Source identification and budget analysis on elevated levels of formaldehyde within ship plumes : a photochemical / dynamic model analysis

C. H. Song, H. S. Kim, R. von Glasow, P. Brimblecombe, J. Kim, R. J. Park, and J. H. Woo School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju, 500-712, Korea School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK Department of Atmospheric Sciences, Yonsei University, Seoul, 120-749, Korea School of Environmental Sciences, Seoul National University, Seoul, 151-742, Korea Department of Advanced Technology Fusion, Konkuk University, Seoul, 143-701, Korea

More recently, elevated levels of HCHO in heavy ship-traffic corridors (or large HCHO vertical columns over the oceans impacted by ship emissions) were detected by the ESA/ERS-2 GOME (Global Ozone Monitoring Experiment) sensor (Marbach et al., 2009).The elevated HCHO levels along the heavy ship-traffic corridors are of fundamental importance because they can greatly perturb the atmospheric oxidation cycle (or O 3 /HO x /N x O y photochemistry) in ship-influenced MBL.In turn, the perturbed oxidation cycle can also affect the global radiation budget, for example, by possibly enhancing the O 3 mixing ratios in the MBL.Moreover, if the main source of MBL HCHO production is from atmospheric CH 4 oxidation, the removal of CH 4 can also indicate re-Figures

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Full duced atmospheric warming, as CH 4 is a major global warming gas (Hansen and Sato, 2001;IPCC, 2007).According to global chemistry-transport modeling (CTM) studies by Bey et al. (2001) and Lawrence et al. (2001), approximately ∼50% of the global CH 4 chemical losses is removed within the planetary and marine boundary layers (PBL and MBL), even when ship NO x emissions are not considered.The model-predicted fractions of global CH 4 destruction in the MBL could be even higher if ∼21% of the anthropogenic global NO x emissions from ocean-going ships are considered (cf.Corbett and Koehler, 2003).Recently, Hoor et al. (2009), in their multiple global 3-D CTM ensemble study, reported active CH 4 destruction within the TBL.However, their study was carried out without rigorous consideration of the complexity and non-linearity of ship-plume photochemistry in the global CTM simulations.Overall, it is important to understand such marine boundary chemical processes in terms of the atmospheric photochemistry and chemistry-climate interactions.
Another important issue that should be addressed is the possible levels of HCHO in ship-influenced MBL.It is difficult to retrieve accurate HCHO mixing ratios in the MBL from satellite sensors, such as GOME, SCIAMACHY, and OMI, mainly because of (1) difficulty in accurately estimating the air-mass factor (AMF) and (2) interference by ship-emitted aerosols during the retrieval of HCHO columns over the heavy ship-traffic ocean corridors (Marbach et al., 2009).Hence, there is some controversy regarding the HCHO levels within the heavy ship traffic corridors (Marbach et al., 2009).Considering these arguments, in this numerical analysis of ship-plume photochemistry, an attempt was made to answer two important scientific questions: (1) what is the main (or dominant) atmospheric HCHO generation process, and (2) how much can HCHO be generated or the HCHO mixing ratios increases by the atmospheric HCHO generation processes within the ship plumes?
In order to explore these issues, first of all, three likely sources of the elevated HCHO levels in the ship plumes were assumed and then examined: (1) primary HCHO emission from ships; (2) secondary HCHO production via the atmospheric oxidation of Nonmethane volatile organic compounds (NMVOCs) emitted from ships; and (3) atmo-Introduction

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Full spheric oxidation of CH 4 within the ship plumes.A ship-plume photochemical/dynamic model developed previously by Kim et al. (2009) was used to help answer to these scientific questions.In addition, in terms of the ship-plume photochemical/dynamic model development and its applications, this is a companion study of Kim et al. (2009) with particular focus on ship-plume HCHO.This study first discusses the ship-plume case from the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) campaign as a base study case because this case has been analyzed most thoroughly (hereafter, called "ITCT 2K2 ship-plume case"), and various likely ship-plume cases at different latitudinal locations of the global ship tracks (hereafter, these are called "constructed cases") were then explored.The impacts of the ship-plume photochemistry on the MBL photochemical cycles, global climate, and marine eco-system in the global ship corridors are further discussed based on the discussions in this study.
Although the UBoM 2K8 model has three operational modes, the three modes share the same gas-phase photochemistry, heterogeneous reaction schemes, and aerosol physics and dynamics.In this study, the last photochemical modeling mode was used.

Model validation
The details of the ship-plume modeling components of the UBoM 2K8 model were explained previously by Song et al. (2003a,b) and Kim et al. (2009), and are not repeated here.However, the model has two fundamental components to treat: (1) atmospheric Introduction

