Long-term change in the contributions of various source regions to surface ozone over Japan

The relative contributions of various source regions to the long-term (1980–2005) increasing trend in surface ozone (O 3 ) over Japan were estimated by a series of tracer-tagging simulations using a global chemical transport model. The model well simulated the observed increasing trend of surface O 3 including its seasonal variation and geographical features in Japan and demonstrated the relative roles of different source regions in forming this trend. Most of the simulated increasing trend of surface O 3 over Japan (~ 97 %) was explained as the sum of trends in contributions of different regions to photochemical O 3 production. The increasing trend in O 3 produced in China accounted for 36 % of the total increasing trend and those in the other northeast Asian regions (the Korean Peninsula, coastal regions in East Asia, and Japan) each accounted for about 12–15 %. Furthermore, the contributions of O 3 created in the entire free troposphere and in West, South, and Southeast Asian regions also increased; and their increasing trends accounted for 16 and 7 % of the total trend, respectively. The impact of interannual variations in climate, in methane concentration, and in emission of O 3 precursors from different source regions on the relative contributions of O 3 created in each region estimated above was also investigated. The variation of climate and the increase in methane concentration together caused the increase of photochemical O 3 production in several regions, and represented about 19 % of the total increasing trend of surface O 3 over Japan. The increase in emission of O 3 precursors in China caused an increase of photochemical O 3 production not only in China itself but also in the other northeast Asian regions and accounted for about 46 % of the total increase in surface O 3 over Japan. Similarly, the relative impact of O 3 precursor emission changes in the Korean Peninsula and Japan were estimated as about 16 and 4 % of the total increasing trend, respectively. The O 3 precursor emission change in regions other than northeast Asia caused increases in surface O 3 over Japan mainly through increasing photochemical O 3 production in West, South, and Southeast Asia and the free troposphere, and accounted for about 16 % of the total.


Introduction
Tropospheric ozone (O3) plays multiple roles in the atmosphere.O3 itself is an oxidant and photodissociates to generate the hydroxyl radical which strongly oxidizes many atmospheric compounds including various air pollutants and thus removes them from the atmosphere.In contrast, high levels of O3 are a major air pollutant due to adverse effects on human health, natural vegetation, and agricultural produce (Wang and Mauzerall, 2004;Mauzerall et al., 2005;US EPA, 2006;Silva et al., 2013).Moreover, tropospheric O3 is a major greenhouse gas in the atmosphere, and reduction of its amount was recently recognized as an effective measure to mitigate near-term climate change (UNEP and WMO, 2011;Shindell et al., 2012).Therefore, the spatial and temporal variations in tropospheric O3 have been always a matter of scientific and public concern.
An increasing trend in tropospheric O3 concentration has been observed during recent decades at many locations in East Asia including Taiwan (Chou et al., 2006;Chang and Lee;2007;Li et al., 2010;Lin et al., 2010), mainland China (Lu and Wang, 2006;Ding et al., 2008;Xu et al., 2008;Wang et al., 2009;Zhang et al., 2014), and South Korea (Susaya et al., 2013;Lee et al., 2014;Seo et al., 2014).The increase rates of O3 in those East Asian regions significantly vary depending on location and season in the range of about 0.3-3 ppbv/yr; however, the increases are generally larger than the trends in tropospheric O3 for other regions in the world (Cooper et al., 2014).Japan is no exception, with an increasing trend found in various observations of O3 over the past approximately 40 years.Routine ozonesonde measurements since 1970 at three Japanese sites of Sapporo (43° N), Tsukuba (36° N), and Kagoshima (32° N) showed an increasing trend of O3 concentration in the lowermost troposphere up to about 1990 and relatively stable thereafter, with largest increase near the ground and discernible about 300 hPa height and below (Logan et al., 1999;Oltmans et al., 2006).With an air mass classification method based on backward air trajectories, Naja and Akimoto (2004) showed that a significant amount of the air masses reaching these ozonesonde sites in Japan spend substantial time over polluted regions in East Asia.The O3 levels in these regionally polluted air masses increased from the 1970s to the 1990s, mainly due to large increases in nitrogen oxide (NOx = NO + NO2) emissions over China in the 1990s.Oltmans et al. (2013) analyzed a rather short period of data (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) obtained at the Ryori (39° N) surface site in north-eastern Japan and showed an increase into the mid-1990s followed by relatively little change.Other ground-based observations at a mountain site (Mt.Happo; 43° N, 1850 m asl) and three sites in the marine boundary layer along the west coast of Japan [Rishiri (45° N), Tappi (41° N), and Sado (38° N)], where few sources of pollutants exist nearby, obtained under the monitoring network of EANET (the Acid Deposition Monitoring Network in East Asia) also showed increasing trends of O3 concentrations at least until the mid-2000s (Tanimoto, 2009;Tanimoto et al., 2009;Parrish et al., 2012).
In addition, analysis of long-term observations by the ambient air quality monitoring network mainly established in urban-suburban regions in Japan also showed continuous increases of surface O3 from the mid-1980s until the present (Ohara and Sakata, 2003;Ohara et al., 2008;Kurokawa et al., 2009;MOE Japan, 2013;Wakamatsu et al., 2013;Akimoto et al., 2015).However, simultaneous observations of O3 precursors such as NOx and nonmethane hydrocarbons (NMHCs) by this monitoring network revealed their decreasing trends in the same period (MOE Japan, 2013), which seemed inconsistent with the increasing trend of O3 over Japan.These observed features of O3-related atmospheric species in Japan suggest that there should be an influence of transboundary transport from outside of Japan on the recent increasing trend in O3.The influence of transboundary transport on surface O3 in East Asia was examined in several studies (Sudo and Akimoto, 2007;Li et al., 2008;Nagashima et al., 2010;Wang et al., 2011).Nagashima et al. (2010) demonstrated that the O3 transported from outside of Japan accounted for more than 70 % of surface O3 over Japan in the cold season (October-March) during 2000-2005, and most was attributable to O3 from distant sources outside East Asia and from the stratosphere.In the warm season (April-September), the contribution of domestically created O3 in Japan to surface O3 over Japan increased significantly (about 20-40 %), the short range intra-regional transport of O3 from other parts of East Asia still contributed about 25 %, and long range inter-regional transport of O3 from outside East Asia and the stratosphere particularly in spring could account for about half of surface O3 over Japan.
