Research findings should not be ignored!

Facets of urban air pollution as of 2018: Diesel engines and related exhaust gases, microphysics of particles and health effects 

Statement by

Prof. Dr. Hartmut Herrmann
Atmospheric Chemistry

Department Leibniz Institute for Tropospheric Chemistry (TROPOS)



The diesel emissions scandal continues to stir up emotions in Germany. Nowadays, urban measurement stations run by the German Federal Environmental Agency (UBA) only rarely show limit-exceeding particle mass values (PM10), however, exceedances of NO2 limits have been identified for multiple cities, especially those located in Western Germany [1]. Following the judgement of the German Federal Administrative Court of February 27, 2018 [2], municipalities may now implement restricted access zones for diesel vehicles. Having said that, implementation of such regulations is likely to be only possible for single road sections rather than entire urban districts, let alone cities.


Nitric oxides, diesel technology and diesel exhaust purification


There is a common factor among the functionality of a diesel engine and its exhaust purification and diesel – NO2 emissions: A diesel engine exhibits higher pressure and temperature values during combustion, promoting NO to emerge as a by-product of the reaction between nitrogen and oxygen that occurs during the combustion process.

Using an “Oxycat”, which is applied in EURO-2 and EURO-3 especially, all components of the exhaust gas are oxidized – CO and hydrocarbons are combusted to CO2 in particular. When an Oxycat is in place, NO as well is oxidized to NO2. EU pollutant standards of EURO-4 and beyond apply a diesel particulate filter (DPF). Depending on the DPF operating mode, NO2 may be utilized for soot oxidation during DPF cleaning. To this end, the DPF is heated to allow NO2 to react with carbon to form NO and CO2.

The diesel engine offers a valuable and highly efficient motor technology that, by use of appropriate exhaust purification technologies (SCR, [3]), is capable of complying with the latest emission standards. Public events discussing the future of diesel technology have been carried out twice with our involvement [4, 5], identifying and debating viable approaches and solutions for the future of diesel engines, while also pointing out possible alternatives. My colleague J. Wolfrum has given an excellent overview on the topic [6]. In this respect, it should be noted that the option of reducing NOx emissions from diesel engines by applying software updates only is to be viewed critically [7]. Hardware retrofitting from EURO-4 to EURO-6 will likely prove to be a more effective solution; however, albeit technically quite feasible, this involves significant expenses [8].



Diurnal cycles of urban particle pollution

Notions and visual representations that are absurd in nature have been circulating on how diurnal cycles of urban air pollution come about [9]. Some of these concepts are at least simplistic, or also entirely false. Particle pollution that occurs within cities is influenced by various factors, depending on particle size. Needless to say, so-called diesel particles, which lie within a size range of 50-100nm in diameter (Dp), are primarily linked to traffic emissions. For diurnal cycles of medium sized particles (accumulation mode, approx. 80 nm < Dp < approx. 900 nm), there is a variety of different processes that play a role, such as the condensation of semi-volatile gaseous molecules, which are partially formed by chemical reactions in the atmosphere in the first place. For larger particles (coarse dust), a significant urban contribution can be attributed to resuspension. Resuspension, by the way, is primarily the result of traffic, as opposed to mere heat-driven “air rollers”. There is an extensive body of literature available on the topic, ranging from textbooks [10] to classical studies [11] as well as very recent works, such as studies on the urban dynamics of nanoparticles [12]. Related commentary and statements addressed to the interested public should not ignore these scientific data.


Air pollution and related health effects


Present-day atmospheric research has been motivated by the atmospheric interconnections within the climate and earth system and possible adverse effects of air pollution on human health. A broad range of new, recent and classical studies, such as the Harvard “Six Cities Study” [13], have clearly shown and continue to demonstrate that health effects resulting from air pollution do exist and cannot be denied.

Air pollution has a clearly detrimental effect on human health. Recent studies have shown that around 3.3 million global premature deaths can be attributed to PM2.5 air pollution – for the EU, this amounts to around 350.000 cases of premature deaths. This is the equivalent of about 70.000 premature deaths for Germany. Latest figures announced by the WHO in early May 2018 indicate significantly higher numbers [14]. Related statements such as – and I quote directly – “The risk to health is largely overestimated and exaggerated. Alleged horror scenarios implying cases of death due to fine dust and nitric oxide are purely populist. No man or woman has yet died of the fine dust levels we have measured.” [9] are out of touch with scientific findings of the disciplines involved and are often expressed by individuals whose expertise, unfortunately, lies neither in air pollution nor in health research. For further information on the discussion of health hazards and some of the parties involved, please refer to a recent and insightful interview with Wolfgang Hien [15]. Criticisms directed at the present particle mass related EU limits for PM10 and PM2.5 are not uncalled for, given the fact that mass-based limits are unable to capture differences in chemical composition and therefore varying impacts on health. In this context, I should like to appeal for the utilization of metrics that are better capable of taking into account both differing chemical constituents and, in regard to ROS formation, related effects on the human body, as well as other resulting factors [16]. This debate has not reached its conclusion, and I, as the author of the text at hand, am involved in the debate, as part of my involvement in the ProcessNet Community Committee for Particulate Matters as co-chairperson in particular [17]. Concerning the debate on metric improvement it should, however, be noted that the setting EU limits, with all its consequences, is dependent on factors that are based on epidemiological evidence [18].

