Emissions - Ozone Precursors

"…During the [RDE] tests, the Mercedes C-Class C220d actually emitted no NOx at all during the test while the BMW 5 Series 520d Touring emitted just 1mg per kilometre…."

Source: https://www.express.co.uk/life-style/cars/1090903/diesel-car-engine-UK-fuel-NOx-emissions?fbclid=IwAR3ofgxpzHfRF35oBc-LxczKbpW79bN9R0jciuRvwDl1s2abmkd4uJhcA0Q

"...the average NOx recording for Euro 6d TEMP-standard diesel cars, over the course of a variety of tests including lab conditions, the real world and cold-weather, was 20mg/km...."

Source: https://www.motoringresearch.com/car-news/diesel-cars-zero-nox/?fbclid=IwAR02cKSdUNzxuq1PUCTrMjZufpKhc0MHtXfDvv0GsCI7tElJNMiu6JoT5lI

"...In the test, the cars not only significantly below the tolerance value of 168 mg / km for diesel cars, which is applied in the road test (RDE method), but also below the current Euro 6 WLTC limit of 80 mg / km...." (translated)

Source: https://presse.adac.de/meldungen/adac-ev/technik/neue-diesel-pkw-sauberer-als-vorgeschrieben.html

"…The ACEA tests found that all the Euro 6d-Temp type-approved cars performed well below the NOx threshold of the real driving emissions (RDE) test…"

Source: https://fleetworld.co.uk/new-diesels-can-help-cities-meet-air-quality-targets/?fbclid=IwAR10WGIXfnUFClSJsLAbY4_f_qCXios4NOOhD5KH3XrA0dI6RpUo0uwmgJ0

"...Let us assume that there is no appeal and no new legislation, meaning vehicles must meet 80 mg/km on the RDE test. Taking a sample of 30 RDE-certified diesel cars tested by Emissions Analytics on its independent EQUA Index test, we conclude that up to 90% of the vehicles would still be compliant…."

Source: https://mailchi.mp/emissionsanalytics/european-court-annuls-real-driving-emissions-limits-the-potential-consequences

"...When looking at Nitrogen Oxide (NOx) specifically, Emissions Analytics found average emissions were 48mg/km, meaning the cleanest diesels are getting close to the average NOx emissions from new petrol vehicles, which is 36mg/km...."

Source: https://www.fleetnews.co.uk/news/environment/2018/06/22/new-diesels-found-to-be-71-cleaner-than-petrol-models

"...Due to significantly lower HC emissions from diesel cars, the ozone forming potential...was generally significantly lower for this category of vehicle compared to petrol vehicles. ...Comparing the ozone forming potential averaged for the new petrol cars with the average for the new diesel cars, yields a difference of more than one order of magnitude...

...Note that the evaporative emissions from the vehicles were not included in the evaluation of the ozone forming potential, neither was this done for evaporative losses from fuel distribution and refuelling. As these emissions were higher for petrol cars, the ozone formation for these cars is underestimated....

...The ozone forming potential was considerably lower for diesel cars, i.e., roughly one order of magnitude lower than for petrol cars. This is mainly due to the low level of HC emissions for diesel cars compared to petrol cars. Should the evaporative emissions have been taken into account, the relative differences would have been even greater...."

Source: Environmental and Health Impact From Modern Cars, May 2002 and August 2002 (DEER Conference); Ecotraffic (Peter Ahlvic)

"...The diesel is distinguished by its low emissions of carbon monoxide and hydrocarbons. In addition, unlike gasoline engines, diesel engines do not emit evaporative volatile organic compounds (VOCs), which contribute to the production of ozone in the atmosphere. The advantage of the diesel also includes very low cold-start emissions.

In Europe and elsewhere, the diesel engine is relied upon to meet greenhouse gas emission reduction commitments and, particularly, to reduce carbon dioxide emissions. A comparison of vehicles equipped with a three-way catalyst gasoline engine versus a diesel engine meeting the same performance standards evidenced overall reduced greenhouse gas emissions of 45 percent for the diesel, with even better performance under real driving conditions. Diesel engines also can run on new bio-diesel fuels, making further greenhouse gas reductions possible.

