Evaluation of Photochemical Pollution during transport of Air Pollutants in spring over the East china sea

We conducted intensive observations of ozone, CO, NOx (=NO and NO2), NOy (total odd nitrogen species including particulate nitrate) and total nitrate (the sum of gaseous HNO3 and particulate nitrate) at Cape Hedo, Okinawa, Japan, from 19 March to 3 April, 2009, to investigate ozone production during long-range transport from the Asian continent. Ozone production efficiency (OPE) was used to evaluate photochemical ozone production. OPE is defin ed as the number of molecules of ozone produced photochemically during the lifetime of a NOx molecule. OPE is calculated by the ratio of the concentration increase of ozone to that of NOz (= NOy–NOx). Average OPE during observation was estimated to be 12.6±0.5, but concentrations of ozone increased nonlinearly with those of NOz. This non-linearity suggests that OPE depends on air mass origin and NOz concentrations. There were very different values of OPE for the same air mass origin, so that only the air mass origin alone does not control OPE. OPE was low when NOz concentration was high. We examined the correlation between NOz and CO/NOy ratios, which we used instead of the ratio of non-methane hydrocarbons (NMHCs) to NOx. The CO/NOy ratios decreased with increasing NOz concentrations. These results indicate that competition reactions of OH with NMHCs and NO2 are the rate determining steps of photochemical ozone production during longrange transport from the Asian continent to Cape Hedo, for high concentrations of nitrogen oxides.


IntroductIon
Recent remarkable economic progress in East Asia has caused an increase in NO x (=NO + NO 2 ) emissions (e.g.Akimoto, 2003).Ohara et al. (2007) developed an emission inventory for Asia (Regional Emission Inven tory in Asia: REAS), and reported that Asian NO x emissions increased by 176% from 1980 to 2003.In particular, NO x emissions in China showed a marked increase of 280%.More recently, Kurokawa et al. (2013) updated REAS emission inventory (REAS ver. 2) and estimated that NO x emissions in Asia and China increased by 54 and 89%, respectively, from 2000 to 2008.Zhang et al. (2007) estimated a NO x emission increase by 70% in China from 1995 to 2004.Satellite observations show that NO 2 column densities increas ed by 95% from 1996 to 2004 (Zhang et al., 2007).
Atmospheric pollutants such as NO x emitted in East Asia are transported with chemical transformations and can affect the environmental quality of other regions directly and indirectly (e.g.Takiguchi et al., 2008).A representative effect is photochemical pollution such as ozone.Suthawaree et al. (2008) performed continu ous observations of ozone and CO at Cape Hedo, Oki nawa, Japan.They described longrange transport of ozone and CO from the Asian continent using back ward trajectory analyses, and suggested photochemical production of ozone during the transport because of a positive correlation between ozone and CO.Episodic pollution of photochemical ozone was observed over Japan in spring (89 May 2007).Ohara et al. (2008) concluded that this episode was strongly influenced by transboundary air pollutants such as nitrogen oxides and volatile organic compounds emitted on the Asian continent.Longterm monitoring of springtime ozone at a site on Mount Happo, Japan, has shown increase at a rate of ~1 ppbv (part per billion by volume) y -1 .One reason for this increase is considered to be an indirect effect caused by rapidly increasing anthropo genic emissions of NO x from East Asia (Tanimoto, 2009).
