Summer ozone in the northern Front Range metropolitan area: weekend–weekday effects, temperature dependences, and the impact of drought

Abstract

Abstract. Contrary to most regions in the US, ozone in the northern Front Range metropolitan area (NFRMA) of Colorado was either stagnant or increasing between 2000 and 2015, despite substantial reductions in NO x emissions. We used available long-term ozone and NO x data in the NFRMA to investigate these trends. Ozone increased from weekdays to weekends for a number of sites in the NFRMA with weekend reductions in NO 2 at two sites in downtown Denver, indicating that the region was in a NO x -saturated ozone production regime. The stagnation and increases in ozone in the NFRMA are likely due to a combination of decreasing NO x emissions in a NO x -saturated environment and increased anthropogenic volatile organic compound (VOC) emissions in the NFRMA. Further investigation of the weekend-weekday effect showed that the region outside of the most heavily trafficked Denver area was transitioning to peak ozone production towards NO x -limited chemistry. This transition implies that continued NO x decreases will result in ozone being less sensitive to changes in either anthropogenic or biogenic VOC reactivity in the NFRMA. In contrast to anthropogenic VOCs, biogenic VOCs are unlikely to have increased in the NFRMA between 2000 and 2015, but are temperature dependent and likely vary by drought year. Ozone in the NFRMA has a temperature dependence, albeit smaller than many other US locations, consistent with biogenic VOC contributions to ozone production in the region. We show that while ozone increased with temperature in the NFRMA, which is consistent with a NO x -saturated regime coupled to temperature-dependent VOCs, this relationship is suppressed in drought years. We attribute this drought year suppression to decreased biogenic isoprene emissions due to long-term drought stress. Thus, while anthropogenic NO x and VOCs likely dominate ozone production regimes in the NFRMA, biogenic VOCs may also impact regional ozone and its temperature dependence.

Changes in summertime ozone in Colorado during 2000–2015

“…Conversely, it appears that local O 3 production has increased under declining NO x . Similar conclusions were derived from a recent analysis of trends in weekday/weekend differences in O 3 and NO x at selected NCFR sites by Abeleira and Farmer (2017). Their study found that "most sites in the NFRMA were NO x -saturated, but are transitioning to, and in one case may already have reached, the peak P(O 3 ) cross-over point between NO x -saturated and NO x -limited regimes".…”

Spatiotemporal Variations and Factors of Air Quality in Urban Central China during 2013–2015

“…Its concentration begins to increase gradually from January and demonstrates a downward trend after reaching a maximum in June (79.6 ± 22.8 µg/m 3 in 2014) or August (96.3 ± 18.1 µg/m 3 in 2015), and the minimum of the whole year is in November (30.0 ± 9.7 µg/m 3 in 2014) or December (28.2 ± 5.9 µg/m 3 in 2015). Temperature directly influences O 3 production by interfering with the photochemical reaction speed and the volatile organic compounds (VOCs) emissions [40,41]. The local emissions of precursors (NO x , VOC, and CO) also have an important effect on the regulation of O 3 variability.…”

What Are the Principal Factors Affecting Ambient Ozone Concentrations in Czech Mountain Forests?

“…O 3 has a temperature-dependent chemistry (Pusede et al, 2015), and temperature was used in some studies as a surrogate to indicate O 3 formation via the O 3 -temperature association, both at individual measuring sites and on a greater regional scale (Oikonomakis et al, 2018). The O 3 -temperature relationship originates in: (1) temperaturedependent biogenic VOC emissions, (2) thermal decomposition of PAN to HO x and NO x , (3) increased likelihood of favorable meteorological conditions for ozone formation (Abeleira and Farmer, 2017). However, there are major uncertainties in the mechanisms underlying the temperature-dependent changes in O 3 concentrations, their interactions, and relative contributions in rural and remote regions (Romer et al, 2018).…”

“…However, there are major uncertainties in the mechanisms underlying the temperature-dependent changes in O 3 concentrations, their interactions, and relative contributions in rural and remote regions (Romer et al, 2018). Ozone chemistry regimes are shifting as precursor emissions are changing (Abeleira and Farmer, 2017). Moreover, meteorology-dependent O 3 chemistry implicates the impact of ongoing climate change on O 3 .…”

“…Moreover, meteorology-dependent O 3 chemistry implicates the impact of ongoing climate change on O 3 . Increasing temperature is expected to increase O 3 concentrations (Abeleira and Farmer, 2017). Additionally, the projected rise in global precursor emissions over the twenty-first century is also assumed to have a strong effect on O 3 throughout the world (Vingarzan, 2004).…”