Environmental benefits of the diesel also include lower emissions of greenhouse gases in the fuel production and distribution process, as well as smaller refueling and evaporation losses at the pump...."


"...There are other environmental advantages of diesels as well: near-zero evaporative hydrocarbon emissions due to the extremely low vapor pressure of diesel fuel, low cold-start emissions, and low in-use emissions deterioration...."

Source: http://www.epa.gov/otaq/reports/adv-tech/420r04002.pdf

"...As tailpipe emissions are reduced due to improved exhaust emission control systems, the hydrocarbons produced by evaporation of the gasoline during distribution, vehicle refueling, and from the vehicle, become more and more significant. A recent European study found that 40% of man-made volatile organic compounds came from vehicles..."

Source: http://www.faqs.org/faqs/autos/gasoline-faq/part2/

"...Though the sites are well removed from large urban source regions, the observations show a clear anthropogenic influence on the hydrocarbon levels at these rural sites...."

Source: http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JOEEDU000129000012001085000001&idtype=cvips&gifs=yes

This brings up another factor that's typically not considered when evaluating how "green" a specific vehicle is. These evaporative emissions from gasoline vehicles are in addition to the tailpipe emissions of the FTP 75 testing:

The FTP 75 emissions do not include evaporative emissions:

"...During the FTP, tailpipe exhaust is collected in three bags: the so-called cold bag for the first 5 cycles of driving, the stabilized bag for the next 13, and the hot bag for the 5 repeat cycles following the hot soak. For regulatory purposes, the measured mass emissions from each bag are substituted in a prescribed equation to determine the emission rate per unit of travel (in this case, grams per mile) of each regulated emission.

Evaporative emissions, including those resulting from leaks of liquid fuel, are measured separately using a variable-temperature SHED (sealed-housing-for-evaporative-determination) facility; i.e., an instrumented temperature-controlled room in which the test vehicle is housed...."

Source: Ozone-Forming Potential of Reformulated Gasoline, Commission on Geosciences, Environment and Resources; excerpt available at http://www.nap.edu/books/0309064457/html/74.html

"...In cities, as much as 95 percent of all CO emissions may come from automobile exhaust..."

Source: http://www.epa.gov/air/airtrends/aqtrnd01/summary.pdf

"...The primary source of CO from vehicles is the incomplete combustion of gasoline in engine cylinders. ...CO emissions by diesel vehicles are minimal, primarily because of the excess air used in the diesel combustion cycle...."

Source: The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska (2002), Board on Environmental Studies and Toxicology (BEST), Board on Atmospheric Sciences and Climate (BASC), Transportation Research Board (TRB), excerpt available at http://www.nap.edu/books/0309084849/html/19.html

"...gasoline vehicles emit the large majority of CO..."

Source: EPA Hazard Assessment Document for Diesel Engine Exhaust (Page 43), available at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=29060

"...Compared to spark-ignition engines, diesel engines typically produce very low NMOG emissions..."

Source: EPA, "Draft Regulatory Impact Analysis: Tier 3 Motor Vehicle Emission and Fuel Standards." http://www.epa.gov/otaq/documents/tier3/420d13002.pdf (Section 1.4.2, Page 1-29 (49 of 532))

"...Gasoline automobiles are by far the largest contributors to the carbon monoxide and hydrocarbon inventories...

...Jon Van Gerpen, the agricultural and biological sciences department head at the University of Idaho, said untreated diesel vehicles are in the same category-they release comparable emissions-as gas-powered cars with fully functioning catalytic converters. Van Gerpen, an expert in the mechanics of compression ignition engines and their emissions, said that by putting an oxidation catalyst on a diesel engine, CO and HC emissions actually fall below measurement thresholds...."

Source: http://www.biodieselmagazine.com/article.jsp?article_id=286&q=&page=all

"...The results suggest that the emissions of gasoline-powered vehicles still dominate the hydrocarbon distribution in northern mid-latitude urban areas..."