Therefore, the influence of O3 from source regions outside and inside East Asia and the stratosphere should be considered to explain the cause of the increasing trend in surface O3 over Japan.The rapid increase in O3 precursor emissions in East Asia in recent decades (Ohara et al., 2007;Kurokawa et al., 2013) was demonstrated as a major cause of the increasing trend of springtime O3 over Japan by comparing regional chemical transport model (CTM) simulations of recent decades with and without the East Asian O3 precursor emission increases during the period (Kurokawa et al., 2009;Tanimoto et al., 2009).However, they only showed the springtime O3 case and it was unclear whether the relationship held in other seasons.Moreover, the relative contributions of individual countries or regions in East Asia have not been well examined, particularly concerning increased surface O3 over Japan.
Here, we investigated the cause of the continuous increase in surface O3 over Japan reported in the above literature, focusing on the relative contributions of various source regions over the globe, particularly the contributions of individual regions in East Asia, with a long-term simulation of a global CTM using the tagged tracer method.Using the same model and method, Nagashima et al. (2010) showed such relative contributions of regions inside and outside East Asia on surface O3 over Japan as average values for the early 2000s.The current study investigated the temporal evolution of the relative contributions of each region for the 26 years of 1980-2005.

Model description
In this study, we employed a chemistry climate model (CCM), CHASER (Sudo et al., 2002), developed for the atmospheric chemistry research in the troposphere.The basic setting of the model was almost identical to that used by Nagashima et al. (2010).However, the horizontal resolution was modified from T63 (about 1.9° by 1.9° grid spacing in longitude and latitude) to T42 (about 2.8° by 2.8°), because longer simulation period was necessary than in the previous study, and so the cost of computation was reduced in the present study by selecting lower horizontal resolution.There were 32 vertical layers with the top layer set at approximately 40 km altitude.A detailed tropospheric photochemistry consisted of 113 chemical reactions and 27 photodissociation involving O3, HOx, NOx, methane (CH4), CO and NMHCs calculated the temporal evolution in the concentrations of 53 chemical species.The gas and liquid phase oxidation of sulfur dioxide (SO2) and dimethyl sulfide to form the sulfate aerosol was also included in the model.The concentrations of O3 and some nitrogen compounds (NOx, HNO3, and N2O5) above the tropopause that should affect tropospheric chemistry were assimilated into the monthly mean output data of stratospheric CCM, because the version of CHASER used was unable to calculate several chemical processes, such as halogen-related chemical reactions, which are indispensable for realistic representation of Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-1087, 2017 Manuscript under review for journal Atmos.Chem.Phys.Published: 19 January 2017 c Author(s) 2017.CC-BY 3.0 License.such chemical compounds in the stratosphere.For the transport of chemical species, a semi-Lagrangian advective transport scheme (Lin and Rood, 1996;van Leer, 1997) and vertical convective transport associated with cumulus convection process were considered.The model also included dry and wet deposition of chemical species.
In this study, we conducted tracer-tagging simulation by using two different setups (fullchemistry and tracer-transport setups) of CHASER.The full-chemistry setup calculated the actual temporal change in the concentration of chemical species through the abovementioned chemical and physical processes and outputted the chemical production and loss tendencies of O3 and related species.Then, the tracer-transport setup used the outputted chemical tendencies to calculate the temporal change in the concentration of hypothetical O3 tracers.In the following subsection, the calculation procedure is briefly described.

Forcings for long-term simulation
Long-term simulation was performed for the period 1980-2005.To drive the physical properties of the model for this 26-year period, the temperature and horizontal wind velocities in the model were assimilated into the National Center for Environmental Prediction/National Center of Atmospheric Research (NCEP/NCAR) 6-hour reanalysis data (Kalnay et al., 1996) of the corresponding year, and sea surface temperature and sea ice data of the Hadley Centre's Sea Ice and Sea Surface Temperature (HadISST) data set (Rayner et al., 2003) were used in the model.
The monthly mean stratospheric O3 data of Akiyoshi et al. (2009) was used for the assimilation above the tropopause for this period.These data were the output of a stratospheric CCM simulation according to the hindcasting scenario for 1980-2004 (REF1 scenario) of the CCM validation activity (CCMVal) (Eyring et al., 2005), and included an interannual variation (IAV) associated with the 11-year solar cycle and large declines after 1982 and 1991 due to the El Chichon and Pinatubo eruptions, respectively, in addition to a continuous decreasing trend during the whole period.Although the simulated declines of stratospheric O3 due to the two large volcanic eruptions were somewhat overestimated, the simulated IAVs in stratospheric O3 reasonably well represented those observed with a total ozone mapping spectrometer (TOMS) from satellites (Akiyoshi et al., 2009).Incidentally, the stratospheric O3 data of 2004 were used for 2005.
