Attribution of the Present-Day Aerosol Direct Radiative Forcing to

ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2013, VOL. 6, NO. 5, 369374

Attribution of the Present-Day Aerosol Direct Radiative Forcing to Anthropogenic Emission Sectors CHANG Wen-Yuan State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China Received 30 January 2013; revised 6 March 2013; accepted 11 March 2013; published 16 September 2013

Abstract In this study, a general circulation model coupled with a gas-phase module and an aerosol chemistry module was employed to investigate the impacts of anthropogenic emission sectors on aerosol direct radiative forcing at the top of atmosphere (TOA) in the present-day climate. The predictions were based on the emission inventories developed in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). Six emission sectors—agriculture, open biomass burning, domestic activities, industry, energy generation, and transport—were considered, with a special focus on nitrate aerosol that shows large uncertainties in current models. The results show that the energy sector accounts for the largest contribution (−222 mW m−2) to global aerosol radiative forcing, with substantial negative forcing from sulfate. Inclusion of nitrate results in the transport sector yielding a global nitrate radiative forcing of −92 mW m−2 and an internally mixed aerosol radiative forcing of −85 mW m−2, which is opposite to the positive radiative forcing predicted in the past, indicating that the transport emissions could not be a potential control target to counteract climate warming as expected before. The maximum change in nitrate burden is found to be associated with agricultural emissions, which accounts for about 75% of global ammonia gas (NH3) emissions. Agricultural emissions account for global nitrate radiative forcing of −186 mW m−2 and internally mixed aerosols direct radiative forcing of −149 mW m−2. Such agricultural radiative forcing exceeds the radiative forcing of the industrial sector and is responsible for a large portion of negative radiative forcing over the Northern Hemisphere.  Keywords: anthropogenic aerosols, radiative forcing, emission sector Citation: Chang, W.-Y., 2013: Attribution of the present-day aerosol direct radiative forcing to anthropogenic emission sectors, Atmos. Oceanic Sci. Lett., 6, 369‒374, doi:10.3878/j.issn.1674-2834.13.0016.

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Introduction

Aerosol particulates affect both the Earth’s energy budget and climate by scattering or absorbing solar radiation as well as by acting as cloud condensation nuclei that change the optical properties of clouds (Intergovernmental Panel on Climate Change (IPCC), 2007). In heavily populated regions, since the preindustrial time till the Corresponding author: CHANG Wen-Yuan, [email protected]

present day, direct radiative forcing of aerosols has become comparable or even higher than that of long-lived greenhouse gases (LGHGs, e.g., CO2); however, aerosol radiative forcing takes negative values and masks a substantial portion of CO2 radiative forcing (Taylor and Penner, 1994; Andreae et al., 2005; Chang et al., 2009). Conventionally, climate mitigation policies focus on LGHGs. However, their long atmospheric residence times postpone current human actions of controlling LGHGs emissions to make an effect at least in the future few decades. Aerosols are short-lived species with life times of days to weeks, and are primarily determined by anthropogenic emissions. Aerosol-induced radiative forcing responds quickly to changes in emissions of their precursors and primary particulates, and hence, it has been suggested as an alternative choice to mitigate global warming (Jacobson, 2002; Bond and Sun, 2005). Aerosol precursors and primary particulates have various sources of emission and are often coemitted with other pollutants from various sectors. The complex nonlinear chemical reactions that result in aerosol radiative forcing must be associated with emissions from all the sectors. In the past, the traditional climate researches evaluated radiative forcing of aerosol chemical species only (Liao and Seinfeld, 2005; Hoyle et al., 2009; Chang and Liao, 2009) and ignored the chemical coupling between species emitted from various sectors. Meanwhile, air quality legislations have been formed without considering the climatic effects of air pollutants. Elimination of aerosols can benefit human health but exposes the regional climate to full greenhouse gas effects, exacerbating regional warming. Appropriate environmental policies should consider the benefits of synergies between air quality management and climate policies (Shindell et al., 2008). Recent studies have begun to quantify the contributions of sector emissions to aerosol radiative forcing in the present-day situation (Koch et al., 2007a) and projected future scenarios (Koch et al., 2007b; Unger et al., 2010; Shindell and Faluvegi, 2010). Koch et al. (2007a) evaluated the sulfate and carbonaceous aerosols emitted from different sectors and regions. Unger et al. (2010; hereafter referred to as U10) studied emission sectors more elaborately, and evaluated radiative forcing due to aerosols, O3, and LGHGs, and extended the results for future emissions until 2100. Bauer and Menon (2012; hereafter referred to as B12) claimed that the contributions of emission sectors to aerosol radiative forcing vary greatly among countries.

