Oxidation processes in the eastern Mediterranean atmosphere: evidence from the modelling of HO<sub><i>x</i></sub> measurements over Cyprus

Abstract

Abstract. The Mediterranean is a climatically sensitive region located at the crossroads of air masses from three continents: Europe, Africa, and Asia. The chemical processing of air masses over this region has implications not only for the air quality but also for the long-range transport of air pollution. To obtain a comprehensive understanding of oxidation processes over the Mediterranean, atmospheric concentrations of the hydroxyl radical (OH) and the hydroperoxyl radical (HO2) were measured during an intensive field campaign (CYprus PHotochemistry EXperiment, CYPHEX-2014) in the northwest of Cyprus in the summer of 2014. Very low local anthropogenic and biogenic emissions around the measurement location provided a vantage point to study the contrasts in atmospheric oxidation pathways under highly processed marine air masses and those influenced by relatively fresh emissions from mainland Europe.The CYPHEX measurements were used to evaluate OH and HO2 simulations using a photochemical box model (CAABA/MECCA) constrained with CYPHEX observations of O3, CO, NOx, hydrocarbons, peroxides, and other major HOx (OH + HO2) sources and sinks in a low-NOx environment (< 100 pptv of NO). The model simulations for OH agreed to within 10 % with in situ OH observations. Model simulations for HO2 agreed to within 17 % of the in situ observations. However, the model strongly under-predicted HO2 at high terpene concentrations, this under-prediction reaching up to 38 % at the highest terpene levels. Different schemes to improve the agreement between observed and modelled HO2, including changing the rate coefficients for the reactions of terpene-generated peroxy radicals (RO2) with NO and HO2 as well as the autoxidation of terpene-generated RO2 species, are explored in this work. The main source of OH in Cyprus was its primary production from O3 photolysis during the day and HONO photolysis during early morning. Recycling contributed about one-third of the total OH production, and the maximum recycling efficiency was about 0.7. CO, which was the largest OH sink, was also the largest HO2 source. The lowest HOx production and losses occurred when the air masses had higher residence time over the oceans.

Diurnal variability, photochemical production and loss processes of hydrogen peroxide in the boundary layer over Europe

“…These limitations will be discussed in the 10 context of the HUMPPA campaign. This campaign is particularly suitable for this purpose since two other studies have been published that specifically address the temporal evolution of H2O2 using a box model (Crowley et al, 2018) and the contribution of entrainment to the early morning increase of O3 (Ouwersloot et al, 2012) for this particular campaign. Results from HUMPPA will also apply to the other campaigns discussed here.…”

“…Previous measurements of HO2 reported for DOMINO and HUMPPA are therefore an upper limit as they are affected by a fractional measurement of RO2. Crowley et al (2018) suggest that on average 50 % of the daytime 25 HO2 signal during HUMPPA is due to a RO2 interference caused by high NO additions. During later campaigns (PARADE, HOPE, CYPHEX) the reduction of NO addition resulted in significantly smaller RO2 interferences of the order 12 to 15 % (Mallik et al, 2018).…”

“…during HUMPPA. In a recent modelling study, Crowley et al (2018) determined the contribution of RO2 to the measured HO2 during the daylight hours to 20 be of the order of 50 %. If we correct the HUMPPA data for this potential interference, median daytime HO2 is reduced from 24 pptv (14 to 35 pptv for the 25 -75 % percentiles) to 12 pptv (7 -17.5 pptv).…”

“…Median nighttime 30 production was ~ 0 pptv/s for DOMINO, PARADE, HOPE and CYPHEX, but between 0.01 and 0.02 pptv/s during HUMPPA, due to elevated HO2 concentrations during the night (~ 20 pptv in Fig. S2d), which is most likely an artefact due to a RO2 interference of the HO2 measurements (Crowley et al 2018). The total uncertainty is calculated by error propagation:…”

The community atmospheric chemistry box model CAABA/MECCA-4.0

“…Becker et al (2000) improved the treatment of the diffuse actinic flux and corrected an implementation error of Meier et al (1982). The extension to the use of multiple latitudes was introduced within the development of the model CLaMS (McKenna et al, 2002). The possibility to calculate diurnally averaged photolysis rates was introduced for the simplified fast chemistry setup used in multi-annual CLaMS simulations (Pommrich et al, 2014).…”