The long-term variation of the emissions of O3 precursors (NOx, CO, and NMHCs) and SO2 were taken from multiple emission inventories.For anthropogenic emissions in Asia, the Regional Emission inventory in ASia (REAS ver.1.2) (Ohara et al., 2007) was used for the whole simulation period ; the REAS emission data were available for each year in the period.Kurokawa et al. (2009) used these emission data with a regional air quality model representing well the interannual variability of surface O3 over Japan for similar period  to the present study.For anthropogenic emissions outside Asia, a combination of three versions of EDGAR (Emission Database for Global Atmospheric Research) emission data was used: EDGAR-HYDE (Van Aardenne et al., 2001) for 1980and 1990;EDGAR v3.2 (Olivier and Berdowski, 2001) for 1990and 1995;andEDGAR v3.2 Fast Track 2000 (FT2000) (Olivier and Berdowski, 2001) for 2000.Because several emission sectors considered in EDGAR v3.2 were not considered in EDGAR-HYDE, the emissions for 1990 in EDGAR-HYDE were generally smaller than in EDGAR v3.2.Therefore, we used EDGAR Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-1087, 2017 Manuscript under review for journal Atmos.Chem.Phys.Published: 19 January 2017 c Author(s) 2017.CC-BY 3.0 License.v3.2 data for 1990, and also scaled them to estimate emission data for 1980 rather than simply using EDGAR-HYDE data for 1980.For that, we scaled EDGAR v3.2 data for 1990 so that the ratio (r) of the difference between 1980 (f1) and 1990 data (f2) and their average in EDGAR-HYDE [i.e., r = (f2 − f1)/(f1 + f2)/2] equaled the corresponding ratio (R) calculated from 1990 data in EDGAR v3.2 (F2) and 1980 data scaled from it (F1) [i.e., R = (F2 − F1)/(F1 + F2)/2].We calculated F1 from the known values of f1, f2, and F2 using the equation r = R. Since EDGAR emission data were not available for each year but for every 10 or 5 years in the simulation period, the emissions for intermediate years were interpolated, and FT2000 data used for years after 2000.The vegetation fire emission data developed in the REanalysis of the TROpospheric chemical composition over the past 40 years project (RETRO) (Schultz et al., 2008) were used for O3 precursor emissions from biomass burning for the whole land area.RETRO data were available for each year until 2000 in the simulation period, and data for 2000 were used for years after 2000.Historical transition of the atmospheric concentrations of carbon dioxide, nitrous oxide (N2O), and CH4 were prescribed with those used in Nozawa et al. (2005), which were somewhat old estimations of the historical evolution in greenhouse gas concentrations, but not much different from recent estimations such as for the Representative Concentration Pathways (RCPs) (Meinshausen et al., 2011).The difference in the concentrations between both estimations were generally within a couple of percent in the simulation period.
The linear trends of NOx and NMVOC annual emissions used in this study in the simulation period of 1980-2005 are shown in Fig. 1.The long-term trends of emissions of both species showed generally similar geographical features to each other; large decrease trends in central Europe, Scandinavia, western Russia, and Kazakhstan, whereas there were widely spread increasing emissions in West, South, Southeast, and East Asia, almost all Africa and Central and South America except for inland Brazil.In North America, NOx emission generally decreased in the simulation period except for the west coast and New England area of the USA, but that of NMVOC mostly increased with a few patchy exceptions.The trends of NOx and NMVOC emissions mentioned above were mainly due to the change in anthropogenic emissions, while the change in biomass burning emissions led to a discernible trend in several regions such as inland Brazil and the south of Sahel.
The long-term evolution of annual emissions of NOx and NMVOC over several source areas in the Northern Hemisphere is shown in Fig. 2. Because the emission data were the combination of three different datasets outside Asia, there were somewhat discontinuous changes at the joint years (1990 and 1995) in European and North American emissions.The emissions of NOx and NMVOC over Europe had peaks around 1990 and generally decreased afterward.Over North America, both species showed small long-term trends: slight decreases in NOx and slight increases in NMVOC emissions.The emissions of both species over China greatly increased during the whole period.The NOx emissions were about 4.0 times larger in 2005 than 1980 and correspondingly NMVOC was 2.5 times larger, which made emissions of both species for China equal to or even surpassing those for Europe or North America in 2005.The emissions of both species over the Korean Peninsula increased approximately 2.8 times during this period.However, those over Japan showed no such increase: NOx emission decreased until 1995 and thereafter remained stable, whereas NMVOC emissions went up until 1995 and then slightly decreased.

Tracer tagging
We conducted a 26-year simulation using the full-chemistry setup of CHASER with all the forcings mentioned above, followed by another 26-year simulation with the tracer-transport setup of CHASER which calculated the concentration of hypothetical O3 tracers, each tagged with a particular region in the model domain.The procedure to tag a tracer with each region in the second simulation was the same as used by Nagashima et al. (2010) and a brief description follows.In the second simulation, the transport and dry deposition of each O3 tracer were calculated same as in the first simulation, however the chemical development of tracers was calculated using the chemical production (P) and loss frequencies (L) of the extended odd oxygen family [Ox = O3 + O + O( 1 D) + NO2 + 2NO3 + 3N2O5 + PANs + HNO3 + other nitrates] calculated and archived in the first simulation.In the first simulation, 3D fields of P and L were outputted every 6 hours.Each O3 tracer could be lost chemically everywhere in the model domain at the frequency of L, but could be chemically produced only inside its tagged region.In the stratosphere over the tropopause defined by the lapse rate, the concentration of O3 tracer tagged with the stratosphere was assimilated into the same stratospheric O3 data as used in the first simulation, but the concentration of the tracers tagged with the region in the troposphere were all set to zero.The calculated concentration of each tagged O3 tracer at a given location represents the contribution of O3 produced in each source region and transported to that location.
