The turbulent future brings a breath of fresh air

Near-surface air pollution is a serious health hazard, estimated to cause seven million premature deaths globally each year. High pollution episodes arise from a combination of emissions and unfavorable meteorological conditions within the planetary boundary layer (PBL), where turbulent mixing dilutes surface-emitted aerosols and trace gases. The height of the PBL, strongly controlled by the boundary layer lapse rate, modulates near-surface pollutant accumulation—shallower PBLs intensify surface concentrations. Recent research indicates a boundary layer feedback: high emissions of absorbing aerosols (e.g., black carbon, BC) heat the atmosphere aloft, reduce the lapse rate, suppress turbulence, and shallow the PBL, further exacerbating pollution episodes. Scattering aerosols (e.g., sulfate) can cool the surface and increase stability, while greenhouse gases (e.g., CO₂) warm the surface and may destabilize the PBL over land. However, the relative roles of these drivers and their net effects on turbulence, PBL height, and surface pollution remain unclear across regions and time scales. This study investigates how perturbations in BC, sulfate (SO₄), and CO₂ affect turbulence and PBL height, and whether these changes relate to near-surface aerosol concentrations, with a particular focus on land regions and an in-depth analysis over Eastern China, including the North China Plain.

Prior studies have linked aerosol-PBL interactions to severe pollution, especially in China, where absorbing aerosols intensify atmospheric stability aloft and suppress turbulence, leading to higher surface concentrations. Observations and modeling have shown that the impact of BC depends on its vertical distribution: heating aloft reduces the lapse rate and shallows the PBL, whereas near-surface heating can enhance convection. Scattering aerosols primarily act via shortwave reflection, cooling the surface and tending to stabilize the lower atmosphere, though effects can be regionally complex and weaker than those from absorbing aerosols. Greenhouse gases increase surface temperatures and can destabilize the PBL over land while potentially shallowing marine PBLs via rapid adjustments. Reported trends in observed and reanalysis-derived PBL height over China and India suggest aerosol emission changes are important co-drivers of PBL evolution, with evidence of increased PBL height during periods of lower aerosol emissions and reversals when BC emissions increased with urbanization. Nevertheless, disentangling emission versus meteorological controls remains challenging, motivating multi-year, multi-model studies.

The study employs idealized perturbation experiments using the Community Earth System Model version 2 (CESM2) with the Community Atmosphere Model version 6 (CAM6). Two configurations were used: (1) atmosphere-only with fixed sea surface temperatures (fsst) and (2) fully coupled (cpl) simulations with interactive ocean, land, and cryosphere. CAM6 was run on a 0.9°×1.25° grid with 32 vertical levels. Perturbations relative to year-2000 baseline emissions (CMIP6) were: doubled CO₂ concentration (CO₂×2), tenfold increase in anthropogenic black carbon emissions (BC×10), and fivefold increase in anthropogenic sulfate emissions (SO₄×5). Perturbations were imposed instantaneously; fsst simulations ran 15 years (last 10 analyzed). Coupled simulations were spun up (≈60 years), then branched to baseline and perturbed runs for 50 years plus an additional 20 years for analysis. Hourly model output was used to compute stability and turbulence metrics: Brunt–Väisälä frequency (N) and gradient Richardson number (Ri). In CAM6, PBL height is diagnosed as the first level where Ri exceeds a critical value of 0.3 (noting known GCM challenges in very shallow, stable boundary layers). BC surface concentration was used as a proxy for near-surface aerosol pollution due to health relevance and availability of hourly output. To assess resolution sensitivity, corresponding downscaling experiments were performed with the Weather Research and Forecasting (WRF) model v4.3 over Eastern China and Southeast Asia at 45 km and 15 km (and a 5 km inner East China domain in one description) resolutions. WRF used CESM2-provided meteorological boundary conditions (updated every 6 h), spectral nudging in the outer domain, prescribed aerosol 3-D fields from CESM2 in the radiation scheme (RRTM with aer-opt), and specified physics schemes (WSM6 microphysics, Grell–Freitas cumulus, Mellor–Yamada–Janjic TKE boundary layer, Monin–Obukhov surface layer, Unified Noah land surface). WRF was run without interactive chemistry. Additionally, historical and future PBL height trends were analyzed from CMIP6 ensemble simulations: linear trends over 1850–2014 (historical) and future periods (2015–2045, 2015–2100) across SSP scenarios, in conjunction with population data (SEDAC). Over Eastern China, wintertime (Nov–Feb) diurnal cycles of temperature profile, Ri, PBL height, and BC concentration were examined to infer timing and mechanisms of feedbacks.

