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Abstract. Gas-phase ethene ozonolysis experiments were conducted at room temperature to determine formic acid yields as a function of relative humidity (RH) using the integrated EXTreme RAnge chamber-Chemical Ionisation Mass Spectrometry technique, employing a CH3I ionisation scheme. RHs studied were


Introduction
Organic acids are ubiquitous in the gas and aerosol phase, and are common constituents of global precipitation (Keene and Galloway, 1983).Organic acids have been measured in urban, rural, marine and remote areas (Talbot et al., 1988;Kawamura et al., 2001;Chebbi and Carlier, 1996).The contribution of organic acids to the acidity of precipitation and subsequent effects on aquatic and terrestrial ecosystems has been documented by Keene and Galloway (1986).Formic and acetic acid can dominate free acidity of precipitation thereby having an influence on pH-dependent chemical reactions and even OH cloud chemistry (Jacob et al., 1986).Low molecular weight organic salts -presumably the product of organic acid dissolution -are present in the fine fraction of aerosols, whose physical properties, namely hygroscopicity, possess relatively low critical supersaturations, allowing the activation of cloud droplets and subsequently affecting the total indirect forcing (Yu, 2000).Introduction

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Full Sources of carboxylic acids include biogenic and anthropogenic primary emissions, biomass burning and hydrocarbon oxidation, though their relative fluxes are poorly constrained (Chebbi and Carlier, 1996;Paulot et al., 2011).The major sinks of carboxylic acids are dry and wet deposition as a result of their low reactivity towards OH and NO 3 .However, the chemical loss via reaction with OH is poorly constrained resulting from the uncertainty in the reported rate coefficient (Atkinson et al., 2006).The modelled atmospheric lifetime of formic acid has been calculated to be 3.2 days (Paulot et al., 2011).
Global models under predict formic acid concentrations (von Kuhlmann et al., 2003;Rinsland et al., 2004;Paulot et al., 2011) especially in the marine boundary layer where [HC(O)OH] can be underestimated by a factor of 10-50, this discrepancy has been attributed to missing sources such as higher biogenic emissions during the growing season (Rinsland et al., 2004) and ageing of organic aerosols (Paulot et al., 2011).Also, the oxidation of VOC precursors leading to the production of formic acid has been suggested to be a significant source (Arlander et al., 1990), for instance the ozonolysis of ethene.Ethene emissions have been estimated to be about 15 Tg yr −1 (EDGAR, 1996) with about 162 Gmol yr −1 from the oceans (Paulot et al., 2011), and the presence of a major formic acid-producing reaction channel would therefore be of major importance to atmospheric chemical modelling.This study focuses on the production of formic acid from ethene ozonolysis.Intuitively, monitoring the products of this reaction ought to be easier than many ozonolysis reactions since the first-generation products possess a carbon number of one and are likely to be of maximum volatility.However, there still remains considerable inconsistencies in formic acid yields reported in the literature (Orzechowska and Paulson, 2005;Neeb et al., 1997;Wolf et al., 1997).Ozonolysis proceeds via a 1,3-cycloaddition across the olefinic bond to produce a primary ozonide, the decomposition of which forms a carbonyl moiety and a Criegee biradical each with unit yield (Fig. 9, Scheme 1).Introduction

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Full It is the fate of the Criegee biradical that determines the end product yield and this has provoked much attention in the atmospheric chemistry community (Johnson and Marston, 2008 and references therein), here the mechanisms highlighted shall focus on acid production pathways.It was first suggested by O'Neal and Blumstein (1973) that the Criegee biradical may isomerise to form a dioxirane intermediate, leading to the formation of carboxylic acids, as detailed by Orzechowska and Paulson (2005), this hypothesis is supported by the theoretical calculations of (Cremer et al., 1998) (Fig. 9, Scheme 2).
Formic acid may also be produced from bimolecular reactions.Calvert et al. (1978) suggested that in the presence of water, acid production can be significantly enhanced via reaction of the stabilised Criegee radical with water (Fig. 9, Scheme 3).The formation of HC(O)OH via Fig. 9 (Scheme 3) has been further supported by the theoretical results of Hatakeyama et al., 1981;Crehuet et al., 2001 andAnglada et al., 2002.Minor pathways such as cross reactions of reactive intermediates can form secondary ozonides, for instance reaction between Criegee biradicals and carbonyls, which have been suggested to lead to the formation of acids (Neeb et al., 1996).
Despite the importance of these formic acid-producing channels, there have been relatively few experimental determinations of HC(O)OH yields from the ozonolysis of ethene.Wolf et al. (1997) and Orzechowska and Paulson (2005) report high formic acid yields (0.36) for ethene ozonolysis conducted in dry conditions compared with that of Neeb et al. (1997) (0.01) (see Table 1).Discrepancies in yields reported under humid conditions also exist as Neeb and co-workers obtain 0.42 at 20 % RH yet Orzechowska and Paulson report 0.33 at 65 % RH.Wolf et al. (1997) and Orzechowska and Paulson (2005) both use indirect analytical techniques to quantify acid yields, whereas Neeb et al. (1997) use FTIR.All the techniques have the potential for significant errors as a result of sampling efficiency, spectral overlap and low sensitivity.This study aims to resolve the discrepancy by quantifying acid yields using the highly selective and sensitive technique, Chemical Ionisation Mass Spectrometry (CIMS).Introduction

