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.

Abstract. 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 massesPublished by Copernicus Publications on behalf of the European Geosciences Union. = 0.68), but were stronger in air masses measured after the shorter transport time from the east (r 2 = 0.73).

Abstract. Lagrangian particle dispersion models (LPDMs) in backward mode are widely used to quantify the impact of transboundary pollution on downwind sites. Most LPDM applications count particles with a technique that introduces a so-called footprint layer (FL) with constant height, in which passing air tracer particles are assumed to be affected by surface emissions. The mixing layer dynamics are represented by the underlying meteorological model. This particle counting technique implicitly assumes that the atmosphere is well mixed in the FL. We have performed backward trajectory simulations with the FLEXPART model starting at Cyprus to calculate the sensitivity to emissions of upwind pollution sources. The emission sensitivity is used to quantify source contributions at the receptor and support the interpretation of ground measurements carried out during the CYPHEX campaign in July 2014. Here we analyse the effects of different constant and dynamic FL height assumptions. The results show that calculations with FL heights of 100 and 300 m yield similar but still discernible results. Comparison of calculations with FL heights constant at 300 m and dynamically following the planetary boundary layer (PBL) height exhibits systematic differences, with daytime and night-time sensitivity differences compensating for each other. The differences at daytime when a well-mixed PBL can be assumed indicate that residual inaccuracies in the representation of the mixing layer dynamics in the trajectories may introduce errors in the impact assessment on downwind sites. Emissions from vegetation fires are mixed up by pyrogenic convection which is not represented in FLEXPART. Neglecting this convection may lead to severe over-or underestimations of the downwind smoke concentrations. Introducing an extreme fire source from a different year in our study period and using fire-observation-based plume heights as reference, we find an overestimation of more than 60 % by the constant FL height assumptions used for surface emissions. Assuming a FL that follows the PBL may reproduce the peak of the smoke plume passing through but erroneously elevates the background for shallow stable PBL heights. It might thus be a reasonable assumption for open biomass burning emissions wherever observation-based injection heights are not available.

Table 1: Gas phase reactions # labels reaction rate coefficient reference G1000 UpStTrG O 2 + O(1 D) → O(3 P) + O 2 3.3E-11*EXP(55./temp) Sander et al. (2011) G1001 UpStTrG O 2 + O(3 P) → O 3 6.E-34*((temp/300.)**(-2.4))*cair Sander et al. (2011) G1002a UpStG O 3 + O(1 D) → 2 O 2 1.2E-10 Sander et al. (2011) * G1002b UpG O 3 + O(1 D) → O 2 + 2 O(3 P) 1.2E-10 Sander et al. (2011) G1003 UpStG O 3 + O(3 P) → 2 O 2 8.E-12*EXP(-2060./temp) Sander et al. (2011) G1004 UpG O 2 + O + → O + 2 + O(3 P) k_Op_O2(temp,temp_ion) Fuller-Rowell (1993) G1101 UpG O + 2 + e − → 2 O(3 P) 2.7E-7*(300./temp_elec)**.7 Fuller-Rowell (1993) G2100 UpStTrG H + O 2 → HO 2 k_3rd(temp,cair,4.4E-32,1.3, 7.5E-11,-0.2,0.6) Sander et al. (2011) G2101 UpStG Table 1: Gas phase reactions (... continued) # labels reaction rate coefficient reference G3002 UpGN N + 2 + e − → .88 N + 1.12 N(2 D) 1.8E-7*(temp_elec/300.)**(-0.39) Swaminathan et al. (1998) G3003 UpGN N(2 D) + e − → N + e − 3.8E-12*(temp_elec)**.81 Swaminathan et al. (1998) G3100 UpStGN N + O 2 → NO + O(3 P) 1.5E-11*EXP(-3600./temp) Sander et al. (2011) G3101 UpStTrGN N 2 + O(1 D) → O(3 P) + N 2 2.15E-11*EXP(110./temp) Sander et al. (2011) G3102a UpStGN N 2 O + O(1 D) → 2 NO 7.25E-11*EXP(20./temp) Sander et al. (2011) G3102b StGN N 2 O + O(1 D) → N 2 + O 2 4.63E-11*EXP(20./temp) Sander et al. (2011) G3103 UpStTrGN NO + O 3 → NO 2 + O 2 3.E-12*EXP(-1500./temp) Sander et al. (2011) G3104 UpStGN NO + N → O(3 P) + N 2 2.1E-11*EXP(100./temp) Sander et al. (2011) G3105 UpStGN NO 2 + O(3 P) → NO + O 2 5.1E-12*EXP(210./temp) Sander et al. (2011) G3106 StTrGN NO 2 + O 3 → NO 3 + O 2 1.2E-13*EXP(-2450./temp) Sander et al. (2011) G3107 UpStGN NO 2 + N → N 2 O + O(3 P) 5.8E-12*EXP(220./temp) Sander et al. (2011) G3108 StTrGN NO 3 + NO → 2 NO 2 1.5E-11*EXP(170./temp) Sander et al. (2011) G3109 UpStTrGN NO 3 + NO 2 → N 2 O 5 k_NO3_NO2 Sander et al. (2011) * G3110 StTrGN N 2 O 5 → NO 2 + NO 3 k_NO3_NO2/(2.7E-27*EXP(11000./ temp)) Sander et al. (2011) * G3111 UpGN N(2 D) + NO → N 2 + O(3 P) 6.70E-11 Fuller-Rowell (1993) G3112 UpGN N(2 D) + O 2 → NO + O(3 P) 6.20E-12*(temp/300.) Duff et al. (2003) G3113 UpGN N(2 D) + O(3 P) → N + O(3 P) 6.90E-13 Fell et al. (1990) G3114 UpGN N(2 D) + O 3 → NO + O 2 0.80E-16 Sander et al. (2003) G3115 UpGN NO + O(3 P) → NO 2 k_3rd(temp,cair,

Active fire observations with satellite instruments exhibit a well-documented increase of the detection threshold with increasing pixel footprint size, i.e., distance from the sub-satellite point. This results in a viewing angle-dependent, negative bias in gridded representations of the observed Fire Radiative Power (FRP), which in turn is frequently being used for climate monitoring of biomass burning and for pyrogenic emission inventories. We present a method based on quantile mapping to alleviate this bias and apply it to the gridded-FRP from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite instruments. The gridded-FRP observations are corrected with a correction function that depends on the satellite viewing angle and the magnitude of FRP in each grid cell. Assuming the fire observations at nadir to be the best representation of the truth, we derive a correction function by mapping cumulative distribution function (CDF) of off-nadir gridded-FRP to the CDF of near-nadir gridded-FRP. The method can be directly applied to correct the negative bias in gridded-FRP observations at a grid resolution of 1 ∘ or more. The performance of the correction methodology is confirmed through comparisons with co-located Visible Infrared Imaging Radiometer Suite (VIIRS) satellite observations: After bias correction, the gridded-FRP observations from both satellites agree better than before, particularly over savanna, tropical forests, and extra-tropical forests. Experiments with the Global Fire Assimilation System (GFAS) show that the impacts of the bias-corrected MODIS/Aqua gridded-FRP observations and VIIRS/Suomi-NPP gridded-FRP observations on regional FRP analyses are comparable, which confirms the potential for improving fire emission inventories and climate monitoring based on FRP.