Simulation of current and future tropospheric chemistry with the Earth System Model EMAC

<p>The increasing future methane (CH<sub>4</sub>) leads to changes in the lifetime of CH<sub>4</sub> and in the Hydroxyl radical (OH) and (O<sub>3</sub>) mixing ratios and distribution in the lower atmosphere. With increasing CH<sub>4</sub> the lifetime of CH<sub>4</sub> and the O<sub>3</sub> mixing ratios in the troposphere will increase, the tropospheric OH mixing ratios will decrease (Winterstein et al., 2019; Zhao et al., 2019). The CH<sub>4</sub> changes, together with the future Nitrous oxide (N<sub>2</sub>O) and temperature increase, will lead to a different tropospheric chemistry. For example, substances as acetone (CH<sub>3</sub>COCH<sub>3</sub>), ethane (C<sub>2</sub>H<sub>6</sub>), formic acid (HCOOH) or peroxy acetyl nitrate (PAN) will change their distribution and mixing ratios.</p><p>In different studies we could show that EMAC (ECHAM/MESSy Atmospheric Chemistry, Jöckel et al., 2010) has the ability to simulate some of the mentioned tropospheric substances in comparison to results of the GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) instrument, used on board of the research aircrafts Geophysica and HALO during the STRATOCLIM (July/August 2017) and WISE (August to October 2017) campaigns (Johansson et al., 2020; Wetzel et al., 2020).   </p><p>In this study, we will additional show the first results of the simulated future changes of tropospheric chemistry (especially with focus on CH<sub>3</sub>COCH<sub>3</sub>, C<sub>2</sub>H<sub>6</sub>, HCOOH and PAN and the upper troposphere) related to the future increase of CH<sub>4</sub>, N<sub>2</sub>O and temperature change as a result of climate change. For these we use different EMAC simulations from the project ESCiMo (Earth System Chemistry Integrated Modelling, Jöckel et al., 2016).</p><p>We will present some results of the comparison of EMAC to GLORIA and results with regard to the future development of the (upper) tropospheric chemistry in EMAC.    </p>

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

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.

Regional and intercontinental pollution signatures on modeled and measured PAN at northern mid-latitude mountain sites

Peroxy acetyl nitrate (PAN) is the most important reservoir species for nitrogen oxides (NOx) in the remote troposphere. Upon decomposition in remote regions, PAN promotes efficient ozone production. We evaluate monthly mean PAN abundances from global chemical transport model simulations (HTAP1) for 2001 with measurements from five northern mid-latitude mountain sites (four European and one North American). The multi-model mean generally captures the observed monthly mean PAN but individual models simulate a factor of ~ 4–8 range in monthly abundances. We quantify PAN source-receptor relationships at the measurement sites with sensitivity simulations that decrease regional anthropogenic emissions of PAN (and ozone) precursors by 20 % from North America (NA), Europe (EU), and East Asia (EA). The HTAP1 models attribute more of the observed PAN at Jungfraujoch (Switzerland) to emissions in NA and EA, and less to EU, than a prior trajectory-based estimate. The trajectory-based and modeling approaches agree that EU emissions play a role in the observed springtime PAN maximum at Jungfraujoch. The signal from anthropogenic emissions on PAN is strongest at Jungfraujoch and Mount Bachelor (Oregon, U.S.A.) during April. In this month, PAN source-receptor relationships correlate both with model differences in regional anthropogenic volatile organic compound (AVOC) emissions and with ozone source-receptor relationships. PAN observations at mountaintop sites can thus provide key information for evaluating models, including links between PAN and ozone production and source-receptor relationships. Establishing routine, long-term, mountaintop measurements is essential given the large observed interannual variability in PAN.

Analysis of elevated spring-time levels of Peroxy Acetyl Nitrate (PAN) at the High Alpine research sites Jungfraujoch and Zugspitze

Largest atmospheric peroxy acetyl nitrate (PAN) mole fractions at remote surface sites in the Northern Hemisphere are commonly observed during the months April and May. Different formation mechanisms for this seasonal maximum have previously been suggested: hemispheric-scale production from precursors accumulated during the winter months, increased spring-time transport from up-wind continents, increased regional-scale production in the atmospheric boundary layer from recent emissions. The two high Alpine research sites Jungfraujoch (Switzerland) and Zugspitze (Germany) exhibit a distinct and consistent spring-time PAN maximum, too. Since these sites intermittently sample air masses of free tropospheric and boundary layer origin, they are ideally suited to identify the above mentioned PAN formation processes and attribute local observations to these. Here we present a detailed analysis of PAN observations and meteorological conditions during May 2008 when PAN levels were especially elevated at both sites. Highest PAN concentrations were connected with anti-cyclonic conditions, which persisted in May 2008 for about 10 days with north easterly advection towards the sites. A backward dispersion model analysis showed that elevated PAN concentrations were caused by the combination of favourable photochemical production conditions and large precursor concentrations in the European atmospheric boundary layer. The results suggest that the largest PAN values in spring 2008 at both sites were attributable to regional-scale photochemical production of PAN in the (relatively cold) planetary boundary layer from European precursors whereas the contribution of inter-continental transport or free tropospheric build-up was of smaller importance for these sites.