Source: Borbon, et al. (2013), "Emission ratios of anthropogenic VOC in northern mid-latitude megacities: observations vs. emission inventories in Los Angeles and Paris." Journal of Geophysical Research: Atmospheres, Volume 118, Issue 4. Pages 1585-2083, http://www.greencarcongress.com/2013/04/vocs-20130419.html

"...Diurnal ambient VOC data also suggest that LDGV [light-duty gasoline vehicle] tailpipe emissions remained the dominant source of ambient VOCs in the SoCAB in 2009...."

Source: Pang et al., "Trends in selected ambient volatile organic compound (VOC) concentrations and a comparison to mobile source emission trends in California's South Coast Air Basin." Atmospheric Environment, Volume 122, December 2015, Pages 686-695, http://www.sciencedirect.com/science/article/pii/S1352231015304350

"...However, recent OHVT-sponsored research has shown that ozone is present in higher concentrations during weekends than on weekdays in California and other urban U.S. locations, even though ozone precursors are present in lower concentrations on weekends. This is especially true of heavy-duty vehicles - they contribute about 50% less of ozone precursor emissions on weekends than they do on weekdays. The data are beginning to suggest that in urban locations, NMOCs are more important than NOx in determining ozone concentrations. These findings, if demonstrated to be correct, would suggest that reducing NOx emission reductions actually increase ozone concentrations in urban locations...."

Source: U.S. Department of Energy, "Plain Talk on Trucks", September 2001

"...NOx controls in Southern California (and other urban U.S. locations) are counterproductive for reducing ambient ozone, and they actually increase ambient ozone levels. Were it not for large concurrent HC emission reductions on weekends, weekend ozone would be even higher than it is, and the weekend/ weekday ozone difference would be even larger...."

Source: http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2002/session5/2002_deer_lawson.pdf

"...Gasoline exhaust and gasoline vapor account for ~80 percent of ambient NMHC in on-road samples and at regional air monitoring locations suggesting that gasoline emissions are responsible for the majority of ozone found in the SoCAB....

...an ozone disbenefit will result if NOx emissions are decreased at current levels of VOC until ambient NOx levels are decreased by roughly 90 percent to about 10-12 ppb...."

Source: Eric Fujita, et al, "Weekend/Weekday Ozone Study in the South Coast Air Basin." http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2002/session5/2002_deer_fujita.pdf

"...modeling suggests that peak O3 will increase unless NOx mixing ratios are decreased by approximately 90% from current weekend levels...." ["IMPLICATIONS" on page 1]

Source: Fujita, E.M.; Stockwell, W.R.; Campbell, D.E.; Keislar, R.E.; Lawson, D.R., "Evolution of the Magnitude and Spatial Extent of the Weekend Ozone Effect in California's South Coast Air Basin, 1981-2000." J. Air & Waste Manage. Assoc. 2003, 53 (7), 802-815, http://www.tandfonline.com/doi/abs/10.1080/10473289.2003.10466225#.VQR4FI7F9ro

"...Lawson is openly sharing data that shows NOx emissions in the L.A. Basin on Sundays are approximately 50 percent less relative to the emissions of NOx on Wednesdays. Conversely, ozone formation in the L.A. Basin on Sundays is approximately 50 percent higher compared to Wednesdays...

...urban areas all across the country experience NOx reductions of 50 percent on Sundays relative to Wednesdays..."

Source: http://www.biodieselmagazine.com/article.jsp?article_id=851&q=&page=all

"...Some experts - most notably Lawson and Eric Fujita of the Desert Research Institute in Nevada, both former California air pollution officials - believe that regulators could keep ozone in check better by slowing the pace of nitrogen oxide reductions while doing more to cut volatile organic compounds...."

Source: hhttp://articles.latimes.com/2004/may/24/local/me-smog24

"... Since the percent derived here is 35 percent, this implies that a UAM derived MIR factor for CO would then be 1.75 (i.e., 35/20) times 0.07 or 0.12 grams ozone per gram CO...."

