Methane Interannual Distribution over Peninsular Malaysia from Atmospheric Infrared Sounder Data : 2003 – 2009

Methane (CH4) is a significant greenhouse gas (GHG's) with a relatively short atmospheric lifetime of about 12 years, and is released to the atmosphere by biological processes occurring in anaerobic environments. The CH4 is second in importance only to CO2 with regard to its environmental effects, and its relative global warming ability is 23 times that of CO2 over a time horizon of 100 years. The interannual distribution of atmospheric CH4 has been studied in Peninsular Malaysia during the period 2003–2009 using Atmosphere Infrared Sounder (AIRS) data, onboard NASA's Aqua Satellite. The analysis of CH4 above five dispersed stations in the study area shows that the high CH4 growth rates observed at the end of each year can be attributed to the increased emissions from biomass burning and wetlands, and the reduced hydroxyl (OH) sink. In particular, we observe a quasi-biennial variation in CH4 emissions in Peninsular Malaysia, with varying magnitudes in peak emissions occurring in 2004, 2006, and 2008. The seasonal variation in the CH4 fluctuated significantly between northeast (NEM) and southwest (SWM) monsoon seasons. The CH4 value in the NEM season was higher than in the SWM season, and higher in the north regions, above the latitude 4°, than in the rest of area throughout the year. To study the CH4 distribution over peninsular Malaysia for 2009, monthly CH4 maps were generated using the Kriging interpolation technique. The AIRS data and satellite measurements are able to measure the increase in the atmospheric CH4 concentrations over different regions.


INTRODUCTION
Many GHGs take place naturally in the atmosphere, such as water vapor, methane, nitrous oxide and carbon dioxide, while others are synthetic.Methane (CH 4 ) is a potent greenhouse gas and it's second in importance only to CO 2 with relative global warming ability 23 times that of CO 2 over a time horizon of 100 years.This mean, for the same mass, that CH 4 emission will have 23 times the influence on temperature of CO 2 emission (Yashiro et al., 2008).Fortunately, it has a much shorter atmospheric lifetime, about 12 years.It is also formed and released to the atmosphere by biological processes occurring in anaerobic environments.CH 4 is emitted from a diversity of both human-connected (anthropogenic) and natural sources.Human-connected activities include agriculture, biomass burning, fossil fuel production, waste management, and animal husbandry (enteric fermentation in livestock and manure management).Domestic ruminant animals, especially cattle and sheep, emit about 15% of all methane (Hill, 2004).
Natural sources comprise wetlands, oceans, non-wetland soils, gas hydrates, permafrost, termites, freshwater bodies, and other sources such as wildfires.Wetlands are the single largest source of CH 4 into the atmosphere, because of their water saturated soils, and account for 20-40% of the global CH 4 source, of which tropical wetlands account for 50-60% of this global wetland CH 4 source (Bloom et al., 2012).In the earth's atmosphere, the dominant infrared absorbing and emitting gases are carbon dioxide (CO 2 ), which causes 9-26% of the greenhouse effect; water vapor, which causes about 36-70% (not including clouds); methane (CH 4 ), which causes 4-9%; and ozone (O 3 ), which causes 3-7%.Clouds have different effects on radiation from water vapor due to it's composed of liquid water or ice (Blais et al., 2005).
Owing to dominated by marshes and swamps, tropical wetlands emit large amounts of CH 4 into the atmosphere compared to northern peatlands.What's more, CH 4 emissions are significantly higher in open peatlands than in forested peatlands, which is most likely related to the increase in temperature and water table depth (Melling et al., 2005).Climate affects the exchange of CH 4 between ecosystems and the atmosphere by influencing CH 4 oxidation, production, and transport in the soil.The net CH 4 exchange depends on soil, vegetation characteristics and ecosystem hydrology (Spahni et al., 2011).
The rice paddies in the tropics are a major, seasonally varying CH 4 emission source.Although a lot of field observations of CH 4 emission from rice paddies have been prepared under a variety of soils, agricultural practices and climates in Asia, precise estimates of CH 4 from global or regional rice paddies via-scaling have been difficult due to large differences in temporal and spatial variability in soils, agricultural practices and climate.However a large range of emissions was given by the IPCC (IPCC, 2007 and references therein), both the seasonal maximum and the monthly total, of the Asian CH 4 emissions are still not well known (Xiong et al., 2009).
