Satellite derived trends in NO2 over the major global hotspot regions during the past decade and the

Satellite derived trends in NO 2over the major global hotspot regions during the past decade and their inter-comparisonSachin D.Ghude a ,*,R.J.Van der A b ,G.Beig a ,S.Fadnavis a ,S.D.Polade aa Indian Institute of Tropical Meteorology,Pune,IndiabRoyal Netherlands Meteorological Institute,De Bilt,The NetherlandsTrends in tropospheric column NO 2over newly developing regions.a r t i c l e i n f oArticle history:Received 26November 2008Received in revised form 16January 2009Accepted 18January 2009Keywords:Tropospheric NO 2TrendsGOME and SCIAMACHY measurements Pollutiona b s t r a c tWe assessed satellite derived tropospheric NO 2distribution on a global scale and identified the major NO 2hotspot bined GOME and SCIAMACHY measurements for the period 1996–2006have been used to compute the trends over these regions.Our analysis shows that tropospheric NO 2column amounts have increased over the newly and rapidly developing regions like China (11Æ2.6%/year),south Asia (1.76Æ1.1%/year),Middle East (2.3Æ1%/year)and South Africa (2.4Æ2.2%/year).Tropospheric NO 2column amounts show some decrease over the eastern US (À2Æ1.5%/year)and Europe (0.9Æ2.1%/year).We found that although tropospheric NO 2column amounts decreased over the major developed regions in the past decade,the present tropospheric NO 2column amounts over these regions are still significantly higher than those observed over newly and rapidly developing regions (except China).Tropospheric NO 2column amounts show some decrease over South America and Central Africa,which are major biomass burning regions in the Southern Hemisphere.Ó2009Elsevier Ltd.All rights reserved.1.IntroductionNitrogen oxides (NO x ¼NO þNO 2)is an important contributor to air pollution.It is now well known that high amounts of nitrogen oxides emitted from industries (point sources)and motor vehicles lead to surface ozone production (Sillman et al.,1990;Fowler et al.,1998)and exceedances of the ozone standard (Frost et al.,2006).Tropospheric NO 2distribution derived from the satellite instru-ments GOME and SCIAMACHY provided an opportunity to observe urban-scale NO 2pollution globally.It is already evident that local changes in the NO 2column correlate well with the high concen-tration levels at the surface (Leue et al.,2001;Richter and Burrows,2002)and can be proportional to the changes in local emissions (Martin et al.,2003).Several comparisons of these NO 2distribu-tions with ground based observations and aircraft measurements have recently been made (Petritoli et al.2004;Schaub et al.,2006).Some recent studies have highlighted areas of intense pollution over industrialized regions (Beirle et al.,2003;Van der A et al.,2006;Blond et al.,2007;Ghude et al.,2008),emissions frombiomass burning,soil emissions (Jaegle´et al.,2005;Van der A et al.,2008)and NO x production from lightning (Boersma et al.,2005;Martin et al.,2007;Van der A et al.,2006).Similarly,Jaegle´et al.(2005)used GOME measurements to derive a global a posteriori emission inventory.Recent studies,using combined GOME and SCIAMACHY measurements,have shown decreasing NO 2trends over the eastern United States (US)(Kim et al.,2006)and Europe (Richter et al.,2005).While strong increasing trends are observed over China (Richter et al.,2005;Van der A et al.,2006).Ghude et al.(2008)have shown increasing trends with substantial heterogeneity over congested industrial areas (emission hotspots)in India.However,there are several other newly developed/developing regions (newly industrialized countries)in Asia,America (North and Central),Europe,Africa and South America,where industrial and vehicularactivities have increased during the past decade (Bo _zyk,2006).Most of these previous trend studies were focused on particular regions such as China or the US and Europe.More recently,Van der A et al.(2008)have shown positive trends over some of the large cities and newly developed/developing regions over the globe during the period 1996–2005on a 1 by 1 grid cell.However,much of the focus of this study was to identify the source category by analyzing the seasonal variation in the time series of each grid cell.In this study,for the first time,we have comprehensively reviewed the trends in satellite tropospheric column NO 2amounts in many regions of the world (global NO 2hotspot regions),and*Corresponding author.Tel.:þ9102025893600.E-mail address:sachinghude@tropmet.res.in (S.D.Ghude).Contents lists available at ScienceDirectEnvironmental Pollutionjournal homepage:/locate/envpol0269-7491/$–see front matter Ó2009Elsevier Ltd.All rights reserved.doi:10.1016/j.envpol.2009.01.013Environmental Pollution 157(2009)1873–1878made a relative comparison.To