the air / water interface 
 is perhaps the most widespread surface 
 in earth s environment . cloud droplets , lakes , seas , and oceans 
 all exhibit very large surfaces that play a crucial role in the exchange 
 of matter between the hydrosphere and the atmosphere . the understanding 
 of the specific processes occurring at such interfaces is thus of 
 importance from an atmospheric point of view . environmental air / water 
 interfaces often show the presence of surfactants , which are known 
 to impact the physical processes there . for instance , the transfer 
 between the aqueous phase and the gas phase is restrained in the presence 
 of an insoluble organic coating , which is able to inhibit both water 
 evaporation and solubilization of trace gases . moreover , organic coated air / water interfaces are expected to increase 
 the surface concentrations of a variety of hydrophobic compounds , 
 possibly impacting their phase transfers . these coated interfaces can also play a role in the removal of atmospheric 
 gases . for example , dry deposition on organic coated water surfaces 
 is suspected to constitute an important sink for ozone in the marine 
 boundary layer , both through its reactivity with the components of 
 the coating material and through increased solubility in this thin 
 organic layer . these air / water surfaces are furthermore 
 exposed to solar light for much of the time and specific photochemical 
 processes are therefore expected to occur there . one 
 interface of particular interest is the sea surface microlayer ( sea - sml ) , 
 defined as the uppermost 1 m to 1 mm of the surface of the 
 ocean , which is mainly composed of biogenic organics species such 
 as lipids , hydrocarbons , proteins or polysaccharides . it was shown 
 that light absorbing compounds , such as the natural chromophoric dissolved 
 organic matter ( cdom ) , are more concentrated in the sea - sml than in 
 the bulk water , and therefore it has been speculated 
 that photosensitized reactions could be particularly important in 
 this region , although these processes are still poorly understood 
 and neglected in atmospheric models . contrary to what was previously thought , the sml 
 is widespread over the sea surface and stable under wind speeds up 
 to 6.6 m s. evidence 
 also shows rapid reformation , within minutes , of the perturbed sml . in direct contact with the atmosphere , the role 
 the sml plays in the physical and chemical regulation of emission 
 and uptake of volatile organic compounds ( vocs ) and trace gases is 
 still to be clarified . light absorbing 
 species within the sml can participate in heterogeneous light - induced 
 or light - enhanced reactions with trace gases at the interface , as 
 has been shown for no2 reacting with humic acids or for 
 ozone with chlorophyll . these photochemical 
 reactions can also be a source of gaseous reactive species , as demonstrated 
 for the light induced release of volatile halogen radicals above organic 
 films . in another context , the sml sampled in a 
 commercial rice field , highly enriched in pesticides with concentrations 
 up to 10 times higher than in the subsurface waters , showed 
 an enhanced degradation for the pesticide thiobencarb compared to 
 the sampled subsurface water . organic 
 coatings can thus have significant impacts on the interfacial processes 
 compared to pure water surfaces , particularly if photosensitizers 
 show a propensity to partition to this organic layer . recently , 
 spectrometric measurements of the gas phase above an 
 irradiated real sea - sml sample or synthetic nonanoic acid sml enriched 
 with humic acids , showed the formation of a wide variety of functionalized 
 vocs . the photosensitized formation of unsaturated 
 compounds from a synthetic 1-octanol interfacial layer was also evidenced . these surface photochemical pathways could constitute 
 a still unaccounted for abiotic source of vocs in the marine boundary 
 layer and in all atmospheric environments where large air / water interfaces 
 are present . in particular the fate of common organic acids , who are 
 relatively unreactive in the gas - phase , could present an ubiquitous photochemical substrate and a source 
 of vocs in the sml , where such photochemical reactions would compete 
 with , for example , oxidation by hydroxyl radicals . nevertheless , the 
 chemical formation routes of these vocs are still poorly understood 
 as well as the role of the organic coating on the concentration of 
 photosensitizers at the interface . this work presents the study 