Southeast Asia is one of the most heavily populated regions of the world with a vibrant mixture of cultures, and its important contribution to regional and global pollution because of increasing anthropogenic emissions associated with biogenic emissions from large tropical forests.In Malaysia, it is considered one of Southeast Asia tropical country, industrialization; urbanization and rapid traffic growth have contributed significantly to economic growth.Pockets of heavy pollution are being created by emissions from major industrial zones, a dramatic increase in the number of residences, office buildings, manufacturing facilities, increases in the number of motor vehicles and trans-boundary pollution.Besides that, Malaysia is situated in a humid tropical zone with heavy rainfall and high temperatures (Tangang et al., 2007;Mahmud and Kumar, 2008), the cloudy conditions cover the study area becomes the obstacle to acquire a high quality and resolution for satellite data.
Over the past three decades, the abundances of the atmosphere gases were obtained from a lot of sources such as Balloons, airplane and sparsely distributed measurement sites.The observations were mostly limited to the surface and more sensitive to sources and sinks with best accuracy from ground and aircraft, but the major shortfall is not being able to make daily global variations evaluation (Rajab et al., 2011a).The Satellite remote sensing has very good global coverage increase our capability to access the influence of human activities on the chemical composition of the atmosphere and on the climate changes (Dousset and Gourmelon, 2003).Satellite measurements have been used in several studies which have provided some evidence for the transport or traces of atmospheric components from the surface to the upper atmosphere (Kar et al., 2004;Park et al., 2004 and reference therein;Li et al., 2005;Randel and Park, 2006;Xiong et al., 2009).
The first new generation of meteorological advanced sounders for operational and research use was Atmospheric Infrared Sounder (AIRS), one of several instruments onboard the Earth Observing System (EOS) Aqua spacecraft launched May 4, 2002 (Fishbein et al., 2007a).From 705 km above the Earth's surface the AIRS measures the integrated impact of numerous atmospheric molecules emitting and absorbing radiation at various temperatures throughout the atmospheric path from the surface to the instrument.Providing information for several greenhouse gases is one goal of the AIRS instrument, new Insights into Weather and Climate for the 21st Century, and study the water and energy cycle (Marshall et al., 2006).
In this study the analysis of CH 4 mixing ratio was investigated for the period 2003-2009 in Peninsular Malaysia using the retrieved AIRS Level 3 monthly product (AIRX3STM) Version 5 data.The CH 4 Satellite data were evaluated over five stations; Subang, Penang, Kuantan, Johor, and Kota Bharu, respectively, for the study period.The monthly CH 4 maps were generated using Kriging Interpolation technique to analyze its distribution on 2009 for study area.This interpolation technique produced high correlation coefficient R and low root mean square error.

STUDY AREA
The study area is peninsular Malaysia is located, between 1° to 7° latitudes north and 99° to 105° longitudes east, south of Thailand, north of Singapore and east of the Indonesian island of Sumatra.An area Fig. 1, covering 3.575 × 105 km 2 , with a center at Pahang (102°E and 4°N) was selected for this study.The central dimensions of the study domain are 550 km E-W and 650 km N-S.The Titiwangsa Mountain is a range from the Malaysia-Thai border in the north running approximately south-southeast over a distance of 480km forms the backbone of the Peninsula and separating the western part from the western part.Surrounding the central high regions are the coastal lowlands (Rajab et al., 2011b).
Version 5 Leve-3 data are available at http://disc.sci.gsfc.nasa.gov/AIRS/data-holdings/by-data-product,as well as auxiliary data including the corresponding location and time along the satellite track in a Hierarchical Data Format (HDF) format on daily basis.Using the location information, CH 4 data was gridded monthly at Geospatial Resolution of 1° × 1° (lat × lon).