broadly categorize major NO2 source areas associated with human activities(linked with indus-trial and vehicular emissions)and large biomass burning events,we used the Earth’s nightlight image and globalfire count image (Duncan et al.,2003)as a proxy for the pattern of anthropogenic and large biomass burning emissions.Unlike previous trend studies (which have used NO2time series on a0.5 by0.5 (Richter et al., 2005)and1 by1 (Van der A et al.,2008)(except Ghude et al., 2008)grid to estimate the trends on each grid),here we analyze 11years of tropospheric NO2data averaged over these major global NO2hotspot regions(Fig.1a)to estimate the region resolved trends. We also compare tropospheric NO2column amounts with the oil and coal consumption in some of these area source regions.We will further demonstrate that despite increases in tropospheric NO2 amounts in the developing world(south Asia and newly industri-alized countries)during the past decade,the NO2column amounts over these regions(with the exception of China)are still lower than those observed over developed countries in the past decade.2.Methods2.1.Satellite measurementsMeasurements of tropospheric NO2from space have been available since the Global Ozone Monitoring Experiment(GOME)aboard ERS-2and SCIAMACHY on the Envisat platform became operational.The ultraviolet/visible nadir measurements of SCIAMACHY are similar to those from GOME,with the main difference being the improved spatial resolution(60Â30km2vs.320Â40km2) (Richter et al.,2005).From the observed spectral features(around425–450nm) NO2column amounts are retrieved with a differential optical absorption spec-troscopy technique.The same retrieval techniques were used for both instru-ments.More details on the retrieval are discussed in Boersma et al.(2004)and Van der A et al.(2006).The local overpass times of GOME(10:30a.m.)and SCIAMACHY(10:00a.m.)are close enough to avoid systematic differences in their time series(Van der A et al.,2006).Richter et al.(2005)and Van der A et al. (2006)have shown that the bias between both data sets is smaller than the minimum error(0.5Â1015molecules/cm2)in the data and,therefore,the bias can be ignored.The GOME data from March1996to December2002and the SCIAMACHY data from January2003to December2006(available at the TEMIS project website,http://www.temis.nl)were used in the present study.Monthly mean tropospheric NO2observations with an estimated cloud fraction less than about0.2were used to derive the linear trends in this study.Van der A et al. (2006)suggested that the effect of scattering aerosol on the derived trend in NO2 can be ignored.Further,Boersma et al.(2004)showed that satellite derived cloud fractions are also sensitive to aerosols having a high single scattering albedo. Therefore,an increase in the cloud fraction due to higher concentration of aerosols leads to a similar air mass factor correction for aerosols.Therefore,its effect on NO2slant columns would be compensated by increased cloud fraction. In work done by Boersma et al.(2004),it was observed that the chances of finding significant change(percent change in NO2slant columns)from strong increases in absorbing aerosols are small.Hence,we do not expect a significant effect(percent change in NO2slant columns)of scattering or absorbing aerosols on NO2column amounts.For detailed error estimates and kernel information see Boersma et al.(2004).2.2.Trend analysisTo analyze the tropospheric NO2time series obtained from GOME and SCIA-MACHY measurements,a multifunctional regression model has been used in this work,details of which are given elsewhere(Ziemke et al.,1997)in detail and hence described here briefly.Linear trends and two-sigma errors are computed by regression model.Error estimates are according to Neter et al.(1985).The general expression for the regression model equation can be written as followsqðtÞ¼aðtÞþbðtÞ,TrendðtÞþResidðtÞHere,a(t)and b(t)are the time-dependent12-month seasonal and trend coefficients respectively.Resid(t)represents the residues or noise.The model performs multiple regression analysis of NO2time series over the regions of interest.Monthly mean tropospheric NO2columns are averaged over the regions shown in Fig.1a to obtain the time series.These time series are then subjected to the multifunctional regression model.Monthly trend coefficients obtained from the model(for a region) are then averaged to get a single annual trend coefficient.Similarly,corresponding monthly two-sigma error values are averaged to get a single annual two-sigma error estimate.3.Results and discussion3.1.Global distribution of NO2area sourcesSpatial distribution of tropospheric NO2columns derived from the SCIAMACHY measurements(average2003–2006)is shown in Fig.1a,which illustrates many