Changes in tropospheric air quality related to the protection of stratospheric ozone in a changing climate

Abstract

Ultraviolet (UV) radiation drives the net production of tropospheric ozone (O₃) and a large fraction of particulate matter (PM) including sulfate, nitrate, and secondary organic aerosols. Ground-level O₃ and PM are detrimental to human health, leading to several million premature deaths per year globally, and have adverse effects on plants and the yields of crops. The Montreal Protocol has prevented large increases in UV radiation that would have had major impacts on air quality. Future scenarios in which stratospheric O₃ returns to 1980 values or even exceeds them (the so-called super-recovery) will tend to ameliorate urban ground-level O₃ slightly but worsen it in rural areas. Furthermore, recovery of stratospheric O₃ is expected to increase the amount of O₃ transported into the troposphere by meteorological processes that are sensitive to climate change. UV radiation also generates hydroxyl radicals (OH) that control the amounts of many environmentally important chemicals in the atmosphere including some greenhouse gases, e.g., methane (CH₄), and some short-lived ozone-depleting substances (ODSs). Recent modeling studies have shown that the increases in UV radiation associated with the depletion of stratospheric ozone over 1980-2020 have contributed a small increase (~ 3%) to the globally averaged concentrations of OH. Replacements for ODSs include chemicals that react with OH radicals, hence preventing the transport of these chemicals to the stratosphere. Some of these chemicals, e.g., hydrofluorocarbons that are currently being phased out, and hydrofluoroolefins now used increasingly, decompose into products whose fate in the environment warrants further investigation. One such product, trifluoroacetic acid (TFA), has no obvious pathway of degradation and might accumulate in some water bodies, but is unlikely to cause adverse effects out to 2100.

Fig. 1

Simplified schematic of tropospheric photochemistry. UV-C radiation (100–280 nm) in the stratosphere generates ozone, O₃, some of which is transported to the troposphere. UV-B radiation initiates tropospheric chemistry by photo-dissociating O₃ and generating highly reactive hydroxyl radicals (OH). These react with many compounds emitted by human activities and natural processes, e.g., carbon monoxide, methane, volatile organic compounds including halocarbons, and others, all generalized in the figure as RC (reduced compounds). By removing these compounds, the concentration of OH controls the self-cleaning capacity of the atmosphere. Nitrogen oxides (NO_x = NO + NO₂) catalyze the photo-oxidation by regenerating OH via reaction of NO with HO₂. This coupling of the NO_x and HO_x (OH + HO₂) cycles also leads to autocatalytic production of O₃, often in amounts larger than lost initially via its UV-B photolysis, since NO_x and HO_x molecules can cycle many times before being removed. The cycles are terminated by reaction with OH to make nitric acid, (HNO₃) or by reaction of HO₂ to inorganic or organic peroxides, e.g., hydrogen peroxide (H₂O₂). Other products, depending on the reduced compounds being oxidized by OH, could include partly oxidized organics, secondary organic aerosols (SOA), sulfuric acid (H₂SO₄), and trifluoroacetic acid (TFA)

Fig. 2

Global tropospheric ozone (Tg) estimated by multi-model assessments (MMM, ACCMIP, TOAR) and observations (OBS, for the year 2000). Future projections are for one Shared Socioeconomic Pathway (SSP370 scenario). From IPCC 2021 Ch. 6 [11]

Fig. 3

Regional and local trends in tropospheric ozone at the surface and lower atmosphere. From IPCC 2021 Ch. 6 [11]

Fig. 4

Composition of PM_2.5 over the contiguous United States calculated with a chemistry-transport model. Particles produced by UV photochemical reactions (secondary aerosols) include sulfate (SO₄), ammonium nitrate (NH₄NO₃) and secondary organic aerosols (SOA) from anthropogenic or biogenic precursors (SOA_AVOC and SOA_BVOC, respectively), and account for more than half of the total PM_2.5, compared to directly emitted particles (primary aerosols) such as dust, soot, and sea spray. VOC, volatile organic compounds. From Pye et al. [46]