ACQUISTION AND SPECIFICATION
AIRS is a continuously operating cross-track scanning sounder, consisting of a telescope that feeds a scale spectrometer.The AIRS instrument views the atmospheric infrared spectrum in 2378 channels with a nominal spectral resolving power λ⁄∆λ ranging from 1086 to 1570 covering more than 95% of the earth surface and returning about three million spectra daily, in the 3.74-4.61μm, 6.20-8.22μm and 8.8-15.4μm infrared wavebands at a nominal spectral resolution, also includes four visible/near-IR (Vis/NIR) channels between 0.40 and 0.94 μm, with a 2.3-km FOV (Rajab et al., 2010).
The AIRS/AMSU/HSB is designed to operate in synchronism, and its science objective mission is to understand the dynamics of climate, operational numerical weather forecasting, determination of the factors that control the global energy and water cycles, inquisition of atmospheresurface interactions, and diagnosis of the effects of increased carbon dioxide, methane, ozone and other greenhouse gases.Its data will be used to improve numerical weather predictions and to support climate-related studies (Aumann et al., 2003).AIRS measures nearly 200 channels in the absorption band of CH 4 , 71 channels near 7.6 µm are used for CH 4 retrieval, and they are most sensitive to the middle to an upper troposphere.AIRS CH 4 products include not only the CH 4 profile but also the information content.The atmospheric temperature-humidity profiles, emissivity, and surface skin temperature required to derive CH 4 are acquired from retrievals using separate AIRS channels and the AMSU (Xiong et al., 2008).
The L3 data are created from the L2 data product by binning them in 1° × 1° grids.Level 3 products are statistical summaries of geophysical parameters that have been temporally aggregated and spatially re-sampled from lower level data products (e.g., Level 2 data) (Kopacz et al., 2010).There are three AIRS Level 3 data products separately derived from Microwave-Only (MW-Only) retrievals and combined Infrared/Microwave (IR/MW) retrievals: daily, weekly and monthly as summarized in Table 1.Each product provides separate ascending (daytime) and descending (nighttime) binned data sets.
The spatial footprint of the infrared channels is 1.1° in diameter, which corresponds to about 15 × 15 km in the nadir.Spatial coverage and calibration targets are supplied by the scan head assembly, including the scan mirror and calibrators.The rotating external mirror scans the underlying Earth scene 49° × 49° nadir, in 90 integration periods, and provides two views of dark space during each scan.Table 2 shows AIRS technology-specifications.

MATERIALS AND METHODS
This research has been carried out for seven-year data from January 2003 to December 2009.In order to evaluate and analysis the CH 4 distribution over the study area, we selected five stations dispersed across Peninsular Malaysia; Subang, Penang, Kuantan, Johor, and Kota Bharu.Results from the analysis of the retrieved for the CH 4 obtained from AIRS ascending (AIRX3STM) Level-3 data.The AIRS standard CH 4 products are derived from the IR stage of the combined IR/MW retrieval.The effective CH 4 volume mixing ratio is produced at three levels between 390 mb (height 7500 m) and 160 mb (height 1300 m).This study were used effective CH 4 volume mixing ratio (CH 4 ) (ppmv) at a height of 7500 metres.Generally, 84 monthly L3 ascending granules were downloading to obtain the desired output.Extract the AIRX3STM product's files from the Satellite using the AIRS website, and saves in HDF-EOS4 files; this is a convenient file extension that can be easily extracted data from it and arrange in table using MS Excel.
Data including the corresponding location and time along the satellite track in a HDF (Hierarchical Data Format) format on monthly basis.Map of the study area was conducted by using Photoshop CS and SigmaPlot 11.0 software to analyze the carbon dioxide data distribution along the study period.To better assess the impacts and distribution of CH 4 over Peninsular Malaysia the maps of CH 4 was generated by using Kriging interpolation technique for the year 2009.The CH 4 data were obtained from 1° × 1° (latitude × longitude) spatial resolution ascending orbits.