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Full thermo-chemical, photo-chemical and heterogeneous reactions; and (2) turbulent dispersion of air pollutants emitted from a ship.For the latter, a "Gaussian-based" turbulent dispersion scheme was adopted from the Offshore and Coastal Dispersion (OCD) model with some modifications (Hanna et al., 1985;Song et al., 2003a;Kim et al., 2009).For the former, a modified Lurmann chemical mechanism was used, in which 255 atmospheric gas-phase reactions coupled to heterogeneous condensations and aerosol micro-physics were considered (Lurmann et al., 1986;Song et al., 2003b).In particular, several turbulent ship-plume dispersion schemes within the MBL were reviewed extensively by Faloona (2009).According to his study, the parameterizations from the OCD model would be the best for accurately describing the turbulent dispersion of ship plumes within the MBL, which was confirmed by Kim et al. (2009) using the ITCT 2K2 ship-plume case.
Along with the turbulent dispersion scheme, the full capability of the ship-plume photochemical/dynamic model was validated comprehensively using a data set from the ITCT 2K2 ship-plume measurements by comparing the model-predicted data with the aircraft-measured ship-plume concentrations of NO x , NO y , O 3 , HNO 3 , and H 2 SO 4 as well as the ozone production efficiency (OPE) (Parrish et al., 2004;Kim et al., 2009;Xuan et al., 2009).Figure 1 shows a schematic diagram of the airborne ship-plume experiment using NOAA WP-3D aircraft during the ITCT 2K2 campaign, which were carried out approximately 100 km off the coast of California.The NOAA WP-3D aircraft traversed the ship-plume eight times from transects A to H at an angle of 59 • between the ship-plume centerline and the flight pathways.In Fig. 1, A − and A • represent two imaginary transects where the ship-plume photochemistry near the ship stack is intended to be investigated.According to Kim et al. (2009), the ship-plume photochemical/dynamic model can predict the ship-plume composition accurately.This was confirmed again in Fig. 2 with further multiple sensitivity model runs, in which the NO x emission rates from the ship were varied over a range of 2.6-13.3g s −1 , together with the primary NMVOC and HCHO emissions (both the primary NMVOC and HCHO emissions are discussed further in Sect.2.2).As shown in Fig. 2, there is good agreement Introduction

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Full between the model-predicted and observed concentrations of NO x , O 3 , and HNO 3 .
For this set of model validations, the concentrations of one primary pollutant (NO x ) and two secondary species (O 3 and HNO 3 ) were compared because the photochemistry of these three species is coupled closely with one another and is also related strongly to the production of OH radicals, which play a key role in CH 4 and NMVOC oxidation in the MBL.Here, the blue and red shadows represent the ranges of the model-predicted species concentrations under moderately stable and stable MBL conditions, respectively.Further details regarding the measurement uncertainties, selection of the MBL stability, and data analysis were discussed previously by Kim et al. (2009).

Model simulations
According to Chen et al. (2005), the ship being investigated in this study was equipped with a 6,707 kW engine.With this power, the total rate of NMVOC emission from the ship can be estimated to be 0.93 g s −1 using the NMVOC emission factor of 0.5 g kWh  Ltd. (2005;hereafter, abbreviated ECDGE & Entec).On the other hand, the total NMVOC emission rate from the ship can also be estimated from a ratio of NO x to NMVOCs in the ship emissions.Again, according to ECDGE & Entec (2005), the ratio was typically 29.6:1.0(NO x :NMVOCs) on a mass basis.Total NMVOC emission rates of 0.09-0.45g s −1 were obtained from the estimated NO x emission rate from the ship (2.6 to 13.3 g s −1 ).From these estimations, the total NMVOC emission rate could range between 0.09 g s −1 and 0.93 g s −1 .Although the estimated values have significant uncertainty, they appear to be consistent with the globally-averaged ship NMVOC emission rate of 0.75 g s −1 estimated by Endresen et al. (2003).Table 1 was constructed from these values.In Table 1, the chemical speciation and fraction of NMVOCs emitted from the ship were determined from the ship-plume data reported by Cooper et al. (1996), EPA (2000), Endresen et al. (2003), ECDGE & Entec (2005), and Houyoux (2005).The fraction of primary HCHO in the NMVOC emissions varies Introduction

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Full considerably from ship to ship, fuel-type, age of the ship, engine-speed etc. HCHO fractions of 0.4-10.0%were reported based on the Lloyd Register (1995), EPA (2000), Houyoux (2005), andMarbach et al. (2009).Unless noted otherwise, the mid-point values from Table 1 were used (making their summation unity) in this numerical study of the ship-plume photochemical/dynamic modeling.
As discussed by Kim et al. (2009), the ship-plume dispersion is governed primarily by the stability class of MBL.Based on the NOAA WP-3D aircraft measurements of the meteorological variables in/around the ship plume, Kim et al. (2009) suggested that the stability class of the ship-going MBL would range from moderately stable (E) to stable (F).In general situations, within the heavy ship-traffic MBL over the remote oceans, the stability class of the MBL frequently ranges from neutral (D) to stable (F) (cf.Frick and Hoppel, 2000;Song et al., 2003a).Therefore, three stability classes (instead of the two stability classes for the base case study) were considered in this numerical shipplume photochemistry study of the constructed cases at different latitudinal locations: (1) neutral (D); (2) moderately stable (E); and (3) stable (F) stability classes.
In this numerical ship-plume modeling study, a background photochemical box model was also run to consider the diurnal changes in HCHO mixing ratios in the background air using both the background meteorological conditions and chemical composition.This calculation was carried out by running the background photochemical box model until a pseudo steady-state in the background chemical composition had been reached.
The background HCHO mixing ratios calculated were then mixed with the ship-plume volumes.By doing this, a discontinuity problem in HCHO mixing ratios, which typically occurs at the interface between the edge of the ship-plumes and the background air, can be avoided.With the exception of HCHO, the fixed average concentrations measured by the NOAA WP-3D aircraft in the background were used in the model simulations.
One of the key issues of this study is CH 4 oxidation.The CH 4 oxidation rates are controlled mainly by the levels of OH radicals, which were found to be a function of the stability class of the MBL (Song et al., 2003a;Kim et al., 2009).The levels of Introduction