Photochemical ozone production is nonlinear against the precursors of ozone such as NO x and nonmethane hydrocarbons (NMHCs).Ozone in the troposphere is generated in the photolysis of NO 2 : NO 2 +hv (λ<420 nm) → NO + O ( 3 P), (1) where M represents thirdbody molecules such as N 2 and O 2 .The inverse of reactions ( 1) and ( 2) regener ates NO 2 and destroys ozone: Photochemical ozone production is substantially controlled by the reaction of NO with peroxy radicals (RO 2 ; R means an organic group): Reactions (1)(6) form the photostationary state of NO x during daytime.Peroxy radicals are generated by the reaction of OH with NMHCs and CO: OH Reactions (4)(10) form a chain reaction centered on RO x ( = OH + HO 2 + RO 2 ) radicals to generate tropo spheric ozone.NMHCs act as a propagator of the chain reaction.On the other hand, NO x acts as a terminator as well as propagator of the chain reaction while NO x is the direct precursor of ozone.NO 2 reacts with OH to generate nitric acid: In the case of low NO x mixing ratios, the ratedeter mining steps of the photochemical ozone production are reactions (4) and ( 6), so that ozone production rates rise when NO x concentrations increase.In the case of high NO x mixing ratios, the ratedetermining steps are shifted to the peroxy radical regeneration (reactions ( 7) and ( 9)).Reactions ( 7) and ( 9) have a competing reac tion (11), which acts as a terminator of the chain reac tion.The increase of NO x concentrations makes favor able for reaction (11), so that ozone production rates decrease.An important factor of photochemical ozone production is the NMHCs/NO x ratio, because the ozone production rate is determined with respect to the bran ching ratio between reactions ( 7) and (11).
To clarify behavior of ozone for longrange transport, it is necessary to discuss the ozone production rate as well as the ozone concentration level.Ozone produc tion efficiency (OPE) is an important parameter for addressing ozone production rate (e.g.Ge et al., 2013;Chou et al., 2009;Kleinman et al., 1994).OPE is defined as the "number of ozone molecules per NO x molecule oxidized".Fig. 1 shows ozone production chain reactions in view of nitrogen oxides.Nitrogen oxides are emitted as NO x and ozone is photochemical ly produced by the NO x RO x chain reaction in Fig. 1.Ozone is produced as long as the chain reaction pro ceeds.On the other hand, NO 2 reacts with OH and per oxyacyl radicals (RC(O)OO) to produce HNO 3 and PANs, respectively.Total odd nitrogen species (NO y ) other than NO x such as HNO 3 and PANs are called NO z (i.e.NO z = NO y -NO x ), which is more stable than NO x and does not contribute directly to ozone production.The reactions of NO z production are equivalent to the termination reaction of photochemical ozone produc tion.In summary, a NO x molecule produces several ozone molecules via oxidation from NO x to NO z and the number of ozone molecules is defined as OPE.Thus, OPE is expressed as the ratio of concentration incre ment of ozone (Δ[O 3 ]) to that of NO z (Δ[NO z ]): To diagnose ozone production during longrange trans port from the Asian continent, we performed intensive observation of NO 2 using a laserinduced fluorescence technique at the National Institute for Environmental Studies Cape Hedo Atmosphere and Aerosol Monitoring Station (CHAAMS), Okinawa, Japan.Ozone, CO, NO, NO y and total nitrate (TN = the sum of gaseous HNO 3 and particulate nitrate) are observed continuously at CHAAMS.We investigated OPE during the transport using data observed, and discussed the contributing fac tor when determining the value of OPE.

