Covid-19 Lockdowns Reveal That Automobiles Are a Major Source of Atmospheric Ammonia Pollution.

[NNadir]

I came across this paper today: COVID-19 Lockdowns Afford the First Satellite-Based Confirmation That Vehicles Are an Under-recognized Source of Urban NH3 Pollution in Los Angeles Hansen Cao, Daven K. Henze, Karen Cady-Pereira, Brian C. McDonald, Colin Harkins, Kang Sun, Kevin W. Bowman, Tzung-May Fu, and Muhammad O. Nawaz Environmental Science & Technology Letters 2022 9 (1), 3-9.

In my long term advocacy of nuclear energy as being the only sustainable form of energy compatible with human development goals and environmental justice, I have often been "challenged" by my critics with complaints that nuclear energy can't power cars. Some of the weakest complainants in this respect think they are being witty and make remarks, replete with the sarcasm emoji - emojis are very popular means of expressing laziness, and often, a complete lack of depth - about putative "nuclear powered cars."

I hear less of this in recent times than I used to hear, possibly because of the popular enthusiasm for the unsustainable and morally dubious "fix" for the car CULTure's environmental impact represented by the band-aid theory of electric cars.

Whatever.

I am not concerned with sustaining the car CULTure so much as I am concerned as sustaining the world environment.

I happen to believe, to the extent that self propelled vehicles are required for some limited purposes - far below the requirements of the suburban nightmare in which I personally hypocritically live - that the internal combustion engine under the right conditions (and certainly not with petroleum based fuels) may be environmentally superior to electric cars. Often in energy discussions, particularly about so called "renewable energy" the conditional word could appears with unwarranted frequency, mostly to encourage wishful thinking. A rhetorically better description of the best of such schemes, including those I promote, is feasible. Much of what is feasible is unlikely, often for cultural reasons that have nothing to do with technological considerations.

However no system of substitutions in energy technology is without risk; the task is not to eliminate risk but to reduce it.

The paper listed above suggests a profound limitation on internal combustion engines by revealing something that has previously been difficult to discern, at least until the Covid lockdowns, the effect of the automobile and diesel engines on producing ammonia pollutants.

From the text of the paper:

Atmospheric ammonia contributes to ambient air pollution primarily through formation of inorganic components (ammonium sulfate and ammonium nitrate) of fine particulate matter (PM2.5). While the largest sources of NH3 are agricultural at national to global scales, (1) NH3 is also emitted from vehicles. In light- and medium-weight vehicles using catalytic converters, the air-rich fuel ratios that are optimal for reducing emissions of NOx produce NH3 emissions. (2−4) More recently, diesel engines of heavy duty vehicles use urea for selective catalytic reduction (SCR), leading to NH3 emissions. (5) Emissions of NH3 in urban areas are of concern because of the efficient formation of ammonium nitrate in NOx-rich environments and the potential for exposing large populations to PM2.5; for example, in the United States, despite vehicle emissions being more than an order of magnitude smaller than agricultural emissions, they are estimated to lead to similar numbers of premature deaths (∼15,000) per year. (6,7)

There is, however, considerable discussion around the magnitude of vehicle NH3 emissions in the United States and internationally. Early studies identified emissions from light and heavy duty vehicles equipped with catalytic converters as a missing source in inventories (3,8,9) and potentially a dominant source of NH3 in urban environments worldwide. (10−13) Other work has found more limited evidence of vehicle NH3 emissions in urban environments. (14,15) However, many of these studies predate the adoption of SCR systems by heavy duty vehicles that has led to increasing NH3 emissions. (16) More recent research suggests U.S. vehicular NH3 emissions are actually twice as high as national inventories. (17−21) Evidence for vehicle NH3 emissions has come from field measurements near roadways or in tunnels using isotope signatures (21−23) or correlations of NH3 with combustion tracers such as CO, CO2, and NOx, (19,24−26) from laboratory studies using chassis dynamometers, (4,27) and from open-path mobile measurements (28,29) of in-use vehicle operations over a range of conditions. (19)

A challenge with characterizing vehicle emissions over an entire metropolitan area is reconciling emission factors that vary on the basis of vehicle age, road grade, temperature, and operating conditions. Inverse modeling approaches based on ambient concentration are appealing in this regard. Top-down estimates of vehicle emissions of NH3 throughout western Los Angeles (LA) were first made using NH3 and CO measurements from aircraft. (30) While remote-sensing instruments have been used to identify NH3 emissions from agriculture, (31,32) industry and fertilizer production, (33) biomass burning, (34) and other natural sources, (35) there has not yet been a satellite-based measurement of NH3 specifically linked to transportation emissions...

So the authors utilized the reduced traffic associated with Covid lockdowns to get a handle on automotive emissions of ammonia.

The following graphic shows what they saw:



The caption:

Figure 1. CrIS NH3 column concentrations (a) during March 1–15 and (b) during March 16–31, as well as (c) their difference (difference = (b) – (a)). (d–f) Same as panels (a)–(c), respectively, but for TROPOMI NO2 column concentrations (qa > 0.75). (g–i) Same as panels (a)–(c), respectively, but for GC-simulated NH3 column concentrations from Run2 (see Table S1 and Text S2). (j–l) Same as panels (a)–(c), respectively, but for GEOS-Chem-simulated NO2 column concentrations from Run2 (see Table S1 and Text S2). R is the spatial correlation coefficient between CrIS NH3 and TROPOMI NO2 columns within the red box in each period; p is the corresponding significance level. The red box defines the western LA domain (33.80–34.20°N, 118.50–118.05°W) on which we focus in this study. The red numbers in panels (c), (f), (i), and (l) are percentage decreases of NH3 and NO2 columns within western LA between these two periods observed by CrIS and TROPOMI and simulated by GEOS-Chem, respectively. The upward (△ ) and downward (▽ ) triangles indicate the locations of the downtown LA site and Riverside Municipal ARPT site, respectively, in Figure S2.

They conclude:

Globally, ∼20% of the burden of disease associated with PM2.5 exposure is estimated to stem from NH3 emissions from agriculture. (50) The socio-economic benefits of controlling agricultural NH3 have been highlighted in studies comparing the cost of mitigating agricultural emissions to the air quality benefits. (51,52) Given the apparent under-recognized magnitude of transportation NH3 emissions coupled with the potential of this source for contributing to PM2.5 exposure in populated areas, re-evaluating the health impacts of transportation emissions is warranted. If these emissions have been severely underestimated by a factor of 1.8–4.9, as the findings of this work support, then the health impacts of vehicle NH3 emissions in the United States that have been estimated at ∼15,000 premature deaths annually (6,7) may be commensurately underestimated. Meanwhile, the U.S. EPA’s NH3 monitoring network (AMoN) is designed to measure NH3 from agricultural sources and does not include any urban sites. (53) While previous studies based on in situ measurements have pointed out that vehicle emissions are an underestimated source of NH3 in urban areas, it is laborious to use such approaches to monitor vehicle NH3 emissions with extensive spatiotemporal coverage. Therefore, methods for tracking this urban source using remote sensing observations are valuable and essential....

In a sense, this is unsurprising, because the Haber-Bosch process for ammonia manufacture on which the world's food supply now depends is a high pressure high heat process, close to the conditions obtained in an automotive spark or diesel engine. The SCR process for reducing NOx pollution in diesels utilizes urea, so this is also undoubtedly a contributor, as the authors note.

In any case, this is a revealing paper.

Have as nice a Sunday as one can have with a brutal war so prominent in the news.