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Full OH radicals can also be affected by several other factors, such as NO x emission rate, NMVOC emission rates, NMVOC chemical speciation, and ship location.As discussed above, there is some uncertainty in the ship NO x emission rates, which would range from 2.6 g s −1 to 13.3 g s −1 in the base ship-plume case (Chen et al., 2005;Kim et al., 2009).In addition, in general the NO x emission rates from ships must vary from ship to ship, ship-type, ship-speed and engine-type (cf.EPA, 2000;ECDGE & Entec, 2005;Houyoux, 2005).As discussed previously, the total NMVOC emission rates were also variable, and there is large uncertainty, even in the chemical speciation of NMVOCs emitted from ships (see Table 1).Moreover, the OH radical mixing ratios are a strong function of the meteorological variables, such as solar radiation, relative humidity and temperature (Song et al., 2003a,b).Therefore, in order to consider these situations, several cases were constructed by changing the factors that can affect the OH levels within the ship plumes.The cases are presented in Sect.3.2.For example, the most active/vigorous ship-plume photochemistry would be expected in the "stable, tropical MBL" with large NO x emission rates, producing the highest OH levels.This study first discusses the ITCT 2K2 ship-plume case as a base study case, and then explores various likely constructed cases at different latitudinal locations.The results from the constructed case studies are discussed in Sect.3.2.

Results and discussions
In this section, the budget of HCHO within the ITCT 2K2 ship-plume as a base case

Base-case study
As mentioned in Sect. 1, the following three possible sources of the elevated HCHO levels were considered in this numerical study: (1) primary HCHO emission from ships; (2) secondary HCHO production via atmospheric oxidation of NMVOCs emitted from ships; and (3) atmospheric oxidations of CH 4 within the ship plumes.Of these three possible sources, the last source, CH 4 oxidation, may be significant.
In theory, two atmospheric reactions could initiate photochemical HCHO production via the atmospheric CH 4 oxidation: (1) CH 4 +OH ((R1) in Table 2) and ( 2) CH 4 +O( 1 D) (with two reaction pathways, refer to (R2) and (R3) in Table 2).Of these three reactions, the latter two would be of secondary importance because O( 1 D) radicals mainly react with the more abundant H 2 O molecules in the MBL, producing OH radicals (i.e., O( 1 D)+H 2 O → 2OH).The former could be primarily responsible for photochemical HCHO production within the MBL.Numerical sensitivity analyses of the reactions (i.e., switching the reactions ON and OFF) were carried out to determine the impact of the reactions on HCHO production in the ship-going MBL.
Background HCHO mixing ratios of approximately 210-430 pptv were reported in the MBL, e.g. over the Southern Indian Ocean (SIO) by Wagner et al. (2001).In addition, enhanced OH concentrations due to ship NO x emissions were suggested by several global CTM simulation studies (e.g., Lawrence and Crutzen, 1999;Hoor et al., 2009).Although both measurement and modeling studies can provide circumstantial evidence for the possibility of active HCHO production or generation in a ship-going MBL, they are limited in that: (1) the measurements were made only over a limited location like the SIO; and (2) such Eulerian modeling framework with a coarse grid resolution tends to over-predict the OH radical concentrations by skipping the complex, non-linear in-plume photochemistry.Therefore, it can produce excessive CH 4 oxidation by OH radicals (Lawrence and Crutzen, 1999;Song et al., 2003a;von Glasow et al., 2003;Kim et al., 2009).In this numerical study, a single, ordinary ship-plume situation was considered, focusing more on how fast the CH 4 and/or NMVOCs can Introduction

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Full CH 4 +OH reaction ON+NMVOC emission with primary HCHO emission; and (Background): only with the HCHO background mixing ratios.As shown in Fig. 3, neither the direct emission of HCHO from the ship nor atmospheric oxidation of NMVOCs emitted from the ship is the major factor that can completely explain the elevated levels of HCHO within the ship plume.Instead, the enhanced rate of the atmospheric oxidation of CH 4 is the most likely cause for the enhanced HCHO levels.Figure 3a and b, the first and the second rows in Fig. 3, present the changes in the transect-averaged ship-plume HCHO mixing ratios with respect to the ship-plume travel time (Fig. 3a, first row) and differences between the averaged ship-plume HCHO mixing ratios and modelpredicted background HCHO mixing ratios (Fig. 3b, second row), respectively.Both the transect-averaged ship-plume HCHO mixing ratios and the differences are a function of the stability class of the MBL.In other words, both ship-plume HCHO mixing ratios and their differences increase as the MBL changes from neutral (D) to stable (F).The increases are due mainly to the atmospheric CH 4 oxidation (see CASE I in Fig. 3).The increased CH 4 oxidation is certainly due to the increased OH radical concentrations inside the ship plume (this will be shown in Fig. 5).
In addition, there was almost no difference between CASEs I and II.Although the "total NMVOC" emission rates are up to 5% of the ship NO x emission rates in the simulations, the emission rates of the "individual" NMVOC species in the model simulations (refer to Table 1) are small because they are chemically-lumped (or chemically-splitted) into eight individual NMVOC species in the simulations.The relatively low emissions Introduction