1 Site description
The CHAAMS observation site is at latitude 26° 52ʹN, longitude 128°15ʹE, and an elevation 60 m above sea level (Fig. 2).Details of the site are describ ed else where (Suthawaree et al., 2008;Takiguchi et al., 2008;Takami et al., 2007;Kanaya et al., 2001).Briefly, it is located near Cape Hedo, the northernmost point of the Okinawa main island and is about 100 km from Naha City, which is the largest city on that island.The site is about 20 km from the center of Kunigami Village, pop ulation ~6000.Horizontal distance from the site to the seashore is only about 200 m to the northwest.The site is in the remote marine boundary layer and is suitable to diagnose air pollution by longrange transport from East Asia.Observation was carried out from 19 March to 3 April 2009.

2 measurement Species
Ozone, CO and NO were measured using commer cially available instruments based on UV absorption (Model 49i; Thermo Fisher Scientific), a nondisper sive infrared photometer (Model 48C; Thermo Elec tron) and NOozone chemiluminescence (Model 42i TL; Thermo Fisher Scientific), respectively.For the CO measurement, zero air generated from a heated Pt catalyst (Model 96, Nippon Thermo) was measured during the first 20 min of every hour, to check the zero point of the CO analyzer (Suthawaree et al., 2008).NO y was observed by a molybdenum catalyst followed by NOozone chemiluminescent detection (Sadanaga et al., 2008a;Williams et al., 1998).The fraction of particulate nitrate in NO y reaches 50% at CHAAMS.A large part of particulate nitrate around CHAAMS is in coarse mode (Takiguchi et al., 2008), which would mainly originate from uptake of gaseous HNO 3 on the surface of sea salt.The NO y measured at CHAAMS includes a large portion of particulate nitrate because a molybdenum catalyst reduces NaNO 3 quantitatively to NO (Sadanaga et al., 2008b).The detection limits of ozone, NO, CO and NO y are 1 (according to the opera tion manual), 0.045 (3σ), 17 (3σ) and 0.045 (3σ) ppbv, respectively.
TN was measured by the scrubber difference/NO ozone chemiluminescence method (Yuba et al., 2010;Sadanaga et al., 2008a;Tanner et al., 1998).Briefly, ambient air was passed through a Teflon filter to remove particles and then introduced to an annular denuder coated with NaCl to remove gaseous HNO 3 before entering the other molybdenum catalyst followed by the NOozone chemiluminescent detector.The NO y -TN concentration was measured by this method.The TN concentration can be obtained by subtracting the NO y -TN concentration from the NO y concentration: The detection limit of TN is the same as that of HNO 3 analyzer reported previously (Sadanaga et al., 2008a).The detection limit depends on NO y concen tration and was estimated to be 71 pptv with 10min integration time (2σ) under NO y concentration of 5 ppbv (Sadanaga et al., 2008a).
NO 2 was measured using a laserinduced fluores cence technique (LIF).The detailed principle and ins trumentation of the LIF are described elsewhere (Sada naga et al., 2004;Matsumoto and Kajii, 2003).Briefly, ambient air was introduced to the fluorescence detec tion cell through an orifice of diameter 0.254 mm.In this cell, a second harmonic of the solidstate pulsed Nd:YAG laser (Awave532 nm8W10 kHz; Advanced Optowave) was irradiated to excite NO 2 molecules.The YAG laser has a repetition rate of 10 kHz, pulse width 76 ns, beam diameter 1 mm and maximum out put 7 W during observation.Fluctuation of laser power was monitored outside the detection cell using a pho todiode (S12265BQ; Hamamatsu) to correct the sensi tivity of this LIF system fluctuated by the laser power.The cell was pumped by a rotary pump (RV8; Edwards) to reduce quenching of excited NO 2 .Pressure in the cell was approximately 330 Pa, measured using a capaci tance manometer (Model 720; Setra).
Fluorescence emitted from the excitation volume is focused onto a photocathode of a dynodegated pho tomultiplier tube (R928P; Hamamatsu) through two lenses and a sharpcutoff filter (R62; Asahi Techno glass), which was used to cut off scattered light to the photomultiplier tube.In addition, the photomultiplier tube was timegated to distinguish between scattered light and fluorescence using a normally off dynode gating system (C139256; Hamamatsu).The gating system was controlled by a positive transistortransis tor logic (TTL) pulse, which is generated by a delay/ pulse generator (DG535; Stanford Research Systems).The negative output signal from the photomultiplier tube is led to an amplification/discrimination unit (C9744; Hamamatsu) to convert to positive TTL puls es.The number of these pulses in a single gate period is counted by a photon counting board (M8784; Hama matsu), that is slotted in a master computer (Dimen sion 8250; Dell).Typical gate timing for photon count ing is between 0.3 and 3.1 μs after the laser pulse.The detection limit of the NO 2 LIF instrument during inten sive observation is 53 pptv (parts per trillion by vol ume) at S/N = 2, with an integration time of 60 s and laser power 7 W.
NMHCs were analyzed by a Gas Chromatograph Flame Ionization Detector (GCFID) (HP 6890; Hewlett Packard) (Kato et al., 2004(Kato et al., , 2001)).Ambient air samples were collected into 6 L stainless canisters for NMHCs analyses.The sampled air of 500 cm 3 was concentrated into a threestage preconcentrator (Entech7000; Entech) prior to the injection into the GCFID.GC column was HP1 (60-m length, 0.32-mm inner diameter and 1-μm film thickness).Initially GC oven temperature was kept at -50°C (for 8 min) and then increased to 40°C at the rate of 5°C min -1 , after that further increased to 150°C at 15°C min -1 .NMHC concentrations were cali brated with 1 ppmv standard gas containing 58 species (PAMSJ58; Sumitomoseika).Detection limits of the NMHCs are in the range of 13 pptv with 213% accu racy and 215% precision.