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Specifically, some studies attempted to quantify aerosol radiative forcing from specific emission sectors such as transportation (Berntsen and Fuglestvedt, 2008; Fuglestvedt et al., 2008), coal-fired power plants (Shindell and Faluvegi, 2010), and household fuel burning (Aunan et al., 2009). In this study, sector-based assessment of aerosol radiative forcing is reconsidered in six major anthropogenic emission sectors according to the comprehensive emission inventories developed in support of IPCC Fifth Assessment Report (AR5). This work specifically focuses on the role of nitrate, which had not been investigated fully in previous studies. Nitrate is a typical secondary inorganic aerosol; its main precursors are ammonia (NH3) and nitric acid (HNO3), which is mainly derived from nitric oxides (NOx). Nitrate formation is sensitive to temperature and relative humidity conditions, and is also closely linked to sulfate and O3 chemistry as these affect nitrate formation by consuming NH3 and NOx (Bauer et al., 2007). The nitrate involvement in the transport sector may reduce the effectiveness of emission controls to counteract climate warming, because elimination of transport emission decreases both the black carbon (BC) warming and nitrate cooling. Other sectors such as fossil fuel and agriculture emit significant amount of ammonia gas that can also affect the radiative forcing of nitrate substantially. This work aims at testing the degree of changes in aerosol direct radiative forcing at the top of atmosphere (TOA) induced by anthropogenic emission sectors in the present-day climate (of the year 2000), using a general circulation model coupled with an online chemistry module.

(Lamarque et al., 2010). These emission inventories contain data on the annual global emissions of major precursors and primary particulates, and identify 12 subsectors within a half-degree global resolution. In this work, emissions in the year 2000 are considered and the subsectors are grouped into six sectors: agriculture (AGR), open biomass burning (BMB), domestic activities (DOM), industry (IND), energy generation (ENE), and transport (TRA). The transport sector involves emissions from on-road transport and international shipping, but ignores those from the aircraft. The baseline simulation was performed with all sector emissions prescribed. Six sensitive experiments were carried out; in each experiment, emissions from one sector were eliminated. All experiments were continuous integrated for three years. Differences between the baseline and the relevant sectoral experiment in the last year predictions show the aerosol direct radiative forcing and aerosol burdens attributed to a given sector. In this study, heterogeneous reactions of SO2 and HNO3 on wet anthropogenic aerosols are considered, but those on natural aerosols (mineral dust and sea salt) are ignored. Natural aerosols were found to have important effects on anthropogenic aerosol burdens (Liao and Seinfeld, 2005), but uncertainties exist among current models regarding their global burdens. As a primary study, the results presented here have intentionally excluded the effects of natural aerosols, to figure out the impacts of anthropogenic emissions on aerosol radiative forcing. This study can be used as a reference for the potential model studies that do not consider the effects of natural aerosols burdens fully.

