Analysis of Emission-Driven Changes in the Oxidation Capacity of the Atmosphere in Major European Urban Cities

Abstract

Air quality is highly affected by emissions generated by human activities. Anthropogenic emissions in Europe have gradually been reduced thanks to a combination of factors, including restrictive regulation and policy implementation, fuel switching, technological developments, and improved energy efficiencies. Many measures have been specifically introduced to meet the NO₂ limit values for the protection of human health, primarily targeting traffic emissions. Due to NO_X reduction policies in Europe, NO₂ levels have been generally reduced, but O₃ concentrations have been found to increase. This phenomenon would cause changes in the oxidant capacity of the atmosphere, promoting the formation of tropospheric oxidants. In order to observe the occurrence in Europe, the Community Multiscale Air Quality Modeling System (CMAQ) is applied in this study. We examine the variation for two months representative of winter and summer conditions (January and July) in the period of 2007–2015. In addition to observing the general situation, we selected 67 European large cities (those with more than 0.5 million inhabitants in 2015) to analyze the trends of pollutants along with NO_X emission changes by means of statistical methods. Additionally, we characterized the phenomenon of general changes in the oxidation capacity of the atmosphere dividing into five groups. As a general result, we found that NO_X emissions and NO₂ concentrations decreased in the period in both seasons. On the other hand, concentrations of O₃ and other main tropospheric oxidants (OH and NO₃)  increased in winter but decreased in summer. However, the OH radical also shows an increase in urban areas in summer. Finally, HNO₃ shows a negative trend in general.

Questions and Answers

Questioner 1::

Oriol Jorba.

Question::

What is the sensitivity of your results to different meteorological years?

Answer::

Meteorology is very important for ambient concentrations of pollutants due to influences on chemical reactions and vertical mixing. As we focused on the meteorology of 2015 to isolate the effect of the emission changes in this study, the potential influence of meteorological factors was not studied. According to the previous study (Saiz-Lopez et al., 2017), the meteorological factors between 2007 and 2014 were statistically insignificant. Therefore, we thought that the changes during the study period in meteorological conditions were supposed to be small. However, we would like to include this factor in further studies to confirm specific and possible differences.

Questioner 2::

Joachim Fallmann.

Question::

How are urban areas represented/parametrized? Canopy Model? How are urban areas resolved considering coarse resolution? I guess sometimes only ~1 grid cell.

Answer::

In this study, we used CMAQ model with a 12 × 12 km resolution and the domain covers the whole European continent. As a result, urban areas are represented as one grid cell of the resolution. Due to this coarse resolution, pollutant concentrations are supposed to be represented as urban background sites, which are also used for the model assessment in the study. According to the assessment of the model, the CMAQ model can reproduce the concentrations of atmospheric chemical species in Europe, including urban areas with reasonable correlation coefficients for NO₂ and O₃ (0.60 and 0.44 for NO₂ in January and July, respectively, and 0.59 and 0.68 for O₃ in January and July, respectively). This enables us to investigate changes in the oxidation capacity associated with other relevant chemical species.