Source: CARB (ICF)

"...CARB focuses on reducing NOx. But atmospheric scientist Gary Whitten of ICF notes that if the tradeoff of reducing NOx is to increase hydrocarbon [HC] and carbon monoxide [CO] emissions [which is exactly what is happening with regulations which favor gasoline engine emission profiles and disfavor diesel emission profiles, thus, intentionally or not, encourage the gasoline vehicle market while discouraging the diesel vehicle market], the environment would be poorly served. The reason, according to Whitten, is that a reduction in hydrocarbon and carbon monoxide emissions has a much greater beneficial impact on ozone formation than an equivalent reduction in NOx. Whitten concludes, "The effectiveness of THC for reducing ozone in these simulations must be as much as 8 times better than NOx reductions on an equal percentage of the mobile emissions basis."..."

Source: The Other Gasoline Crisis: Speeding Up the Shift From MTBE to Ethanol, David Morris and Jack Brondum, Institute for Local Self-Reliance, September 2000; http://www.ilsr.org/columns/2000/0900.html

"...That has the EPA focusing on cutting nitrogen oxides, a method that scientists say reduces smog slowly and inefficiently...."

Source: http://www.usatoday.com/weather/news/2003-10-15-smog-usat_x.htm

"...studies into the "weekend ozone effect" in California indicate that NOx reductions trigger increased ozone levels. Thus, future diesel NOx emission standards may be in fact counterproductive-meaning less clarity for clean air policy makers...."

Source: Dieselnet, "The Log", January 14, 2004; http://www.dieselnet.com/

"...Moreover, CO is a recognized precursor to urban ozone (or smog) that appears to be growing in importance relative to VOC. For example, the newest California LEV II regulations allow CO-to-VOC emissions ratios of up to 100-to-1 even though the ozone-forming potential of CO is only about 50-to-1 relative to exhaust VOC. As a consequence of such emissions ratios, the carbon monoxide from such vehicles can generate more urban ozone than their exhaust VOC emissions...."

Source: AIR QUALITY IMPACTS OF ETHANOL IN CALIFORNIA GASOLINE, By Dr. Gary Whitten, Systems Applications International, Inc.

"...emissions inventories of CO consistently show that the mobile contribution, especially in troublesome urban areas, is often as high as 90 percent....

...Carbon monoxide is a major ozone precursor. New studies...have shown that CO can be equivalent to 25 to 50 percent of the mobile-related contribution from volatile organic compounds (VOC)...."

Source: http://egov.oregon.gov/ENERGY/RENEW/Biomass/docs/FORUM/Whitten2004.pdf

"...Simplified chemistry:

UV radiation (hv), acting on atmospheric NO2 creates ozone (O3):

NO2 + hv ---> NO + O

O + O2 ---> O3

But ozone is also destroyed (titrated) if NO is still present:

O3 + NO ---> NO2 + O2

Why does O3 build up? If volatile organic compounds (VOCs) are also present, NO is consumed by other reactions and is not available to titrate the O3:

VOC + NO ---> NO2 + other products ..."

Source: http://www.nws.noaa.gov/ost/air_quality/Davidson.pdf

"...reducing NOx emissions causes the "ozone production efficiency" (OPE) of NOx to increase, offsetting the decrease in NOx emissions...

...OPE lower with higher NOx emissions..."

Source: from a presentation by Dr. James F. Meagher, NOAA's Air Quality Program Manager, at the "East Tennessee Ozone Study 2006 Science Workshop", May 2006, Ozone Formation - Is All NOx Created Equal?

"...The atomic oxygen produced by the photolysis of NO2 is very reactive and readily combines with O2 in the air to form O3. In the presence of NO, however, the O3 will immediately decompose, regenerating the nitrogen dioxide. This nitrogen dioxide photolytic cycle is summarized in the following three reactions.

NO2 + hv --> NO + O

O + O2 --> O3

O3 + NO --> NO2 + O2

Hence, while the presence of NO2 is required to form O3, the nitrogen dioxide photolytic cycle by itself does not generate net ozone, and cannot explain ozone accumulation....