details of the global distribution of tropospheric NO2.Significantly enhanced levels of tropospheric NO2have been observed mostly over the regions linked with the bright(human-made light)areas as seen in the Earth’s nightlight image(Fig.1b,available at ).Human-made lights observed in Fig.1b predominantly highlight regions of densely clustered industrial activity or populated places on the Earth’s surface,which can be used as a proxy for the pattern of anthropogenic emissions(Toenges-Schuller et al.,2006).In Fig.1a these regions appear as a source of NO2(area source)on a global scale.Many NO2hotspots in these regions can be precisely identi-fied as localized sources of NO2and most are found to be associated directly with large cities,industrial areas or large coal based thermal power plants(Van der A et al.,2006,2008;Ghude et al., 2008).In addition to localized sources,NO2emissions from ships can also be captured by the satellite observations.The highly fre-quented ship routes in the Indian and North Atlantic Ocean can be seen in Fig.1a(by an apparent increase in tropospheric NO2levels over these ship routes).NO2emissions associated with shipping based on emission inventories also showed(not shown here)NO2 emission over these routes(ftp:///pub/emissions).Fig.1a also depicts significant tropospheric NO2column amounts over a few regions,particularly tropical regions of the Southern Hemisphere where human settlement in the visible range of satellite measurements(corresponding to dark regions)are not observed to be very dense in Fig.1b.Over Central Africa,enhanced NO2column amounts can be seen,however NO2sources are not as sharply localized as in the populated areas of the Northern Hemi-sphere.Similarly,enhanced levels of tropospheric NO2over some regions in South America(mostly Brazil,Bolivia and Paraguay)and Australia(northern and eastern)can also be seen in Fig.1a.This implies that the observed NO2column amount over these regions is not linked largely with industrial or vehicular sources.It can be seen from Fig.1c(thefigure is adapted from Duncan et al.,2003 which shows the global distribution offire counts)that these regions are mostly dominated by large yearly biomass burning events.In Central Africa,large biomass burning events can be seen mostly during the June to September period,and during the dry season in South America and Australia(Duncan et al.,2003).This indicates that large biomass burning is probably the dominant source of NO2in these regions.Over the Sahara,Siberia,central Australia,some parts of South America etc.,where large human activities and biomass burning events are absent,tropospheric NO2 levels less than w50Â1013molecules/cm2have been observed.Fig.1c also shows the areas in the northern tropical and extra topical region where significant biomass burning is observed.A noticeable amount of biomass burning is seen over south Asia,the Middle East,south east and east Asia,while it is negligible over the eastern US and Europe.This suggests that the enhanced levels of tropospheric NO2over south Asia could be partly from biomass burning(the contribution of biomass burning to the total NO x emission in India is20%(Garg et al.,2002)).On the other hand,over most of the region of China(except south east China),the US and Europe,it is almost dominated by industrial and vehicular activities.3.2.Trends in tropospheric NO2over major source regionsIt can be seen from Fig.1a,b,c that the enhanced tropospheric NO2column amounts observed over much of Europe,the US,China,S.D.Ghude et al./Environmental Pollution157(2009)1873–1878 1874S.D.Ghude et al./Environmental Pollution157(2009)1873–18781875Fig.1.(a)Mean tropospheric NO2column(in1Â1013molecules/cm2)for the year2003–2006as derived from SCIAMACHY measurements(there are data points higher than 450Â1013molecules/cm2however,for clarity,the scale is capped).The inset illustrates highly frequented ship routes in the Indian and North Atlantic Oceans.(b)Earth’s nightlight image(source:).(c)Fire counts binned into1Â1 grid boxes using the ATSR World Fire Atlas(August1996to December2000)(figure is adapted from Duncan et al.,2003).Japan,Mexico,the Middle East and south Asia are mostly due to the large anthropogenic activities.Whereas,enhanced tropospheric NO 2column amounts observed over South America and Africa are dominated by large biomass burning events.To estimate and compare the trends over these major NO 2source regions,combined GOME and SCIAMACHY measurements are used.In Fig.2observed trends (along with its two-Sigma error)for the period 1996–2006are shown.East China is one of the fastest growing industrial areas in Asia (Zhang et al.,2007),which is reflected in a large NO 2growth rate of about 11Æ2.6%per year.Van der A et al.