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Full of the individual NMVOCs are then diluted rapidly and concurrently, resulting in minimal impacts of NMVOC emissions from the ship.More importantly, the dilution of NMVOCs actually occurs in the early ship-plume stage, where oxidation is not yet very active due to the depletion of OH radicals (this will be discussed further below).Due to these two reasons, the simulations with and without NMVOC emissions (CASEs I and II) produced similar results.In this sense, ship-emitted NMVOCs are not responsible for the HCHO enhancements "inside the ship plumes".However, the diluted NMVOCs would contribute to HCHO production "in the background air".Wagner et al. (2002) reported that approximately ∼22.7% of HCHO could be produced from the background NMVOCs over the SIO.
Here, there are two important issues, particularly near the ship stack (approximately, within the ship-plume travel time of ∼50 min): (1) the levels of the transect-averaged ship-plume HCHO are below the background HCHO levels near the ship stack (CASE I and II) and ( 2) the primary HCHO emitted from the ship could make a significant contribution to the levels of the averaged ship-plume HCHO (CASE III).The former is basically due to ozone titration near the ship stack.The ozone titration near the ship stack is well-known and has been discussed in many papers (e.g., Song et al., 2003a;von Glasow et al., 2003;Kim et al., 2009).Ozone titration then leads to OH depletion near the ship stack, and causes an imbalance between HCHO production and destruction inside the ship plume (i.e., HCHO production rate HCHO destruction rate).
Therefore, the levels of HCHO near the ship stack fall below the background HCHO levels inside the ship plume (hereafter, called "HCHO depletion").However, when the primary HCHO emission from the ship is considered (CASE III, the most realistic case), the primary HCHO compensates for the depletion of HCHO near the ship stack and the HCHO levels decrease in a pseudo-exponential manner.These dynamic changes in the HCHO budget are certainly due to the "non-linear" characteristics of the ship-plume photochemistry.
In order to analyze the HCHO budget in more detail for CASE III (the most realistic case), the fractions of the ship-plume HCHO mixing ratios and enhanced ship-plume Introduction

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Full HCHO mixing ratios are depicted in Fig. 3c and d, the third and the forth rows in Fig. 3, respectively.As shown in Fig. 3c and d, until a ship-plume travel time of ∼20 min, primary HCHO emission is the main source of the HCHO enhancement inside the ship plume.The primary HCHO contributes continuously to ship-plume HCHO generation until ∼50 min after the ship puff is released.However, CH 4 oxidation becomes the major contributor after a ship-plume travel time of ∼50 min.In particular, after a shipplume travel time of 100 min, almost all the HCHO enhancement is due to atmospheric CH 4 oxidation.
Figure 4 shows the cross-sectional profiles of the HCHO mixing ratios in the ITCT 2K2 ship-plume at ten transects (including two imaginary transects A − and A • ).Two issues should be noted in Fig. 4: (1) the contribution of primary HCHO emissions to HCHO in the cross-sectional HCHO profiles of the ship plume depends strongly on the stability of the MBL and (2) the cross-sectional profiles of the HCHO mixing ratios initially show a minimum in the plume core; the duration of this effect depends on the stability of the MBL.Again, these "non-Gaussian" HCHO profiles across the ship plume are important evidence of the non-linear ship-plume photochemistry (Kim et al., 2009).The contribution of primary HCHO to the total HCHO profiles last longer, as the MBL becomes more stable.The non-Gaussian shapes of the cross-sectional HCHO profiles also last longer as the MBL becomes more stable (because ship plumes are spread more narrowly when the MBL becomes more stable, as shown in Fig. 4).However, as ship-plume ages photochemically and dynamically, the cross-sectional HCHO profiles become Gaussian and the contribution of primary HCHO to the profiles of HCHO mixing ratios reaches a minimum.In addition, the extent of the enhancements of the HCHO levels increases, as the MBL becomes more stable.For example, the peak HCHO mixing ratios are elevated above the background HCHO levels by ∼170 pptv (at transect F) under stable MBL conditions, whereas they are elevated only by ∼70 pptv (at transect D) under neutral conditions.These enhancements in the HCHO levels were used in Sect.3.2 as an indicator of the degree of activity of the HCHO-related photochemistry inside the ship plume.Introduction

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Full Figure 5 represents the OH radical concentrations, instantaneous chemical lifetime of CH 4 (τ CH 4 ), and the net instantaneous HCHO production rate (P HCHO ) at ten shipplume transects under moderately stable MBL conditions.The quantities in Fig. 5 were defined as follows: (1) (2) (3) where k i are the thermal reaction rate constants (cm 3 molecules −1 s −1 ), and the values for k i are shown in Table 2. k NMVOCs,i and Φ i are the reaction rate constant (cm 3 molecules −1 s −1 ) for HCHO production from the oxidation of NMVOC species i by OH radicals and the HCHO yield from the oxidation of NMVOC species i , respectively.D CH 4 , D HCHO , and F HCHO denote the rates of CH 4 and HCHO destruction and HCHO formation, respectively.Another possible oxidation reaction of CH 4 would be CH 4 +Cl in the MBL (Sander and Crutzen, 1996;Vogt et al., 1996;Wagner et al., 2002;von Glasow et al., 2003).However, these reactions were not considered in this study because there was no clear evidence of halogen-mediated photochemistry found in this specific ship-plume case (Parrish et al., 2004;Chen et al., 2005;Kim et al., 2009).Introduction

