Editing IPCC:AR6/WGI/Chapter-6 (section)

=== 6.1.1 Importance of SLCFs for Climate and Air Quality ===

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The atmospheric lifetime determines the spatial and temporal variability, with most SLCFs showing high variability, except methane and many HCFCs and HFCs that are also well-mixed (as a consequence methane is discussed together with other well-mixed greenhouse gases (GHGs) in Chapters 2, 5, and 7). In contrast to well-mixed GHGs, such as CO <sub>2</sub> , methane and some HFCs, the radiative forcing effects of most SLCFs are largest at regional scales and climate effects predominantly occur in the first two decades after their emissions or formation. However, changes in their emissions can also induce long-term climate effects, for instance by altering some biogeochemical cycles. Therefore, the temporal evolution of radiative effects of SLCFs follows that of emissions, that is, when SLCF emissions decline to zero their atmospheric abundance and radiative effects decline towards zero. The total influence of individual SLCF emissions on radiative forcing and climate includes their effects on the abundances of other forcers through chemistry (chemical adjustments).

SLCFs can affect climate by interacting with radiation or by perturbing other components of the climate system (e.g., the cryosphere and carbon cycle through deposition, or the water cycle through modifications of cloud properties via cloud condensation nuclei or ice nuclei). SLCFs can have either net warming or net cooling effects on climate. In addition to altering the Earth’s radiative balance, many SLCFs are also air pollutants with adverse effects on human health and ecosystems. SLCFs are of interest for climate policies (e.g., methane, HFCs), and are regulated as air pollutants (e.g., aerosols, ozone) or because of their deleterious influence on stratospheric ozone (e.g., HCFCs). The list of SLCFs assessed in this chapter and their effects are provided in Table 6.1.

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'''Table 6.1 |''' '''Overview of SLCFs of interest for Chapter 6.''' For each SLCF, its source types, lifetime in the atmosphere, and associated radiatively active agent is given. Source type can be primary (emitted) and/or secondary (formed through multiple atmospheric mechanisms). Unless otherwise noted, the stated lifetime refers to tropospheric lifetime.* Climate effect of increased SLCFs is indicated as ‘+’ for warming and ‘–’ for cooling. ‘Direct’ is used for SLCFs exerting climate effects through their radiative forcing and ‘Indirect’ for SLCFs which are precursors affecting the atmospheric burden of other climatically active compounds. Other processes through which SLCFs affect climate are listed where applicable. The World Health Organization (WHO) guidelines for air quality (AQ) are given, where applicable, to show which SLCFs are regulated for air-quality purposes.

{| class="wikitable"
|-
| '''Compounds'''

| '''Source Type'''

| '''Lifetime'''

| '''Direct'''

| '''Indirect'''

| '''Climate Forcing'''

| '''Other Effects on Climate''' <sup>a</sup>

| '''WHO AQ Guidelines''' <sup>b</sup>

|-
| '''CH''' <sub>4</sub>

| Primary

| ~9 years

~12 years (perturbation time)

| CH <sub>4</sub>

| O <sub>3</sub> , H <sub>2</sub> O, CO <sub>2</sub>

| +

|
| No <sup>c</sup>

|-
| '''O''' <sub>3</sub>

| Secondary

| Hours to weeks

| O <sub>3</sub>

| CH <sub>4</sub> , secondary organic and sulphate aerosols

| +

| Ecosystems

| 100 μg m <sup>–3</sup>

<sup>8-hour mean</sup>

|-
| '''NO''' <sub>x</sub> '''(= NO + NO''' <sub>2</sub> ''')'''

| Primary

| Hours to days

|
| O <sub>3</sub> , nitrate aerosols, CH <sub>4</sub>

| +/–

| Ecosystems

| 40 μg m <sup>–3</sup>

<sup>annual mean</sup> 200 μg m <sup>–3</sup>

<sup>1-hour mean</sup>

|-
| '''CO'''

| Primary + Secondary

| 1 to 4 months

|
| O <sub>3</sub> , CH <sub>4</sub>

| +

|
| No

|-
| '''NMVOCs''' <sup>**</sup>

| Primary + Secondary

| Hours to months

|
| O <sub>3</sub> , CH <sub>4</sub> , organic aerosols

| +/–

|
| No

|-
| '''SO''' <sub>2</sub>

| Primary

| Days (trop.) to weeks (strat.)

