Heat waves and air pollution episodes pose a serious threat to human health and may worsen under future climate change. In this paper, we use 15 years (1999–2013) of commensurately gridded (1° x 1°) ...surface observations of extended summer (April–September) surface ozone (O₃), fine particulate matter (PM2.5), and maximum temperature (TX) over the eastern United States and Canada to construct a climatology of the coincidence, overlap, and lag in space and time of their extremes. Extremes of each quantity are defined climatologically at each grid cell as the 50 d with the highest values in three 5-y windows (∼95th percentile). Any two extremes occur on the same day in the same grid cell more than 50% of the time in the northeastern United States, but on a domain average, co-occurrence is approximately 30%. Although not exactly co-occurring, many of these extremes show connectedness with consistent offsets in space and in time, which often defy traditional mechanistic explanations. All three extremes occur primarily in large-scale, multiday, spatially connected episodes with scales of >1,000 km and clearly coincide with large-scale meteorological features. The largest, longest-lived episodes have the highest incidence of co-occurrence and contain extreme values well above their local 95th percentile threshold, by +7 ppb for O₃, +6 μg m−3 for PM2.5, and +1.7 °C for TX. Our results demonstrate the need to evaluate these extremes as synergistic costressors to accurately quantify their impacts on human health.
Knowledge of the atmospheric chemistry of reactive greenhouse gases is needed to accurately quantify the relationship between human activities and climate, and to incorporate uncertainty in our ...projections of greenhouse gas abundances. We present a method for estimating the fraction of greenhouse gases attributable to human activities, both currently and for future scenarios. Key variables used to calculate the atmospheric chemistry and budgets of major non‐CO2greenhouse gases are codified along with their uncertainties, and then used to project budgets and abundances under the new climate‐change scenarios. This new approach uses our knowledge of changing abundances and lifetimes to estimate current total anthropogenic emissions, independently and possibly more accurately than inventory‐based scenarios. We derive a present‐day atmospheric lifetime for methane (CH4) of 9.1 ± 0.9 y and anthropogenic emissions of 352 ± 45 Tg/y (64% of total emissions). For N2O, corresponding values are 131 ± 10 y and 6.5 ± 1.3 TgN/y (41% of total); and for HFC‐134a, the lifetime is 14.2 ± 1.5 y.
Key Points
A new method proposed for projecting non‐CO2 GHG with uncertainty
Enables the community to evaluate the importance of different processes
Independent evaluation of natural and anthropogenic GHG emissions
Stratosphere–troposphere exchange (STE) is an important source of
tropospheric ozone, affecting all of atmospheric chemistry, climate, and air quality. The study of impacts needs STE fluxes to be ...resolved by latitude and month, and for this, we rely on global chemistry models, whose results diverge greatly. Overall, we lack guidance from model–measurement metrics that inform us about processes and patterns related to the STE flux of ozone (O3). In this work, we use modeled tracers (N2O and CFCl3), whose distributions and budgets can be constrained by satellite and surface
observations, allowing us to follow stratospheric signals across the
tropopause. The satellite-derived photochemical loss of N2O on annual
and quasi-biennial cycles can be matched by the models. The STE flux of
N2O-depleted air in our chemistry transport model drives surface
variability that closely matches observed fluctuations on both annual and
quasi-biennial cycles, confirming the modeled flux. The observed tracer
correlations between N2O and O3 in the lowermost stratosphere
provide a hemispheric scaling of the N2O STE flux to that of
O3. For N2O and CFCl3, we model greater southern hemispheric
STE fluxes, a result supported by some metrics, but counter to the prevailing theory of wave-driven stratospheric circulation. The STE flux of O3, however, is predominantly northern hemispheric, but evidence shows that this is caused by the Antarctic ozone hole reducing southern hemispheric O3 STE by 14 %. Our best estimate of the current STE O3 flux based on a range of constraints is 400 Tg(O3) yr−1, with a 1σ uncertainty of ±15 % and with a NH : SH ratio ranging from 50:50 to 60:40. We identify a range of observational metrics that can better constrain the modeled STE O3 flux in future assessments.
Using Aura Microwave Limb Sounder satellite observations of stratospheric nitrous oxide (N2O), ozone, and temperature from 2005 through 2021, we calculate the atmospheric lifetime of N2O to be ...decreasing at a rate of -2.1 ± 1.2 %/decade. This decrease is occurring because the N2O abundances in the middle tropical stratosphere, where N2O is photochemically destroyed, are increasing at a faster rate than the bulk N2O in the lower atmosphere. The cause appears to be a more vigorous stratospheric circulation, which models predict to be a result of climate change. If the observed trends in lifetime and implied emissions continue, then the change in N2O over the 21st century will be 27 % less than those projected with a fixed lifetime, and the impact on global warming and ozone depletion will be proportionately lessened. Because global warming is caused in part by N2O, this finding is an example of a negative climate–chemistry feedback.