Fig. 5

Sites at which air pollution can affect interactions mediated by olfactory or visual cues between plants and their associated community. A Effects of pollutants on signal-emitting organisms. B The degradation of VOCs by air pollutants and formation of reaction products and secondary organic aerosol (SOA). C Effects on the signal receiving organisms, e.g., pollinating insects. In addition, exposure to air pollution can influence the interactions between herbivores and plants. D From [134], reproduced with permission

Fig. 6

Relative change in global concentrations of tropospheric OH from 1980 to 2020, estimated with three different Earth System Models (ESMs). The net change in OH (rightmost column) has contributions from increased emissions of nitrogen oxides and other precursors of tropospheric ozone (ΔNOx); increased emissions of methane (ΔCH₄); accumulation of ozone-depleting substances now regulated under the Montreal Protocol (ΔODSs); emissions of particulate matter and its precursors (ΔPM); and other undifferentiated changes attributed to underlying climate change (e.g., water vapor), as well as interactions among these separate factors (ΔOther) (modified from [141])

Fig. 7

Interacting effects of UV-B radiation and climate change on tropospheric concentrations of OH and on the lifetime of very-short-lived substances (VSLSs). Effects of climate change include more frequent wildfires and thawing of permafrost soils with the formation of thermokarst lakes, which are important sources of CO and CH₄, respectively. Increased emissions of CO and CH₄ tend to decrease the tropospheric OH concentration, which in turn results in longer lifetime of VSLSs and, thus, a higher probability of stratospheric ozone depletion

Fig. 8

Schematic of the circulation of air into the stratosphere and its return. The general circulation is slow relative to movements in the troposphere. Tropopause folds (shown with a dotted blue line) occur sporadically and inject stratospheric air into the troposphere

Fig. 9

Drivers of tropospheric ozone concentration (redrawn with permission and assistance from Guang Zeng from Fig. 13, in [158]). The deposition amount is directly related to the concentration of O₃ at the surface of the Earth. The two panels represent the output of different chemistry–climate models considered in the study. STE, stratospheric–tropospheric exchange

Fig. 10

Atmospheric degradation pathways and corresponding yields of TFA for HFCF-123 (A), HFC-134a (B) and HFO-1234yf (C) representing three generations of important CFC replacements. Approximate atmospheric lifetimes for the chemical species involved are indicated in parenthesis. Species marked by an asterisk have significant competing fates in the atmosphere [205, 206]

Fig. 11

Atmospheric degradation of HCFO-1233zd. The OH-initiated oxidation of the product, CF₃CHO, is a minor source of TFA. Based on [207] and [208]

Fig. 12

Yields of TFA from selected individual chlorofluorocarbon (CFC) replacement compounds, and their estimated global emissions. Also included are selected compounds not under the purview of the Montreal Protocol and Amendments. Error bars represent both experimental uncertainties and upper and lower yield ranges due to competing reaction channels that depend on environmental conditions. Yields of TFA from individual compounds are estimated based on evaluations of the available literature as described in Online Resource SI Sect. 4. Note split scale for the emission for HFC-134a, which is much higher than those of other compounds

Fig. 13

Concentration of trifluoroacetate in composite precipitation samples from eight sites in Germany from February 2018 to February 2020. The y-axis is on a binary logarithmic scale (log₂) and the solid horizontal bar is the median, the box indicates the upper and lower quartiles, the whiskers the upper of lower quartile – or + the interquartile range × 1.5, and the data symbols are the outliers. From Fig. 27 in [220]

Fig. 14

Maximum possible annual emissions of TFA from plant protection products in Germany for active ingredients that can theoretically form TFA. Data based on mean sales volume of each of the active substances over the three years 2016, 2017 and 2018 (Figure from [259])

Fig. 15

A log-probability cumulative frequency plot of no observed effect concentrations (NOEC) of trifluoroacetic acid salt compared to various environmental concentrations in water. The dashed vertical green line indicates the NOEC for TFA-Na salt in microcosms is a toxicology-based criterion