Here’s the full Q&A with researcher Jianhua Xu, whose team studied past and future emissions of nitrous oxide.
Q. Why did you decide to do this study?
A. Nitrous oxide (N2O) is one of the six greenhouse gases under the United Nations Framework Convention on Climate Change (UNFCCC) and is the third largest contributor to climate change just after carbon dioxide (CO2) and methane (CH4). Recently, it has been identified as the single most important ozone-depleting substance in the 21st century. Therefore, reducing its emissions is believed to offer the combined benefits of mitigating climate change and protecting the ozone layer. This motived us to study N2O.
We then did some homework on the emission sources of N2O. Globally, the relative contributions from the various sources has remained steady over the years, with agricultural activities being the largest contributor (60 percent), followed by industrial processes and fuel combustion, each contributing about 10 percent . In China, agricultural activities are the largest (81 percent) sources as well, while the contribution of industrial processes and combustion were about 9.8 percent and 9.1 percent in 2007. Further investigation showed that the most cost-effective N2O control options are in the industrial sectors. Then we focused on N2O emissions from industrial sector.
China is the largest developing country with incredible growth of industrial chemical outputs in recent years. China is becoming the leading global industrial N2O emitter with its emissions increasing six fold from 14.8 Gg in 1994 to 106 Gg in 2005. During the same period, emissions from all industrialized countries in Annex I to the UNFCCC decreased by almost half from 549 Gg to 317 Gg. In this context, we realized that studying N2O emissions and reductions from industrial sector in China might be of practical policy implications.
Q. What are the most important findings from your study?
A. First, China has become the world’s largest industrial N2O emitter. From 1990 to 2012, industrial N2O emissions in China grew some 37-fold to 174 Gg, while global industrial N2O emissions decreased by 41 percent to 379 Gg and the total industrial N2O emissions from Annex I countries decreased by 71 percent to 171 Gg. By 2009, the emissions from China surpassed those from the European Union and United States for the first time.
Second, reducing nitrous oxide (N2O) emissions offers the combined benefits of mitigating climate change and protecting the ozone layer. The total cumulative mitigation potential for China within the period 2013-2020 is about 1.54 Tg, the equivalent of reducing all the 2011 greenhouse gases from Australia in terms of mitigating climate change, or the 2011 halocarbon ozone-depleting substances from China in terms of protecting the ozone layer.
Third, the major future mitigation potential lies in adipic acid production sector. However, the mitigation prospect is not good due to the current ineffective policies.
Q. The Montreal Protocol phased out chlorofluorocarbons (CFCs) and led to the still-ongoing but largely successful recovery of the ozone layer. If N2O emissions continue to rise, how will it affect this recovery?
A. This study focused on the N2O emissions from industrial sectors and its potential risk on the ozone layer, but paid no attention to the total anthropogenic N2O emissions and its impacts on the recovery of the ozone layer. Nevertheless, the authors would like to make their efforts, by quoting some published data from previous studies, to illustrate the magnitude of the problem.
N2O possesses a small ozone-depleting potential (ODP) of only 0.017, which is around one-sixtieth of CFC-11, a typical CFC regulated under the Montreal Protocol. However, its mild ODP could be quite insidious because the current anthropogenic N2O emissions is much larger than the past and future CFC emissions worldwide — especially in the context that 95 percent of the CFCs have been eliminated so far, and the abatement of HCFCs is ongoing due to the successful implement of Montreal Protocol.
This makes anthropogenic N2O emissions the single most important of the anthropogenic ozone-depleting substances (ODS) emissions today and throughout the 21st century. In 2006, global total anthropogenic N2O emissions were estimated to be 6.9 Tg N2O-N with its average atmospheric concentration of 319.8 ppb1. If unregulated, the total anthropogenic N2O emissions are projected to be 10.3 ± 2.0 Tg N2O-N 1 (RCP8.5 scenario in IPCC simulation) with its atmospheric concentration reaching (345-368) ppb in 20502. By then, the ODP-weighted N2O emissions are in excess of 30 percent of the peak CFC ODP-weighted emissions of 19873.
From a scientific standpoint, "recovery of the ozone layer" means the global average stratospheric ozone level returns to the 1980 or earlier levels in the coming decades4. With a supposed fixed atmospheric N2O level, a complete recovery is projected to occur at around 2025-20285. However, the increase in atmospheric N2O level is confirmed to reduce in ozone concentration, for example, a doubled N2O mixing ratio is suggested to cause a reduction in ozone mixing ratios of maximally 10 percent in the middle stratosphere in a recent simulation6. Thus, the increase of atmospheric N2O level could delay the complete recovery of the ozone layer, with the most possible time frame of a decade according to the latest simulation5, although drawing down CFCs under the Montreal Protocol has provided all possible relief.
