This chapter should be cited as: Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, 2013: Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Coordinating Lead Authors: Gunnar Myhre (Norway), Drew Shindell (USA)

Anthropogenic and Natural Radiative Forcing

Rapid industrialization and urbanization in developing countries has led to an increase in air pollution, along a similar trajectory to that previously experienced by the developed nations. In China, particulate pollution is a serious environmental problem that is influencing air quality, regional and global climates, and human health. In response to the extremely severe and persistent haze pollution experienced by about 800 million people during the first quarter of 2013 (refs 4, 5), the Chinese State Council announced its aim to reduce concentrations of PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 micrometres) by up to 25 per cent relative to 2012 levels by 2017 (ref. 6). Such efforts however require elucidation of the factors governing the abundance and composition of PM2.5, which remain poorly constrained in China. Here we combine a comprehensive set of novel and state-of-the-art offline analytical approaches and statistical techniques to investigate the chemical nature and sources of particulate matter at urban locations in Beijing, Shanghai, Guangzhou and Xi'an during January 2013. We find that the severe haze pollution event was driven to a large extent by secondary aerosol formation, which contributed 30-77 per cent and 44-71 per cent (average for all four cities) of PM2.5 and of organic aerosol, respectively. On average, the contribution of secondary organic aerosol (SOA) and secondary inorganic aerosol (SIA) are found to be of similar importance (SOA/SIA ratios range from 0.6 to 1.4). Our results suggest that, in addition to mitigating primary particulate emissions, reducing the emissions of secondary aerosol precursors from, for example, fossil fuel combustion and biomass burning is likely to be important for controlling China's PM2.5 levels and for reducing the environmental, economic and health impacts resulting from particulate pollution.

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High secondary aerosol contribution to particulate pollution during haze events in China

Secondary organic aerosol (SOA) accounts for a significant fraction of ambient tropospheric aerosol and a detailed knowledge of the formation, properties and transformation of SOA is therefore required to evaluate its impact on atmospheric processes, climate and human health. The chemical and physical processes associated with SOA formation are complex and varied, and, despite considerable progress in recent years, a quantitative and predictive understanding of SOA formation does not exist and therefore represents a major research challenge in atmospheric science. This review begins with an update on the current state of knowledge on the global SOA budget and is followed by an overview of the atmospheric degradation mechanisms for SOA precursors, gas-particle partitioning theory and the analytical techniques used to determine the chemical composition of SOA. A survey of recent laboratory, field and modeling studies is also presented. The following topical and emerging issues are highlighted and discussed in detail: molecular characterization of biogenic SOA constituents, condensed phase reactions and oligomerization, the interaction of atmospheric organic components with sulfuric acid, the chemical and photochemical processing of organics in the atmospheric aqueous phase, aerosol formation from real plant emissions, interaction of atmospheric organic components with water, thermodynamics and mixtures in atmospheric models. Finally, the major challenges ahead in laboratory, field and modeling studies of SOA are discussed and recommendations for future research directions are proposed.

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The formation, properties and impact of secondary organic aerosol: current and emerging issues

Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high-time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

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Evolution of Organic Aerosols in the Atmosphere

The present paper reviews existing knowledge with regard to Organic Aerosol (OA) of importance for global climate modelling and defines critical gaps needed to reduce the involved uncertainties. All pieces required for the representation of OA in a global climate model are sketched out with special attention to Secondary Organic Aerosol (SOA): The emission estimates of primary carbonaceous particles and SOA precursor gases are summarized. The up-to-date understanding of the chemical formation and transformation of condensable organic material is outlined. Knowledge on the hygroscopicity of OA and measurements of optical properties of the organic aerosol constituents are summarized. The mechanisms of interactions of OA with clouds and dry and wet removal processes parameterisations in global models are outlined. This information is synthesized to provide a continuous analysis of the flow from the emitted material to the atmosphere up to the point of the climate impact of the produced organic aerosol. The sources of uncertainties at each step of this process are highlighted as areas that require further studies.

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Organic aerosol and global climate modelling: a review