 of photochemical reactions in a synthetic 
 sml , composed of a saturated carboxylic acid , nonanoic acid ( na ) , 
 in the presence of known photosensitizers ( 4-benzoylbenzoic acid and 
 imidazole-2-carboxaldehyde ) . it focuses on the fundamental processes 
 leading to the formation of functionalized and unsaturated compounds 
 in the gas and condensed bulk phases . vis spectroscopy and 
 surface fluorescence experiments were performed to elucidate the behavior 
 of the photosensitizer at the coated interface . photoproducts were 
 observed in parallel in the gas phase by online mass spectrometry 
 and in the aqueous phase by off - line liquid chromatography coupled 
 to high resolution mass spectrometry . chemical mechanisms which explain 
 the observed product distributions are proposed and discussed in detail . uv  vis spectra 
 were acquired on a cary 60 uv  vis spectrophotometer ( agilent 
 technologies ) using 1 cm quartz cuvettes . the absorption spectra of 
 a 0.2 mm 4-benzoylbenzoic acid ( 4-bba ) aqueous solution at ph = 7.0 
 and of neat na were first recorded . next , 5 ml of the 4-bba aqueous 
 solution was mixed with 5 ml of neat na . the biphasic system was separated 
 by centrifugation at 4500 rpm for 2 min with fisher bioblock scientific 
 sigma centrifuge using a 15 ml eppendorf centrifuge tube . a similar 
 experiment was performed without 4-bba in the aqueous phase to check 
 for changes in the na spectrum due to the presence of dissolved water . fluorescence 
 of imidazole-2-carboxaldehyde ( ic ) at the air  water interface 
 has been studied by glancing angle laser - induced fluorescence ( galif ) . aqueous interface was induced using 
 the unfocused output of a nd : yag - pumped optical parametric oscillator 
 ( opo ) set at 285 nm , with 2 mj per 5 ns pulse . the laser beam 
 impinged the liquid sample surface at an angle > 85 from the 
 surface normal . the fluorescence emission signal was collected using 
 a liquid light guide suspended 6 mm above the impinging laser 
 beam and transmitted through a monochromator to a photomultiplier 
 tube . more details about experimental protocols , signal treatment , 
 and surface adsorption isotherm measurements are provided in the supporting information . transient absorption 
 experiments were carried out using the fourth harmonic ( 266 nm ) of 
 a quanta ray gcr 130 - 01 nd : yag laser , exciting the sample in a right - angle 
 geometry with respect to the monitoring light beam . individual cuvette samples ( 3.5 ml volume ) were used for a 
 maximum of two consecutive laser shots in order to avoid possible 
 substrate degradation . the transient absorption at a preselected wavelength 
 was monitored by a detection system consisting of a pulsed xenon lamp 
 ( 150 w ) , monochromator , and a photomultiplier ( 1p28 ) . a spectrometer 
 control unit was used for synchronizing the pulsed light source and 
 programmable shutters with the laser output . the signal from the photomultiplier 
 was digitized by a programmable digital oscilloscope ( hp54522a ) . a 
 32 bits risc - processor kinetic spectrometer workstation was used to 
 analyze the digitized signal . stock solutions of na and 4-bba were 
 prepared in milli - q water and mixed in an appropriate volume to obtain 
 the desired concentrations just before each laser flash photolysis 
 ( lfp ) experiment . the second - order rate constant for the quenching 
 of triplet state 4-bba ( 4-bba * ) in the presence of na was obtained 
 from the slope of the best - fit linear correlation between the pseudo - first - order 
 decay constant of 4-bba * monitored at 540 nm vs concentration of na 
 in aqueous solution . all experiments were performed at ambient temperature 
 ( 295  2 k ) in aerated solutions . the 
 experiments were performed using a 5 cm path length , 2 cm i d , 14 ml 
 cylindrical quartz cell with a gas inlet near each end ( starna , uk ) 
 13 cm away from the output lens of a xenon lamp ( 150 w xe , lot - quantumdesign , 
 france ) . the lamp was equipped with a 1 cm water filter in front of 
 the reaction cell to avoid excessive heating . the cell was half - filled 
 with 7 ml of the aqueous phase under study ( pure water or an aqueous 
 0.1 mm 4-bba solution ) in order to maximize the surface to volume 
 ratio ( 1.4 cm ) . a theoretical concentration of 
 2 mm of na was achieved by the addition of 2.5 l of neat na . the solution was then slightly stirred to promote na spreading over 
 the surface and was left to sit for 20 min in the dark before starting 
