On the long-term impact of emissions from central European cities on regional air quality

Abstract

Abstract. For the purpose of qualifying and quantifying the impact of urban emission from Central European cities on the present-day regional air quality, the regional climate model RegCM4.2 was coupled with the chemistry transport model CAMx, including two-way interactions. A series of simulations was carried out for the 2001-2010 period either with all urban emissions included (base case) or without considering urban emissions. Further, the sensitivity of ozone production to urban emissions was examined by performing reduction experiments with −20 % emission perturbation of NO x and/or non-methane volatile organic compounds (NMVOC).The modeling system's air quality related outputs were evaluated using AirBase, and EMEP surface measurements showed reasonable reproduction of the monthly variation for ozone (O 3 ), but the annual cycle of nitrogen dioxide (NO 2 ) and sulfur dioxide (SO 2 ) is more biased. In terms of hourly correlations, values achieved for ozone and NO 2 are 0.5-0.8 and 0.4-0.6, but SO 2 is poorly or not correlated at all with measurements (r around 0.2-0.5). The modeled fine particulates (PM 2.5 ) are usually underestimated, especially in winter, mainly due to underestimation of nitrates and carbonaceous aerosols.European air quality measures were chosen as metrics describing the cities emission impact on regional air pollution. Due to urban emissions, significant ozone titration occurs over cities while over rural areas remote from cities, ozone production is modeled, mainly in terms of number of exceedances and accumulated exceedances over the threshold of 40 ppbv. Urban NO x , SO 2 and PM 2.5 emissions also significantly contribute to concentrations in the cities themselves (up to 50-70 % for NO x and SO 2 , and up to 60 % for PM 2.5 ), but the contribution is large over rural areas as well (10-20 %). Although air pollution over cities is largely determined by the local urban emissions, considerable (often a few tens of %) fraction of the concentration is attributable to other sources from rural areas and minor cities. For the case of Prague (Czech Republic capital), it is further shown that the inter-urban interference between large cities does not play an important role which means that the impact on a chosen city of emissions from all other large cities is very small. At last, it is shown that to achieve significant ozone reduction over cities in central Europe, the emission control strategies have to focus on the reduction of NMVOC, as reducing NO x (due to suppressed titration) often leads to increased O 3 . The influence over rural areas is however always in favor of improved air quality, i.e. both NO x and/or NMVOC reduction ends up in decreased ozone pollution, mainly in terms of exceedances.

Validation of gas phase chemistry in the WRF-Chem model over Europe

“…In agreement with the results of Huszar et al (2012), Tuccella et al (2012) or Huszar et al (2016), the correlation values are substantially higher for ozone than for other species (Fig. 3).…”

“…The lower daily amplitude of summer ozone daily cycle (Fig. 5) is nearly the same as the amplitude described by Huszar et al (2012) and Huszar et al (2016), with the missing early-morning decrease, too. In winter, there is a difference between specific chemical mechanisms; simulation with CBMZ scheme produces a double daily cycle, similarly to Huszar et al (2012), who invoked the CB-IV chemical mechanism.…”

“…4) at rural stations are captured with similar accuracy as in Huszar et al (2012) or Huszar et al (2016), with a little improvement for ozone, but with model annual cycle of SO 2 shifted to lower values. In general, simulations with either chemical mechanism overestimate ozone, especially for suburban and urban stations and in the nighttime (Fig.…”

“…The findings of Mar et al (2016) were similar -they have shown that the differences between two simulations without aerosol-radiative effects are small, but not zero. The lower daily amplitude of temperature is probably caused by overpredicted cloudiness in the model (also seen in Huszar et al, 2016), which can further cause lower daily amplitude of ozone concentrations. Higher convective precipitation, as was mentioned before, lowers concentration of water-reactive gases due to wet scavenging, so it can be one of the possible reasons for negative biases of SO 2 , and partly of NO 2 , in the convective season.…”

“…However, even with the fully on-line approach, lots of model inaccuracies still occur in resulting values of gas and aerosol concentrations in validation studies (e.g. Huszar et al, 2012;Tuccella et al, 2012;Huszar et al, 2016). These inaccuracies are caused by simplification used in model design as well as by unreliable model inputs, mainly regarding the emission sources.…”