|
| Sulphate and nitrate aerosols, <sub></sub> O <sub>3</sub>

| –

| Ecosystems

| 20 μg m <sup>–3</sup>

<sup>24-hour mean</sup> 500 μg m <sup>–3</sup>

<sup>10-minute mean</sup>

|-
| '''NH''' <sub>3</sub>

| Primary

| Hours

|
| Ammonium Sulphate, Ammonium Nitrate

| –

| Ecosystems

| No

|-
| '''HCFCs'''

| Primary

| Months to years

| HCFCs

| O <sub>3</sub>

| +/–

|
| No <sup>c</sup>

|-
| '''HFCs''' <sup></sup>

| Primary

| Days to years

| HFCs

|
| +

|
| No <sup>c</sup>

|-
| '''Halons and Methylbromide'''

| Primary

| Years

| Halons and Methylbromide

| Stratospheric O <sub>3</sub>

| +/–

|
| No <sup>c</sup>

|-
| '''Very Short-lived Halogenated Species (VSLSs)'''

| Primary

| Less than 6 months

|
| O <sub>3</sub>

| –

|
| No <sup>c</sup>

|-
| '''Sulphate aerosols'''

| Secondary

| Minutes to weeks

| Sulphate

|
| –

| Clouds

Ecosystems

| as part of PM <sup>d</sup>

|-
| '''Nitrate aerosols'''

| Secondary

| Minutes to weeks

| Nitrate

|
| –

| Clouds

Ecosystems

| as part of PM <sup>d</sup>

|-
| '''Carbonaceous Aerosols'''

| Primary + Secondary

| Minutes to Weeks

| BC, OA

|
| +/–

| Cryo, Clouds

Ecosystems

| as part of PM <sup>d</sup>

|-
| '''Sea spray'''

| Primary

| Day to week

| Sea spray

|
| –

| Clouds

Ecosystems

| as part of PM <sup>d</sup>

|-
| '''Mineral dust'''

| Primary

| Minutes to Weeks

| Mineral dust

|
| +/–

| Cryo Cloud

Ecosystems

| as part of PM <sup>d</sup>

|}

\* <sup></sup> for lifetimes reported in this table, it is assumed that the compounds are uniformly mixed throughout the troposphere, however, this assumption is unlikely for compounds with lifetimes &lt;1 year and, therefore, the reported values should be viewed as approximations ( [[#Prather--2001|Prather et al., 2001]] ).

\** Some NMVOCs are biogenic volatile organic compounds (BVOCs).

<sup>a</sup> Clouds: effect on clouds through aerosol–cloud interactions, Ecosystems: effect on ecosystems through changes in radiation and deposition, Cryo: effect on planetary albedo through deposition on snow and ice; <sup>b</sup> [[#Krzyzanowski--2008|Krzyzanowski and Cohen (2008)]] ; <sup>c</sup> regulated through Kyoto/Montreal protocols; <sup>d</sup> for Particulate Matter with diameter &lt;2.5 µm (PM <sub>2.5</sub> ): 10 µg m <sup>–3</sup> annual mean or 25 µg m <sup>–3</sup> 24-hour mean (99th percentile) and for Particulate Matter with diameter &lt;10 µm (PM <sub>10</sub> ): 20 µg m <sup>–3</sup> annual mean or 50 µg m <sup>–3</sup> 24-hour mean (99th percentile).

As depicted in Figure 6.1, emissions of SLCFs are governed by anthropogenic activities and sources from natural systems (see Section 6.2 for details). Atmospheric chemistry in this context is both a source and a sink of SLCFs. For instance, ozone and secondary aerosols are exclusively formed through atmospheric mechanisms (Sections 6.3.2 and 6.3.5 respectively). The hydroxyl (OH) radical, the most important oxidizing agent in the troposphere, acts as a sink for SLCFs by reacting with them and thereby influencing their lifetime (Section 6.3.6). Through SLCF radiative forcing and feedbacks (Section 6.4), key climate parameters, such as temperature, hydrological cycle and weather patterns are perturbed. Climate change also influences air quality (Section 6.5). As depicted in Figure 6.1, SLCFs affect both climate and air quality, hence SLCF mitigation has linkages to both issues (Section 6.6). Socio-economic narratives including air-quality policies determine future projections of SLCFs in the five core Shared Socio-economic Pathways (SSPs): SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 (described in Chapter 1), and in addition, a subset of SSP3 scenarios make it possible to isolate the effect of various SLCF mitigation trajectories on climate and air quality (Section 6.7).

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