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Experimental
Experiments were conducted in the dark in the 123 L Teflon®-coated EXTRA chamber, described in detail elsewhere (Leather et al., 2010, 2011and McGillen et al., 2011), and shown in Fig. 1.FVMQ o-ring seals within the end flanges ensured that the chamber was leak tight, whilst reinforcement ribs afford maximum pressures of 3750 Torr.
Seven sample ports are incorporated into the end flanges, to enable simultaneous sampling by a range of analytical techniques.The EXTRA chamber can be operated over the temperature range 193-473 K and pressure range 1-3750 Torr.Temperature control also allowed two day bake out cleaning procedures to be performed between experiments.Despite the volume of EXTRA being modest (surface:volume = 0.12) a combination of 100 % Teflon® surfaces, and temperature and pressure control results in a system of minimal wall losses with respect to oxidants and condensable hydrocarbons.
Quantitative ozone decay measurements were taken at 10 s time intervals, after allowing 5 min mixing time.Absolute ozone concentrations were measured using a Monitor Labs Inc. Ozone Analyzer (model 8810) through UV absorption at 254 nm (supplied from a mercury discharge lamp).Ozone was produced by flowing purified compressed air or oxygen (BOC, zero grade) through a UVP ozone generator (97-0067-02) into the chamber containing an atmosphere of nitrogen (BOC, oxygen free).The first-order decay rate of ozone with respect to walls and thermal decomposition using this continuous sampling configuration was found to be 6.94 × 10 −6 s −1 and thus were considered which enabled formic acid to be detected selectively at m/z = 173 (Slusher et al. 2004).
Ions were detected with a quadrupole mass spectrometer in a three-stage differentially pumped vacuum chamber, as shown in Fig. 2. A sample of the ion molecule gas flow containing reactant ions is drawn into the collision dissociation chamber through a 0.38 mm aperture which was held at a potential of −0.17 V to focus charged reactant molecules.The collision dissociation chamber was pumped by a molecular drag pump (Alcatel MDP-5011) backed by a scroll pump (ULVAC DISL-100) and held at approximately 20 Torr.The ions were further focused by an octopole ion guide, stainless steel with a 1.00 mm aperture held at −0.36 V and passed into a second chamber containing the further octopole ion guide and passed into the rear chamber via a stainless steel plate with a 1.00 mm aperture held at −0.48 V which contained the quadrupole mass filter (ABB Extrel, Merlin).This second and rear chamber were each pumped by a turbomolecular pump (Varian 81-M) backed by the molecular drag pump (Alcatel MDP-5011).Under typical operating conditions the rear chamber was held at a pressure of approximately 9 × 10 −6 Torr.Ions were detected using a channeltron (Dtech 402A-H) via negative ion counting.Gaseous reagents were added to the chamber at a known flow rate and duration using calibrated 1179 MKS mass flow controllers.The chamber contained an atmosphere of nitrogen preceding the addition of reagent gases.Ethene was introduced from a dilute ethene/nitrogen gas mixture.Ozone was produced by flowing oxygen through a UVP ozone generator (97-0067-02).Introduction

