Sell me on reducing NOx.

Discussion in 'Emissions' started by RobertSmalls, Jul 30, 2010.

  1. RobertSmalls

    RobertSmalls Ecodriver

    As I understand it, there are perhaps four major pollutants coming from our collective tailpipes.

    CO2, a greenhouse gas that I try hard to emit less of.
    There's CO which is lethal in large concentrations, but I figure it's harmless in small doses.
    HC, which is toxic, but eventually degrades in the ecosystem. I emit a little of on a cold start. But my HC emissions should be very small otherwise, because I keep my PCV system in good condition, and I don't fill my EVAP canister with liquid fuel. I chose an electric lawnmower partially so as not to have to deal with HC emissions there.

    I've made up my mind on those three pollutants. I'm not so sure about NOx. It's a major contributor to smog and acid rain. But Buffalo, NY, where I live, very seldom sees smog. Same with the thousand miles of small towns, farms, and forests downwind of me before my pollution passes over the Atlantic. Acid rain? I'm not worried. The soil downwind of me will survive a pH adjustment, even if it means we get different kinds of trees in some areas.

    The reason I bother to ask is, we all make decisions on NOx versus CO2. Running an engine at heavy throttle, low RPMs creates more NOx and less CO2. So does lean burn. New vehicles can only achieve better EPA bins at the expense of FE and CO2.

    So, given my geographic location, is the SULEV CVT Insight a greener car than the LEV Insight with lean burn? What's the range and timescale of damage done by NOx? Are there any other reasons I should reconsider my prioritizing CO2 over NOx?
  2. Chuck

    Chuck just the messenger

    My understanding (?) is NO2 warms the atmosphere more than CO2, but it dissipates quicker.
  3. RobertSmalls

    RobertSmalls Ecodriver

    Thanks, Chuck. Hmm... N2O has a GWP 310 times that of CO2 on a per kg basis, but you emit FAR less of it.

    A 70mpg LEV emits 127g/mi CO2 and less than 0.05g NOx/mi.
    A 60mpg SULEV emits 148g/mi CO2 and less than 0.02g NOx/mi.

    Assuming that 100% of the NOx is N2O, we're looking at the equivalent of 142g/mi for the LEV lean burn car versus 154g/mi for the SULEV.

    Okay, from a GWP perspective, I made the right choice, but SULEV isn't the worst thing ever invented - the numbers are closer than the mpg alone would lead you to believe. Plus, the reduction in smog is worth something, albeit not much to me.

    Speaking of GHG, each 12-oz can of R134a you leak out of your car's A/C has the GWP of 50 gallons of gasoline. I'm happy to find that my car has no A/C. I find that that stuff does inevitably leak out.
  4. Yaris Hilton

    Yaris Hilton Half a Bubble Off Plumb

    N2O isn't a major product of gasoline-air combustion. NO is mostly what's produced. It gets photo-oxidized in air by sunlight to NO2, the brown stuff in smog. Catalytic converters, however, produce significant amounts of N2O in the exhaust. NO is an important signalling compound in many biological processes. We really don't know what the long term effects of breathing small amounts of it are. NO2 reacts with water to produce nitric acid, and is an important component of acid rain. Nitrates are a macronutrient for plants, but the acid falling in rain has seriously leached the calcium and magnesium out of the topsoil in large areas of the world.
  5. wxman

    wxman Well-Known Member

    NOx and vapor-phase hydrocarbons (e.g., NMHC, VOC) actually work synergistically to produce ground-level ozone (i.e., "smog"). In simplest terms, NOx creates the ozone, but hydrocarbons allow it to accumulate in the atmosphere. In effect, the concentration of hydrocarbons in the atmosphere determines the "ozone production efficiency" of NOx (actually, NO2, NOx = NO + NO2; NO2 splits into NO + O in the presence of sunlight; the "O" combines with molecular oxygen (O2) in the atmosphere to produce ozone (O3)). NOx in and of itself will NOT produce net ozone since the NO that's produce in the photolysis of NO2 is available to immediately react with ozone in the atmosphere to regenerate NO2 and O2, and the cycle starts all over again (known as the NOx photolytic cycle). Hydrocarbons interfere with this cycle thus allowing O3 to accumulate.

    There is actually significant controversy about a regulatory approach of mandating greater reductions in NOx than NMHC (which is what the current Tier 2 emission regs do). A phenomenon known as the "weekend ozone effect" has shown empirically that relatively greater reductions in ambient levels of NOx than ambient levels of NMHC can actually INCREASE ambient ozone levels in some locations (metropolitan areas mostly). The weekend effect was initially noticed decades ago in Southern California where, on average, ozone levels increased significantly on weekends when ambient NOx levels were greatly reduced compared to week days.

    Based on these studies, it appears the regulators should re-emphasize NMHC/VOC controls more than NOx controls since the benefit of the current regulatory approach is questionable, and there possibly may even be a disbenefit in some locations.

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