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Full As discussed above, CH 4 is destroyed/oxidized, producing HCHO directly and/or CH 3 O 2 radicals indirectly.CH 3 O 2 radicals are converted to HCHO via (R11) and (R12), as shown in Table 2 (Finlayson-Pitts and Pitts, 2000;Wagner et al., 2002).Both CH 3 OOH and CH 3 OH are also eventually converted to HCHO via (R8) and (R9) in Table 2.However, the production rates of both "reservoir" species (CH 3 OOH and CH 3 OH) were subtracted from the HCHO production rate (F HCHO ) due to the long chemical lifetimes of both species and the relatively short time period of the ship-plume photochemical/dynamic modeling (see Eq. 3).However, the ship-plume situations inherently create high NO x conditions (HO x -limited situations), which limits the formation of the both reservoir species.Therefore, under such high NO x conditions, one mole of CH 4 produces one mole of HCHO, and at a pseudo steady-state, the rates of CH 4 destruction are approximately equal to the rates of HCHO production, as expressed in Eq. (3-1), when As discussed previously, HCHO production from NMVOC oxidation inside the ship plume is also minor compared to that from CH 4 oxidation (D CH 4 ), due to the rapid and concurrent individual species dilutions and the OH radical deletion.This was confirmed in Fig. 6.
In the current framework of the analysis, two points should be considered: (1) the changes in the quantities inside and outside the ship-plume; and (2) the imbalance betweenD CH 4 (≈F HCHO ) and D HCHO inside the ship plume.The former was examined to compare the characteristics of CH 4 destruction and HCHO formation inside and outside the ship-plume.In the latter, the difference between F HCHO and D HCHO can be regarded as the net instantaneous HCHO production rate (P HCHO ), i.  2).This is because the ship plume encounters relatively low NO x conditions in this ship-plume photochemical stage due to dilution.On the other hand, the negative P HCHO values until ∼20 min after the ship plume is released indicate net HCHO destruction, mainly via active (R4) and (R5) in Table 2, which are due to OH depletion during the early ship-plume photochemical stage.
Figure 5 presents the cross-sectional variations of the quantities defined in Eqs. ( 1)-( 5).They vary depending on the stability classes of the MBL.However, their general characteristics are similar.Therefore, only the results from the moderately stable MBL case are shown in Fig. 5. First of all, the peak OH radical concentrations increase, even up to ∼1.7×10 7 molecules cm −3 .These levels of OH radicals are ∼2.8 times higher than the background levels of OH (∼6.1×10 6 molecules cm −3 , also refer to Chen et al, 2005), indicating ∼2.8 times faster atmospheric CH 4 oxidation inside the ship plume than that in the background (out-plume) MBL.This was confirmed in terms of τ CH 4 .τ CH 4 is approximately ∼1.1 yr in the background, but as low as ∼0.45 yr inside the ship plume.τ CH 4 at the peak of the ship plume was approximately ∼41% lower than τ CH 4 in the background under the moderately stable MBL conditions.Under stable MBL conditions, τ CH 4 was shortened even further (not shown).Here, it should be emphasized once again that τ CH 4 is a function of the OH radical concentration.Our ship-plume model simulations can successfully regenerate, e.g., ∼8 yr of CH 4 lifetime (approximately, globally-averaged value at daytime), when the OH radical concentration is 1.02×10 6 molecules cm −3 , as reported from the Oslo global CTM simulation by Karlsd óttir and Isaksen (2000).
As mentioned above, the imbalance between D CH 4 (≈F HCHO ) and D HCHO (i.e., P HCHO ) inside the ship plume was also examined.Basically, it is the imbalance that enhances Introduction

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Full the ship-plume HCHO levels (or depletion of the HCHO mixing ratios close to the ship stack).As shown in Fig. 5, P HCHO inside the ship plume has values above the background P HCHO , indicating excessive HCHO formation inside the ship plume.The background P HCHO ranges from ∼0.02 to ∼0.03 pptv s −1 , whereas the ship-plume P HCHO increases up to ∼0.08 pptv s −1 .Such an imbalance leads to large increases in the HCHO levels inside the ship plume, as shown in Fig. 4.However, some depletion at the ship-plume center is also found near the ship locations between transects A − and B. Again, this is due mainly to the retarded F HCHO caused by OH depletion.Figure 7 shows the time-dependent changes in the transect-averaged OH radical concentrations ([OH]), net instantaneous transect-averaged chemical lifetime of CH 4 (τ CH 4 ), and net instantaneous transect-averaged HCHO production rate (P HCHO ) under moderately stable and stable MBL conditions.All values shown in Fig. 7 were averaged over the transects of the ship plume.In Fig. 7, one could also find a very dynamic, non-linear nature of the ship-plume photochemistry.Near the ship stack, both [OH] and P HCHO are very low, whereas τ CH 4 are high.However, as ship plume travels, [OH] recovers and P HCHO increases accordingly.In contrast, τ CH 4 decreases.Then, after 50-80 min, both [OH] and P HCHO decrease again.

