Idle Speculation About Acid Deposition & Mercury

Walt, who lives in South Carolina, asked some air chemistry questions about California, and got me going....

I note from acid rain data that California doesn't seem to have any acid rain. Why is that? Aren't there any power plants in CA?

Also: Why is mercury deposition by rainfall concentrated in south Florida and the gulf coast? Is it from TX/LA oil refineries, or does it come from tropical ocean air?

These are really-interesting, thought-provoking, isopleth maps that Walt pointed out! I answered:

These inorganic ions do not correlate precisely with acid rain: rain acidity depends on anions like carbonate as well. In general, the West doesn't get much acid rain because of the buffering presence of alkaine dust in the rain. There is much less of that dust in the Northeast, and so acid rain is more of a problem there.

The sulfur comes mostly from coal combustion (which has a lot of sulfur - much more than most petroleum), and so sulfate deposition closely-tracks the power-plant region of the Ohio Valley (particularly around Wheeling, WV), but also noticeable around Price, Utah, a coal-mining area in the Green River Valley. The mystery, in my mind, is why there isn't more deposition in the Tennessee River Valley, where there are also large power plants. Is the coal cleaner there? Or does the sulfur come down far enough downwind to obscure the source location? But then why does Price, Utah stand out (unless the source is actually from farther south, like the Navajo Power Plant?)

Nitrate deposition is more erratic, and depends, in part, on whether an area is ammonia-rich, or not. Ammonia emissions closely track soil fecundity and seasonality. There are two, common, depositable forms of nitrate: nitric acid and ammonium nitrate. Nitric acid can deposit directly to surfaces as a gas, or dissolve directly into rain, and deposit that way. Ammonium nitrate is very volatile, and small changes in temperature and humidity can strongly affect how much of it is present. Deposition of ammonium nitrate occurs as particles.

Ammonia-rich means nothing more than total ammonia (TA) molar concentration is more than twice the total sulfur (TS) molar concentration: [TA] > 2*[TS] (ref.: Seinfeld & Pandis, p.538). Under ammonia-rich conditions, rather large amounts of nitrates are sometimes formed, particularly if its humid and sunny. A runaway heterogeneous reaction occurs whereby ammonia dissolves into aerosol water, more than neutralizing the sulfate present, raising the pH and drawing nitric acid in. At times, you see huge spikes in ammonium nitrate concentrations, expressed as Extremely Hazy Days and Nights (the kind of nights that disoriented novice pilot JFK, Jr. and caused him to crash near Martha's Vineyard), but it's a finicky process, and easily derailed. The process is also affected by the presence of sea-salt aerosol. Ammonium nitrate (particulate) concentrations are low to non-existent under ammonia-poor conditions.

So, nitrate and ammonium deposition tends to track:

• humid areas
  
• fertile areas
  
• urban areas, where there are more nitrogen emissions
  

The West sees less deposition of nitrates as a whole than the East, but there are exceptions, like the Central Valley, where there's lots of ammonia, lots of nitrogen emissions, and where it's humid and fecund enough in the winter.

I haven't followed mercury issues. My understanding is that there is a significant marine source of that volatile metal, but I also understand that people are puzzled why Florida and the Gulf Coast get so much deposition. The area is not downwind of a lot of the coal combustion, and so it's a mystery. But there's a reason, I'm sure.

Walt then asked:

OK. Referring to the sulfate isopleths, I notice that California has very little. Being a populous state, I would expect coal plants there to put sulfate into the air; but I don't see that. Whats up?

Nitrate: Is this mostly from combustion or from agriculture?

Ammonium: Is this released by plants, or is it particulate fertilizer?

Haze: Is ammonium nitrate a major cause of haze?

I answered:

Actually, there is very little coal combustion in California (very little coal in California at all). The comparatively small, but numerous power plants here are powered mostly by natural gas, most of which is derived from either Alberta, or Texas (with some exceptions, like dual gas/oil in at least one of San Diego's power plants). In a broad sense, no coal combustion means little sulfur in the air (with the notable exception of petroleum extraction and refining in the SF Bay, LA area, Kern Co., and Santa Barbara Co.)

It's not an iron-clad rule, though. There are some exceptions, of course: they burn nasty sulfur-rich petroleum-based fuels from Indonesia and elsewhere in Hawaii and the Pacific Islands (mostly out of necessity, given the remote locations). The air isn't that bad, though, because the islands are compact.

The nitrate derives principally from transportation emissions, but with an agricultural component. Oxidized nitrogen can deposit anytime as nitric acid, but it can sometimes, erratically, deposit as ammonium nitrate. Ammonia can deposit anytime as ammonia (but only sometimes as ammonium nitrate).

The ammonia can come from either plants, or from fertilizer, or both. It would be interesting to have a tracer study and see which dominates in a place like the Central Valley (my guess would be fertilizer, but I could be wrong).

Haze is hydrated aerosol deriving from numerous sources: elemental carbon, organic carbon, sea-salt aerosol, sulfates and nitrates. Sometimes you see huge spikes in particulate concentrations, however. In California, these spikes occur in the winter, when it's humid, and can prevent you from seeing even short distances. These haze spikes are principally, sometimes almost exclusively, ammonium nitrate.

Two more points occur to me....

Deposition is dependent on available surface area, particularly inviting leaf surface area, with its numerous, intricate stomata. Because the East is lusher and has a lot more leaf surface area than the West, all else being equal, it will receive higher deposition.

Also, these wintertime haze spikes of which I speak are different than California's famous summertime smog. Smog aerosol is generally ammonia-poor and tends to be more acidic than ammonium-nitrate aerosol. Smog aerosol contains a lot of nasty oxidants that accumulate due to rapid, solar-powered reactions of nitrogen oxide and hydrocarbon precursors.

Plus, there's plain, old wildfire smoke.

So, no matter what the season, California can generate a visibility-robbing aerosol to soot (sorry, suit).

Then, I started looking harder at the mercury question. It's actually pretty interesting, and has contradictory aspects that make me think the chemists don't have a strong handle on it yet.

The deposition map for mercury tends to mirror the U.S. annual rainfall map. That's interesting: unlike mercury, inorganic ions get rained out, so the deposition isn't proportional. Tripling rainfall will increase inorganic ion wet deposition by a lesser factor, say 1.5, because the cleansing effect of the rain strips out the available ions. In a single rainstorm, the last bit of rainfall is much cleaner than the first bit of rainfall.

With mercury, that seems to be less true - perhaps not true at all. Perhaps there is a larger reservoir of mercury in the air, so that no matter how much it rains, there is still much more mercury around. Now, I wonder about the chemistry of mercury! Are there multiple species??

Checking further, the reservoir species is probably elemental mercury (~ 0.5 ppb), and the removable species is either mercuric oxide or mercuric chloride, or both. Oxidation seems to be rapid when water is around (like in a rainstorm - otherwise the depositable mercury would rain out), but the apparent long residence time of mercury in the atmosphere (several months) suggests either the natural or anthropogenic source emissions for mercury have been underestimated (meaning the residence time is actually shorter), or oxidation just about ceases in the absence of water. In any event, the information looks kind of contradictory to me. Strange stuff!

Anyway, like Mark Twain says, this is the value of science: investing a little knowledge yields a huge dividend of idle speculation!