...ROG [VOC] reductions (with constant NOx) always lead to a slowing of the ozone production process and lower peak ozone concentrations. NOx reductions (with constant ROG) can lead to a speeding up of the ozone process, and can increase or decrease peak ozone values depending on the ROG-to-NOx ratio.

Thus, whereas ROG control is never detrimental, NOx control can be detrimental, particularly in the central cores of urban areas...."

Source: Cooper, D. C.; Alley, F. C.,"Air Pollution Control, A Design Approach." Third Edition, pages 595 and 600

"...Our analyses of weekday/weekend differences in ozone precursor emissions show that different emission reductions of ozone precursors than normally take place each weekend will be required before significant reductions in ambient ozone can be achieved in the Atlanta area. The principal limitation of our analysis is that current weekend emission reductions are not identical to those that might occur in the future as a result of emission control programs. Regional-scale changes in ozone levels potentially could result from combined urban and nonurban reductions of ozone precursor emissions. However, it is sobering that Atlanta sites exhibited so much difference between the weekend reductions of ozone precursors compared with much smaller changes in ozone levels. Any expectation of future declines of regional ozone concentrations due to precursor reductions in the metropolitan area should be further investigated...."


"...NOx concentration reductions on Sundays exceeded the Sunday reductions of CO and hydrocarbon concentrations in all cities that were studied... (Page 101)

...all areas exhibited ozone concentration increases for data averaged over the period 10 am through 5 pm, with median Sunday ozone levels ranging from 100 to 115 percent of the Wednesday concentrations... (Page 104)

...none of the metropolitan study areas exhibited significant reductions of weekend ozone levels, nor were significant ozone decreases observed downwind of the study areas on weekends...." (Page 118)


"...Much lower emissions of VOC, CO, and NOx on weekends, with larger NOx reductions than VOC and CO reductions

- Up to 80% fewer trucks and buses and ~15% less light-duty traffic on roads in urban areas on weekends...

...Projected emission reductions from 2005- 2010 similar to today's weekend reductions of ozone precursor concentrations (future NOx emission reductions > VOC emission reductions)...

...In urban areas, NOx reductions increase ozone production; VOC (and CO) reductions decrease ozone production. For ozone production, these emission reductions offset each other....

...Downwind ozone levels do not appear to be sensitive to changes in NOx emissions (downwind ozone has traditionally been thought to be sensitive to changes in NOx emissions)..."

"...PM nitrate is reduced less than 3%...

...NOx reductions in urban areas currently do not reduce, and usually increase, ambient ozone...

...One state reviewer's comment: 'The over-arching conclusion that lower weekend concentrations of ozone precursors do not lead to lower ozone concentrations is impossible to dispute and has far reaching regulatory implications.'..."

Source: Weekday and Weekend Air Pollutant Levels in Ozone Problem Areas in the United States, Dr. Douglas R. Lawson, et al, NREL, August 23, 2005, DEER 2005; http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2005/session2/2005_deer_lawson.pdf

"...CARB especially has been vigorously resisting the conclusions of independent researchers and offering other hypotheses to explain the weekend effect, including a change in the timing of emissions on weekends or carryover of pollution from increased driving on Friday and Saturday evenings.[10] Most of these explanations are not persuasive, or they merely obfuscate the debate. In fact, CARB's views have failed to pass the rigors of scientific peer review. The July 2003 issue of the Journal of the Air & Waste Management Association (JAWMA) devoted a special section to studies of the weekend effect, several of which are cited here. The journal's reviewers rejected CARB's submission.

EPA has similarly resisted the implications of the weekend effect. The technical documentation for EPA's proposed off-road diesel rule, released last May, approvingly cites the CARB paper that was later rejected by JAWMA.[11] Therein lies another irony: when EPA in 1999 promulgated a rule requiring a 90-percent reduction in NOx emissions from automobiles, the agency's own analysis concluded that the rule would increase ozone in many areas of the country.[12]..."

