Further points to consider:
- Nation-wide, NOx emissions have been reduced by 52%, and ambient NO2 levels have decreased by 49% (annual) since 2000, yet ambient O3 levels have declined by only 17%, according to EPA's own data (
https://www.epa.gov/air-trends/air-quality-national-summary). VOC emissions have decreased by 19%, which correlates better with the decline in ambient O3 levels. This suggests the intensive regulatory focus on NOx emission reductions has been less effective than anticipated for ground-level ozone reductions since there are many metropolitan areas that are still in non-attainment with the ozone NAAQS, some still classified as "extreme" nonattainment, especially with the NAAQS reduction from 75 ppb to 70 ppb in 2015.
- Some area may not be able to achieve attainment with the ozone NAAQS because of high biogenic NOx emissions anyway (Oikawa, P. Y. et al. "Unusually high soil nitrogen oxide emissions influence air quality in a high-temperature agricultural region."
Nat. Commun. 6:8753 doi: 10.1038/ncomms9753 (2015),
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4659929/).
A multi-year study done in Israel showed that days with virtually no anthropogenic NOx emissions (during a religious holiday - "Day of Atonment"; 83%-98% decrease in ambient levels), ozone levels INCREASED by 8 ppbv at urban core, and only decreased by 5 ppbv downwind. Nighttime ozone levels rose by 20 ppbv at urban sites and 30 ppbv downwind (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)).
Another recent study openly critical of higher-than-regulated NOx emissions from diesel engines in Europe (Karl et al. "Urban eddy covariance measurements reveal significant missing NOx emissions in Central Europe."
Scientific Reports, Article number: 2536 (2017),
https://www.nature.com/articles/s41598-017-02699-9) suggests that ambient NOx levels would have to decrease to ~2 ppbv before conditions transition to NOx-limited with respect to ozone formation in Innsbruck, Austria. Many areas have natural (biogenic) NOx levels at or above 2 ppbv.
- The correlation between lower ambient NOx levels and nitrate PM2.5 (nitrate aerosols) appears to be highly non-linear. The "weekend effect" mega-study (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)) showed that a nearly 50% decrease in ambient NOx levels on weekends resulted in only a 2.6% decrease in ambient nitrate aerosol concentration on average.
This has been supported by a more recent study by University of Colorado Boulder and NOAA (Womack, C. C. et al. ( 2019). "An Odd Oxygen Framework for Wintertime Ammonium Nitrate Aerosol Pollution in Urban Areas: NOx and VOC Control as Mitigation Strategies."
Geophysical Research Letters, 46,
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL082028 (Abstract)). This study found that ambient NH4NO3 levels (a major component of wintertime ambient PM2.5) are responsive to VOC controls, but NOT responsive to NOx controls, and reducing NOx emissions may actually INCREASE ambient PM2.5 levels.
- Secondary organic aerosol (SOA) production may dramatically increase as NOx emissions are decreased according to a study by Carnegie-Mellon University - Yunliang Zhao et al. "Reducing secondary organic aerosol formation from gasoline vehicle exhaust,"
Proceedings of the National Academy of Sciences, May 2017,
http://www.pnas.org/content/early/2017/06/13/1620911114 . According to the "Acknowledgement" section of that paper, this study was co-funded by CARB and EPA. CARB appears to be ignoring the results of its own funded study by requesting the further dramatic decrease in NOX emissions from diesel trucks.
According to that study, changing the NMOG:NOx ratio from 4:1 to 10:1 increases SOA production by a factor of ~8 (SOA yield increased from 0.06 to 0.46 as NMOG:NOx ratio increased from 4:1 to 10:1). NOx artificially lowers SOA production from NMOG/VOC emissions. This has the potential of offsetting the expected reduction in nitrate PM (nitrate aerosols; from NOx emission reductions) with a potentially more toxic form of PM2.5 (organic aerosols; from NOx emission reductions).
- No areas in the U.S. are currently in nonattainment with the NO2 NAAQS according to EPA. The last area to be classified nonattainment with the NO2 NAAQS was Los Angeles/South Coast Air Basin, and that area was reclassified attainment (maintenance) in 1998. The current design value for that region is 33 ppb, or slightly more than half of the 53 ppb (0.053 ppm) NAAQS (
https://www3.epa.gov/airquality/greenbook/ndtc.html).
EPA recently concluded that the current NAAQS values of 53 ppb (annual) and 100 ppb (1-hour) are adequate and no changes to the current NO2 NAAQS are warranted (
https://www.epa.gov/no2-pollution/p...-air-quality-standards-naaqs-nitrogen-dioxide). From a public health perspective, there certainly is no urgency to further reduce ambient NO2 levels at this time.