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3

Model and description of emissions

The atmospheric chemistry and aerosol radiative forcing was predicted using the Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) II coupled with a complex gas-phase module and an aerosol chemistry module. The aerosol chemistry module is responsible for simulating gas-phase species and aerosols of sulfate, nitrate, ammonium, BC, and organic aerosols (OC). These aerosols are assumed to be internally mixed with the aerosol volume weighting factors. The partitioning of ammonia and nitrate between the gas and aerosol phases is determined by the online thermodynamic equilibrium model ISORROPIA (“equilibrium” in Greek). Different from the traditional chemistry transport models, this model builds up a two-way coupled climate-chemistry system, allowing aerosol radiative forcing to affect the climate at each model step. The model has a horizontal resolution of 4° latitude × 5° longitude, with nine sigma vertical layers from the surface to a height of 10 hPa. A “Q-flux” mixed-layer ocean scheme is used to predict the sea surface temperature and sea ice. Detailed descriptions and evaluations of the model can be found in the works of Liao et al. (2003, 2004). The prescribed model emissions were taken from the historical anthropogenic emission inventories prepared for the Coupled Model Intercomparison Project Phase 5 (CMIP5) climate simulations in support of the IPCC AR5

3.1

Results Sector contributions to global aerosol burdens

Before discussing sector contributions, the predicted global aerosol burdens based on emissions from all sectors are compared with the B12 work, which also used the same emission inventories as we used. Both models predict comparable global burdens of sulfate, BC, and OC, indicating a consistent model performance. The main difference is that the global nitrate burden of 0.65 Tg is higher than the B12 prediction by 282%. In this work, the GISS model excludes the uptake reaction of HNO3 on the coarse mineral dust particulates and thus overestimates the fine nitrate particulates formed by gas-phase reactions (Liao and Seinfeld, 2005). According to the study of Liao et al. (2003), the burden ratio of nitrate produced from gas-phase reactions to that from heterogeneous reactions with dust particulates is 0.3. Applying this ratio would decrease the GISS model nitrate burden down to B12 prediction. Heterogeneous reaction with dust also affects sulfate, but the burden ratio is 8.6; therefore, the sulfate burden is comparable between the models. Figure 1 shows the global sector emissions and changes in aerosol burdens between the baseline and the sector elimination predictions. Table 1 lists the global sector emissions and corresponding aerosol burdens as well as TOA direct radiative forcing. Clearly, the sectors

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have different emission strengths for precursors and primary particulates, and strongly affect aerosol burdens. The energy and industrial sectors are found to be the two sources with maximum SO2 emissions. SO2 emission of 52.93 Tg yr−1 by the energy sector accounts for the global sulfate burden of 0.56 Tg; Emission from the industrial sector is almost half of that from the energy sector (27.01 Tg yr−1), which causes a global sulfate burden of 0.31 Tg. Biomass burning is found to generate maximum emissions of primary carbonaceous particulates, yielding the maximum OC and BC burdens of 0.45 and 0.03 Tg, respectively. The domestic sector is the secondary source of emission of carbonaceous aerosols generating from residential cooking processes in East Asia. One should note

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here that, the changes in global BC burdens are disproportional to the changes in BC emissions from various sectors. Since BC is chemically inactive and its global burden is determined by its emissions and depositions, the disproportionate changes in BC burdens might be a result of the different responses of BC depositions to the climate change under different sector radiative forcing. Nitrogen oxides are emitted primarily by the transport sector and have a secondary source in the energy sector. However, the maximum nitrate burden is found to be caused by neither of these sectors but by agricultural emissions. The nitrate burden from the agricultural sector is two times that from the transport sector. The agricultural sector emits 75% of NH3; elimination of this sector

Figure 1 (a) Global sector emissions for aerosol precursors and primary aerosol particulates and (b) the corresponding changes in aerosol burdens (baseline minus sector elimination). The emission units are Tg (C, NO, SO2, NH3) yr−1 and the aerosol burden units are Tg. Table 1

Global sector emissions a, associated aerosol burdens, and direct radiative forcing at the top of atmosphere. Agriculture (AGR)

Bio. Burn (BMB)

Domestic (DOM) Emission (Tg)b

Energy (ENE)

Industry (IND)

