The European heat wave of 2018 and its promotion of the ozone climate penalty in southwest Sweden

The ozone concentration ([O3]), temperature (T) and vapour pressure deficit (VPD) during the 2018 heat wave (HW) was compared with conditions from 2013–2017. The study included one coastal and two inland sites in southwest Sweden. The positive relationship between [O3] and T was stronger in 2018. The average daytime T from April–September was higher by 2.0–2.4°C in 2018. The VPD was strongly and positively affected by the 2018 HW. The daytime mean [O3] was enhanced by 7–12% in 2018. The relationship between hourly daytime [O3] and T, as well as that between the daily maximum [O3] and daily maximum T, was steeper in 2018. The stronger promotion of [O3] by T in 2018 was possibly partly a result of dry conditions causing stomatal closure of vegetation and thus a weaker O3 sink. If HWs like that in 2018 become more common, they can be expected to promote higher [O3] and risk larger health and ecosystem effects.