A semi-Lagrangian view of ozone production tendency in North American outflow in the summers of 2009 and 2010

The Pico Mountain Observatory, located at 2225 m a.s.l. in the Azores Islands, was established in 2001 to observe long-range transport from North America to the central North Atlantic. In previous research conducted at the observatory, ozone enhancement (> 55 ppbv) in North American outflows was observed, and efficient ozone production in these outflows was postulated. This study is focused on determining the causes for high d[O3] / d[CO] values (~1 ppbv ppbv−1) observed in the summers of 2009 and 2010. The folded retroplume technique, developed by Owen and Honrath (2009), was applied to combine upwind FLEXPART transport pathways with GEOS-Chem chemical fields. The folded result provides a semi-Lagrangian view of polluted North American outflow in terms of physical properties and chemical processes, including production/loss rate of ozone and NOx produced by lightning and thermal decomposition of peroxy acetyl nitrate (PAN). Two transport events from North America were identified for detailed analysis. High d[O3] / d[CO] was observed in both events, but due to differing transport mechanisms, ozone production tendency differed between the two. A layer of net ozone production was found at 2 km a.s.l. over the Azores in the first event plume, apparently driven by PAN decomposition during subsidence of air mass in the Azores–Bermuda High. In the second event, net ozone loss occurred during transport in the lower free troposphere, yet observed d[O3] / d[CO] was high. We estimate that in both events, CO loss through oxidation contributed significantly to d[O3] / d[CO] enhancement. Thus, it is not appropriate to use CO as a passive tracer of pollution in these events. In general, use of d[O3] / d[CO] as an indicator of net ozone production/loss may be invalid for any situation in which oxidants are elevated. Based on our analysis, use of d[O3] / d[CO] to diagnose ozone enhancement without verifying the assumption of negligible CO loss is not advisable.

Closing the peroxy acetyl nitrate budget: observations of acyl peroxy nitrates (PAN, PPN, and MPAN) during BEARPEX 2007

Acyl peroxy nitrates (APNs, also known as PANs) are formed from the oxidation of aldehydes and other oxygenated VOC (oVOC) in the presence of NO2. There are both anthropogenic and biogenic oVOC precursors to APNs, but a detailed evaluation of this chemistry against observations has proven elusive. Here we describe measurements of PAN, PPN, and MPAN along with the majority of chemicals that participate in their production and loss, including OH, HO2, numerous oVOC, and NO2. Observations were made during the Biosphere Effects on AeRosols and Photochemistry Experiment (BEARPEX 2007) in the outflow of the Sacramento urban plume. These observations are used to evaluate a detailed chemical model of APN ratios and concentrations. We find that the ratios of APNs are nearly independent of the loss mechanisms and thus an especially good test of our understanding of their sources. We show that oxidation of methylvinyl ketone, methacrolein, methyl glyoxal, biacetyl and acetaldehyde are all significant sources of the PAN+peroxy acetyl (PA) radical reservoir, accounting for 26%, 2%, 7%, 20%, and 45%, of the production rate on average during the campaign, respectively. At high temperatures, when upwind isoprene emissions are highest, oxidation of non-acetaldehyde PA radical sources contributes over 60% to the total PA production rate, with methylvinyl ketone being the most important of the isoprene-derived sources. An analysis of absolute APN concentrations reveals a missing APN sink that can be resolved by increasing the PA+∑RO2 rate constant by a factor of 3.

Near-IR photodissociation of peroxy acetyl nitrate

Measurements of the C-H overtone transition strengths combined with estimates of the photodissociation cross sections for these transitions suggest that near-IR photodissociation of peroxy acetyl nitrate (PAN) is less significant (Jnear-IR≈3x10-8s-1 at noon) in the lower atmosphere than competing sinks resulting from unimolecular decomposition and ultraviolet photolysis. This is in contrast to the photochemical behavior of a related peroxy nitrate, pernitric acid (PNA), that undergoes rapid near-IR photolysis in the atmosphere with Jnear-IR≈10-5s-1 at noon (Roehl et al., 2002). This difference is attributed to the larger binding energy and larger number of vibrational degrees of freedom in PAN, which make 4νCH the lowest overtone excitation with a high photodissociation yield (as opposed to 2νOH in PNA).