Constructed case studies
The ship-plume HCHO photochemistry was investigated using the base-case ship plume (ITCT 2K2 ship plume).The investigation was extended to various ship-track situations by applying the ship-plume photochemical/dynamic model to heavy ship-traffic corridors at three different latitudinal locations in the world.To accomplish this, thirty six possible scenarios were constructed by changing three major variables: (1) latitude, (2) stability class, and (3) NO x emission rate.Table 3 lists the simulation conditions for the ship-plume photochemical/dynamic modeling.As shown in Table 3, the ship-track situations at the three latitudes were considered in order to examine the extent of the Introduction

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Full increase in the HCHO levels (or the degree of activity of the HCHO-related photochemistry in the ship-going MBL): (1) tropical, (2) sub-tropical, and (3) mid-latitude regions.The three latitudinal situations were constructed considering three busy shiptraffic corridors: (1) between Sri Lanka and Sumatra ( 5• N; 90  (Chin et al., 2000;de Gouw et al., 2001;Kamra et al., 2001;Arnold et al., 2009).The details are shown in the footnote of Table 3.
Basically twelve scenario simulations were carried out at each latitude, changing the MBL stability conditions from neutral to stable (i.e., stability classes of D, E, and F) and NO x emission rates from 3 to 12 g s −1 (i.e., 3, 6, 9, and 12 g s −1 ).Numbering for the 12 scenarios was made sequentially from scenarios I-a to III-d (regarding the numbering convention, refer to the footnote of Table 4).The simulations at each latitude were extended further to account for seasonal variations in the HCHO-related photochemistry by carrying out the simulations at four Julian days of 80, 173, 267, and 356, which correspond to the spring equinox, summer solstice, fall equinox, and winter solstice in the Northern Hemisphere, respectively.Therefore, a total of forty eight simulations were carried out at each latitude, and the results are summarized in Table 4.While constructing the scenarios, the changes in the primary NMVOC emissions were excluded, based on our discussions in Sect.3.1.The J(O 3 ) values for the photolysis reaction of O 3 +hν → O( 1 D)+O 2 are presented in Table 3 to show the latitudinal variations in the intensity of solar radiation because the differences in the magnitudes of J(O 3 ) lead to different OH levels.As shown in Table 3, a more polluted background situation was selected for the sub-tropical conditions than for tropical conditions, with an intention of examining the main factor to affect the degree of activity of the HCHO-related photo-Introduction

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Full To examine the extent of the enhancements in the HCHO levels (or the degree of activity of the HCHO-related photochemistry), one variable, the maximum difference of HCHO mixing ratio (∆[HCHO] max ) between peak HCHO level ([HCHO] peak ) inside the ship plume and background HCHO level ([HCHO] back ), was chosen from the scenario simulations, i.e.: Table 4 lists the model-calculated maximum differences for the scenarios.In the scenario simulations, all the first plume puffs were released at 10:30 a.m.LST.Therefore, the peak OH values appeared around noon, after a period of ozone/OH/HCHO depletions.In addition, this is the approximate local pass time of the ESA/ERS-2 GOME satellite sensor (Marbach et al., 2009).This release time was chosen intentionally considering satellite studies.
The background HCHO levels in this study were again calculated by UBoM 2K8 modeling using the method discussed in Sect.2.2.Table 3 lists the calculated background HCHO mixing ratios.The background HCHO mixing ratios are lower than those reported by Wagner et al. (2001).Again, the background HCHO mixing ratios are affected greatly by the background NMVOC levels, and are most sensitive to the isoprene levels (Wagner et al., 2002).However, such levels are highly variable due to their short atmospheric lifetimes.For example, the calculated background HCHO mixing ratios have a higher value (98.0 pptv) in sub-tropical regions despite the less intense solar intensity compared to tropical regions (78.0 pptv).This is because a more polluted background situation in terms of the O 3 and NMVOC mixing ratios was chosen for the sub-tropical conditions, as mentioned previously.The high levels of O 3 and NMVOCs in the sub-tropical background air enhance the levels of the "background HCHO".
In addition, the peak HCHO mixing ratios within the ship plumes can also be estimated by adding the calculated background HCHO levels to the differences in HCHO mixing ratios (∆[HCHO] max ) (refer to Eq. 6).Table 4 shows the differences in HCHO Introduction

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Full mixing ratios (∆[HCHO] max ) estimated from the scenario simulations.The magnitudes of ∆[HCHO] max inside the ship plumes are dominated primarily by the changes in latitude, MBL stability class, and NO x emission rates.∆[HCHO] max becomes larger as the MBL becomes more stable from D to F and as the NO x emission rate increases from 3 g s −1 to 12 g s −1 .Therefore, significant differences are observed in a stable, tropical MBL with a large NO x emission rate.In each latitudinal situation, three ∆[HCHO] max values (minimum, middle, and maximum values) are shown, which were selected from the multiple ship-plume scenario simulations under the spring/fall equinox and winter/summer solstice conditions.As expected, the largest and smallest values of ∆[HCHO] max occur under tropical and mid-latitude conditions, respectively.However, the differences between the minimum and maximum ∆[HCHO] max values would be smaller in tropical regions than in the other two locations due to the minimal difference in sun location between the summer and winter solstice over tropical locations.
As discussed by Marbach et al. ( 2009), the ship-traffic corridor between Sri Lanka and Sumatra is the only location in the world where the HCHO vertical columns can be retrieved from the ESA/ERS-2 GOME sensor through their multi-year high-pass filter algorithm.In addition to several methodological benefits mentioned by Marbach et al. (2009), this is also due to the fact that this ship-traffic corridor is one of the busiest ship tracks in the world with large ship emissions along a very narrow ship track, according to the AMVER data base.Moreover, the HCHO-related photochemistry would be the most active along this busy ship track because the ship track is located in a tropical region (along the latitude of ∼5 • N).Table 4 shows this active HCHO-related photochemistry.The magnitudes of the HCHO increases (∆[HCHO] max ) in the tropical scenarios are 1.3-8.9times larger than those in the other latitudinal scenarios.Also, it is expected that the peak HCHO mixing ratios along the tropical ship track would be enhanced up to ∼553.7 pptv (∆[HCHO] max +background HCHO mixing ratio) due to the increases in both the HCHO enhancements within the ship plumes and in the background air.The peak HCHO mixing ratios can increase even further because our background HCHO mixing ratios shown in Table 3 were calculated simply from the Introduction

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Full UBoM 2K8 model with relatively low background conditions, which are not affected directly by other ship emissions in the narrow ship track.The real background HCHO mixing ratios could be much higher than the ones used in this study due to the effects of the pollutants emitted from other ships in the heavy ship traffic background.