Source: http://www.aei.org/article/energy-and-the-environment/emissions-down-smog-up-say-what

"...While this final rule will reduce ozone levels generally and provide significant ozone-related health benefits, this is not always the case at the local level. Due to the complex photochemistry of ozone production, NOx emissions lead to both the formation and destruction of ozone, depending on the relative quantities of NOx, VOC, and ozone catalysts such as the OH and HO2 radicals. In areas dominated by fresh emissions of NOx, ozone catalysts are removed via the production of nitric acid, which slows the ozone formation rate. Because NOx is generally depleted more rapidly than VOC, this effect is usually short-lived and the emitted NOx can lead to ozone formation later and further downwind. The terms "NOx disbenefits" or "ozone disbenefits" refer to the ozone increases that can result from NOx emission reductions in these localized areas....

...EPA's air quality modeling predicts NOx disbenefits in the areas identified by some studies as "VOC-limited" (e.g., Los Angeles)...." (Page 2-113)

Source: EPA, "Final Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel Engines." Available at http://www.noticeandcomment.com/Final-Regulatory-Analysis-Control-of-Emissions-from-Nonroad-Diesel-Engines-EPA420-R-04-007-%5BDCN-SE03157%5D-fn-43776.aspx

"...Under these conditions NOx reductions are highly effective in reducing ozone while VOC reductions have little effect. Such conditions are called "NOx-limited"....

...When NOx levels are relatively high and VOC levels are relatively low, NOx forms inorganic nitrates (i.e., particles) but relatively little ozone. Such conditions are called "VOC-limited." Under these conditions, VOC reductions are effective in reducing ozone, but NOx reductions can actually increase local ozone under certain circumstances...." (Page 2-41)

Source: EPA, "Regulatory Impact Analysis: Control of Emissions of Air Pollution from Locomotive Engines and Marine Compression Ignition Engines Less than 30 Liters Per Cylinder." http://www.epa.gov/nonroad/420d07001.pdf. See also "EPA Final Regulatory Impact Analysis: Control of Emissions of Air Pollution from Highway Heavy-Duty Engines.", "U.S. EPA Integrated Science Assessment for Oxides of Nitrogen - Health Criteria (First External Review Draft)." U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-07/093, "EPA Final Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel Engines.", Federal Register / Vol. 72, No. 63 / Tuesday, April 3, 2007, Federal Register / 12/17/2014 (Section E, "Ozone Air Quality")

(Note EPA acknowledges that there are no "VOC disbenefits" in "NOx-limited" areas, but there are "NOx disbenefits" in "VOC-limited" areas)

"...EPA Rule Is Making Ozone Smog Worse

...in Los Angeles NOx and VOC decline, respectively, about 20 percent and 15 percent on Saturdays relative to weekdays. Nevertheless, ozone rises about 20 percent. On Sundays, NOx and VOC decline even more--about 35 percent and 20 percent, respectively, relative to weekdays. Yet ozone levels climb higher still, to about 30 percent above weekday levels.

Los Angeles has one of the worst weekend effects in the nation, but the pattern is similar all over the country. In Atlanta, NOx and VOC decrease, respectively, 57 percent and 17 percent on Sundays relative to weekdays, but ozone levels don't change. In Cincinnati, NOx drops 40 percent on weekends, but with no change in ozone.

This is a problem because EPA and state regulators assume that reducing both VOC and NOx is necessary for attaining the federal eight-hour ozone standard, and they have built that assumption into NOx-reduction regulations that are costing Americans billions of dollars each year. weekend-effect research suggests reducing NOx is at best slowing the pace at which ozone declines, and is even making ozone worse in some cities....

...Most VOC comes from gasoline engines, mainly automobiles..."

Source: http://www.heartland.org/Article.cfm?artId=18973

The weekend effect has also been documented in Europe, even in suburban and rural locations:

"...Despite the known weekend decrease in emissions, average ozone exposures increased at weekends for rural, suburban and urban sites...."

Source: http://www.rsc.org/chemistryworld/Issues/2003/May/weekend.asp

"...The extremely important implication of this scientific finding (AN11) and findings from other studies is that because of the ozone-destruction effect of NOx under VOC-sensitive conditions and the widespread occurrence of such conditions (e.g., within intensely urbanized areas), air quality managers should view NOx control with caution...."