RESULTS
Fig. 2 Shows the monthly CH 4 from 2003-2009 for five stations; Subang, Penang, Kuantan, Johor, and Kota Bharu, respectively.The mean and the standard deviation of monthly CH 4 was (1.71 ± 0.03 × 10 -6 ppmv) for the entire period.The CH 4 experience various seasonal fluctuations depend on weather conditions and topography.Seasonal variation in CH 4 fluctuated considerably between northeast monsoon (NEM) and southwest monsoon (SWM) periods.High CH 4 growth rates observed at the end of each year were attributed to a reduced hydroxyl (OH) sink and increased emissions from wetlands and biomass burning.The increased wetland emissions were related to climatic variations involving positive temperature and precipitation anomalies (Langenfelds et al., 2002).
A study by Dlugokencky et al. (1996) indicated that high  CH 4 growth rates observed in later years were attributed to a reduced hydroxyl (OH) sink resulting from the Mount Pinatubo volcanic eruption.Novelli et al. (1999) also pointed out that the main sink of CO and CH 4 is oxidation by OH.
Even though H 2 is itself destroyed by OH, it is also a by product of CH 4 destruction and a net product of OH photochemistry.Thus variation in OH concentration would produce anticorrelation in growth rates of CO and CH 4 (net less due to higher OH).
The CH 4 emissions are affected indirectly by the water table depth, temperature, topography and climatic regions, and increase from north to south by unit area, which is mostly connected to the increase in temperature.In the NEM season, the production of CH 4 is low at low temperatures (resulting from the few sunny hours in April because of the intermonsoon), as occurred during November and April.In open peatlands, which are usually characterised by a water table close to the surface, CH 4 fluxes are significantly high compared with forested peatlands.The late NEM season coincides with minimal OH levels, which is an important component of the CH 4 equation, and the instability of the climatic conditions with tropical cyclones significantly affects the emission of CH 4 in different regions; therefore, CH 4 fluxes have relatively positive results in the late NEM season as a result of reductions in OH, first formed from water vapour collapse by oxygen atoms that come from the splitting of O 3 by ultraviolet radiation (Reiners et al., 1998).In the SWM season, the oxidation of methane by OH; radical chemistry is the major removal mechanism of CH 4 from the atmosphere, and the lack of rice cultivation during the SWM season also contributes to reduction in CH 4 emissions.The positive results in July and August are the result of minimum precipitation; those are the driest months in most districts which lead to low production of OH (Suhaila and Jemain, 2009).
The prolonged wet weather conditions improved the air quality slightly in Malaysia throughout 2003.Unexpected heavy rains occurred in areas such as Penang, reaching 869.1 mm in October.Therefore, it can be observed two peaks of CH 4 coinciding with the impact of the intermonsoon in May and October (DOE, 2003).The air quality in peninsular Malaysia deteriorated slightly throughout 2004 compared with 2003 because of the influence of southwesterly winds, especially on the west coast, simultaneity with the impact of El Nino from May to July (DOE, 2004).We observed an slightly decreases in CH 4 values in most areas, especially in Penang and Subang.The year 2005 shows the lowest value of CH 4 due to the rainfall deficits and the influence of peatland fires in several areas in the state of Selangor.Following a prolonged dry season in the region, coincide with impact of El Nino effects with the beginning of the year and the direct impact of southwesterly wind.In addition, the effects of the land and forest fires in the Riau Province of Central Sumatra, Indonesia, from mid-May until mid-October on the CH 4 values (DOE, 2005).
In 2006, we observed an increase in the CH 4 values from August until November, mainly as a result of sensibly rain that occurred in this period, coupled with direct effect of south-westerly winds and El Nino (DOE, 2006).Favourable weather significantly improved the CH 4 emissions on the east coast in the late months of 2007.The wet weather conditions in 2008 slightly improved the CH 4 values compared to 2007, especially in the east coast due to heavy rainfall (DOE, 2008).
A quasi-biennial variation in CH 4 mixing ratio over Peninsular Malaysia in October is plainly evident in the monthly AIRS CH 4 maps presented in Fig. 3 for 2003, 2004, 2005, 2006, 2007, and 2008.Maximum values of CH 4 over the rice paddies, wetland and open peatlands in the northern regions, up to latitude 4° occur in 2004, 2006, and 2008, with minima in 2003, 2005, and 2007.More careful examination reveals subtle differences in the CH 4 spatial patterns for each of the peak years.Interannual variations are visible for the geographic regions, but none is as pronounced or regular during these six years.