3 Backward trajectory analysis
Backward trajectory analyses were performed using the HYSPLIT 4 model developed by the American National Oceanic and Atmospheric Administration (NOAA) (Draxler and Rolph, 2012;Rolph, 2012).Ini tial altitude and calculation time were set at 500 m and 120 hours, respectively.Origins of air masses reaching the observational site were classified into six groups, based on the last coastline passed.The origins are described in Fig. 2: North China (CH1), Middle China (CH2), South China (CH3), Korea (KR), Japan (JP) and the Pacific Ocean (PO).Air masses that mean dered or did not belong to any of above origins were excluded from analysis.Trajectory data were gathered every six hours: 3:00, 9:00, 15:00 and 21:00 Japan Standard Time (JST).The NO y and HNO 3 concentra tions were averaged into 6hour bins to classify them into air masses identified by backward trajectory.Con centration data of 0:006:00, 6:0012:00, 12:0018:00 and 18:0024:00 correspond to trajectories of 3:00, 9:00, 15:00 and 21:00, respectively.

1 overview
Fig. 3 shows observed data of ozone, NO y , TN, NO, NO 2 and CO from 19 March to 3 April 2009.Average concentrations of ozone, NO y , TN, NO, NO 2 and CO were 50.0, 1.02, 0.44, 0.02, 0.20 and 180.8 ppbv, res pectively.Large peak concentrations of ozone, NO y , TN and CO were observed around 21:00 on 22 March.Backward trajectory analysis indicates that the air mass in this period came from the Asian continent via the Shanghai area.Unfortunately, NO 2 concentrations between 21 and 23 March were missing data because of problems with the laser system.The secondhighest peak concentrations of ozone, NO y , TN and CO during the observation were observed between 1 and 3 April.The backward trajectory analyses indicate that the air masses in these periods originated between CH2 and KR.Concentration variations of NO y were similar to those of TN as shown in Fig. 3(a).The air mass around CHAAMS was wellaged, and these concentration peaks are due to the outflow of Asian pollutants.

2 Estimation of opE
Fig. 4(a) shows daily variations of NO z and ozone concentrations.The highest concentration period of NO z was from 1 to 3 April and ozone concentration during this period was also highest, save for a large peak concentration on 22 March.Fig. 4(b) shows that the ozone concentration was positively correlated with the NO z concentration.The slope of the regression line is defined to be OPE.In this case, OPE during the obs ervation period was estimated at 12.6±0.5.However, ozone concentrations increased nonlinearly with NO z concentrations.This result suggests that OPE is depen dent on air masses and NO z concentrations.Analyses of OPE using air mass classification and NO y concen trations are described in Section 3.3.
In previous studies, OPE was reported to be about 10 in areas of low NO x concentrations (Kleinman et al., 1994;Olszyna et al., 1994).On the other hand, OPEs in the urban areas of New York, Nashville and Beijing were estimated from 24 (Kleinman et al., 2000), 36 (St. John et al., 1998) and 1.06.8 (Ge et al., 2013), res pectively, which are lower than those in the clean atmo sphere.At the remote area in East Asia, for example, averaged OPE was estimated to be 10 at Oki Island, Japan (Jaffe et al., 1996).NO x concentrations at CHAAMS are low, and the OPE of 12.6 around the site is consistent with the earlier reports.OPE estimat ed by this method is inferred under the assumption that deposition velocities of ozone and NO z are the same.In reality, the NO z deposition velocity is larger than that of ozone in most cases (Section 3.4).It should be noted that OPE estimated by this method shows the upper limit (Nunnermacker et al., 1998).