The horizontal and vertical separation of the model domain for the tracer tagging was also the same as used by Nagashima et al. (2010).The troposphere in the model domain was horizontally separated into 22 regions and each horizontal region was further separated vertically between the free troposphere (FT) and the planetary boundary layer (PBL).The stratosphere was considered one separate source region, that is, the model domain was separated into 45 source regions.The 22 regions for horizontal separation are shown in Fig. 1 and each region was assigned a three-letter code (e.g., AMN for North America) which is used in the following sections.For the vertical separation of the source regions in the troposphere, the PBL was defined as the lowest six layers in the model (surface to about 750 hPa), based on the observed and modeled vertical profiles of O3 production.
The long-term tracer-tagging simulation allowed estimation of the long-term variations in contributions of each source region to the O3 concentration at given receptor locations.This is important information to explain the cause of the reported increasing trend in surface O3 over East Asia.However, it should be noted that the tracer-tagging simulation calculates the amount of O3 in a receptor location that was produced chemically in each source region from O3 precursors emitted both from the source region and adjacent source regions.Thus, the contribution of a source region estimated in tracer-tagging simulation should not be fully attributed to emissions of O3 precursors in that source region.Emission sensitivity simulation is another method of estimating the portion of O3 fully attributable to a change in O3 precursor emissions in a source region, and takes the difference of simulated O3 between two model runs with and without perturbed O3 precursor emissions in that source region.The resulting estimations of source contributions by the two methods can differ; however, the differences have not yet been well quantified.Li et al. (2008) reported that the difference between the two methods could be as much as 30 % in source apportionment estimation for one location and time (i.e., Mt.Tai in central eastern China in June 2006).Wang et al. (2011) found somewhat larger differences in the contributions of China to domestic O3 concentration between the two methods for each month of the year, but no discussions were made for O3 over Japan.
Nevertheless, we employed the tracer-tagging simulation to study the cause of reported longterm change in surface O3 over Japan mainly due to its computational efficiency.Thus, the results should be carefully interpreted in terms of the difference between the source regions of chemical O3 production and those of O3 precursor emissions.The computational efficiency resulting from the tracer-tagging approach and relatively coarse horizontal resolution enabled us to make several sensitivity simulations with the different combination of forcings for longterm simulation.In the following sections, the simulation with the full set of long-term forcings described above, hereinafter referred to as "standard" simulation, is initially analyzed.This is then further interpreted using the results of sensitivity simulations; the specific settings of sensitivity simulations are also described.

Results and discussion
3.1 Long-term evolution of surface O3 over Japan Nagashima et al. (2010) validated how well CHASER can reproduce the observed features of surface O3 concentrations by comparing the simulated surface O3 concentrations with observations taken during 2000-2005 at several sites mainly in rural areas in the Northern Hemisphere, and CHASER successfully simulated the annual variation of surface O3 in a variety of regions.In this study, the horizontal resolution of the model differed from that used in Nagashima et al. (2010); however, the model well represented the observed concentrations and seasonal evolutions of surface O3 (Fig. S1 and Table S1 in the Supplement).The surface O3 over Japan has been observed at ambient air quality monitoring stations since the early 1970s when severe air pollution occurred in industrial or urban areas.The monitoring data have been compiled by the Atmospheric Environmental Regional Observation System (AEROS).The number of stations increased since the launch of the system and, for the period of simulation , about 1000 monitoring stations widely distributed throughout Japan except in the southern islands could be used for validation of the model results.The monitoring data of AEROS have been used to examine the long-term variation of surface O3 over Japan in several studies and showed significant increasing trends (Ohara and Sakata, 2003;Ohara et al., 2008;Kurokawa et al., 2009;Akimoto et al., 2015).We validated the simulated surface O3 over Japan with the AEROS data in terms of the long-term variation in the following.
For the validation, the monitoring sites selected had continuously observed the surface O3 during the simulation period .To ensure continuity of sites, we selected monitoring sites with annual mean surface O3 available for every year in the simulation period.The annual mean data at a monitoring site was calculated as the average of monthly means when available for more than 9 months, the monthly mean was calculated from daily means when available for over 19 days per month, and the daily mean was calculated from hourly means when available for more than 19 hours per day.There were 339 sites, located mainly in populated areas of Japan except in the northernmost island (Hokkaido) and southern islands (Nansei Islands).We first calculated the annual mean surface O3 from the observed hourly data at each monitoring site as described above, and then the annual means of all sites were averaged to calculate the observed annual mean surface O3 over Japan.The simulated annual mean surface O3 over Japan was calculated as the average of annual means of the model grids, which included the locations of monitoring sites selected for the validation.Therefore, the model grids including Hokkaido or Nansei Islands were not used to calculate the simulated annual mean.The temporal variations of observed and simulated annual mean surface O3 anomalies during 1980-2005 averaged over Japan are shown in Fig. 3.During the period, the observed annual mean surface O3 over Japan showed a clear increasing trend with a linear increase of about 2.70 ppbv/decade, which was significant at the 5 % risk level.The simulated annual mean surface O3 over Japan also showed a significant increasing trend with a rate of about 2.58 ppbv/decade, which corresponded well to the observed increase in surface O3 over Japan.The value of the linear increasing trend and the observed features of IAVs in surface O3 over Japan -such as a rapid increase from the mid-1980s to the mid-1990s followed by a stagnation of increase for about 7-8 years and a further increase in the past several yearswere reasonably well captured by the model.