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Global model description
The Global Chemistry Transport model CRI-STOCHEM has been used to assess the mass of products formed in the atmosphere using data from this study.CRI-STOCHEM is described in detail in (Utembe et al., 2010 andArchibald et al., 2010).The model used is an updated version of the UK Meteorological Office tropospheric chemistry transport model (STOCHEM) described by Collins et al. (1997), with updates reported in detail in the recent paper of Utembe et al. (2010).STOCHEM is a global 3-dimensional CTM which uses a Lagrangian approach to advect 50 000 air parcels using a 4th-order Runge-Kutta scheme with advection time steps of 3 h.The transport and radiation models are driven by archived meteorological data, generated by the Met office numerical weather prediction models as analysis fields with a resolution of 1.25 The common representative intermediates mechanism (CRIv2-R5) (Jenkin, et al., 2008;Utembe, et al., 2009;Watson, et al., 2008), which represents the chemistry of methane and 22 emitted non-methane hydrocarbons was employed in the model.Each parcel contains the concentrations of 219 species involved in 618 photolytic, gas-phase and heterogeneous chemical reactions, with a 5 min time step.The formation of secondary organic aerosol (SOA) is represented using 14 species, which are derived from the oxidation of aromatic hydrocarbons, monoterpenes, and isoprene (see Utembe et al. 2011).
The surface emissions (man-made, biomass burning, vegetation, oceans, soil and "other" surface emissions) are distributed using two-dimensional source maps.Emissions totals for the base case run for CO, NO x and non-methane hydrocarbons are taken from the Precursor of Ozone and their Effects in the Troposphere (POET) inventory (Granier, et al., 2005) for the year 1998.The emission of aromatic species ortho-xylene, benzene and toluene were taken from Henze et al. (2008).Biomass burning emission of ethyne, formaldehyde and acetic acid are produced using scaling factors from Andreae and Merlet (2001) per mole of CO emitted.NASA inventories are used for aircraft NO x emissions for 1992 taken from Penner et al., (1999).The lightning and aircraft NO x emissions are monthly averages and are 3-dimensional in distribution.The gas-phase rate coefficient of the reaction of ethene with ozone was determined using the absolute method.The rate equation is shown in Eq. (2);

Assessment of instrument sensitivity
where k is the pseudo-first order rate coefficient given by k = k [O 3 ].For each experiment, the slope k was obtained using the linear regression of ln[O 3 ] vs. time for a broad range of alkene concentrations.First-order plots exhibited linear decays, (Fig. 3) having typical R 2 of 0.99, indicating first-order kinetic behaviour.The plot of k vs. initial [ethene] also exhibited a strong linear relationship (R 2 = 0.99), from which the gradient k, the bimolecular rate coefficient for the reaction was determined (Fig. 4).The rate was found to be (1.62 ± 0.14) × 10 −18 cm 3 molecule −1 s −1 , in excellent agreement with the literature recommendation (Atkinson et al., 2001).

Product yields
Product yields were determined in excess ethene conditions, in excess typically by a factor of 300-400.Initial comparison of HC(O)OH signal shows that [HC(O)OH] at RH 30 % exceeds that of RH <1 % by more than a factor of 7, in the initial stages during ethene ozonolysis, which suggests that reaction Fig. 9 (Scheme 3) dominates in the presence of water.
A simple model encapsulating these two Reactions (3 and 4) is compared with measurement data in Fig. 7. Here, the yield of HC(O)OH is defined as Clearly it is not possible to obtain a unique fit to the experimental data as there are no direct measurements of the rate of reaction of the Criegee radical with water (k 3 ).Indeed, estimates for the reaction rate of the Criegee radical with water range over three orders of magnitude (Calvert et al., 2000).However, a ratio between k 3 and k 4 emerges, where k 4 /k 3 is 3.3 × 10 17 molecule cm −3 to obtain an excellent fit to the measurement data.Assuming that k 3 has a maximum value of around 1.5 × 10 −10 cm 3 molecule −1 s −1 (gas kinetic limit) this puts an upper limit on the decomposition rate of the Criegee bi-radical of 5 × 10 7 s −1 , similarly, if k 3 is around 1.5 × 10 −17 cm 3 molecule −1 s −1 as suggested by indirect measurements then k 4 is only 5 s −1 , much lower than theoretical (Ryzhkov and Ariya, 2004) estimates.Indeed, Ryzhkov and Ariya (2004) suggest a value of k 4 between 5 × 10 5 s −1 and 5 × 10 2 s −1 , which provides a range for k 3 of 1.5 × 10 −12 cm 3 molecule −1 s −1 to 1.5 × 10 −15 cm 3 molecule −1 s −1 .Introduction