3 Variation of opE with air masses
As described in the preceding section, correlation between ozone and NO z was nonlinear.The plot of Fig. 4(b) seems to be composed by several linear relation ships between ozone and NO z .This intensive observa tion can be divided into the following five periods by air mass origins obtained by backward trajectory anal yses: 19 through 21 March (period A, PO air mass ori gin), 23 through 25 March (period B, KRJP air mass origin), 25 through 28 March (period C, CH1CH2 air mass origin), 28 through 31 March (period D, JP air mass origin) and 1 through 3 April (period E, CH1CH2 air mass origin).Fig. 5 shows relationships between ozone and NO z within each category.Linear correla tions were obtained for each period.OPEs ranged widely, from 4.7 to 24.2.As seen in Fig. 5(c) and (e), OPEs were very different between periods C (OPE = 14.56) and E (OPE = 4.83), despite the fact that air masses in both periods had from the same origin, CH1 CH2.Therefore, OPE is not determined by air mass origin only.
It is noteworthy that OPE decreases when the NO z concentration is high.For example, OPE for period A  was estimated at 14.16.However, OPE decreased to 2.03 in the same period when using only data of NO z concentrations more than 1 ppbv.As described above, OPE for period E was much lower than that for period C. The NO z concentration for period E was much great er than that for period C, while the air masses originat ed from CH1CH2 in both periods.In the previous studies, OPEs in polluted air were lower than those in the clean atmosphere (Section 3.2).
To investigate the relationship between OPE and the NO z concentration, we examined correlation between NO z and NMHCs/NO x ratios (Sadanaga et al., 2012).As described in Section 1, an important factor of pho tochemical ozone production is the NMHCs/NO x ratio, which indicates that ozone production rate is deter mined with respect to the branching ratio between reac tions ( 7) and ( 11).Unfortunately, NMHC concentrations were not continuously observed during the study period, so we used CO concentrations instead.CO concentra tions have good correlation with those of NMHCs origi nating from combustion processes.Indeed, there was good correlation between CO and NMHCs measured at CHAAMS from 16 to 20 March and from 4 to 13 April 2006 (Fig. 6).Table 1 shows NMHCs measured in spring 2006.NO y concentrations were used instead of NO x to analyze the relationship between NO z concentra tions and NMHCs/NO x ratios.The lifetime of NO x is so short that most NO x emitted from the Asian continent is oxidized to NO y during transport to CHAAMS.To dis cuss photochemical ozone production during the trans port process, the use of NO y instead of NO x is more appropriate.
CO/NO y ratios decreased with increasing NO z con centrations (Fig. 7(a)).Considering the mechanisms of photochemical ozone production, decreases in both   CO/NO y ratio and OPE with increasing NO z suggest that competition reactions of OH with NMHCs and NO 2 (i.e., reactions ( 7) and ( 11)) are the rate determin ing steps for photochemical ozone production during the transport to CHAAMS, at least for high concentra tions of nitrogen oxides.We also investigated the rela tionships between NO z concentrations and CO/NO y ratios for periods B and C, which have similar NO z concentrations but different OPEs.The CO/NO y ratios for period B were higher than those for period C (Fig. 7(b)), whereas OPE for period B was lower than that for period C (Fig. 5(b) and (c)).Reasons for this result may be differences of air mass origin, NMHC compo sition, meteorological conditions and others, but we cannot draw clear conclusions at this time.It is expect ed that the accumulation of NMHC as well as NO z and ozone data will clarify OPEs during transport over East Asia.

4 Influence of transport time on opE
As described in Section 3.2, OPE estimates were inferred under the assumption that deposition veloci ties of ozone and NO z are the same.Thus, these depo sition velocities might influence OPE values.The depo sition velocity of NO z depends on its constituents.The main components in NO z observed around CHAAMS were reported to be gaseous nitric acid and coarse par ticulate nitrate (Takiguchi et al., 2008), whose deposi tion velocities are greater than that of ozone (Lovett, 1994).Indeed, about half of NO y was total nitrate in this observation.These results indicate that the deposi tion velocity of NO z is greater than that of ozone.Therefore, the OPE values estimated by this method show the upper limit as described in Section 3.2 and may be overestimated.The difference of deposition velocities influences the apparent OPEs more strongly in the case of longer transport time from the Asian continent to CHAAMS.From this viewpoint, the relationship between OPEs and transport time was investigated.The transport time was defined as the air mass travel duration from the last Asian continental coastline to CHAAMS.This is also the time spent over the sea, and can be calculated from the backward trajectory analysis.We categorized the transport time into "short" (029 hours), "middle" (3059 hours), and "long" (60 hours or more) groups.
OPEs can be affected by air mass origins and NO z concentration as well as transport time, so the relation ship between OPEs and transport time should be  investigated over the same periods.Fig. 8 shows cor relations between OPEs and transport time in periods C (Fig. 8(a)) and E (Fig. 8(b)), when both air masses originated from CH1CH2.In period C, transport times were long and fell into only the middle and long groups.OPE in the middle group was in agreement with that in the long group within statistical errors.In period E, transport times were short and fell into only the short and middle groups.An OPE difference was not observed with statistical significance between the short and the middle groups.Given these results, we concluded that OPE is more strongly affected by NO z concentration and air mass origin than by NO z deposi tion during longrange transport.

