Characterization of daytime sources of nitrous acid (HONO) is crucial to understand atmospheric oxidation and radical cycling in the planetary boundary layer. HONO and numerous other atmospheric trace constituents were measured on the Mediterranean island of Cyprus during the CYPHEX (CYprus PHotochemical EXperiment) campaign in summer 2014. Average volume mixing ratios of HONO were 35 pptv (±25 pptv) with a HONO ∕ NO_x ratio of 0.33, which was considerably higher than reported for most other rural and urban regions. Diel profiles of HONO showed peak values in the late morning (60 ± 28 pptv around 09:00 local time) and persistently high mixing ratios during daytime (45 ± 18 pptv), indicating that the photolytic loss of HONO is compensated by a strong daytime source. Budget analyses revealed unidentified sources producing up to 3.4 × 10⁶ molecules cm⁻³ s⁻¹ of HONO and up to 2.0 × 10⁷ molecules cm⁻³ s⁻¹ NO. Under humid conditions (relative humidity > 70 %), the source strengths of HONO and NO exhibited a close linear correlation (R² = 0.72), suggesting a common source that may be attributable to emissions from microbial communities on soil surfaces.

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, NO_x, hydrocarbons, peroxides, and other major HO_x (OH + HO₂) sources and sinks in a low-NO_x 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 HO_x production and losses occurred when the air masses had higher residence time over the oceans.

During the summertime CYPHEX campaign (CYprus PHotochemical EXperiment 2014) in the eastern Mediterranean, multiple volatile organic compounds (VOCs) were measured from a 650 m hilltop site in western Cyprus (34° 57′ N/32° 23′ E). Periodic shifts in the northerly Etesian winds resulted in the site being alternately impacted by photochemically processed emissions from western (Spain, France, Italy) and eastern (Turkey, Greece) Europe. Furthermore, the site was situated within the residual layer/free troposphere during some nights which were characterized by high ozone and low relative humidity levels. In this study we examine the temporal variation of VOCs at the site. The sparse Mediterranean scrub vegetation generated diel cycles in the reactive biogenic hydrocarbon isoprene, from very low values at night to a diurnal median level of 80–100 pptv. In contrast, the oxygenated volatile organic compounds (OVOCs) methanol and acetone exhibited weak diel cycles and were approximately an order of magnitude higher in mixing ratio (ca. 2.5–3 ppbv median level by day, range: ca. 1–8 ppbv) than the locally emitted isoprene and aromatic compounds such as benzene and toluene. Acetic acid was present at mixing ratios between 0.05 and 4 ppbv with a median level of ca. 1.2 ppbv during the daytime. When data points directly affected by the residual layer/free troposphere were excluded, the acid followed a pronounced diel cycle, which was influenced by various local effects including photochemical production and loss, direct emission, dry deposition and scavenging from advecting air in fog banks. The Lagrangian model FLEXPART was used to determine transport patterns and photochemical processing times (between 12 h and several days) of air masses originating from eastern and western Europe. Ozone and many OVOC levels were ∼ 20 and ∼ 30–60 % higher, respectively, in air arriving from the east. Using the FLEXPART calculated transport time, the contribution of photochemical processing, sea surface contact and dilution was estimated. Methanol and acetone decreased with residence time in the marine boundary layer (MBL) with loss rate constants of 0.74 and 0.53 day⁻¹ from eastern Europe and 0.70 and 0.34 day⁻¹ from western Europe, respectively. Simulations using the EMAC model underestimate these loss rates. The missing sink in the calculation is most probably an oceanic uptake enhanced by microbial consumption of methanol and acetone, although the temporal and spatial variability in the source strength on the continents might play a role as well. Correlations between acetone and methanol were weaker in western air masses (r² = 0.68), but were stronger in air masses measured after the shorter transport time from the east (r² = 0.73).

Simultaneous measurements of O₃, CO, NO_x, CH₄, and light nonmethane hydrocarbons were made over the Bay of Bengal (BoB) during 28 October to 17 November 2010 to study the role of chemistry and dynamics. The measurements revealed large variability in O₃ (11 to 60 ppbv) and CO (45 to 260 ppbv). Estimated south to north latitudinal gradients in O₃ (3.95 ppbv/°) and CO (16.56 ppbv/°) were significantly higher than those observed during earlier campaigns. Hybrid Single‐Particle Lagrangian Integrated Trajectory simulated back air trajectories were used to classify these measurements into pollution plumes from nearby sources (India‐Bangladesh region and Southeast Asia), influenced by long‐range transport and pristine marine air from the Indian Ocean. Interspecies correlations were used to identify emission signatures in these air masses, e.g., chemical proxies suggested influence of biofuel/biomass burning in NE‐BoB and E‐BoB air masses. Principle component analysis indicated contributions of ship emissions to NO_x levels over the BoB. Influences of fire from the Myanmar and Thailand regions are shown to be the potential contributor to enhanced CO levels (>250 ppbv) over the BoB during 14–15 November. Diurnal variations in surface O₃ revealed effects of advection, entrainment, and photochemistry. A chemical box model simulated the photochemical buildup in O₃ in polluted air masses and daytime destruction in pristine oceanic air masses.