Q. There is a lot of focus on CO2 emissions’ effects on climate change. What would happen to our climate if we dramatically reduced CO2 emissions but allowed N2O to rise unchecked?
A. This study focused on the N2O problem raised by production processes of industrial chemicals, which account for roughly 10 percent of total anthropogenic N2O emissions. Another equally important sector that contributes 10-20 percent to the global N2O burden is fuel combustion, including biomass burning, and stationary and mobile combustion. In the biomass burning process, like forest-to-cropland conversion, wildfire and bio-waste combustion, the organic N in the biomass, coupled with the ambient atmospheric N2, are oxidized to convert into N2O with an inadequate supply of oxygen. In stationary and mobile combustion, N2O is generated in the incomplete catalytic conversion of nitrogenous tail gas (e.g. NOx) to reduce tailpipe emissions of atmospheric pollutants. The utilization of renewable energy avoids the consumption of biomass, diesel, gasoline and fossil fuel that contain nitrogenous substances, thus makes for N2O mitigation.
Global warming will not be alleviated if subjecting CO2 to dramatic reduction but leaving N2O unregulated, although CO2 is and will always be the largest contributor to global radiative forcing in climate change regime. Currently, N2O is the third most important greenhouse gas with its net global radiative forcing (0.17 ± 0.03 W m–2) roughly 6 percent that of CO2 (1.63-2.01 W m–2), and the total anthropogenic N2O emissions are projected to keep ascending by 58 percent and the global average N2O abundance by 13 percent to 20501. Over 60 percent of global anthropogenic N2O emissions reside in agricultural activities, like nitrification and denitrification of applied synthetic N fertilizer and manure, where improving fertilizer-use efficiency, applying nitrification inhibitors and controlled-released fertilizers are regarded as the most cost-effective control options7 but are still less available in most agriculture-dominant developing countries. Undoubtedly, the N2O issue will be more intractable in the future. If it remains unchecked, the increased radiative forcing could finally make up the net climate benefit from CO2 abatement.
Q. Are there technological solutions N2O-producing industries can implement now to reduce their emissions? How costly are they?
A. Yes, there are a few feasible technologies to abate industrial N2O emission. In nitric acid production, N2O is generated as an unwanted exhaust gas from the catalytic oxidation of ammonia. There are now three types of N2O abatement technologies available: primary, secondary and tertiary, all of which depend on the location where the supplementary N2O abatement unit is installed. For the primary technology, a N2O inhibition unit is installed inside the ammonia oxidation reactor to suppress the generation of N2O during ammonia oxidation. For the secondary technology, a N2O decomposition unit is installed inside the ammonia oxidation reactor, and N2O is removed immediately after its generation by ammonia oxidation. For the tertiary technology, a N2O decomposition unit is installed after the NOx absorption tower, and tail-gas N2O is removed downstream after NOx absorption.
In adipic acid production, N2O is generated when intermediates cyclohexanol and cyclohexanone are oxidized by nitric acid. There are two types of N2O abatement technologies available currently, thermal destruction and catalytic decomposition. The former breaks N2O down into N2 and O2 under adiabatic ultra-temperature (>1000 °C) conditions; while the latter convert N2O into NOx or N2+O2 by catalytic reduction using the reducing agent at relatively lower temperatures (<500 °C).
Some mature N2O abatement technologies have been recommended by the Kyoto Protocol’s Clean Development Mechanism (CDM) methodologies and have been applied in over 50 projects. However, the cost of abatement technologies, including investment costs, operational revenues and CDM-related transaction costs, is not affordable for many plants without the economic returns from carbon market as compensations. For example, the world’s average investment costs of N2O abatement technology was EUR 8 million, and the operational revenues remained another EUR 1 million per year. In China, the investment with the lowest cost was US$10 million, and the net present value (NPV) was minus US$30 million (NPV<0 means the project is not profitable) for entire 21-year operation. Given the considerable total cost, voluntary mitigation cannot be expected if new abatement equipment is declared ineligible for generating CDM credits, or tradable projects in carbon markets are narrowed or shut down.
In my understanding, the availability of abatement technology is not an issue in the current situation, but the design and implementing effective policies and regulatory programs really are.