concluSIonS
Intensive observations of ozone, CO, NO x , NO y and TN were carried out at CHAAMS, Okinawa, Japan, from 19 March to 3 April 2009.Average concentra tions of ozone, NO y , TN, NO, NO 2 and CO were 50.0, 1.02, 0.44 0.02, 0.20 and 180.8 ppbv, respectively.The concentration of NO z was positively correlated with that of ozone.OPE was estimated from the regression line slope.Average OPE was estimated at 12.6±0.5 during the observation period, and it was confirmed that ozone concentration increased nonlinearly with that of NO z .Both OPE and NO x concentrations obtained dem onstrate that CHAAMS is in a remote area.
We investigated OPEs for various air masses arriv ing at CHAAMS, via classification according to air mass origin and period using backward trajectory ana lyses.OPEs were very different between the periods 2528 March (OPE = 14.56) and 13 April (OPE = 4.83), despite the same air mass origin (CH1CH2).There fore, air mass origin is not the sole determinant of OPE.OPE tends to be small when NO z concentration is high, regardless of air mass origin.This suggests that NO z concentration influences OPE.We investigated the relationship between NO z and ratios of CO/NO y , which were used instead of NMHCs/NO x ratios.The CO/NO y ratios decreased with increasing NO z concentrations, so the competition reactions of OH with NMHCs and NO 2 could be the rate determining steps for photo chemical ozone production during transport from the Asian continent to CHAAMS, at least for high concen trations of nitrogen oxides.
The OPE obtained herein may be overestimated, because the deposition velocity of NO y is generally greater than that of ozone.The component in NO y that has the largest deposition velocity is TN, so the appar ent value of OPE may increase with transport time from the Asian continent to CHAAMS.We categorized transport time into short (029 hours), middle (3059 hours), and long (60 hours or more) groups, and com pared the group OPEs with the same period and air mass origin.There were no significant differences of OPEs between groups.We concluded that OPE is most affected by NO z concentration and that the influence of the NO z deposition velocity during transport was not great during the observation period.

Fig. 1 .
Fig. 1.Mechanism of ozone production chain reactions in view of nitrogen oxides.

Fig. 2 .
Fig. 2. Map of East Asia and CHAAMS location.Dashed lines indicate classification of air mass origin (see Section 2.3).

Fig. 3 .
Fig. 3. (a) Daily variations of ozone (gray line), NO y (solid line) and TN (dashed line) concentrations.Each plot repre sents 10min average values.(b) Daily variations of NO (solid line) and NO 2 (gray line) concentrations.Each plot represents 10min average values.(c) Daily variation of CO concentration.Each plot represents 1h average values.A to E mean periods A to E (see section 3.3).

Fig. 4 .
Fig. 4. (a) Daily variations of ozone (dashed line) and NO z (solid line) concentrations.Each plot represents 10min aver age values.A to E mean periods A to E (see section 3.3).(b) Relationship between ozone and NO z concentrations.Solid curve shows just guide to eye.

Fig. 5 .
Fig. 5. Relationships between ozone and NO z concentrations from 19 to 21 March (period A, PO air mass origin), 23 to 25 March (period B, KRJP air mass origin), 25 to 28 March (period C, CH1CH2 air mass origin), 28 to 31 March (period D, JP air mass origin) and 1 to 3 April (period E, CH1CH2 air mass origin).Solid lines represent regression lines.N means the number of data.

Fig. 6 .
Fig. 6.Correlation between CO and NMHC concentrations measured at CHAAMS.ppbvC means parts per billion by vol ume carbon basis.Solid line is the regression line.N means the number of data.

Fig. 7 .
Fig. 7. Relationship between the CO/NO y ratios and NO z concentrations for (a) entire observation period, and (b) peri ods B (open circles) and C (filled circles).

Fig. 8 .
Fig. 8. Correlation between NO z and ozone concentrations measured at CHAAMS in periods C and E, categorized by transport time: short (029 hours), middle (3059 hours), and long (60 hours or more) groups.Solid lines represent regres sion lines.Errors show two standard deviations (2σ).N means the number of data.

table 1 .
Summary of measured NMHCs at Cape Hedo in spring 2006.Evaluation of Photochemical Pollution during Transport of Air Pollutants 243