 the irradiation . a gas flow at a flow rate of 200 sccm ( standard cubic 
 centimeter per minute ) was continuously introduced into the cell , 
 with 80 sccm of the outgoing gas flow sampled for analysis by a switchable 
 reagent ion - time of flight - mass spectrometer ( 8000 sri - tof - ms , ionicon 
 analytik gmbh , innsbruck , austria ) . the experiments were performed 
 either under purified compressed dry air ( df ultrafilter , donaldson , 
 usa ) or dry nitrogen . for the experiments performed under nitrogen , the aqueous solution was deoxygenated before na addition by bubbling 
 nitrogen for 25 min . the analysis of the gas phase products 
 was performed using both h3o and no ionization modes to provide information about the product functionalities . ions used for identification of functional groups in the no mode are listed in table s2 ( supporting information ) . only qualitative results and normalized signals 
 are discussed for the no reagent mode as reaction rates 
 for most of the products with no are unknown . fragmentation 
 patterns for the main products under our experimental conditions were 
 carefully assessed using liquid standards diluted in 7 ml of water 
 in the reactor and analyzed under the same conditions as the samples , 
 as shown table s3 ( supporting information ) . concentrations were theoretically determined from the spectra 
 taken in h3o ionization mode , corrected for 
 the reaction branching ratios when these were known . the global uncertainty 
 on the absolute concentrations can be estimated to be about 40% . background spectra were taken before each experiment 
 above pure water , before adding the surfactant and an eventual photosensitizer . the error bars reported represent the standard deviation on repeated 
 experiments ( n  2 ) . the 
 experiments were performed using a similar setup as for the gas 
 phase products analysis . the cell was first filled with 11 ml of the 
 aqueous phase under study ( pure water or an aqueous 0.1 mm 4-bba solution ) 
 and 3.8 l of na was added to achieve a theoretical concentration 
 of 2 mm . the solution was then slightly stirred to promote na spreading 
 over the surface and was left to sit a few minutes in the dark before 
 a first sampling of 4 ml of the solution . the remaining 7 ml filled 
 the cell halfway . a fixed irradiation time of 1 h was used for all 
 the experiments . after irradiation , the solution was stirred again 
 for a few minutes in the dark before a second 4 ml sampling . sampling was performed 
 using a glass syringe through a septum before and after irradiation . the experiments were carried out either under air or under nitrogen . 
 for the experiments performed under nitrogen , the aqueous solution 
 was deoxygenated before na addition by bubbling with n2 for 25 min . aliquots of 500 l of each aqueous sample were 
 diluted adding 500 l of a 1:1 , v : v , water / acetonitrile mixture . next , 200 l of each sample was derivatized for specific carbonyl 
 compounds detection adding 800 l of a 1 mg ml pfbha solution and leaving the obtained mixture at room temperature 
 for 24 h before analysis . all samples were analyzed by reversed phase ultraperformance 
 liquid chromatography coupled to a q exactive high resolution mass 
 spectrometer through heated electrospray ionization [ uplc/()hesi - hrms , 
 thermo scientific , usa ] . a water / acetonitrile solvent gradient was 
 applied . the mass spectrometer was operated in the negative ionization 
 mode for the analysis of the nonderivatized samples and in the positive 
 ionization mode for the analysis of the derivatized ones . reported 
 signal intensities correspond to the total areas of the chromatographic 
 peaks determined from the extracted ion chromatograms of the identified 
 quasi - molecular ions . the analytical uncertainties were determined 
 over the whole experimental period as the relative standard deviation 
 on two series of standard injection : 8% for the nonderivatized standard 
 ( negative ionization mode ) and 23% for the derivatized standard ( positive 
 ionization mode ) . these standard deviation values are reported on 
 the bar graphs . the standard deviation over the three replicates performed 
 for each sample details about ionization conditions , calibration , 
 and data acquisition and processing are provided in the supporting information . the propensity 
 of a photosensitizer to partition to the surface 
 in the presence of an organic surfactant was investigated using uv  vis 
 spectroscopy and glancing angle laser - induced fluorescence spectroscopy . two well - known efficient photosensitizers were used , 4-benzoylbenzoic 