Transport (TRA) 1.47

BC

0.15

2.61

1.95

0.05

1.53

OC

0.70

23.24

7.75

0.37

2.30

1.59

NOx

1.79

11.56

5.89

15.21

10.46

35.06

SO2

0.20

3.85

8.27

52.93

27.01

15.38

NH3

36.50

11.14

0.32

0.03

0.15

0.47

BC

0

+0.03

+0.02

−0.00

+0.01

+0.02

OC

+0.02

+0.45

+0.14

+0.02

+0.05

+0.06

Burden (Tg)

−

+0.40

+0.08

−0.01

−0.08

+0.07

+0.19

SO42−

−0.03

+0.05

+0.05

+0.56

+0.31

+0.12

+

+0.35

+0.05

+0.01

+0.13

+0.11

+0.07

+23

+36

NO3 NH4

TOA direct radiative forcing (mW m−2) BC

−1

+64

+41

−2

OC

−2

−58

−16

−2

−7

−8

NO3−

−186

−40

+6

+45

−22

−92

SO42− Internal mixed

+32 −26 −24 −262 −129 −57 −19 +41 −222 −115 −85 −149 (+19) (+12) (−132) (−56) (−26) (−147)c a Emissions from agricultural sector include emissions due to agricultural activities and agricultural waste burning; from biomass burning sector are emissions due to fire over grasslands and forests; from industry sector include emissions due to industrial activities, waste treatment and disposal activities, as well as processes of solvent production and use; from transportation sector include emissions due to land transport and international shipping activities; and from the domestic and energy sectors are emissions due to residential activities and related to energy generation, respectively. b Emission units are Tg (C, NO, SO2, NH3) yr−1. c Predicted TOA radiative forcing by Bauer et al. (2012) are given in parentheses.

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would reduce the available amount of NH3 required for ammonium nitrate formation. Activities of other sectors, except for biomass burning, accounts for no more than 2% of NH3 emission and therefore their eliminations would not affect the global nitrate burden significantly. In the case of the energy sector, the global nitrate burden decreases slightly by −0.08 Tg, which is consistent with the predictions of U10 who used different emission inventories. The reason is that sulfate reacts strongly with NH3 because of its preferential neutralization and lower vapor pressure; energy sector releases huge amount of SO2 that forms sulfate and reduces the amount of NH3 available for reaction with HNO3. 3.2 Sector contributions to global aerosol direct forcing The annual TOA radiative forcing of different emission sectors follows the aerosol burdens. The energy sector yields the maximum aerosol radiative forcing of −222 mW m−2, with a substantial contribution from sulfate, which is consistent with previous conclusions (Koch et al., 2007a; Bauer et al., 2012). B12 reported aerosol radiative forcing from the energy sector to be −132 mW m−2, which is lower than this finding by 40%. This discrepancy might primarily be due to the ignorance of the uptake of acidifying precursors by dust particulates; in addition, B12 model used a decoupled gas-phase ozone chemistry scheme and offline fields of oxidants and nitric acid, which could have generated some differences in nitrate radiative forcing. During this study, nitrate reduction in the energy sector generates a positive forcing of 45 mW m−2; U10 also reported such positive forcing of nitrate, but it was weaker by 50%. Considering geographical distribution, the energy sector yields significant radiative forcing of sulfate and nitrate over the continents of the Northern Hemisphere (figure not shown), but radiative forcing of nitrate is more limited than that of sulfate over populated regions, such as eastern China and eastern North America, and over higher latitudes of Eurasian continent. As found in the study, the industrial sector yields aerosol radiative forcing of −115 mW m−2, which is stronger than the B12 prediction of −56 mW m−2. Despite the overestimated nitrate burden, the difference in radiative forcing might be partly due to ignoring the waste treatment emissions by B12 who argued that this sector has a small impact on global burden. However, in this work, contribution of this sector to industry emissions is considered because it shows considerable emissions distributed over some grid cells. The industrial sector induces a nitrate direct forcing of −22 mW m−2, which is opposite to the positive forcing of 47 mW m−2 reported by U10 as a result of the decrease in global nitrate burden in their study. It should be noted here that the energy sector causes positive forcing of nitrate both in this study and U10 study models. Thus, the opposite sign in nitrate forcing of the industrial sector is not only due to the overestimated nitrate burden in this study. Actually, industrial emissions in U10 are up to 63.2 Tg (SO2) yr−1 and leads to