(2006,2008)reported similar trends up to 29%per year over some of the cities in eastern China.Power plants are the largest coal consumers in China and are believed to be the largest contributors to NO x emission (Streets et al.,2003).In the past 11years,thermal power generation increased by approximately 125%(NBS,2006)and the vehicular population increased by approximately 90%in China.This trend can be attributed to increased consumption of oil and coal in China in the last decade as has been reported by Zhang et al.(2007).As can be seen from Fig.3,oil consumption in China has increased from w 3.6million barrels per day to w 7.2million barrels per day during 1996–2006,which is an approximate increase of 100%.Similarly,coal consumption during the same period has been observed to increase by about 72%(EIA,2008).Over south Asia (India,Pakistan,Bangladesh and Nepal)a positive trend of about 1.76Æ1.1%per year is clearly seen.This is quite consistent with the recently reported trends over India (as a whole)of about 1.67%per year (Ghude et al.,2008).This also implies that the increasing levels of tropospheric NO 2column amounts over south Asia are mostly dominated by emissions from the Indian region,while a small contribution appears to be from Pakistan,Bangladesh and Nepal.This trend can be attributed to increasing emissions from the industrial and transport sectors due to the rapid economical development in India (Garg et al,2002;Akimoto,2003;Ghude et al.,2008).The growth in the coal based thermal power sector and vehicular population in India during 1997–2007was about 40and 50%,respectively.Coal contributes about 45%and transport contributes about 32%of total NO x emissions in India (Garg et al.,2002).About 70%of all Indian coal consumption is utilized for power generation.Oil and coal consumption in south Asia has increased by approximately 46%(w 2.07million barrels per day in 1996to 3.02million barrels per day in 2006)and 56%,respectivelyduring past decade (Fig.3).Apart from China and south Asia,an increasing trend of about 2.3Æ1%per year over the Middle East region (Iran,Iraq,Saudi Arabia,Kuwait,Quatar,UAE and Turkey)and 2.4Æ2.2%per year over South Africa is observed,which is found to be somewhat higher than that observed over south Asia.The growth in oil and coal consumption in South Africa has been increased by w 20%each during the past decade.Middle Eastern countries have also become large consumers of transportation fuels in the last decade.Oil consumption in Middle Eastern countries has increased from w 3.8million barrels per day to w 5.6million barrels per day during 1996–2006,an approximately 45%growth in the last decade (EIA,2008).Oil is mostly consumed for transportation,infrastructure,petrochemicals,and electricity in this region.Many of these Middle Eastern countries have price controls and subsidies that provide inexpensive fuel for their residents,which encourage energy waste and inefficiency.Automobile sales in some of these countries have tripled in the last 4years as cars have become more readily available to a growing middle class population and fuel costs remain low.Recent EIA (2008)reports indicate that Saudi Arabia has increased its oil consumption from 1.33million barrels per day in 1996to 2.14million barrels per day in 2006,an increase-1135791112T r e n d i n % p e r y e a rS o u t h A s iA u s t r a l i C h i n S o u t h A f r i c M i d d l e E a M e x i c W e s t e r n U E a s t e r n U S o u t h A m e r i c E u r o p J a p a C e n t r a l A f r i c -20246810(±2.6)Fig.2.Linear trend (per year)over the selected area source region for tropospheric NO 2in the period 1996–2006derived from the GOME and SCIAMACHY satellite observation.(For the light grey bars the trend is not significant at the two-sigma level in the time series.)U .S .E u r o p eC h i n aJ a p a nM i d d l e E a s tM e x i c oS o u t h A s i aA u s t r a l i aS o u t h A f r i c a246810121416182022aM i l l i o n b a r r e l s p e r d a yRegionsU .S .E u r o p eC h i n aJ a p a nM i d d l e E a s tM e x i c oS o u t h A s i aA u s t r a l i aS o u t h A f r i c a50010001500200025003000bM i l l i o n s h o r t t o n sRegionsFig.3.