 acid ( 4-bba ) and imidazole-2-carboxaldehyde ( ic ) which are both aromatics possessing a carbonyl function and therefore 
 appropriate as a proxy for cdom . ic , 
 which is formed in situ in aerosols , can induce aerosol growth and has recently been observed in aerosol particles 
 in the field . first , the uv  vis 
 absorption spectrum of an aqueous solution of 4-bba at ph 7 was recorded , 
 as shown in figure 1 ( black ) . neat nonanoic acid was then added to this solution , and 
 aqueous and organic phases were separated by centrifugation . the absorption 
 spectrum of each phase was registered separately , and displayed in figure 1 ( dashed lines ) . changes in the absorption spectrum of the two phases show an almost 
 quantitative removal of 4-bba from the aqueous phase . indeed , approximately 
 98% of the 4-bba migrated from the aqueous phase into the organic 
 phase after seconds of contact , suggesting that the partitioning of 
 the photosensitizer toward the organic layer is highly favored . vis 
 spectra of aqueous and na solutions before and after 
 equilibration with the other phase in the presence and absence of 
 4-bba . blue and red arrows show the change in the absorption spectra 
 for the aqueous phase and the organic phase , respectively , in the 
 presence of 4-bba . the black line shows the absorption spectrum of 
 an aqueous solution of 4-bba at 0.2 mm . the red lines show the absorption 
 spectra of the organic phase after separation of the two phases ; the 
 solid line when no 4-bba was present in the system , the red dashed 
 line when in the presence of 4-bba . the blue lines show the absorption 
 spectrum of the aqueous phase after separation of the phases ; the 
 solid blue line when the system contained no 4-bba , the blue dashed 
 line when 4-bba was present in the system . the galif measurements probed the fluorescence of ic at the 
 surface 
 of an aqueous solution as a function of its solution concentration . the intensity of this ic surface fluorescence using pure water as 
 a substrate was compared with the ic surface fluorescence measured 
 at the surface of a 1.5 mm solution of na . this bulk concentration 
 provides near - monolayer na coverage at the air  water interface . the adsorption isotherms of ic at the aqueous  air interface , 
 derived from these measurements and displayed in figure 2 , clearly show an enhanced 
 fluorescence ( 50 ) in the presence of a na coating at the surface . this difference in fluorescence can solely be attributed to changes 
 in the surface concentration of ic demonstrating the propensity of 
 the photosensitizer to partition to the organic surface layer , since 
 ( a ) the bulk fluorescence yield of ic in nonanoic acid is lower than 
 in pure water ( figure s1 , supporting information ) and ( b ) the enrichment of a compound at the surface seems to depend 
 on its solubility in the monolayer . that show enhanced fluorescence of ic in the 
 presence of a near monolayer of 1-octanol . both spectroscopic techniques 
 confirm the enhanced concentration of the photosensitizers used here 
 at the air / water interface in the presence of an organic coating , 
 leading very probably to an enhanced and specific photochemistry there . fluorescence 
 intensity (  = 335 nm ) of ic at the air / water 
 interface as a function of the bulk concentration of ic , measured 
 at a pure water surface (  ) and at a nonanoic acid coated surface 
 (  ) . lfp 
 excitation of a 0.21 mm 4-bba aqueous solution at ph 7.0 produces 
 a transient absorption spectrum presenting two intense absorption 
 peaks at 410 and 540 nm , illustrated in figure s2 ( supporting information ) , which are attributed to the triplet 
 state of 4-bba ( 4-bba * ) . the transient 
 species decays uniformly with a pseudo - first - order k540nm = 4.76  0.11  10 s , 
 and no residual absorbance was observed after about 5 s . when 
 4-bba * is dissolved in neat na , its transient spectrum presents two 
 maxima at 350 and 560 nm , shown in figure 3 . after relaxation of the triplet state , 
 approximately 2 s after the laser impulse , two intense bands 
 centered at 360 nm and at 580 nm are observed . the absorption decay 
 in this pure na solution was followed at 560 nm and fitted well a 
 double exponential equation , suggesting the presence of two different 
 species . the first , short - lived species can be attributed to the excited 
 state of 4-bba . the second long - lived component may correspond to 
 the ketyl radical of 4-bba . the formation of a 
