SPARTAN instrument will be designed as a stacked filter unit. I found the following image on an Australian Government website with a very similar design scheme. This is good news, as I wasn't sure if our instrument had any precedent (i.e. for using two stacked filters preceded by a impaction plate) Another group has been in charge of the the design stage hence why I did not know more about this general approach.
Stacked filter unit with a very similar design to our own (except that we will have eight columns instead of just one). 
Regarding the coarse and fine filters (separating the "PM2.5" and PM10-PM2.5 mass fractions), we ordered some filters from SPI to do the job: a 47 mm diameter grease-coated membrane filter (part# E4708G-MB with 8um holes, $550 per 100-pack) and a 25mm polycarbonate membrane filter (part # E0425-MB with 0.4 um holes, $70 per 100 pack).

These filters are Nuclepore in spirit but without the eponymous brand name. They are both made of polycarbonate with etched holes to trap coarse and fine particulates, respectively. Notice how much more the 47 mm filters cost. This is due to the grease coating used to prevent bounce-back. I don't know how bad the bounce back is in a straight-shot filter yet it has been argued enough to necessitate the extra cost of the greased filters. I don't know if grease would benefit the smaller size fraction as well. Also the coated disks come in 47 mm diameters only, so we needed to cut them down to size. There were many email discussions about cutting filters in a replicable way, i.e. we need to pre weigh filters accurate to 1 microgram so piece can't be later falling off. The plan is proceed using a sharp steel punch (made courtesy of the workshop in the Dunn basement) sandwiched between two filter papers.

Otherwise the filters look good; the 25 mm filters allows an air flow rate up to 62.5 litres per minute and stable until about 140 degrees Celsius and the coarse filter is much the same. As we are operating these at 4.0 lpm and a temperature between 5-40 degrees, there should be no issue with these parameters.

Wednesday, August 29, 2012

Measuring PM2.5 around the world

If you want an amazingly well-funded study, you'd be hard pressed to outdo sponsorship from the US department of defence (DOD). In that spirit I present the DODs Enhanced Particulate Matter Surveillance Program (EPMSP), which in their words collected
Aerosol and bulk soil samples ... during a period of approximately one year at 15 military sites—including Djibouti, Afghanistan (Bagram, Khowst), Qatar, United Arab Emirates, Iraq (Balad, Baghdad, Tallil, Tikrit, Taji, Al Asad), and Kuwait (Northern, Central, Coastal, and Southern regions). 
That's what SPARTAN equals in terms of proposed site counts: up to 20 depending on future funding.

The DOD sampled each site for 24 hours at 6-day intervals from 2006 to 2007. Each site was measured for PM10, PM2.5, and total suspended particulates (TSP).

Their sampling equipment came from Airmetrics, which certainly give great credit to the company's instrument reliability (SPARTAN's sampling instruments are still in the early stages of design, but we should (or ought to) have something comparable in the near future. The hard deadline to start field testing is March 2013. My focus is to avoid having things come down to the wire).

EPMSP collected samples on Teflon, Quartz, and Nuclepore filters (I keep wanting to spell it Nucleopore!) in the three size frames. The most impressive statistic is the following account of their total assembled data:
The number of laboratory analyses performed in the course of the EPMSP is summarized in Table 2-2, and includes 66,462 analyses on Teflon® membrane, 23,807 on quartz fiber, and several million single particle analyses on Nuclepore®  
Several million samples! I cannot imagine how long that would take SPARTAN to measure, but not certainly not within my stay at Dal. To be fair most of these were samplings of the Nuclepore filters for scanning electron microscopy (SEM) analysis. The Teflon and quartz filters consisted of 'only' a few tenns of thousands of chemical analyses.

The data itself is very useful for our network, such as the PM2.5 levels we might expect to encounter. In Baghdad and Tikrit, Iraq annual levels exceed 100 ug/m3; Recall that acceptably 'clean' air must have fewer than 10 ug/m3 PM2.5 particulates. The TSP in all 15 sites exceeded 100 ug/m3, while up to 600 ug/m3 in Tikrit. Considering trace metal levels, "The USACHPPM 1-year interim Air-MEG of 3.42 μg/m3 for aluminum (Al) was on average exceeded at all 15 sites". These are not a healthy places to breathe.

The study points out that PM2.5/PM10 ratios vary considerably from place to place. In the middle east they apparently lie between 0.21 and 0.60. More anthropogenic combustion usually means the ratio is higher. This large variance supports the need for separate PM2.5 monitoring; PM10 is clearly only weakly related with fine aerosols.

In terms of sand-based composition, the middle east contains similar amounts of aluminosilicates as compared to the United States' more arid desert regions.

The EPMSP study used Nuclepore filters specifically for SEM analysis, as this surface type is good for very size-selective trapping of particulates on a smooth surface. Background signals are small, and the particulates look 'cleaned', like rock species isolated in a museum case. Their methodology has convinced me we should probably be using only Teflon for chemical analysis and gravimmetric weighing.


Aside: I really admire the openness of the data presented here. As a typical example, they write: "Gravimetric results from both Teflon® and quartz fiber filters were processed and are available on the DOEHRS Portal". Compare this to Canada's NAPS raw data access. Military-funded science is surprisingly more open than some other types. Food for thought.

Friday, August 24, 2012

Sampling flow chart

Here's the sampling procedure for our SPARTAN filter network:

Filters will be placed inside a regulated flow apparatus. The prototype design calls for 4 Lpm over a 25 mm diameter filter. The filter is going to be teflon or nucleopore. The former is good because it's easy to clean, the latter is good because volatile nitrates escape less easily. There is no obvious choice, but likely we will end up with teflon.

The steps in the above chart aren't individually complex, but extreme care is needed in order to ensure quality data. If anything goes wrong at any stage (weighing, ion dissolution in water, etc), you can bet the whole goal of the network might be compromised.

Last month I was focused on comparing total PM2.5 aerosol mass with nephelometer bscat measurements. It was important to know as much about what affects the light scattering of aerosols as possible, since the ultimate goal of the network is quantitative PM2.5 mass data (measured in micrograms per cubic meter). One should back spectroscopy data if at all possible (astrophysicists are, for example, limited to spectral data). Even ground nephelometer measurements must be supported by actual weighed filters. Somehow this gives me satisfaction known that practical chemistry has a place among all this modern satellite equipment.  

Tuesday, August 7, 2012

Splitting aerosols

How much more surface area do you obtain from splitting aerosols in half? That is to say a constant volume holds while splitting the aerosol volume like so:
I want to compare surface S1 with surface S2. Since surface is related to volume like so,
then the ration S2/Sbecomes (and accounting for the two spheres in S2),

Hence there is 26% more surface area available if all sphere of volume V divide into spheres of volume V/2.

If the aerosol is split unevenly into fractions x and 1- x (where 0 < x < 1), then
which looks like this:

The equation is if course symmetric about the point x = 0.5.

Monday, August 6, 2012

Climate change is real

Well, duh. Sad, however, that professionals are now reduced to explaining things in terms of gambling. I don't like these odds...

Friday, August 3, 2012

Mars rover

Ok, this Mars rover landing trumps the olympics in my opinion. Sounds pretty awesome what they're trying to do:

UPDATE: ...and they landed! Pretty cool.