Linking Emission Inventories and Ambient Measurements

. Reaction with ozone is an important atmospheric removal process for alkenes. The ozonolysis reaction 17 produces carbonyls, and carbonyl oxides (Criegee intermediates, CI), which can rapidly decompose to yield a 18 range of closed shell and radical products, including OH radicals. Consequently, it is essential to accurately 19 represent the complex chemistry of Criegee intermediates in atmospheric models in order to fully understand the 20 impact of alkene ozonolysis on atmospheric composition. A mechanism construction protocol is presented which 21 is suitable for use in automatic mechanism generation. The protocol defines the critical parameters for describing 22 the chemistry following the initial reaction, namely: the primary carbonyl / CI yields from the primary ozonide 23 fragmentation; the amount of stabilisation of the excited CI (CI*); the unimolecular decomposition pathways, 24 rates and products of the CI; the bimolecular rates and products of atmospherically important reactions of the 25 stabilised CI (SCI). This analysis implicitly predicts the yield of OH from the alkene-ozone reaction. A 26 comprehensive database of experimental OH, SCI and carbonyl yields has been collated using reported values in 27 the literature and used to assess the reliability of the protocol. The protocol provides estimates OH, SCI and 28 carbonyl yields with a root mean square error of 0.13 and 0.12 and 0.14, respectively. Areas where new 29 experimental and theoretical data would improve the protocol and its assessment are identified and discussed. The primary carbonyl yields of vinyl acetate measured to be 0.30±0.04 and 0.70±0.08 for HCHO and CH 3 C(O)OC(O)H respectively by Al Mulla et al. (2010), and 0.20±0.06 and 0.97±0.08 by Picquet-Varrault et al. (2010). These studies suggest a preference for formation of CH 2 OO and the acetate. There are only two 195 compounds reported for ozonolysis of a - b unsaturated acids: acrylic and methacrylic acid. For acrylic acid E ) - CH 3 CHOO would be favoured in cis -but-2-ene 305 ozonolysis. Recent theoretical work (Watson, 2021) looking at POZ fragmentation for a series of disubstituted 2- 306 alkenes (CH 3 CH=CHR), suggests formation of ( E )-RCHOO will be strongly favoured in the ozonolysis of cis - 307 alkenes (87 % for cis -but-2-ene, increasing to 93 % for cis -2-hexene), while there is a roughly equal split from 308 ozonolysis of trans -alkenes. This is in qualitative agreement with the experimental work discussed above but 309 suggests a stronger preference than observed in the direct measurements of the vinoxy radical by Campos-Pineda 310 (2017). For tri-substituted alkenes, Watson (2021) finds a strong preference for formation of ( E )-RCHOO on the 311 Measurements of scavenger removal, or formation of products from the SCI scavenger reaction, are used to determine the SCI yield. Yields measured in such a way must be considered to be lower limits since, under most experimental conditions, a significant fraction of the SCI may undergo (40 %), and 716 HCOOH (5 %). However, ozonolysis experiments (e.g. Nguyen et al., 2016) have generally found HMHP and 717 HCOOH to be the main detected products, with negligible yields of HCHO. Based on results from isoprene 718 ozonolysis chamber experiments, Nguyen et al. (2016) proposed yields from the CH 2 OO + H 2 O reaction of: 719 HMHP (73 %), HCOOH (21 %), HCHO (6 %); and from the (H 2 O) 2 reaction of: HMHP (40 %), HCOOH (54 %), 720 HCHO (6 %). These low HCHO yields are in agreement with earlier work (Hasson et al., 2001b) that determined 721 an HCHO yield of 6 – 9 %.

Estimation of mechanistic parameters in the gas-phase reactions of 1 ozone with alkenes for use in automated mechanism construction 2

Aromatic Hydrocarbon Oxidation: The Contribution of Chamber Oxidation Studies

The chemistry of ground level atmospheric ozone formation is highly complex and nonlinear and has many uncertainties. As a result no chemical model can be relied upon to give accurate predictions unless it has been evaluated by comparison with experimental data. There are essentially two ways in which a photochemical oxidant model can be evaluated. One compares the predictions of the complete model against field data taken during a historic ozone pollution episode. Another approach relies on the evaluation of individual pollutant compounds, separately. In the case of gas-phase chemical mechanisms, this means evaluating the predictions of the mechanism against results of environmental chamber experiments (Carter et al., 1995a).

Evaluation of the Detailed Tropospheric Chemical Mechanism, MCM v3, Using Environmental Chamber Data: Butane and Its Degradation Products

Studies of the UV absorption spectra as well as the kinetics and products of reactions of the following organic peroxy radicals have been carried out: CH3O2, CH2=CHCH2O2, HOCH2CH2O2, CH3CH(OH)CH(O2)CH3, (CH3)2C(OH)CH2O2, (CH3)2CH(OH)CH(O2)(CH3)2, CH3OCH2O2, CH3C(O)CH2O2 and C2H5C(O)O2. These radicals were chosen to be representative of those formed from the degradation of some classes of volatile organic compound (VOC) likely to be important in tropospheric chemistry (i.e. alkanes, alkenes, conjugated dienes, alcohols, ethers, aldehydes and ketones). Kinetic studies of the self reactions and reactions with HO2 were carried out using the molecular modulation technique and the laser flash photolysis technique, coupled with UV absorption spectroscopy. Product studies were carried out by long pathlength Fourier transform infra-red spectroscopy and UV-visible diode array spectroscopy. This work has also allowed investigation of a variety of laboratory sources of peroxy radicals.

Michael E. Jenkin

Papers

Linking Emission Inventories and Ambient Measurements

Estimation of mechanistic parameters in the gas-phase reactions of 1 ozone with alkenes for use in automated mechanism construction 2

Aromatic Hydrocarbon Oxidation: The Contribution of Chamber Oxidation Studies

Evaluation of the Detailed Tropospheric Chemical Mechanism, MCM v3, Using Environmental Chamber Data: Butane and Its Degradation Products

Laboratory Studies of Peroxy Radical Reactions of Importance for Tropospheric Chemistry