 ketyl radical can be explained by electron or hydrogen transfer from 
 organic molecules ( i.e. , na ) , a process which has previously been 
 reported for other benzophenone derivatives . time 
 trend of the transient spectrum obtained upon laser - pulse 
 excitation ( 266 nm , 40 mj ) of 0.21 mm 4-bba in na solution . figure s3 ( supporting information ) reports 
 the rate constant for the pseudo - first - order decay of 4-bba * followed 
 at 540 nm as a function of na concentration in an aqueous solution . the data allows for the determination of the second - order rate constant 
 for the quenching of the triplet state by nonanoic acid k4bba * ,  na = 3.0  0.5  
 10 m s , which 
 is competitive with rate constants reported for aliphatic carboxylic 
 acids and hydroxyl radicals . the photoinduced 
 chemistry occurring at the air / water interface 
 in the presence of na as organic coating and 4-bba as photosensitizer 
 was investigated . gas - phase products were analyzed online while the 
 analysis of bulk condensed phase products was performed off - line , 
 with and without selective derivatization of carbonyl groups . upon 
 irradiation , the prompt formation of gas phase products similarly , 
 a wide variety of photoproducts was detected in the condensed phase 
 after 1 h of irradiation . a complete list of the detected compounds 
 is provided in table s1 ( supporting information ) . nevertheless , it clearly appears that this large variety of photoproducts 
 is formed upon irradiation in the presence and in the absence of 4-bba . the photochemistry of only na in concentrated media , and more specifically 
 at the air / water interface , was recently evidenced and can explain 
 these observations . however , here , the 
 detection of the na-4-bba recombination product ( c23h28o5 ) confirms the existence of photoinduced reactions 
 between the photosensitizer and the organic surfactant . the ratios 
 of the net productions for a selection of photoproducts in the gas 
 and condensed phase in the presence and in the absence of 4-bba are 
 displayed in figure 4 . net production signals observed for the same series of products 
 in both phases are shown in figures s5 and s6 ( supporting information ) . ratio of the signal detected in the presence 
 of 4-bba ( 0.2 mm ) 
 to the signal observed without 4-bba for a series of selected photoproducts 
 in the gas phase and in the aqueous phase , for a 2 mm na aqueous solution . errors reflect the analytical uncertainties ( condensed phase ) or repeatability 
 between experiments ( gas phase ) . ( # ) identified as ketone due to no adduct ; (  ) detected as pfbha derivatives . octanal ( c8h16o ) was one of the main products 
 in both phases . aldehydes and ketones ( < c9 ) , including unsaturated and 
 dicarbonyl compounds , were also detected . octenal ( c8h14o ) was one of the main unsaturated oxygenated compounds observed . 
 in the condensed phase , c9 products with three or more 
 oxygen atoms , such as c9h18o3 and 
 c9h16o3 , were also detected . these 
 compounds can be tentatively identified as na bearing additional hydroxyl 
 and/or carbonyl functions . they were not detected in the gas phase , 
 probably due to their low volatility . small acids ( < c9 ) were detected in both phases but only in rather low abundances . some unsaturated acids were observed as well , the main one being c9h16o2 , identified as nonenoic acid . alkenes could not be evidenced in the condensed phase due to the electrospray 
 ionization limitation for nonpolar compounds , but c4h8 and c5h10 were the most abundant unsaturated 
 products detected in the gas phase . additionally , compounds bearing 
 more than nine carbon atoms were observed in the condensed phase , 
 such as c18h34o4 , c28h22o6 , and c23h28o5 . these molecular formulas can be attributed to the dimer of na , the 
 dimer of 4-bba and , as already mentioned , the combination of na and 
 4-bba , respectively . experiments performed using ic as the photosensitizer 
 showed overall a very similar pattern of product formation , but with 
 lower intensities for the products , probably due to the lower quantum 
 yield of the triplet state of ic compared to 4-bba . interestingly , the formation of certain products is 
 favored in 
 the presence of 4-bba while the formation of others is hindered , as 
 shown in figure 4 . enhanced production is observed in the presence of 4-bba for some 
 oxidized and saturated compounds , such as the c<9 aldehydes ( c8h16o , c5h10o , c4h8o ) and various c9 oxidation products ( c9h18o3 and c9h16o3 ) . photoproducts hindered in the presence of 4-bba are 
