Peroxy acetyl nitrate (PAN) measurements at northern midlatitude mountain sites in April: a constraint on continental source–receptor relationships

Abstract

Abstract. Abundance-based model evaluations with observations provide critical tests for the simulated mean state in models of intercontinental pollution transport, and under certain conditions may also offer constraints on model responses to emission changes. We compile multiyear measurements of peroxy acetyl nitrate (PAN) available from five mountaintop sites and apply them in a proof-of-concept approach that exploits an ensemble of global chemical transport models (HTAP1) to identify an observational “emergent constraint”. In April, when the signal from anthropogenic emissions on PAN is strongest, simulated PAN at northern midlatitude mountaintops correlates strongly with PAN source–receptor relationships (the response to 20 % reductions in precursor emissions within northern midlatitude continents; hereafter, SRRs). This finding implies that PAN measurements can provide constraints on PAN SRRs by limiting the SRR range to that spanned by the subset of models simulating PAN within the observed range. In some cases, regional anthropogenic volatile organic compound (AVOC) emissions, tracers of transport from different source regions, and SRRs for ozone also correlate with PAN SRRs. Given the large observed interannual variability in the limited available datasets, establishing strong constraints will require matching meteorology in the models to the PAN measurements. Application of this evaluation approach to the chemistry–climate models used to project changes in atmospheric composition will require routine, long-term mountaintop PAN measurements to discern both the climatological SRR signal and its interannual variability.

Attribution of ground-level ozone to anthropogenic and natural sources of NO<sub>x</sub> and reactive carbon in a global chemical transport model

“…A set of figures for other HTAP Tier 2 regions, with a complete attribution of surface ozone to all tagged HTAP Tier 1 source regions is available in the Supplementary Material. Simulated PAN is highest in late winter to early spring, consistent with earlier work (Fischer et al, 2014;Fiore et al, 2018). The extra-regional contribution to PAN is also highest in spring, and this is due primarily to anthropogenic NMVOC, also consistent with Fischer et al (2014).…”

“…Given the large variety in model representations of NMVOC chemistry, including PAN formation and decomposition processes (Emmerson and Evans, 2009;Knote et al, 2015) and the large inter-model differences in simulated PAN , the widespread implementation of similar tagging diagnostics in other CTMs may help to provide additional information about the origin and fate of simulated PAN, and more generally about the influence of reactive carbon on atmospheric composition. In combination with routine mountaintop observations of springtime PAN, this may aid understanding of the global PAN budget (Fiore et al, 2018) and other processes responsible for intercontinental transport of air pollution. Better constraints on these chemical and transport processes should also help to reduce inter-model differences in simulated springtime ozone (Figure 1).…”

Attribution of ground-level ozone to anthropogenic and natural sources of nitrogen oxides and reactive carbon in a global chemical transport model

“…The 1 ppb simulated increase in Northern Hemisphere surface ozone in response to a 25 % increase in methane burden is consistent with previous work (HTAP, 2010). The 9.22 Tg increase in tropospheric ozone burden is also consistent with the review of Fiore et al (2008), who derived a sensitivity of 0.11-0.16 Tg(O 3 ) per Tg(CH 4 ) yr −1 emitted based on an analysis of the literature. We calculate 0.13 Tg(O 3 ) per Tg(CH 4 ) yr −1 based on our results.…”

“…Previous work (Lawrence and Crutzen, 1999) has noted the disproportionate influence of ship NO x on tropospheric ozone due to the diffuse and widespread nature of this source over regions that would otherwise have very low NO x mixing ratios. Fiore et al (2008) noted that the response of surface ozone to increased methane was especially strong in ship tracks. Myhre et al (2011) also showed that ship NO x emissions reduce the global methane lifetime much more than terrestrial NO x emissions.…”

“…sphere. In contrast to other forms of reactive carbon, emissions of methane can contribute to ozone formation at any location in the troposphere where photochemical conditions are favourable (Fiore et al, 2008). Despite its low reactivity in comparison to other types of reactive carbon, methane is highly abundant and has been shown to make a large contribution to tropospheric ozone (Wang and Jacob, 1998;Fiore et al, 2008;Stevenson et al, 2013;Butler et al, 2018).…”