Remaining uncertainties
CH 4 oxidation and HCHO production within ship plumes were examined using a shipplume photochemical/dynamic model.Although the ship-plume model used in this study is equipped with state-of-the-science chemical schemes, aerosol micro-physics and dynamics, and heterogeneous reaction parameterizations, it does not include several HCHO-related chemical and heterogeneous processes, e.g.: (1) HCHO uptake by marine aerosols with the production of organic complexes or formic acid (e.g., Jacob et al., 1996;Kieber et al, 1999); (2) heterogeneous recycling of CH 3 OH to HCHO on the surface of marine aerosols (Jaegle et al., 2000); and (3) halogen chemistry (Sander and Crutzen, 1996;Vogt et al., 1996;Wagner et al., 2002;von Glasow et al., 2003).
The heterogeneous HCHO uptake and recycling of CH 3 OH to HCHO were excluded because these issues are rather speculative without any concrete evidence from the field measurements.In addition, as discussed in Sect.3.1, the formation of CH 3 OH would be limited at high NO x situations.Therefore, the heterogeneous recycling of CH 3 OH was excluded.
Halogen chemistry is often active in the MBL and could also be important in ship plumes.If halogen chemistry is active in the ship-influenced MBL, the rate of CH 4 oxidation (rate of hydrogen abstract by Cl, i.e., CH 4 +Cl+O 2 → CH 3 O 2 +HCl) and the rate of the CH 3 O 2 +XO → HCHO+X+HO 2 (where X=Cl, Br, or I) reaction could be enhanced further, yielding even higher HCHO mixing ratios in the ship-going MBL.However, under high NO x conditions, such as ship plumes, bromine chemistry may be of limited importance as BrO mixing ratios might be reduced.In the early plume this can be caused by the reaction NO+BrO → NO 2 +Br (von Glasow et al., 2003).It can also be caused by radical termination reactions of BrO+NO 2 → BrONO 2 .On the 15461 Introduction

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Full other hand, the formation of BrONO 2 can lead to an increase in the release of bromine from sea-salt aerosol as the very photolabile Br 2 is released from sea-salt aerosol upon the reaction of BrONO 2 on bromide containing aerosol (see discussion in Sander et al., 1999).The magnitude of reaction probability of BrONO 2 on sea-salt particles (γ BrONO 2 ,SS ) is somewhat uncertain but Sander et al. (1999) showed that this reaction can be important for an order of γ BrONO 2 ,SS of 0.8.Although the efficiency of the auto-catalytic bromine release from sea-salt particles under the high NO x ship-plume conditions is somewhat uncertain and its effect on HCHO is probably limited, it is still likely that chlorine atoms play an active role in the ship-plume photochemistry, especially for the oxidation of CH 4 .Enhanced concentrations of chlorine in ship plumes can result from acid displacement of HCl from sea-salt aerosol and subsequent release of Cl atoms upon the reaction of HCl with OH (see e.g., Keene et al., 2007).Another pathway for chlorine release is via the reaction of dinitrogen pentoxide (N 2 O 5 ) on chloride containing aerosol particles, resulting in the formation of nitryl chloride (ClNO 2 ) which decomposes to Cl atoms and NO 2 at daytime (Osthoff et al., 2008;von Glasow, 2008).Figure 8 shows the build-up of N 2 O 5 in the ITCT 2K2 base ship-plume case, even during daytime situations due to the high levels of NO 2 and O 3 (a detailed analysis regarding the daytime N 2 O 5 formation inside the ship plumes was reported by Song et al. (2003a)).At this stage, the predicted N 2 O 5 concentrations have not been confirmed by measurements, so that a reliable quantification of a potential Cl atom source from this mechanism is difficult.The rate constant for CH 4 +Cl is about 17 times faster than CH 4 +OH (at T =290 K), in order for Cl atoms to contribute 10% to the oxidation of CH 4 under the high OH conditions in ship plumes as predicted in this study.For this condition, Cl concentrations would have to be on the order of [Cl]=6×10 5 (atoms cm −3 ).Introduction