 the c9 carbonyls , nonanal and nonenal , and in a more general 
 way the compounds presenting a c  c double bond . very similar 
 observations arose from experiments performed in the presence and 
 in the absence of h2o2 as photoinitiator , as 
 can be seen in figure s8 ( supporting information ) , hydroxyl radicals being also able 
 to react quickly with c  c double bonds leading to functionalization 
 and fragmentation . competition for h 
 abstraction between 4-bba * and na * could contribute to the fact that 
 saturated and unsaturated c9 aldehydes , expected to be 
 specific to the reactivity of na alone photochemistry in concentrated 
 media , are hindered in the presence of 4-bba . triplet state 4-bba can react with organics such as na through 
 h - abstraction , to initiate radical chemistry , a pathway supported by the similarity between the series of products 
 observed in these experiments with those obtained using h2o2 as photoinitiator . the 
 hydroxyl radical generated by h2o2 photolysis 
 upon irradiation is expected to react with na only via h - abstraction . have demonstrated 
 that radical - initiated photochemistry of aqueous solutions containing 
 only na readily occurs , and suggested that this is initiated by the 
 formation of an na excited state , followed either by h - abstraction 
 from another na molecule or by a homolytic cleavage releasing a hydroxyl 
 radical . if this hypothesis is true , the formation of c9 aldehydes would be specific to the photochemistry of na alone , as 
 these compounds are formed directly from the excited state of na . the results detailed here suggest that in the presence of 4-bba ( or 
 h2o2 ) , a competition between the photochemistry 
 of na - only chemistry and  traditional  h - abstraction 
 by an excited photosensitizer occurs , with the products derived from 
 the na radical formed after h - abstraction on the carbon chain being 
 favored , and those derived directly from the na excited state hindered . to investigate the role of oxygen in this chemistry , all the experiments 
 were repeated under nitrogen using deoxygenated solutions . the net 
 production of the same selection of compounds under these two conditions 
 is compared in figures s5 and s6 ( supporting information ) , and figure 4 compares 
 the ratios of the signals obtained under these two conditions . the 
 highly oxygenated c9 products are clearly favored when 
 oxygen is more abundant ( e.g. , c9h18o3 and c9h16o3 ) . while on the other 
 hand , the dimers ( e.g. , c18h34o4 ) 
 as well as the unsaturated c9 acid ( c9h16o2 ) appear to be favored in an oxygen - poor environment . the influence of oxygen is less clear for the carbonyl compounds and 
 in particular for those bearing an unsaturation , as illustrated for 
 octenal . from these observations a general mechanism of the 
 photosensitized 
 degradation of na after the initial h - abstraction 
 by the exited state of the photosensitizer , two pathways are expected : 
 the addition of molecular oxygen or radical  radical reactions . the latter explains the formation of na dimers , but also the formation 
 of the observed na-4-bba and 4-bba-4-bba recombination products . this 
 type of fast radical recombination , has been observed previously in 
 photosensitized reactions in aerosols , but occurs only where high radical concentrations are achieved . the presence of such recombination products 
 thus highlights some particular features of the enhanced concentration 
 at the air / water interface . this surface enhanced concentration can 
 also favor radical  radical disproportionation reactions , explaining the formation of nonenoic acid and 
 possibly of other unsaturated compounds . molecular oxygen addition 
 is not expected to be specific to the interface , as oxygen readily 
 dissolves in bulk water , where similar chemistry , leading to the formation 
 of a peroxy radical , has been reported . this peroxy radical is then expected either to fragment , leading 
 to the formation of octanal , or to evolve toward hydroxyl or carbonyl 
 functions . these products can then undergo similar processes as the 
 initial na molecules , leading to the formation of second generation 
 products . radical reactions 
 promoted at the interface and oxygen addition appears thus to play 
 a key role in the distribution of the products . this is highlighted 
 by the fact that recombination and disproportionation products were 
 observed to be favored in an oxygen - poor environment . considering 