The model also well represented the longitudinal differences in the long-term trend of surface O3 in Japan.Figure 4 shows the maps of linear trends of annual mean surface O3 during 1980-2005 calculated from the model simulations and observations at AEROS monitoring sites as selected for Fig. 3.The simulated annual mean surface O3 showed an increasing trend in the whole area including all of Japan and the Korean Peninsula (Fig. 4a).The simulated increasing trend of annual mean surface O3 well exceeded 2.0 ppbv/decade in wide areas of Japan except for Hokkaido, and tended to be greater toward western Japan, which is nearer to the Asian continent.However, the increasing trends of observed annual mean surface O3 at each monitoring site (Fig. 4b) differed greatly from each other even in nearby sites, and there was no apparent longitudinal tendency in trends at individual monitoring sites.However, we averaged the observed annual mean surface O3 at individual monitoring sites at longitudinal intervals (approximately 2.8°) of the model grids as shown by gray rectangles (Fig. 4b) and calculated the long-term trend of averaged monitoring data at each longitudinal band.The calculated increasing trends were clearly larger toward the west, which was consistent with westward rise of the increasing trends of simulated data.
There were seasonal differences in the long-term increasing trend of surface O3 over Japan.The temporal variations of observed and simulated seasonal mean surface O3 anomalies during 1980-2005 averaged over Japan are shown in Fig. 5.The increasing trend of surface O3 over Japan in the monitoring data was greatest in spring (March-May: 4.04 ppbv/year) and was also large in summer (June-August: 3.07 ppbv/year); in contrast, increasing trends were relatively small in fall (September-November: 2.29 ppbv/year) and winter (December-February: 1.28 ppbv/year).Seasonal dependency in the increasing trends of observed surface O3 over Japan has been previously reported (Ohara and Sakata, 2003;Naja and Akimoto, 2004;Parrish et al., 2012).Ohara and Sakata (2003) examined almost the same O3 monitoring data in Japan as used in the present study for the period 1985-1999 and showed year-round increase in surface O3 from 1985-1987 to 1997-1999 with a greater increase in the warm season (March-August) than in the rest of the year.Naja and Akimoto (2004) also reported a larger increase of O3 in the warm season between the period 1970-1985 and 1986-2002 in the boundary layer over Japan by analyzing ozonesonde data at four sites.Parrish et al. (2012) summarized long-term changes in lower tropospheric baseline O3 over the world including two regions in Japan (Mt.Happo and several sites in the marine boundary layer grouped as one region), and showed that the increasing trend of surface O3 was greatest in spring and least in fall in these two regions.In the present study, the simulated increasing trend in seasonal mean surface O3 was also larger in the warm (spring-summer) than in the cold season (fall-winter), consistent with the observed increasing trends.
As described above, our model captured well the basic features of long-term trends in observed surface O3 over Japan, which allowed us to use the simulated data for further analysis on the source of the long-term trend in the next section.

Contributions of O3 production regions
The tracer-tagging simulation for 1980-2005 was conducted to examine the long-term variations of O3 tracers tagged by regions of photochemical production.IAVs in the annual mean concentrations of each tagged O3 tracer averaged over Japan are shown in Fig. 6.The tagged tracers other than FT and stratosphere in Fig. 6 and the following figures represent the contribution of O3 produced in the PBL of different source regions shown in Fig. 1, where contributions of several source regions were grouped into some combined source regions.It should be noted that the model grids used for averaging in these figures differed from those in Figs.3-5.They encompassed almost all of Japan excluding the Nansei Islands in order to examine temporal behavior of tagged O3 tracers in all of Japan (see Fig. 4 for actual areas for averaging).
Domestically created O3 was the largest contribution to surface O3 concentration averaged over Japan during the whole simulation period.The contribution of domestic production had a large IAV and was larger in the last decade than previously.
The second largest contribution was the O3 created in the FT as a whole during almost the entire period.For the FT, the northern mid-latitude regions such as North Pacific (NPC), Europe (EUR), North Atlantic (NAT), North America (AMN), and China (CHN) made leading contributions during the period; however, the increasing trend of these contributions was considerable particularly for CHN and NPC (Fig. S2).Despite such differences among the regional contributions in the FT, we hereafter only considered the total of each regional contribution in the FT, since it was difficult to associate a regional contribution with a particular source region of O3 precursor emissions.The precursors eventually resulted in O3 production in a region in the FT can be transported longer distance due to faster wind speed in the FT and therefore would be influenced by emissions from a wider range of source regions than in the PBL.The total FT contribution showed an increasing trend during the period.
The NOx emission from lightning was an indispensable source of NOx in the FT.The global annual lightning-NOx emission in the current simulation was about 3.1 TgN/year averaged over the entire period and showed a small but significant increase of about 0.012 TgN/year (0.39 %/year).The increase in lightning-NOx emission was a consequence of changes in convection activities due to the change in climate forced into the model during the period (NCEP/NCAR meteorology and HadISST data).However, this increase in lightning-NOx emission was not the main cause of the increase in the contribution of the total FT -because a sensitivity simulation with all emissions, CH4 concentration, and stratospheric O3 fixed at the year 1980 level but with the same temporal evolution in climate showed a quite similar increase in lightning-NOx emission but no significant increasing trend in the total FT contribution.Therefore, the main cause of the increasing trend in the total FT contribution was likely to be factors other than the increase in lightning-NOx emission.
The contribution of stratospheric O3 was also large during the entire period, with considerable temporal fluctuations.The large decreases of stratospheric contribution in the early 1980s and 1990s stemmed from the decline of stratospheric O3 concentration due to the impact of large volcanic eruptions of Mt.El Chichon in 1982 and Mt.Pinatubo in 1991, respectively (Akiyoshi et al., 2009).