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Full Enhanced levels of formaldehyde (HCHO) along global ship corridors were observed from satellite sensors and predicted using global 3-D chemistry-transport models.Three likely sources were investigated to identify the detailed sources of the enhanced levels of HCHO in the ship corridors: (1) primary HCHO emission from ships, (2) secondary HCHO production from NMVOCs emitted from ships, and (3) atmospheric oxidation of CH 4 .By carrying out multiple ship-plume model runs, it was found that CH 4 oxidation via enhanced levels of OH radicals is mainly responsible for the elevated levels of HCHO inside the ship plumes.After an average of ∼200 min after the release of ship plumes, more than ∼91% of the HCHO enhancements within the ship plumes was produced by this atmospheric chemical process for the ITCT 2K2 base case.Various likely ship-plume situations in the global ship corridors were also studied to determine the extent of the enhancements of the HCHO levels in the ship-going MBL.It was found that the ship-plume HCHO levels could increase up to ∼434.9 pptv higher than the background HCHO levels, depending on the latitudinal location of ship plumes, NO x emission rates and the stability of the MBL.
As demonstrated in this study and Kim et al.'s work (2009), the enhanced OH levels inside the ship-plume results in elevated levels of HCHO as well as acidic substances, such as HNO 3 and H 2 SO 4 , mainly through the oxidation of ship-emitted NO 2 and SO 2 .Within the heavy ship-traffic MBL over the oceans, the active production of HNO 3 and H 2 SO 4 can lead to increased formation of particulate nitrate and sulfate, both of which have a negative impact on the global radiative forcing (Hansen and Sato, 2001;IPCC 2007).In addition, the increased H 2 SO 4 mixing ratios within the ship plumes can create favorable conditions for fresh particle formation (i.e., nucleation), which can subsequently grow into accumulation-mode particles and can often form stratocumulus clouds, known for "ship track" (e.g., Radke et al., 1989;Song et al., 2003b;Frick and Hoppel, 2000;Hobbs et al., 2000;Hudson et al., 2000;Phinney et al., 2009).Furthermore, when particulate nitrate in the MBL ship corridor is dry-deposited Introduction

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Full onto the ocean surface, it acts as a fertilizer (nitrogen source) that can subsequently increase the active phytoplankton activity in the ocean (Duce et al., 1991;Prospero et al., 1996;Dentener et al., 2006;Duce et al., 2008).Such active phytoplankton activities could enable ocean surface to take up more CO 2 from the atmosphere, which would enhance the DMS fluxes from the ocean surface.Both processes would lead to global cooling (Charlson et al., 1987;Cropp et al., 2007).Overall, the immense influences of ship-plume photochemistry on atmospheric chemical cycles, global climate changes and marine biota activities as well as the complicated interactions between these changes should be investigated further with a more comprehensive and multidisciplinary research framework, such as European Surface Ocean-Lower Atmosphere Study (SOLAS) (http://www.solas-int.org).Introduction

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Full   Cooper et al. (1996).c The fraction of HCHO was assumed to be from 4% to 10% of the total NMVOC emissions (Houyoux, 2005;Marbach et al., 2009).10% is the upper-limit.d Higher alkanes represent the lumped alkane species with a carbon number >4. e Higher alkenes represent the lumped alkene species with carbon number >3. f Aromatics include branched benzene species such as toluene, xylene, stylene, etc. g The total NMVOC emission rates were estimated based on Endersen et al. (2003) andEntec (2005).Introduction

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Neutral
Moderately stable Stable Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | was first examined because this ITCT 2K2 ship-plume was analyzed most thoroughly by Chen et al. (2005) and Kim et al. (2009) (Sect.3.1).The ship-plume photochemical/dynamic model was then applied to locations at other latitudes (i.e., ship tracks in tropical, subtropical, and mid-latitude regions) to determine the possible extent of the increase in HCHO levels (or degree of activity of the HCHO-related photochemistry) within the global ship corridors (Sect.3.2).Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |be oxidized and how much the HCHO mixing ratios can be enhanced from the CH 4 and/or NMVOCs oxidation or by direct HCHO ship emissions inside the single shipplume.The implications of this work in the global atmospheric photochemistry within a ship-going MBL are then discussed.Figure3shows the budget of HCHO generation (photochemical production+direct emission) inside the ship plume of the ITCT 2K2 ship-plume case.In Fig.3, four cases were considered: (CASE I) CH 4 +OH reaction ON inside the ship-plume; (CASE II) CH 4 +OH reaction ON+NMVOC emission without primary HCHO emission; (CASE III) 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 | Discussion Paper | Discussion Paper | Discussion Paper | Figure 6 compares the two net transect-averaged instantaneous HCHO production rates (P HCHO ) for the three MBL stability conditions.The black solid and red dashed lines represent the two P HCHO expressions defined by D CH 4 −D HCHO and F HCHO −D HCHO in Eq. (5), respectively.The two lines show the differences around the peak P HCHO values.However, the differences are not large (up to ∼17% at the peak 15455 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | chemistry in the ship-going MBL.

Fig. 3 .
Fig. 3. HCHO sources and the budget inside the base-case ship plume (ITCT 2K2 ship-plume): (a) changes in the transect-averaged ship-plume HCHO mixing ratios with respect to shipplume travel times; (b) differences between the transect-averaged ship-plume HCHO mixing ratios and the model-predicted background HCHO mixing ratios; (c) source fraction of the ship-plume HCHO mixing ratios; and (d) source fractions of the enhanced ship-plume HCHO mixing ratios.The first, second, and third columns show the results from the model-simulations under the conditions of neutral (D), moderately stable (E), and stable (F) MBL stability classes, respectively.

Table 1 .
Estimated ranges of the NMVOC mass-fractions and emission rates from ships a .The mass fractions of NMVOCs species were estimated based on EPA (2002), Endersen et al. (2003), European Commission Directorate General Environment And Entec UK Limited (2005), and Houyoux (2005).
a b Chemical speciation was based on

Table 2 .
Some reactions related to HCHO photochemistry in the MBL.

Table 3 .
Simulation conditions for the constructed case studies