 the proposed mechanism , the formation of unsaturated aldehydes , apparently 
 not impacted by the presence of oxygen , also underlines this competition ; 
 while oxygen is required to form the saturated aldehyde on which h - abstraction 
 can then occur , disproportionation reactions leading to the unsaturated 
 aldehyde is favored under nitrogen . proposed mechanism for the photosensitized 
 degradation of na . blue 
 highlighted pathways are expected to be promoted under air and red 
 highlighted pathways at the interface and under nitrogen . the products 
 identified by their chemical formulas ( gray ) correspond to products 
 shown in figure 4 . alkenes , which are not mentioned 
 on the proposed mechanism , could 
 arise from the photolytic cleavage of the carbonyl compounds , but 
 this does not strictly match the observations , as the formation of 
 the main alkenes observed , butene and pentene , appears to be favored 
 under nitrogen ( figure s5 , supporting information ) . another pathway could invoke the cyclization of the na radical 
 leading to the formation of a c5 or c4 lactone 
 and a related c4 or c5 alkyl radical fragment , 
 which ultimately gives rise to butene and pentene ( figure s9 , supporting information ) . this mechanism is in 
 agreement with the observation that these products are favored under 
 nitrogen since the cyclization and disproportionation steps would 
 both be in competition with oxygen addition . in conclusion , 
 this study relates the photochemical production 
 of functionalized and unsaturated compounds at an air / water interface 
 in the presence of a carboxylic acid , nonanoic acid , as surfactant . when a photosensitizer , such as 4-bba or ic , is present in the aqueous 
 solution , it will rapidly partition to the organic surface layer where 
 it can induce radical reactions through a hydrogen abstraction on 
 a na molecule , leading to the formation of a large variety of compounds . the photochemical production and the gas phase release of such 
 unsaturated and functionalized compounds will influence the oxidative 
 capacity of the atmosphere and can lead to secondary aerosol formation . this interfacial photochemistry has potentially a very large impact , 
 especially as both reactants , the photosensitizer , and the substrate 
 can be naturally present at high concentrations at the surface , providing 
 a very favorable venue for this new type of photochemistry . although 
 the photochemistry triggered by the photosensitizer competes with 
 the photochemistry induced by a carboxylic acid at the surface layer , 
 generally it more efficiently leads to oxidized products . moreover , 
 this photochemistry seems to occur with different types of photosensitizers 
 ( 4-bba , ic , h2o2 ) and surfactant organic coatings 
 ( nonanoic acid , octanol , heptanol ) , meaning that these photosensitized 
 processes could be much more generally occurring than previously thought . we expect that this chemistry will apply especially to ocean surfaces , 
 cloud droplets , and evanescent atmospheric droplets , where organics 
 are expected to be enriched at the air interface .
interfaces 
 are ubiquitous in the environment and many atmospheric 
 key processes , such as gas deposition , aerosol , and cloud formation 
 are , at one stage or another , strongly impacted by physical and chemical 
 processes occurring at interfaces . here 
 , the photoinduced chemistry 
 of an air / water interface coated with nonanoic acid  a fatty 
 acid surfactant we use as a proxy for chemically complex natural aqueous 
 surface microlayers  was investigated as a source of volatile 
 and semivolatile reactive organic species . 
 the carboxylic acid coating 
 significantly increased the propensity of photosensitizers , chosen 
 to mimic those observed in real environmental waters , to partition 
 to the interface and enhance reactivity there . 
 photochemical formation 
 of functionalized and unsaturated compounds was systematically observed 
 upon irradiation of these coated surfaces . 
 the role of a coated interface 
 appears to be critical in providing a concentrated medium allowing 
 radical  radical reactions to occur in parallel with molecular 
 oxygen additions . 
 mechanistic insights are provided from extensive 
 analysis of products observed in both gas and aqueous phases by online 
 switchable reagent ion - time of flight - mass spectrometry and by off - line 
 ultraperformance liquid chromatography coupled to a q exactive high 
 resolution mass spectrometer through heated electrospray ionization , 
 respectively .