(a)Oil and (b)coal consumption over the selected regions (1million short tons ¼0.90metric tons).Data source:/country/index.cfmS.D.Ghude et al./Environmental Pollution 157(2009)1873–18781876of almost 60%.Compared to oil consumption the growth in coal consumption is negligible in the Middle East during the same period.This implies that the enhanced level of tropospheric column amounts over this region is probably due to increased consumption of oil during past decade,unlike south Asia and China where it is largely contributed by both oil and coal consumption (Garg et al.,2002;Zhang et al.,2007).A trend of about 1.32Æ1.5%per year has been observed over the NO 2area source region of Australia (in Fig.1a).However,it is not significant at the two-sigma confidence level.The trend over Mexico (0.7Æ2%per year)and the western US (0.65Æ1.5%per year)although it shows some increase,is not significant during this period.Notable regions of decreasing NO 2trends are over the eastern US (w À2%per year),South America (À1.83%per year)and Central Africa (À1%per year).The trend over Europe (w À1%per year)and Japan (À0.4%per year)although showing some decrease,is not significant over this period.It is observed that oil consumption in Japan has decreased by about 10%(Fig.3).Similar reductions in tropospheric NO 2column amounts from 1996to 2002over Europe,the US and Japan have been pub-lished,which are in excellent agreement with the trends shown in Fig.2.When comparing these results with those of Richter et al.(2005)and Van der A et al.(2006,2008),where the global trends are shown for 7years of GOME and 10years of combined GOME and SCIAMACHY data,we see almost the same pattern with few more details.The significant trend that was found over south Asia,the Middle East,South Africa,Mexico,western US,Central America and the Middle Africa is not observed in the results of Richter et al.(2005).Van der A et al.(2008)however showed a similar pattern of trends over these regions (1 by 1 grid cell).However,they did not estimate the region specific (area source)trends and did not make a relative comparison,as we have shown in Figs.1,2and 4.Fig.4shows the tropospheric NO 2column amounts averaged for the periods 1996–1997and 2005–2006for the regions shown in Fig.1a.Tropospheric NO 2column amounts over south Asia were observed to be w 195Â1013molecules/cm 2during 1996–1997and w 218Â1013molecules/cm 2during 2005–pared to the south Asia region,NO 2column amounts during 1996–1997were observed to be higher by about 145%over the eastern US,followed by Europe and Japan (111%),China (79%),South Africa (67%),western US (24%)and South America (24%).However,during 2005–2006,NO 2column amounts were observed to be higher over China (193%),followed by the eastern US (85%),Japan (84%),South Africa (68%),Europe (65%),and the western US (17%)than that observed over the south Asia region.Over the Middle East andMexico,NO 2column amounts during 1996–1997were observed to be slightly higher (w 8%)than that observed over south Asia.During 2005–2006NO 2column amounts over Middle East and Mexico increased slightly to 13and 11%greater than south Asia,respec-tively.Over Australia,the observed NO 2column amounts during 1996–1997and 2005–2006were observed to be less by 10and 15%respectively compared to south Asia.As can be seen from Fig.4,in the Northern Hemisphere,averaged tropospheric NO 2column amounts during 1996–1997over the eastern US,Europe and Japan were nearly equal,followed by China.Elevated tropospheric NO 2column amounts over these regions were mostly linked to human activity (industries and vehicles).As can be seen from Fig.3,combined oil and coal consumption during 1996in these regions was large compared to other regions of the globe.However,an apparent decrease in NO 2column amounts over the eastern US and Europe has been observed in the past decade despite an increase in oil consumption.It has also been observed that there was no significant increase in coal consumption in the US and Europe during the same period (Fig.3).This can be attributed to the intro-duction of cleaner technology and efforts to reduce emissions by the use of catalytic converters on automobile exhaust systems in the US and Europe as has been reported by Richter et al.(2005)and Van der A et al.(2008).Since 1995,the US Environmental Protection Agency has mandated strict emission reduction measures on the industries producing electricity (especially coal burning power plants)in the eastern US (Kim et.al.,2006;Frost et al.,2006).In contrast,tropo-spheric NO 2column amounts over China have increased rapidly during the past decade and were found to exceed the observed levels of tropospheric NO 2over the major contributing regions in the Northern Hemisphere (Fig.4).It is probably due to the fact that compared to coal consumption in the US and Europe in 1996,coal consumption in China has increased by more than 2.5times in the last decade (Fig.3).On the other hand,a relatively small increase in tropospheric NO 2column amounts is observed over south Asia (Fig.4)when compared to the major NO 2hotspot regions in the Northern Hemisphere.It can be seen from Fig.3that combined coal and oil consumption in south Asia was significantly lower between 1996and 2006compared to the US,Europe and China in spite of increases in coal and oil consumption in south Asia during the past decade.These results suggest