In the early 1980s, the combined contributions of far remote regions from Japan in the northern mid-latitude (Remote: EUR, NAT, and AMN) made a significant contribution, the fourth largest, to the surface O3 over Japan and remained at a steady level of contribution during the study period.At the same time, the contribution of CHN significantly increased from the mid-1980s, overtook the contribution of Remote in the early 1990s, and became the largest single regional contribution -excluding the domestic one (i.e., JPN).Moreover, the contributions of O3 produced in the Korean Peninsula (KOR), the coastal regions in East Asia [E-Asia-Seas: NPC, East China Sea (ECS), and Japan Sea (JPS)], and West-South-SouthEast The linear trend (ppbv/decade) of annual mean tagged O3 tracers during the simulation period as well as that of the total O3, which is the sum of all tagged O3 tracers averaged over whole Japan (JPN-ALL) and those averaged over three sub-regions in Japan: western (JPN-W), eastern (JPN-E), and northern (JPN-N) Japan is shown in Fig. 7 (see Fig. 4 for the definition of sub-regions).The trend was calculated from the annual mean concentrations.The increasing trend of total O3 averaged over JPN-ALL was 2.37 ppbv/decade, which was somewhat smaller than estimated in Fig. 3 (2.58ppbv/decade) due to inclusion of model grids in JPN-N for averaging where the simulated increasing trend of O3 was relatively small.The increasing trend of total O3 tended to be greater westward.The absolute contribution of domestically produced O3 in Japan differed among the regions -it tended to be larger in JPN-E than other parts of Japan (Nagashima et al., 2010); however, there were no such regional differences in long-term trends.The westward tendency of larger increasing trends in total O3 over Japan was mainly due to the similar tendency in the trends of the contribution of CHN, KOR, and E-Asia-Seas, which strongly suggested a large impact of intra-regional transboundary air pollution in East Asia.In particular, the increasing trend in the CHN contribution was the largest for all sub-regions in Japan.The increasing trend in the contributions of total FT and WSSE Asia was slightly smaller for JPN-N than for other parts of Japan, which also contributed to the regional differences of the trend in total O3 over Japan.Interestingly, the contribution of Remote showed a small but significant increase only in JPN-N -although emissions of O3 precursors, NOx in particular, in Remote did not increase during the period.Due to the large interannual fluctuation, the linear long-term trend of the stratospheric contribution was non-significant for all regions in Japan.
The linear trend of tagged O3 tracers and total O3 averaged over all of Japan in spring, summer, fall, and winter is shown in Fig. 8.The increasing trends of total O3 in decreasing order were spring, summer, winter, and fall.This is quite consistent with the seasonal differences in the increasing trend of O3 observed at several Japanese sites from the 1990s to 2011 (Parrish et al., 2012).The increasing trend in the CHN contribution was the largest of all contributions in all four seasons and the trend was particularly large in spring.The KOR contribution was also larger in spring than in other seasons, with the trend in summer of low statistical significance due to relatively large IAVs.The contribution of E-Asia-Seas increased significantly in all seasons.Seasonal differences in the increasing trend in the E-Asia-Seas contribution were small, but were slightly larger in the warm (spring-summer) than the cold season (fall-winter).The increasing trend in domestic (JPN) contribution was larger in spring than in summer similarly to the cases of CHN and KOR contributions, but trends in both seasons were non-significant; whereas those in the cold season were significantly larger than in the warm season.The FT and WSSE Asian contributions showed semi-annual change in their increasing trends; larger in summer and winter than in spring and fall.The contribution of Remote showed a significant increasing trend only in winter; conversely that of Central-North (CN) Asian regions [Central Asia (CAS) and East Siberia (ESB)] showed small but significant decreasing trends in the cold season but non-significant trends in the warm season.The seasonal features in each regional contribution described above enabled explanation of the cause of the seasonality of increasing trend in total O3 over Japan as follows.The largest increasing trend of total O3 in spring was predominantly attributed to the large increasing trend in contributions of source regions in northeast Asia (CHN, KOR, E-Asia-Seas, and JPN).The increasing trends in the contributions of CHN, KOR, and JPN were smaller in summer, however, partly compensated by the growth of increasing trends in the FT and WSSE Asian Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-1087, 2017 Manuscript under review for journal Atmos.Chem.Phys.Published: 19 January 2017 c Author(s) 2017.CC-BY 3.0 License.contributions from spring to summer.In the cold season, trends for most regions were smaller than in the warm season, except for JPN.The increasing trend in contributions of northeast Asian regions differed little between fall and winter; however, those of FT, WSSE Asia, and Remote had larger increasing trends in winter than in fall, which made the increasing trend of total O3 in winter larger than in fall.
Table 1 summarizes the linear trends of annual mean tagged O3 tracers and the total O3 averaged over JPN-ALL.The vast majority (about 97 %) of the trend in total O3 was balanced with the sum of those trends in regional contributions with statistical significance.The largest contribution was from the increase of O3 produced in CHN (0.85 ppbv/decade), which corresponded to about 36 % of the increasing trend of total O3.The increasing trend in the contribution of the total FT was also large (0.37 ppbv/decade), representing about 16 % of the total O3 trend.The contributions of northeast Asian regions other than CHN also increased significantly (0.34, 0.29, and 0.27 ppbv/decade for KOR, E-Asia-Seas, and JPN, respectively) and each accounted for about 12-15 % of the total O3 trend.About 7 % of the total O3 trend was attributable to the increasing trend in WSSE Asian contributions (0.16 ppbv/decade).The linear trends in the contributions of remaining regions [CN Asia, Remote, stratosphere, and the others (OTH)] were small and non-significant, and so were not important concerning the cause of reported surface O3 increase over Japan.

