Investigation of reactive gases and methane variability in the coastal boundary layer of the central Mediterranean basin

Paolo Cristofanelli,Paolo Bonasoni,Maurizio Busetto,Francescopiero Calzolari,Claudia Roberta Calidonna,Michela Maione,Daniel Gullì,Damiano Sferlazzo,Ivano Ammoscato,Salvatore Piacentino,Angela Marinoni,Daniele Contini,Adelaide Dinoi,Tatiana Di Iorio,Frank Flocke,Detlev Helmig

doi:10.1525/elementa.216

Abstract

We present a characterization of reactive gases (RG: O3, NO, NO2,SO2, CO) and methane (CH4) variability in the central Mediterranean basin,analyzing in situ measurements at three new permanent WMO/GAW Observatories in Southern Italy: Capo Granitola – CGR (Sicily), Lamezia Terme – LMT (Calabria) and Lecce – ECO (Apulia). At all the measurement sites, a combination of the breeze wind system (especially at CGR and LMT),PBL dynamics, anthropogenic/natural emissions, and photochemistry lead the appearance of well-defined diurnal cycles for the observed RG. According to O3/NOx variability, local emissions appeared to influence CGR and LMT (no NOx data were available for ECO during the period of study) in 4% and 20% of the hourly data, nearby sources in 39% and 40%, remote sources in 31% and 14%, while background O3/NOx were observed in 26% of cases for both the stations. Most of the background O3/NOx were observed during daytime, when offshore air masses usually affected the measurement sites. Local sources of CH4 at CGR can be related to biogenic (oxic) emissions from biomasses along the coastline, while emissions from live stocks can represent a local source of CH4 at LMT. Finally, we provide first hints about the export of O3 from Sicily/Southern Italy to the Mediterranean Sea by comparing simultaneous observations at CGR and Lampedusa (LMP), a small island in the middle of the Strait of Sicily where a WMO/GAW Regional Station is located. In summer,O3 increased by some 7 ppb for transport times lower than 48 h, while no statistical significant differences were observed for travel time longer than 48. This would suggest that photochemical O3 production occurred within air-mass travelling from CGR to LMP, but also that the central Mediterranean MBL represents a O3 sink for relatively aged air-masses.

Highlights

During winter–autumn, the monthly 10th percentiles did not decrease below 20 ppb, while during warm months the 90th percentiles did not exceed 65 ppb, which can be considered indicative of the low impact of NO titration during cold months and photochemistry production in summer
Even if the low mean average values suggest that CGR is not heavy impacted by “fresh” anthropogenic emissions, the summer increase of carbon monoxide (CO) (Figure 3) and NO/NO2 (Figure S2) suggested a seasonal trend in the local/regional emissions, probably related to tourist activities
To provide a better assessment of the impact of airmasses with different anthropogenic emission aging at CGR and LMT, we investigated the variability of O3/NOx ratio that according to Morgan et al (2010) and Parrish et al (2009) can be considered proxies to qualitatively evaluate the proximity to major emission sources and photochemical processing

Summary

Introduction

When, using HYSPLIT back-trajectories, we selected the measurement periods characterized by air-masses only travelling over sea surface before reaching CGR (here not shown), an increase of the winter spring peak was observed (hourly peak value above 0.5 ppb). During summer, daytime average CO values were not statistically different (at the 95% confidence level) between CGR and LMT, supporting the view that when sea breeze affects these measurement sites, the observations were well representative of the regional background.

Results

Conclusion