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many ammonium sulfates rather than ammonium nitrate. Their nitrate loss appears to outweigh the nitrate production from the NOx coemitted from the industrial sector (Unger et al., 2010), and the net loss in nitrate leads to a positive radiative forcing. In this study, the industrial SO2 emission is only 27.01 Tg (SO2) yr−1, and the agricultural NH3 emission of 36.50 Tg (NH3) yr−1 is higher than 17 Tg (NH3) yr−1 reported by U10. Emissions of less SO2 together with more NH3 produce less ammonium sulfate and more ammonium nitrate, resulting in a negative nitrate forcing. Compared to the energy sector, the industrial sulfate emission takes a half negative forcing value corresponding to its half sulfate burden. Sulfate is the largest contributor to the negative forcing in the industrial sector, with a value of −129 mW m−2, which is close to the U10 prediction of −192 mW m−2. The industrial sulfate forcing is also widely distributed over the populated regions. There exist a few reports about agriculture aerosol radiative forcing. In this work, because most nitrate burdens are dependent on NH3 emissions from agricultural activities, the agricultural sector causes the largest nitrate radiative forcing of −186 mW m−2 and imposes the second largest aerosol radiative forcing of −149 mW m−2 after the energy sector. This agricultural aerosol radiative forcing is close to the B12 estimation of −147 mW m−2, which consists of −132 mW m−2 from agricultural activities and −15 mW m−2 from agricultural waste burning. Geographically, the agricultural nitrate forcing spreads over the Northern Hemispheric continents just like the distribution of sulfate radiation forcing in the energy sector. In the past works, transport emissions were reported to induce a positive aerosol forcing and were suggested to be a potential target to counteract global warming. Koch et al. (2007a) estimated that the radiative forcing of sulfate, BC, and OC from the transport sector was 30 mW m−2. B12 estimated the TOA radiative forcing of nitrate and also got a positive forcing in the land transport sector, but with a reduced value of only 8 mW m−2. This study does not distinguish the land transport emissions from the shipping emissions and shows their combined aerosol radiative forcing to be −85 mW m−2, which is higher than −26 mW m−2 reported by B12. Actually, although the transport sector induces a maximum BC positive forcing of 36 mW m−2 in the GISS model, this positive forcing, taking a higher value in North Africa, obviously cannot offset the negative forcing of sulfate and nitrate, which are mainly distributed over Europe and along the shipping tracks in the boreal oceanic area. The domestic sector is the only emission sector that causes a positive aerosol radiative forcing, consistent with the conclusion of Unger et al. (2008) that residential emission is an attractive target to mitigate global warming. The global mean radiative forcing is estimated to be 41 mW m−2, exceeding the estimation of B12 by 242%. However, one must note that the aerosol internal mixture is responsible for such significant warming efficiency of BC in GISS model. With an assumption of aerosol external mixture, the domestic radiative forcing would have decreased to 7 mW m−2. The domestic sector shows a

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maximum BC positive forcing in South Asia. Mixing of BC with the scattering aerosols leads to increased radiative forcing over the Mediterranean, India, and northern China. Thus, the value of BC radiative forcing depends on the degree of mixing of BC with the scattering aerosols and not necessarily on its emissions. 3.3 Contributions of sectors to regional aerosol direct forcing Figure 2 shows the aerosol radiative forcing in sectors over three populated regions: eastern China, Europe, and eastern North America. The energy sector yields an aerosol direct forcing of −0.78 W m−2 over Europe and eastern North America, and an almost doubled radiative forcing of −1.25 W m−2 over eastern China. The energy sector always induces maximum negative forcing of sulfate and, simultaneously, positive forcing of nitrate due to reduced nitrate burden. It implies that desulfurization processes in the energy sector would decrease sulfate but increase nitrate. Although increased nitrate level is disadvantageous to improving air quality, it restricts aggravation of regional warming. However, the industrial sector has a maximum contribution to sulfate, accounting for the maximum negative forcing, and less contribution to nitrate. Thus, the industrial desulfurization reduces sulfate concentration, exposing regional climate to the positive forcing of BC. Eastern China shows the maximum BC positive forcing. The agricultural sector imposes the largest regional negative nitrate forcing over the regions. It suggests that agricultural NH3 emission control could be a more efficient way to change aerosol radiative forcing than managing NOx emissions in other sectors. The transport sector, involving emissions from land transport and shipping, leads to aerosol negative forcing