Source: Southern Oxidant Study (SOS), http://www.ncsu.edu/sos/iii.html

"...The findings from this study suggest that timely attainment of O3 standards will require greater emphasis on reducing VOC emissions in major metropolitan areas....

...From a regulatory perspective, the O3 weekend effect is a significant issue even if peak O3 does not change on weekends despite weekend reductions in O3 precursor mixing ratios....

...Ozone isopleth diagrams, whether generated from simple box models or from three-dimensional grid model simulations, have consistently shown that reductions of VOC emissions do not change peak O3 levels if O3 formation is NOx-limited, whereas reductions of NOx emissions typically increase peak O3 levels if O3 formation is VOC-limited (until a transition to a NOx-limited condition has been achieved, after which further reductions of NOx begin to lower peak O3 levels)....

...the transition from NOx-saturated to NOx-limited O3 formation was estimated to occur downwind of the suburban-rural boundary at a distance corresponding to a mean air-mass travel time of approximately 30 min....

...Differences between downwind and upwind peak O3 levels showed no statistically significant variation among days of the week in any of the four study areas, despite large weekend reductions in mean concentrations of O3 precursors....

...We interpret the observed absence of differences between weekday and weekend peak O3, in combination with significantly lower ambient levels of NOx, as an indication that O3 formation in the urban study areas is VOC-limited. Our analyses of weekday/weekend differences in O3precursors show that different emission reductions of O3 precursors than normally take place on weekends will be required to affect greater reductions in peak O3 in large metropolitan areas...."

Source: Blanchard, Tanenbaum, Lawson; "Differences between Weekday and Weekend Air Pollutant Levels in Atlanta; Baltimore; Chicago; Dallas-Fort Worth; Denver; Houston; New York; Phoenix; Washington, DC; and Surrounding Areas." Journal of the Air & Waste Management Association, Volume 58, December 2008, Pages 1598-1615, http://www.ncbi.nlm.nih.gov/pubmed/19189758 (abstract)

"...We recommend that the priorities for the control of VOC sources include motorcycles, gasoline vehicles, and solvent use because of their larger OFP [ozone formation potential] contributions...."

Source: Junyu Zheng, Min Shao, Wenwei Che, Lijun Zhang, Liuju Zhong, Yuanhang Zhang, and David Streets, "Speciated VOC Emission Inventory and Spatial Patterns of Ozone Formation Potential in the Pearl River Delta, China." Environ. Sci. Technol., Oct. 2009, 43 (22), pp 8580-8586, http://pubs.acs.org/doi/abs/10.1021/es901688e (abstract)

"...A dramatic decrease in primary pollutants emissions (83-98% in NO) causes an 8 ppbv increase in ozone at the urban core. Downwind (27 km), ozone decreases by only 5 ppbv. Nighttime O3 is shown to increase to 20 ppbv at the urban sites and 30 ppbv downwind...."

Source: Ilan Levy, "A national day with near zero emissions and its effect on primary and secondary pollutants." Atmospheric Environment, Volume 77, October 2013, Pages 202-212, http://www.sciencedirect.com/science/article/pii/S1352231013003488 (abstract)

"...in an urban environment such as London, high levels of NOx can suppress O3 production..."

Source: Bohnenstengel et al., "Meteorology, Air Quality, and Health in London." Bulletin of the American Meteorological Society, May 2015, Page 792

"...Ozone (O3) in Chicago did not decline during 2005-2013, despite reductions in NOx emissions.

Since 2009, O3 formation has become more sensitive to volatile organic carbons (VOCs)...."

Source: http://www.luc.edu/media/lucedu/sustainability-new/images420/climatechange/postersccc2015/Response%20of%20Ground%20Level%20Ozone%20to%20Climate%20Change.pdf

"...The excess NOx emissions result in reduced ozone concentrations in almost all countries impacted by the excess emissions..."