The effective CH 4 (CH4_VMR_eff_A) Level-3 monthly AIRX3STM 1° × 1° spatial resolution ascending data were used for mapping CH 4 in 2009. Fig. 4(a) shows that the highest value of CH 4 occurred during the early NEM season (November-January), especially above the northern region above latitude 4° (1.752 × 10 -6 ppmv, at 101.5° × 4.5°), as a result of reduced OH and favourable weather conditions, which increased CH 4 formation.In February, CH 4 decreased to its lowest value in the NEM season (1.688 × 10 -6 ppmv, in Malacca), though it slightly increased to moderate levels in March and increased to slightly high levels in April.This fluctuation in the CH 4 values during the late NEM season was caused by the geographic nature of the areas and climatic variations.
As illustrated in Fig. 4(b) for the SWM season, an increase in the CH 4 value during May and October was observed with an increase in the temperature and precipitation, whereas slightly high to moderate values of CH 4 were predominant from June to September because of the prolonged dry weather conditions that coincided with a moderate to strong El Niño experienced in the region during this period.The highest value that occurred in the SWM season was in May (1.730 × 10 -6 ppmv, at 102.5° × 3.5°), and the lowest value was in August (1.706× 10 -6 ppmv, at 100.5° × 5.5°).
In general, Figs. 2,3,4(a and b) shows that the values of CH 4 in the northern regions, up to latitude 4°, were higher than in the other regions throughout the year because of an increase in CH 4 emission from the abundance of paddies and wetlands, which was related to the weather conditions, water table depth, and topography.In addition, the CH 4 fluxes in open peatlands were high compared with the fluxes in forest peatlands.In addition, in the NEM season, CH 4 had values higher than in the SWM season, and there were regions that had low values in some months, such as Johor in February and September, as a result of low rainfall and temperature.The reduction in the CH 4 values occurred along with the impact of El Niño from June to August.

CONCLUTIONS
As demonstrated here, AIRS' monthly views of atmosphere CH 4 across the study area enable detailed analyses of both the temporal and spatial variations in emissions and the visualization of subsequent transport.We have just begun to Fig. 3.The six maps illustrate the monthly AIRS CH 4 mixing ratio for October of 2003, 2004, 2005, 2006, 2007, and 2008, respectively  investigate the wealth of information contained in the more than six years.The CH 4 values are strongly correlated with weather conditions.The mean and the standard deviation of monthly CH 4 was (1.71 ± 0.03 × 10 -6 ppmv) for the entire period.Seasonal variation in CH 4 fluctuated considerably between NEM and SWM periods.High CH 4 growth rates observed at the end of each year were attributed to a reduced hydroxyl (OH) sink and increased emissions from wetlands and biomass burning.The CH 4 emissions are affected indirectly by the water table depth, temperature, topography and climatic regions.
From monthly CH 4 distribution, the northern regions, up to latitude 4°, were higher than in the other regions throughout the year due to increase of the CH 4 emission from the abundance of paddies and wetlands.In addition, the CH 4 fluxes in open peatlands were high compared with the fluxes in forest peatlands, and CH 4 values in the NEM season was higher than in the SWM season.As a result of low rainfall and temperature, there are areas had low values of CH 4 in some months, such as Johor in February and September.The reduction in the CH 4 values occurred along with the impact of El Niño from June to August.The highest value that occurred in the northern region above latitude 4° on November (1.752 × 10 -6 ppmv, at 101.5° × 4.5°), and the lowest value was on March (1.688 × 10 -6 ppmv, in Malacca).The CH 4 maps were generated using Kriging Interpolation technique.The AIRS data and the Satellite measurements are able to measure the increase of the troposphere CH 4 concentrations over different regions.

Fig. 4 .
Fig. 4. AIRS monthly coverage from the retrieved CH 4 , (a) for NEM season [November to April] and (b) for SWM season [May to October] 2009.