that although rapid urban and indus-trial growth have enhanced NO 2emissions from south Asia,by an apparent increase in tropospheric NO 2levels between 1996and 2006,the tropospheric NO 2column amounts over south Asia remain less than those of the major developed or developing regions (US,Europe,China,Japan and South Africa)in the Northern and Southern Hemispheres.As can be seen from Fig.4,the present levels of tropospheric NO 2column amounts observed over the western US,Mexico and the Middle East is comparable with the levels of tropo-spheric NO 2column amounts observed over south Asia.It is inter-esting to note that although combined coal and oil consumption in South Africa was lower than that of south Asia in 1996and 2006,the tropospheric NO 2column amounts were observed to be higher than that observed over south Asia.This could be due to the fact that in addition to anthropogenic activities,biomass burning (as seen in Fig.1c)is also a major source of emissions in this region.These results imply that despite a decrease in tropospheric NO 2column amounts over the eastern US and Europe (and an apparent increase in the oil and coal consumption in the newly developing countries),the present tropospheric NO 2levels are still significantly higher than that of the developing word,with the exception to China.Tropospheric NO 2column amounts over the Southern Hemisphere (Central Africa and South America),where only biomass burning has been consid-ered important so far,showed some decrease in the past decade over these regions.A possible cause could be the large deforestation in South America (Brazilian Amazonia,Argentina)and Central AfricaSouth AsiaSouth AfricaMiddle East Mexico Western USEastern US EuropeCentral Africa100200300400500600Tropospheric NO 2(x1013 molecules/cm 2)Fig.4.Averaged tropospheric NO 2column amounts over the selected location during 1996–1997(black)and 2005–2006(light grey).S.D.Ghude et al./Environmental Pollution 157(2009)1873–18781877(Achard et al.,2002).It should be noted here that the observed decrease over this region may be partly caused by changes in surface albedo,cloudiness or meteorological conditions,as is discussed in Van der A et al.,(2006).4.ConclusionWith SCIAMACHY tropospheric NO2measurements,we identi-fied major regions of NO2emission(area sources)on a global scale. Further,we broadly distinguished these regions into two cate-gories:(1)regions of NO2emission mostly dominated by human activities(linked with industrial and vehicular sources);and(2) regions of NO2emission mostly dominated by large yearly biomass burning.We used combined GOME and SCIAMACHY measure-ments to evaluate and compare the trends in tropospheric NO2over these global area source regions during the past decade.Notable regions of positive trend are found to be developing regions such as China with the largest trend,followed by the Middle East,South Africa and south Asia.The eastern US shows a significant negative trend in tropospheric NO2levels,while the western US and Mexico show some increase during the past decade.However,most of Europe(major developed industrial regions)do not show signifi-cant negative trends.South America and Central Africa(regions of major biomass burning activity)also show significant negative trends in tropospheric NO2column amounts.Tropospheric NO2 column amounts over China have increased rapidly during the past decade and exceeded the observed tropospheric NO2column amounts over major source(global hotspot)regions in the Northern Hemisphere.Although tropospheric NO2levels over major developed regions(eastern US and Europe)in the Northern Hemisphere show considerable decrease,the present tropospheric NO2levels over these regions are still significantly higher than those of the tropospheric NO2levels observed over south Asia and newly industrialized countries.This study suggests that tropospheric NO2column amounts over most of the industrially developing regions have increased during the past decade.Similarly,tropospheric NO2levels have decreased over the regions where cleaner technology has been introduced in order to reduce emissions.The implementation of measures to reduce emissions may lower the risk of degrading air quality in the short term.Since emissions from newly industrialized countries are rising and may continue to rise for several decades,these regions will have an increasingly large impact on air quality and the situation can be expected to deteriorate in the future.Although tropospheric NO2shows some decrease over the regions in the Southern Hemisphere where only biomass burning has been considered important so far,because of economic growth in South America and Africa this trend may reverse in the future. 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