Impact of IAVs in O3 precursor emissions in different source regions on regional O3 production
The results in the preceding section revealed the relative importance of O3 produced in different regions to the recent increasing trend in surface O3 over Japan.It is noteworthy that this does not indicate the relative importance of the different regions of O3 precursor emissions.For example, there were significant contributions of E-Asia-Seas to the increasing trend in surface O3 over Japan, but there were clearly no large emission sources of precursors in these maritime regions other than navigation.The increasing trend in the contribution of E-Asia-Seas was likely a consequence of increased transport of O3 precursors to this region, which had been emitted in adjacent land areas.However, the tracer-tagging approach cannot distinguish the differences in origins of emissions of precursors that resulted in O3 production in E-Asia-Seas.To further investigate the roles of different regions in the recent increasing trend of surface O3 over Japan, we performed a series of sensitivity simulations with different assumptions for the temporal variation of factors, which would affect the surface O3 over Japan.Each sensitivity simulation consisted of a 26-year simulation with full-chemistry setup of CHASER followed by another 26-year simulation with tracer-tagging setup of CHASER.Initially, a sensitivity simulation was performed that was only forced by the IAVs in the climate (NCEP/NCAR meteorology and HadISST data) but with all emissions of O3 precursors, CH4 concentration, and stratospheric O3 fixed at the year 1980 level; then we gradually added the increase or the IAV of chemical factors as summarized in Table 2.The simulation F, driven by the IAV of all forcings, was identical to the standard simulation; and simulation A was mentioned concerning lightning-NOx emission in the preceding section (3.2).
The linear trends of annual mean total O3 and tagged O3 tracers that had significant effects on the standard simulation averaged over all of Japan in all simulations are shown and compared in Fig. 9. Simulation A showed no obvious increasing trend in total O3 over Japan.The JPN and total FT contributions exhibited increasing trends (0.12 and 0.06 ppbv/decade, respectively), likely due to the IAV of the climate, but they were non-significant.
The increase in atmospheric concentration of CH4 was added in simulation B, because this would have a non-negligible impact on tropospheric O3 (background O3 in particular), as frequently reported (Brasseur et al., 2006;Kawase et al., 2011; HTAP, 2010 and references therein).In the simulations other than A, we used a CH4 concentration increase rate of about 12.3 ppbv/year (0.73 %/year) during 1980-2000 and flattened thereafter.In simulation B, the contribution of the total FT showed a significant increasing trend (0.18 ppbv/decade) as did that of Remote (0.08 ppbv/decade; data not shown).The contributions of several other regions such as CHN, E-Asia-Seas, and WSSE Asia also showed slight increasing trends (approximately 0.01-0.02ppbv/decade), although non-significant.Note that these values included the impact of CH4 increase as well as the IAV of the climate and, consequently, the total O3 in simulation B showed a significant increasing trend of about 0.44 ppbv/decade, representing about 19 % of the increasing trend in total O3 in the standard simulation (2.37 ppbv/decade).
In simulations C-E, the IAVs in emission of O3 precursors in northeast Asian regions were gradually added: CHN, KOR, and JPN, respectively.The increase in emissions of O3 precursors in CHN in simulation C caused a large significant increasing trend in the contribution of CHN itself (0.83 ppbv/decade).Moreover, the emission increase in CHN also had a large impact on the contributions of other regions, in particular, the increase trends in the contributions of KOR and E-Asia-Seas became significant: 0.12 and 0.15 ppbv/decade, respectively.The JPN and the total FT contributions also showed somewhat larger increasing trends in simulation C than in B, but the growth in trends between the two simulations was not as large as those of KOR and E-Asia-Seas.The total effect of the emission increase in CHN on the increasing trend in surface O3 over Japan, assessed using the difference in total O3 trend between simulations B and C, was about 1.08 ppbv/decade and corresponded to about 46 % of the increasing trend in total O3 in the standard simulation.The relative contribution of CHN as a source region of O3 production to the surface O3 increasing trend over Japan was estimated as 36 % in the preceding section (3.2); however, the contribution of CHN as a source region of O3 precursors emission was somewhat (10 %) larger due to the production of O3 outside CHN.It is noteworthy that the slight increasing trend in the contribution of WSSE Asia shown in the CH4 increase in simulation B was smaller in simulation C. The contributions of Remote and the stratosphere showed similar responses.The increase in O3 precursor emissions in CHN seemed to partly offset the increase in influence of long range transport of O3 from such regions.
The increase in emissions from KOR in addition to CHN in simulation D gave rise to a much larger increasing trend in the contributions of KOR itself (0.38 ppbv/decade).Compared with simulation C (0.12 ppbv/decade), about one-third of the increasing trend in the contribution of KOR was attributed to the O3 precursor emission increase in CHN and the rest to emission increase in KOR.Similarly, the emission increase in KOR caused a larger increasing trend in the contributions of E-Asia-Seas in simulation D (0.25 ppbv/decade).We attributed about half of the increasing trend in the contribution of E-Asia-Seas in the standard simulation (0.29 ppbv/decade) to the impact of O3 precursor emission increase in CHN (and partly that of the CH4 increase: 0.15 ppbv/decade) as shown in simulation C, about one-third to that in KOR, and the rest to that in regions other than northeast Asia.By further adding the IAV in the domestic (JPN) emissions in simulation E, the increasing trend in the domestic contribution became significant (0.28 ppbv/decade), implying that the increasing trend in domestically produced O3 was from a combination of multiple factors each of which did not Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-1087, 2017 Manuscript under review for journal Atmos.Chem.Phys.Published: 19 January 2017 c Author(s) 2017.CC-BY 3.0 License.cause a significant increase.The total effect of the emission increase in KOR on the increasing trend in surface O3 over Japan assessed as the difference between simulations C and D was about 0.38 ppbv/decade; and that of the IAV of domestic emission in Japan assessed as the difference between simulations D and E was about 0.09 ppbv/decade; each of which corresponded to about 16 and 4 % of the increasing trend in total O3 in the standard simulation, respectively.