The health effects of air pollution entail socio-economic consequences. The OECD report of 2016 titled “The Economic Consequences of Outdoor Air Pollution” [19] shows that the biophysical impact of air pollution is going to result in immense costs worldwide. Overall, research on the impact of air pollution on human health is a scientific field that is marked by intense and increasing activity [20]. All the same, this does not imply that, at this stage, no clear statements can be made on the topic. Air pollution causes a shortening of the human life span. This also valid for Western countries where relevant existing legislation and effective control of compliance is in place.




Present-day atmospheric research, i.e., atmospheric chemistry and physics is dealing with highly complex processes, many of which may contribute to adverse human health effects. The discussion of these effects must acknowledge and take account of the scientific data available. This cannot be replaced by haphazard statements that are, given the current scientific knowledge, untenable.


References and weblinks

1.:        NO2 data provided by the UBA monitoring network


2.:        Judgement of the German Federal Administrative Court of February 27, 2018



3.:        (a) Selective catalytic reduction


(b) Guan, Bin, et al. "Review of state of the art technologies of selective catalytic reduction of

      NOx from diesel engine exhaust." Applied Thermal Engineering 66.1-2 (2014): 395-414.


4.:        ProcessNet/GDCh/DBG/KRdL Community Committee “Chemistry, Air Quality and Climate“, Special Colloquium „Nitric oxides: Can diesel be saved?“, 14.01.2016, DECHEMA, Frankfurt/Main, Germany.      

Press release:


5.:        ProcessNet/GDCh/DBG/KRdL Community Committee “Chemistry, Air Quality and Climate“, ProcessNet Community Committee for Particulate Matters and GDCh Working Group “Atmospheric Chemistry” (AKAC), Special Colloquium „Nitric oxides: Can diesel be saved?“, 05.12.2017, DECHEMA, Frankfurt/Main, Germany.


6.:        J. Wolfrum, Bunsenmagazin 1 / 2018, pp 4 – 12.




8.:        (a)



9.:        Prof. Dr. Matthias Klingner, Fraunhofer Institute for Traffic and Infrastructure Systems



10.:      Urban Meteorology: Forecasting, Monitoring, and Meeting Users' Needs (2012)

Free download via


11.:      Amato, F., et al. "Spatial and chemical patterns of PM10 in road dust deposited in urban environment." Atmospheric Environment43.9 (2009): 1650-1659.


12.:      Varotsos, C., et al. "An observational study of the atmospheric ultra-fine particle dynamics." Atmospheric Environment 59 (2012): 312-319.


13.:      Dockery, Douglas W., et al. "An association between air pollution and mortality in six US
      cities." New England journal of medicine329.24 (1993): 1753-1759.



14.:      (a)   gesundheit/gesundheitsrisiken-der-bevoelkerung-durch-feinstaub#textpart-5

            (b) Lelieveld, Jos, et al. "The contribution of outdoor air pollution sources to premature mortality on a global scale." Nature 525.7569 (2015): 367.

            (c) World Health Organization. "Ambient air pollution: A global assessment of exposure and burden of disease." (2016).

            (d) Recent figures of the WHO from early May 2018:



15.:  Verharmlosungsdiskurs-fuer-eine-Katastrophe-3984981.html


16.:      (a) Reche, C., et al. "New considerations for PM, Black Carbon and particle number concentration for air quality monitoring across different European cities." Atmospheric Chemistry and Physics11.13 (2011): 6207-6227.

(b) Borm, Paul JA, et al. "Oxidant generation by particulate matter: from biologically effective dose to a promising, novel metric." Occupational and environmental medicine 64.2 (2007): 73-74.




18.:      Strategy Paper by the Processnet Community Committee for Particulate Matters (2014), available at


15.:      Urban Meteorology: Forecasting, Monitoring, and Meeting Users' Needs (2012)

Free download via


16.:      Amato, F., et al. "Spatial and chemical patterns of PM10 in road dust deposited in urban environment." Atmospheric Environment 43.9 (2009): 1650-1659.


17.:      Varotsos, C., et al. "An observational study of the atmospheric ultra-fine particle dynamics." Atmospheric Environment 59 (2012): 312-319.


19.:      The Economic Consequences of Outdoor Air Pollution, OECD, 2016.


20.:      National Academies of Sciences, Engineering, and Medicine. The Future of Atmospheric Chemistry Research: Remembering Yesterday, Understanding Today, Anticipating Tomorrow. National Academies Press, 2017.