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Full In order to further investigate Scheme 3 and obtain experimental evidence to validate the production of hydroxymethylhydroperoxide (HMHP) during ethene ozonolysis, HMHP was synthesised according to Marklund et al. (1971).However, using I − chemistry, the CIMS instrument was not sensitive to the detection of HMHP, though this does not rule out HMHP production and the detection of HMHP could be achieved using an alternative ionisation scheme or an additional analytical technique.Wolf et al. (1997) did not observe enhancement of HMHP in humid conditions and so do not accept Fig. 9 (Scheme 3) to be responsible for acid production as a result of alkene ozonolysis.However, Neeb et al. (1997)  suggest that formic acid is not a major product of ethene ozonolysis and attribute acid production to the decomposition of HMHP on the solid-phase microextraction (SPME) fibre sampling system.This explanation is somewhat paradoxical, since if HMHP decomposition caused spuriously high acid yields in dry conditions, it is uncertain why HMHP was present in the system in the first place given that its formation is dependent on the presence of water (see Fig. 9, Scheme 3).Wolf et al. (1997) also observe large formic acid yields at low RH.However, the formic acid yield that they observe is the sum of primary formic, formic anhydride and HPMF, which could explain the discrepancy under dry conditions.Introduction

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Full CIMS is the most sensitive technique to date used to probe the production of HC(O)OH in the ethene + O 3 system.Whilst CIMS is selective to HC(O)OH there still remains the possibility that formic acid production is enhanced by heterogeneous processes during ethene ozonolysis.Temperature and pressure control allow this system to be baked out during cleanout procedures, producing a small measured k w (wall loss rate coefficient) with respect to ozone and HC(O)OH and so one can expect little impact on HC(O)OH yields from heterogeneous losses.The first-order decay rate of ozone and HC(O)OH with respect to walls were determined to be 6.94 × 10 −6 s −1 and 6.91 × 10 −6 s −1 respectively.Although studies by Neeb et al. (1997)  production, this is not apparent here and so is not concordant with this study.

Loss of CH 2 OO
The dominant loss process for the reaction of the simplest Criegee bi-radical, CH 2 OO (e.g.Taatjes et al., 2008), in the atmosphere on the one hand is not straightforward because of the lack of definitive rate coefficient data.However, it emerges from global model fields that with a rate coefficient of around 1 × 10 reaction with H 2 O should dominate its loss globally.Reaction with NO 2 , NO and SO 2 all compete with water at around the 5 ppbv level (urban environment) if one assumes a rate coefficient of 1 × 10 −12 cm 3 molecule −1 s −1 for these species with CH 2 OO in each case.However, if our previous analysis is correct, a value between 1 × 10 −17 cm 3 molecule −1 s −1 is probably too small, leading to the conclusion that reaction with water dominates non-decompositional loss.This study places an upper limit of about 65 % for the yield of HC(O)OH from the decomposition of CH 2 OO formed in the atmosphere (from ethene ozonolysis and one assumes from ozonolysis of other alkenes, i.e. the stabilised CH 2 OO is formed with a similar excess energy) via reaction with water, assuming a water concentration of around 6 × 10 17 molecule cm −3 .Introduction

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Model results
Data from this study for the ratio of decomposition of CH 2 OO with reaction with water to produce HC(O)OH has been used in the base case global model integration.
In the model there are two photochemical sources of CH 2 OO, ozonolysis of ethene and ozonolysis of isoprene.The base case integration produces the following sources of HC(O)OH (Tg yr −1 ); ozonolysis of ethene (1.6), ozonolysis of isoprene (23.6), from the reaction of OH with acetylene (ethyne) (3.7) and from direct emissions, 5.5 Tg biomass burning and 1.8 Tg anthropogenic sources (combustion) giving a total source of 36.2Tg.Hence the base case produces 28.9 Tg yr −1 from photochemical and 7.3 Tg yr −1 from direct emissions compared with a recent estimate of 48.6 Tg yr −1 from photochemical and 8.1 Tg yr −1 from direct emissions (Paulot et al., 2011).It is clear that the combined production of HC(O)OH from ozonolysis of ethene and isoprene is very important (∼70 %) in the model studies here and all of this arises from the reaction of CH 2 OO (formed from ozonolysis) with H 2 O.Such an assertion is in agreement with other studies such as von Kuhlmann et al., (2003).Loss processes include reaction with OH (3.4), wet deposition (18.3) and dry deposition (14.5), balancing the production processes.Figure 8 shows the surface level yearly average HC(O)OH from the base case integration.A further integration that includes OH recycling following the oxidation of isoprene, as described in Archibald et al. (2010) (as suggested by Paulot et al., 2011).Models underestimate HC(O)OH measurements, especially over the oceans, where in-situ production following the reaction of CH 2 OO with water will be at its peak.
10 Possible sources of CH 2 OO missing from the global model Stable products from isoprene oxidation, methyl vinyl ketone and methacrolein are included but in the simplified chemical scheme, ozonolysis does not yield HC(O)OH.
Using the yields of CH 2 OO from the work of Aschmann et al., (1996) and Grosjean et al., (1993), the ozonolysis of these two species will yield an additional 7.2 Tg yr −1