- Global land response: BC×10 increases atmospheric stability (higher N), reduces turbulence (higher Ri), and lowers PBL height, with strongest effects in high-emission regions (India, East China). SO₄×5 shows a weaker but widespread reduction in turbulence and PBL height over land. CO₂×2 produces the opposite sign over land: reduced stability, increased turbulence, and higher PBL height; over oceans, CO₂ tends to shallow the marine PBL via rapid adjustment, yielding a land–ocean contrast. - Resolution robustness: Regional WRF downscaling reproduces the sign and magnitude of CESM2 responses in PBL height and turbulence over Eastern China, indicating limited sensitivity to model resolution for the scales examined. - Links to surface pollution: Grid cells with the largest increases in near-surface BC under BC×10 also show the strongest reductions in turbulence and PBL height; the association is weaker for SO₄×5 and CO₂×2. Scatter analyses show strong correlations between turbulence/PBL changes and BC concentration changes (e.g., BC×10 Pearson correlation ≈ 0.81 for linked metrics; negative correlations for SO₄×5 and CO₂×2 panels as described in Fig. 2 captions). - Eastern China quantifications: Annual mean PBL height changes averaged over East China: BC×10 reduces PBLH by 7.8% (statistically significant); SO₄×5 reduces PBLH by 1.4%; CO₂×2 increases PBLH by 2.7%. Wintertime PDFs of Ri shift right (more non-turbulent) under BC×10, show minimal change for SO₄×5, and shift left (more turbulent) under CO₂×2. Correspondingly, the frequency of high near-surface BC events increases under BC×10 and SO₄×5, and decreases under CO₂×2. - Diurnal mechanisms (East China, winter): BC×10 induces strong afternoon warming aloft (~above 800 hPa), reduces surface solar radiation and near-surface temperatures, increases stability (Ri), lowers PBL height most around 15:00 local time, and increases near-surface BC, consistent with a BC-driven boundary layer feedback. SO₄×5 cools the surface and increases stability but does not yield statistically significant changes in turbulence or PBL height at hourly scales; increases in near-surface BC are attributed to aerosol microphysical mode shifts (growth/coating), not BL feedback. CO₂×2 warms the surface, decreases stability, increases midday turbulence, and reduces near-surface BC around the same time. - Historical trends (CMIP6 ensemble): Over 1850–2014, PBL height declines coincide with regions of strong aerosol emission increases; ≈83% of the world’s population (year-2100 spatial distribution) reside in areas with historical PBL height reductions. - Future scenarios: Across SSPs, aerosol emissions decline while CO₂ continues to rise. The combined effect (reduced BC and higher CO₂) is projected to enhance turbulence and increase PBL height over land. Under SSP245, ≈60% of the global population in 2100 lives in regions with increased PBL height relative to present. About 6% (>400 million people) live in areas with very high BC (≥95th percentile) where PBL height increases are particularly strong (≥90th percentile). Comparable shares for other SSPs: ≈3% (SSP370), ≈13% (SSP585), and >30% (SSP119). The authors estimate around 10% of people currently in highly polluted regions will experience particularly strong increases in turbulence and PBL height under futures with rising CO₂ and reduced aerosols.

The study addresses how distinct anthropogenic climate drivers—absorbing aerosols (BC), scattering aerosols (SO₄), and greenhouse gases (CO₂)—modify lower-atmospheric stability, turbulence, and PBL height, thereby influencing near-surface pollution episodes. The results corroborate a BC-driven boundary layer feedback: BC heating aloft stabilizes the lower troposphere, suppresses turbulence, shallows the PBL, and elevates surface pollutant concentrations, especially in high-emission regions like East China and India. In contrast, CO₂-induced land surface warming reduces stability, enhances turbulence, and raises PBL height, mitigating high-pollution events over land; over oceans, rapid adjustments shallow the marine PBL, emphasizing a land–ocean contrast. SO₄’s stabilizing influence is weaker and complicated by aerosol microphysical shifts that can increase near-surface BC independently of BL feedback. These findings imply that, beyond direct emission changes, climate-driven alterations in boundary layer dynamics can either exacerbate or alleviate surface air pollution. Historically, rising aerosols likely contributed to widespread decreases in PBL height, consistent with observational and reanalysis indications over Asia. Looking ahead, universal declines in aerosol emissions combined with persistent CO₂ increases should, on balance, increase PBL height and turbulence over land, providing an added positive effect on air quality in many regions, particularly those with historically high BC. However, the magnitude of this benefit varies by scenario and region, and other meteorological processes (e.g., precipitation, clouds) can modulate surface concentrations. The study highlights the need for multi-model, long-term, and higher-resolution analyses to disentangle mechanisms and to better quantify regional air quality–climate interactions.

This work demonstrates that anthropogenic drivers differentially modulate boundary layer dynamics and, through them, near-surface air pollution. Absorbing aerosols (BC) intensify stability, suppress turbulence, and reduce PBL height, aggravating pollution, whereas CO₂ increases over land tend to destabilize the boundary layer, enhance turbulence, and raise PBL height, mitigating severe pollution episodes. Scattering aerosols have weaker and more complex effects. Idealized model experiments, supported by regional downscaling, quantify these relationships globally and in Eastern China, and connect them to historical trends and future SSP scenarios. Importantly, future decreases in BC emissions coupled with rising CO₂ are projected to increase PBL height over many land regions, offering an added positive impact on air quality alongside direct emission reductions. Future research should: (1) undertake multi-model, long-term simulations with higher vertical resolution to better capture shallow stable PBLs; (2) isolate and quantify the separate roles of aerosol microphysics (e.g., SO₂-to-SO₄ coating) versus BL dynamics on surface concentrations; (3) incorporate interactive chemistry and aerosol–cloud interactions; and (4) refine regional assessments linking demographic exposure to evolving PBL dynamics under varying emission pathways.

- Idealized perturbations (BC×10, SO₄×5, CO₂×2) were intentionally exaggerated to improve signal-to-noise, limiting direct quantitative applicability to real-world changes. - GCM-resolution limitations: CESM2/CAM6 cannot resolve turbulence; PBL processes are parameterized, and GCMs struggle with very shallow, stable boundary layers, potentially underestimating BL shallowing from BC. - Known aerosol biases in global models (e.g., vertical BC distribution, total column loads, surface concentrations) may affect stability and feedback estimates. - Use of BC concentration as a proxy for near-surface aerosol pollution omits impacts on other species and gases, and hourly outputs limit comprehensive pollutant metrics. - Attribution challenges: The simulations cannot cleanly separate boundary layer feedback from other meteorological influences (precipitation, clouds, synoptic variability) on surface aerosols. - WRF regional runs lacked interactive chemistry and used prescribed aerosol radiative effects; microphysical mode changes complicate interpretation of SO₄ impacts on surface BC. - Observational comparisons of PBL height are difficult due to heterogeneous datasets and methodologies, adding uncertainty to validation of historical trends.