The IAV in emissions of O3 precursors in northeast Asian regions (CHN, KOR, and JPN) together with the IAV in the climate and the increase in CH4 concentration induced a significant increasing trend in total O3 over Japan with a rate of 1.99 ppbv/decade.This accounted for about 84 % of the increasing trend in total O3 in the standard simulation.The rest of the increasing trend should be regarded as from O3 precursor emission changes in regions other than northeast Asia.The difference between simulations E and F (standard simulation) showed that the emission change in such regions influenced surface O3 over Japan mainly through increasing the O3 production in WSSE Asia and the FT (Fig. 9).

Summary and conclusion
We demonstrated the relative importance of the regions of photochemical O3 production in the global atmosphere on the long-term increasing trend in surface O3 over Japan reported in recent decades by conducting a series of tracer-tagging simulations using the global CTM CHASER.The impact of the IAVs of climate, of CH4 concentration, and of emission of O3 precursors (NOx and NMVOC) in different source regions on regional photochemical O3 production were also investigated.
The observed increasing trend of surface O3 over Japan for 1980-2005 (2.70 ppbv/decade for annual mean over whole Japan) was successfully reproduced by the model (2.58 ppbv/decade) including an obvious tendency of increase toward western Japan and to be greater in the warm (spring-summer) than in the cold season (fall-winter).
The absolute contribution of each photochemical O3 production region to the surface O3 over Japan represented by the concentrations of tagged O3 tracer showed different temporal evolution by regions.The contributions of all Asian regions except the northern part (i.e., CHN, KOR, E-Asia-Seas, JPN, and WSSE) as well as those of the total FT exhibited significant increasing trends during the period.The increasing trend in the contribution of domestically produced O3 in Japan (i.e., JPN) did not differ much among the different regions in Japan.However, there was a tendency in the increasing trends in contributions of CHN, KOR, and E-Asia-Seas to be large toward western Japan, which was a main cause of the same tendency in the increasing trend in total O3 and suggested a large impact of intra-regional transboundary air pollution in East Asia.
The trends in contributions of most O3 production regions, except JPN, were larger in the warm than in the cold season, providing a basis for the seasonality in the increasing trend in total O3 over Japan.Thus, the larger increasing trend of total O3 in spring than in summer was mainly due to the same tendency in increasing trends in the contributions of northeast Asian regions (CHN, KOR, and JPN), although this was partly compensated by larger increasing trends in the FT and WSSE Asia contributions in summer than spring.In the cold season, the contributions of FT, WSSE Asia, and Remote had larger increasing trends in winter than in fall, which led to a larger increasing trend in total O3 in winter than in fall.
The sum of the trends in contributions of O3 production regions with sufficient statistical significance accounted for most (about 97 %) of the increasing trend in total O3 over Japan (2.37 ppbv/decade).The largest portion was attributed to the increasing trend of O3 produced in CHN (36 %; 0.85 ppbv/decade), followed by that in the total FT (16 %; 0.37 ppbv/decade).The increasing trend in contributions of the other northeast Asian regions (KOR, E-Asia-Seas, and JPN; 0.27-0.34ppbv/decade) each accounted for about 12-15 % of the total O3 trend, and the majority of the rest of the total O3 trend (7 %; 0.16 ppbv/decade) was attributable to WSSE Asia.
We further investigated the impact of the IAV of controlling factors, such as climate, CH4 concentration, and emission of O3 precursors, on photochemical O3 production in different source regions and its influence on the long-term increasing trend in surface O3 over Japan through a series of sensitivity simulations that gradually added the IAV of these factors.The IAV of the climate and the increase in CH4 concentration together caused the increase of photochemical O3 production in several regions and resulted in the significant increasing trend in surface O3 over Japan (0.44 ppbv/decade) and represented about 19 % of the increasing trend in surface O3 in the standard simulation.The increase in emission of O3 precursors in CHN led to the increase of photochemical O3 production in northeast Asian regions including CHN itself, KOR, JPN, and E-Asia-Seas; and the resulting increasing trend in surface O3 over Japan (1.08 ppbv/decade) accounted for about 46 % of that in the standard simulation.The relative contribution of CHN to the surface O3 increasing trend over Japan as the source region of O3 precursor "emission" was 10 % larger than as the source region of O3 "production" due to production of O3 outside of CHN.Then, the impact of the O3 precursor emission change in KOR and JPN on the increasing trend in surface O3 over Japan (about 0.38 and 0.10 ppbv/decade, respectively) corresponded to 16 and 4 % of the increasing trend in total O3 in the standard simulation, respectively.The rest of the increasing trend in total O3 in the standard simulation (about 16 %) was attributed to O3 precursor emission change in regions other than northeast Asia, mainly through increasing the photochemical O3 production in WSSE Asia and the total FT.

Figure 1 .
Figure 2. Temporal evolution of emissions of (a) NOx and (b) NMVOC averaged over several source areas in the Northern Hemisphere depicted in Fig. 1.

Table 2 .
Summary of the sensitivity simulations and the standard simulation 820 Precursor emissions in the Rest Of the World (ROW) other than CHN, KOR