HC(O)OH based on global model estimates.
Monoterpenes are included in the model but assumed to react as either α-pinene or β-pinene and in the simplified mechanism used do not form CH 2 OO. Lee et al. (2006) have measured the yield of HC(O)OH from ozonolysis of a series of monoterpenes and found that for α-pinene (RH = 4.1 %) the yield was 7.5 and for β-pinene (RH = 6.3 %) the yield was 4 %.Using these data provides an additional 1.7 Tg yr −1 of HC(O)OH from α-pinene and 0.15 Tg yr −1 from β-pinene.However, the yield of nopinone (the co-product to CH 2 OO formation) from β-pinene ozonolysis was around 20 % (Lee et al., 2006) and assuming that under high water vapour levels most of the CH 2 OO yields HC(O)OH, leads to an increased estimate of 0.75 Tg yr −1 .Indeed, Larsen et al., (2001) reports HC(O)OH yields from ozonolysis of α-pinene (28 %) or β-pinene (38 %), using these yields produces 2.6 Tg yr −1 from α-pinene and 2.9 Tg yr −1 from βpinene.Therefore around 8 Tg yr −1 would be the total estimated by the global model if α-pinene and β-pinene are taken to represent all monoterpenes.In the Global model it is estimated that the total monoterpene emission is 127 Tg yr −1 and these emissions are distributed more globally than those of isoprene (e.g.there is a significant Northern Hemisphere high latitude emission).Hence, monoterpenes could be an important part of the "missing" source of HC(O)OH, further work to investigate the yield of HC(O)OH from ozonolysis of monoterpenes as function of RH is therefore warranted.Introduction

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Conclusions
This study has confirmed that the yield of HC(O)OH from the ozonolysis of ethene has a strong water dependence, rising rapidly with additional water.Assuming a simple two channel model for the fate of the CH 2 OO radical it has been possible to estimate the ratio of the rate coefficient for the reaction with water compared (k 3 ) with decomposition (k 4 ).Such an analysis suggests that k 3 probably ranges between 1 × 10 −12 -1 × 10 −15 cm 3 molecule −1 s −1 and as such will indeed be the dominant loss process, other than decomposition, for this radical in the atmosphere.Global model integrations confirm that this reaction between CH 2 OO with water is responsible for over half the production of HC(O)OH.However, HC(O)OH is still underestimated by the model.Unless there are missing biological sources, one is tempted to conclude that the myriad missing short-lived alkenes that could all contribute to CH 2 OO production could provide the missing source, particularly in the marine boundary layer where Reaction (3) will be at its highest rate.Further analysis shows that monoterpene oxidation and the ozonolysis of methyl vinyl ketone and methacrolein could contribute around 15 Tg yr −1 to the HC(O)OH budget.Introduction

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Full  Full  Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | negligible with respect to the timescale of the experiments.Quantitative concentration-time profiles of HC(O)OH were determined using CIMS.The CIMS was coupled to the EXTRA chamber through a sample port via 70 cm of 1/8" o.d.PFA tubing.CIMS sampled through a critical orifice at a flow rate of 0.8 SLM at 760 Torr and ∼296 K with a residence time of 0.1 s in the sample line preceding the ion molecule regionDiscussion Paper | Discussion Paper | Discussion Paper | HC(O)OH was detected using I − as the reagent ion.I − was generated by combining a 1.5 STP l min −1 flow of N 2 and a 1 sccm flow of 0.5 % CH 3 I/H 2 O/N 2 gas mixture and passing it through a Po(210) Nuclecel ionizer (NRD Inc.).HC(O)OH was ionised by I − via an adduct reaction, Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