Source: Chossiere et al., "Public health impacts of excess NOx emissions from Volkswagen diesel passenger vehicles in Germany." Environ. Res. Lett. 12 (2017) 034014, http://iopscience.iop.org/article/10.1088/1748-9326/aa5987/meta;jsessionid=96AE8E6A30F223478A30A86AB75FDD8E.ip-10-40-1-105

"...In some countries (Belgium, Netherlands, United Kingdom) the extra NOx emissions from LDDVs have actually led to less ozone being formed. Thus, for these countries a reduction in NOx emissions from diesel cars, as simulated, would result in 70 to 90 more cases of ozone related premature deaths...."

Source: Jonson et al. "Impact of excess NOx emissions from diesel cars on air quality, public health and eutrophication in Europe." Environmental Research Letters, 2017, http://iopscience.iop.org/article/10.1088/1748-9326/aa8850/meta;jsessionid=8D525FADE41748F42E78538997B39BB3.ip-10-40-1-105

"...Our flux data clearly demonstrate that the weekend effect of ozone production found in Innsbruck [Austria] is largely a direct result of lower weekend NOx emissions...."

Source: Karl et al. "Urban eddy covariance measurements reveal significant missing NOx emissions in Central Europe." Scientific Reports 7, Article number: 2536 (2017), https://www.nature.com/articles/s41598-017-02699-9

Anyone still convinced that diesel NOx is the primary culprit in urban ozone (smog)?

Of course, as suggested in some references above, the weekend ozone effect shouldn't come as that much of a surprise, since the chemistry would predict that increases in ambient HC (VOC) concentrations relative to ambient NOx concentrations would result in increases in ambient O3 concentrations (ground-level ozone - GLO):

GLO production:

The ozone production pathway for CO would be:

CO + OH --> H + CO2 (oxidation by OH)

H + O2 + M --> HO2 + M (formation of a peroxy radical)

HO2 + NO --> OH + NO2 (rxn with NO to form NO2; regeneration of OH radical)

NO2 --> NO + O (photodissociation of NO2)

O + O2 + M --> O3 + M (creation of O3)

For any generic hydrocarbon RH, the ozone production pathway would be:

RH + OH --> R + H2O (oxidation by OH)

R + O2 + M --> RO2 + M (formation of a peroxy radical)

RO2 + NO --> RO + NO2 (rxn with NO to form NO2)

NO2 --> NO + O (photdissociation of NO2)

O + O2 + M --> O3 + M (creation of O3)

Source: Michael E. Chang, School of Earth & Atmospheric Sciences, Georgia Institute of Technology (personal communications)

So hydrocarbons are responsible for converting NO into NO2 by producing peroxy radicals (RO2./HO2.). Peroxy radicals therefore compete with O3 in the NO into NO2 conversion process. Without the influence of HCs (or CO; CO behaves as a HC in the ozone production pathway), NOx is rather benign with respect to ozone formation, no matter how high the concentration of NOx becomes:

From Air Pollution, Its Origin and Control; Wark, Warner, Davis (Page 476):

[O3] = kI[NO2]/[NO] (I is the intensity of the sunlight.)

Empirical data show that kI = ~25 micrograms/m3 or about 0.015 ppm (15 ppb).

NOx emissions from diesel engines equipped with DPF are approximately 50% NO (Source: Karlsson, "Measurement of Emissions from Four Diesel Fuelled Passenger Cars Meeting Euro 4 Emission Standards." AVL, 2005, http://www20.vv.se/fud-resultat/Publikationer_000001_000100/Publikation_000076/MTC5505_Slutraport.pdf).

Thus, if [NO2]/[NO] = 1, [O3] = (0.015)(1) = 0.015 ppm or 15 ppb, well below the 2015 NAAQS eight-hour standard of 70 ppb (near the natural background (pre-industrial) levels of 10 - 20 ppb).

And, as previously suggested, diesel emissions of HC and CO are very low while the HC and CO emissions from gasoline engines are relatively much higher. Thus, the typical emissions profile of diesel engines (near-zero HC and CO emissions, relatively higher emissions of NOx) could be expected to deplete GLO (i.e., "smog") in urban areas.