Previous aerosol monitoring in the United States

Looking over a few aerosol papers now:

Here's a paper that measures the content of PM_2.5 from JGR: Seasonal composition of remote and urban fine particulate matter in the United States

So what did they find? Well, for starters just about anything you can imagine is in a aerosol, including salts, various forms of carbon, heavy metal (i.e. this paper), and acids (SO2, NO2, HCl, etc). Here the authors narrowed their radar to "ammonium sulfate, ammonium nitrate, particulate organic matter, light-absorbing carbon, mineral soil, and sea salt".

A problem one is faced when sampling is "do I choose a weighted average based on population density or maintain a uniform geographic scattering?" The answer here would appear to be geared to the former; there was a roughly 50/50 split between urban and rural analysis (176 vs 168) since there is a even divide of urban and rural populations (but increasingly urban).

The first two aerosols listed ((NH₄)₂SO₄ and (NH₄NO₃) are specific chemicals. The four thereafter (POM, LAC, MS, SS) are categories, not individual species. What is inside them (which are, in turn, inside aerosols)?

Sulfate and Nitrate Anions 

Determined "from ion chromatography using a nylon filter... preceded with a sodium carbonate coated denuder". The anions sulfate and nitrate are assumed to be neutralized by the ammonium cation (but not necessarily true: other cations could associate with these species eg Na or CaCO3). Relative to the total aerosol mass, sulfate accounted for 40%, up to 60%. Nitrates were smaller, at 10-20%.

Mineral Soil (MS)

For mineral soil we have as components Al2O3, SiO2, CaO, K2O, FeO, Fe2O3, and TiO2. These are found via X-ray fluorescence (XRF).

Light-absorbing carbon (LAC)

For LAC we have a combination of 'black' carbon (aka BC or 'soot') and 'brown' carbon.  Black carbon exists as an aggregate of 10-50 nm carbon particles. These structures are skeletally seen as elemental carbon, much of it graphite (plus other trappings found in the structure: aromatic hydrocarbons and some other aliphatics). Since carbon is the backbone of soot, elemental carbon (EC) is another means of defining these particles. This alternate classification is a functional one rather than chemical or physical. The paper states that "We use “LAC” instead of “EC” based on the recommendation of Bond and Bergstrom [2006] [proposing an absorption cross section of α(550 nm) = 7.5(1.2) m² g⁻¹] and to avoid possible improper classification". But 'improper' is subjective. From the 2006 paper by Andreae et al Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols

Both “BC” and “EC” can only be regarded as “proxies” for the concentration of soot carbon, whose accuracies depend on the similarity between atmospheric soot and the species used for calibration. If atmospheric soot were pure graphite and all the methods were calibrated against graphite, “BC” and “EC” readings would give exactly the mass concentration of soot carbon as intended. Since, however, graphite is only a trace component of atmospheric soot, “BC” and “EC” measurements usually give different results, which have possibly little in common with the “true” mass concentrations of atmospheric soot par ticles. However, in the literature these discrepancies are usually disregarded and the terms “BC” and “EC” are used interchangeably as synonyms for soot carbon.

Not to go off-topic but some of the contention could stem from debates in application of BC/EC to climate change models. Climate change is something everyone wants to throw in their two cents, and debating exact light-scattering parameters lays open some targets. Ultimately the definition hovers close to the notion of that carbon which absorbs light.

Brown carbon absorbs shorter wavelengths than black carbon (less than 550 nm, into the UV spectrum), and appears brown in color. It is composed of "light-absorbing organic matter in atmospheric aerosols of various origins, e.g., soil humics, HULIS (humic-like substances), tarry materials from combustion, bioaerosols"

How is LAC measured? Thermo-analysis is one way, whereby the sample is progressively heated and all carbon is converted to CO₂ and quantified. The original paper discussed here uses thermal desorption from quartz fibres. But since the point of defining LAC is to obtain the light-absorbing component, cross-sectional visible light absorption is an intuitive alternate method of measure. The problem is, however, that light absorption methods have higher measurement errors/lower detection limits. Andreae et al discuss this problem comparing various absorption instruments.

Relative mass to total PM2.5 weight is small, about 3-15% (errors are large due to the small amounts of LAC present in aerosols)

Particulate organic matter (POM):

Particulate organic matter was assumed to related to total molecular weight of 1.8 per unit mass total carbon (changed from 1.4 from an earlier publication). This value is empirical and can vary widely (e.g. 1.2 to 2.6). The 'additional' mass comes from O, N, and H and varies with aerosol age/location. Carbon in POM is assumed to be equivalent to organic carbon (OC) and is determined by thermal desorption techniques. In other words the difference between POM and LAC in this study is merely the temperature at which the carbon is oxidized to CO2; a threshold temperature must be established and therefore it is somewhat an arbitrary division (cutoff of about 650 Celsius).

POM constitutes a relatively large percentage of total PM2.5 mass (40-75%).

Sea Salt (SS)

Sea salt concentrations were determined by measuring chlorine ion concentrations and multiplying by the factor 1.8. Since only one ion represents the total salt ion concentration, there is a chance errors are present if chlorine escapes from reaction with gaseous nitric acid.

Sea salt constitutes a medium percentage of total PM2.5 mass: 10-20% in coastal and <5% in continental (land-locked areas).

Results/Conclusions

At this stage I'm as interested in the results as the methods. Nevertheless the rural/urban divide piques my interest  as this is my project's direction, i.e. to compare and contrast 'clean' country air and 'dirty' city air. I use quotation marks because sometimes a city can be cleaner than the country, especially if near a farm, waste depot, factory, etc. And cities can be very clean if electrically-driven public transportation dominates (such as the solar-powered city Freiburg, Germany). Here are the comparisons

Ammonium sulfate: no strong difference in city/country divide, but east had much more sulfate than western US. Likely this is because of the higher coal power plant density on the eastern half.

Ammonium nitrate: Urban levels were much higher than rural (4-5 times) and increased in the mid-western US, reflecting significant fertilizer use. High urban levels are not easily explained. The paper says maybe it's due to elevation differences (between rural/urban measurement methods?)

POM: Very much higher in urban environments (2-5 times), likely due to fuel combustion and outdoor cooking (my guess since maxima is in summer).

LAC: Urban LAC was much higher that rural levels. LAC is a combustion product, hence emissions are concentrated with human population.

Soil: Higher levels in 'dustier' areas, i.e. varies on the region of US (i.e. Death Valley is high in aerosol soil content). No strong differences were seen in city vs. country, within a given landscape.

Sea salt: Tied to proximity with oceans, not cities


Aside: Another publication (this one by Vandereli Martins et al) shows what PM2.5 light absorption looks like (for 250 to 2500 nm light):