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over the regions in this study; however, B12 predicted aerosol positive forcing from the land transport sector over Europe and eastern North America and negative forcing over eastern China. Their regional aerosol forcing reduced further, close to zero (Fig. 9 in B12), after adding up the radiative forcing from the shipping sector. B12 did not report the regional sector radiative forcing values and hence a quantitative comparison is impossible; however, it indicates that the emissions from the transport sector involving land transport and shipping may not be as important a factor as expected before to abate the regional climate warming. The domestic sector yields a positive aerosol radiative forcing across the globe. However, the domestic radiative forcing is −0.1 W m−2 over eastern China, +0.37 W m−2 over Europe, and +0.15 W m−2 over eastern North America. Signs of these regional radiative forcing are opposite to those of the B12 prediction (Fig. 9 in B12). Because the domestic sector primarily consists of emissions of chemically inactive BC, the radiative forcing discrepancy between models is probably due to the differences in BC burdens across regions, as determined by regional climate feedbacks, in the absence of domestic radiative forcing. Similarly, elimination of biomass burning emissions increases negative radiative forcing in both sulfate and nitrate over eastern China, implying an increase in aerosol burdens. Climate feedbacks on the aerosol dispersion or deposition would likely account for the increases in regional aerosol burdens. Impacts of climate feedbacks on sector radiative forcing are not clear so far. These feedbacks are highly model dependent and only few studies have discussed this issue. More researches with models coupled with advanced chemistry modules are necessary in the future.

Figure 2 Aerosol TOA direct radiative forcing associated with anthropogenic emission sectors across (a) the globe, and (b) over eastern China (22–42°N, 102.5–117.5°E), (c) Europe (38–54°N, 7.5°W–22.5°E), and (d) eastern North America (30–50°N, 112.5–82.5°W). Units: W m−2.

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4


Conclusions

This work evaluates the direct radiative forcing of aerosols at the TOA associated with six anthropogenic emission sectors in the present-day climate. In line with previous researches, the energy sector accounts for the largest aerosol direct radiative forcing of −222 mW m−2 because of its substantial sulfate burdens. The agricultural sector has the maximum ammonia gas emissions and consequently could change the global ammonium nitrate burden significantly. The industrial and agricultural sectors account for comparable global aerosol direct radiative forcing of −115 and −149 mW m−2, respectively, with substantial contributions from their respective dominant aerosols of sulfate and nitrate. Over regions of eastern China, Europe, and eastern North America, the agricultural sector yields more negative aerosol radiative forcing than the industrial sector. Inclusion of nitrate allows the transport sector to induce a global aerosol direct forcing of −85 mW m−2, in contrast to the results of previous studies, which suggests that transport emission induces a positive radiative forcing and is a potential target to counteract climate warming. The domestic sector induces a significant positive forcing of 41 mW m−2 only with when the aerosols mixed internally. The research uncertainties include the impacts of mineral dust on nitrate chemistry, ignorance of indirect effects of aerosols, and impacts of model-dependent climate feedbacks on aerosol burdens. The six emission sectors also need to be separated into more subsectors to identify further the impacts of anthropogenic activities on climate, which will be carried out in the next work. Acknowledgments. This work has been supported by the National Basic Research Program of China (973 Program, 2010CB950804).

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