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longitude and 0.83 • latitude and on 12 vertical levels extending to 100 hPa.Full details of the model version employed are given in Derwent et al. (2008).Discussion Paper | Discussion Paper | Discussion Paper | Dilute mixtures of HC(O)OH in deionized water were injected into the Chamber with no other gases present and the HC(O)OH.I − signal was monitored.From a linear plot of [HC(O)OH] vs. HC(O)OH.I − signal it is estimated that the sensitivity for HC(O)OH was 2.39 × 10 6 molecule cm −3 for a signal to noise ratio of one and a time constant of 1 s.Discussion Paper | Discussion Paper | Discussion Paper | 6 Rate coefficient determination Figure 5 shows two temporal profiles of the formic acid produced.The curve passing through the [HC(O)OH] experimentally determined values utilises the literature retrieved rate coefficient of 1.58 × 10 −18 cm 3 molecule −1 s −1 (Atkinson et al., 2001) and the line of best fit is obtained by varying the branching ratio to HC(O)OH formation.HC(O)OH yields were quantified as a function of relative humdity (RH), as summarised in Table 2.The formic acid yield appears to increase from RH <1-30 % although between 20-30 % RH there is a levelling off.The levelling off of HC(O)OH production (Fig. 6Discussion Paper | Discussion Paper | Discussion Paper | suggests that Fig. 9 (Scheme 3) does indeed operate.At low RH the lifetime of the Criegee radical with respect to reaction with water is long and is dominated by decomposition, as RH increases so this loss process becomes significant and at very high RH will dominate the loss, leading to a levelling off in yield of HC(O)OH.It is possible to model the HC(O)OH yield as a function of RH if it is assumed that the Criegee radical has one of two fates, decomposition (Reaction 4) or reaction with H 2 O to form HC(O)OH (Reaction 3 i.e.Fig. 9, Scheme 3).
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | report a time lag between d[HC(O)OH]/dt and −d[O 3 ]/dt indicating secondary heterogeneous HC(O)OH Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , reduces the amount of in situ production of HC(O)OH by 6.7 Tg.Such a reduction arises because the increase in OH reduces the amount of isoprene (leading to a 6.5 Tg reduction in HC(O)OH production) and to a lesser extent ethene (leading to a 0.2 Tg reduction in HC(O)OH production).Hence, with OH recycling, decomposition of CH 2 OO (from isoprene and ethene ozonolysis) via reaction with water accounts for 63 % of HC(O)OH production.The CRI-STOCHEM model has one of the most detailed Chemistry schemes for a global model, but there will be other sources of CH 2 OO that are not included in this model (e.g. the multitude of short-lived alkenes that are not included) and therefore the reaction of CH 2 OO with H 2 O would appear to dominate the in situ formation of HC(O)Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Adding the monoterpene, methyl vinyl ketone and methacrolein yields (∼15 Tg yr −1 ) with the base case estimate produces a photochemical yield of around 44 Tg yr −1 , close to the biogenic estimate of Paulot et al., (2011).Furthermore, all 1-alkenes (Johnson and Marston, 2008) can undergo ozonolysis to yield CH 2 OO and subsequently HC(O)OH.Hence there are myriad small sources of HC(O)OH that will contribute to global HC(O)OH.

Fig. 6 .
Fig. 6.A comparison of the experimentally determined HC(O)OH yields as a function of RH.

Figure 7 :
Figure 7: Modelled HC(O)OH yields as a function of RH %.

Figure 8 :
Figure 8: The annual mean surface formic acid derived from the base case model run.

Fig. 8 .A
Fig. 8.The annual mean surface formic acid derived from the base case model run.
detect high HMHP yields during ethene ozonolysis though they suggest that secondary chemistry through heterogeneous processes led to acid Orzechowska and Paulson (2005)ork(Anglada et al., 2002)suggests that HC(O)OH is produced via the formation of HMHP through a Criegee intermediate water complex, and HC(O)OH yields increase as a result of increasing relative humidity, which supports the observation of this study.The formic acid product yields obtained in this study are in good agreement withNeeb et al. (1997), across the range of RH studied.However, this work disagrees with dry yields reported byWolf et al. (1997)and byOrzechowska and Paulson (2005)(see Table of literature from earlier comment).Both studies utilise an indirect method of detection of HC(O)OH, which involves a sampling step.Orzechowska and Paulson

Table 1 .
Formic acid yields previously reported, to the best of our knowledge.

Table 2 .
A summary of the experimentally determined HC(O)OH yield obtained in this study, errors quoted are at the 1σ level of sensitivity calibrations.