Changes in the stratosphere‐troposphere exchange (STE) of ozone over the last few decades have altered the tropospheric ozone abundance and are likely to continue doing so in the coming century as ...climate changes. Combining an updated linearized stratospheric ozone chemistry (Linoz v2) with parameterized polar stratospheric clouds (PSCs) chemistry, a 5‐year (2001–2005) sequence of the European Centre for Medium‐Range Weather Forecasts (ECMWF) meteorology data, and the University of California, Irvine (UCI) chemistry transport model (CTM), we examined variations in STE O3 flux and how it perturbs tropospheric O3. Our estimate for the current STE ozone flux is 290 Tg/a in the Northern Hemisphere (NH) and 225 Tg/a in the Southern Hemisphere (SH). The 2001–2005 interannual root‐mean‐square (RMS) variability is 25 Tg/a for the NH and 30 Tg/a for the SH. STE drives a seasonal peak‐to‐peak NH variability in tropospheric ozone of about 7–8 Dobson unit (DU). Of the interannual STE variance, 20% and 45% can be explained by the quasi‐biennial oscillation (QBO) in the NH and SH, respectively. The CTM matches the observed QBO variations in total column ozone, and the STE O3 flux shows negative anomalies over the midlatitudes during the easterly phases of the QBO. When the observed column ozone depletion from 1979 to 2004 is modeled with Linoz v2, we predicted STE reductions of at most 10% in the NH, corresponding to a mean decrease of 1 ppb in tropospheric O3.
Stratospheric ozone depletion from halocarbons is partly countered by pollution‐driven increases in tropospheric ozone, with transport connecting the two. While recognizing this connection, the ozone ...assessment's evaluation of observations and processes have often split the chapters at the tropopause boundary. Using a chemistry‐transport model we find that air‐pollution ozone enhancements in the troposphere spill over into the stratosphere at significant rates, that is, 13%–34% of the excess tropospheric burden appears in the lowermost extra‐tropical stratosphere. As we track the anticipated recovery of the observed ozone depletion, we should recognize that two tenths of that recovery may come from the transport of increasing tropospheric ozone into the stratosphere.
Plain Language Summary
The world has made great strides in phasing out the halocarbons that drive ozone loss, such as the chlorofluorocarbons 11 and 12. While living with the well‐documented depletion of the ozone layer, we are now watching the slow recovery (increase) of stratospheric ozone over this century after our phaseout of halocarbon production and use. Projecting this recovery date also depends on the impact of other changing greenhouse gases on stratospheric chemistry as well as changes in tropospheric ozone. Both observations and models identify tropospheric ozone as increasing due to air‐quality pollution in the lower atmosphere. Here, using a global chemistry‐transport model, we find that this ozone increase carries over into the stratosphere at rates affecting the recovery expected from the decay of atmospheric halocarbons. This process is inherently included in our chemistry‐climate models but is not diagnosed as such. The ozone assessments need consider that what happens in the troposphere does not stay in the troposphere, complicating our of interpretation of ozone changes over this century.
Key Points
Our world has seen damaging levels of ozone depletion over the past several decades, and these will continue into this century
Currently, ozone column changes are a combination of halogen‐driven stratospheric decreases counteracted by pollution‐driven tropospheric increases
Tropospheric increases spillover into the stratosphere, stratospheric depletion can thus be underestimated, and the observed recovery of assumed halogen‐driven loss is overestimated
Measurements from actinic flux spectroradiometers on board the NASA DC-8 during the Atmospheric Tomography (ATom) mission provide an extensive set of statistics on how clouds alter photolysis rates ...(J values) throughout the remote Pacific and Atlantic Ocean basins. J values control tropospheric ozone and methane abundances, and thus clouds have been included for more than three decades in tropospheric chemistry modeling. ATom made four profiling circumnavigations of the troposphere capturing each of the seasons during 2016–2018. This work examines J values from the Pacific Ocean flights of the first deployment, but publishes the complete Atom-1 data set (29 July to 23 August 2016). We compare the observed J values (every 3 s along flight track) with those calculated by nine global chemistry– climate/transport models (globally gridded, hourly, for a mid-August day). To compare these disparate data sets, we build a commensurate statistical picture of the impact of clouds on J values using the ratio of J -cloudy (standard, sometimes cloudy conditions) to J -clear (artificially cleared of clouds). The range of modeled cloud effects is inconsistently large but they fall into two distinct classes: (1) models with large cloud effects showing mostly enhanced J values aloft and or diminished at the surface and (2) models with small effects having nearly clear-sky J values much of the time. The ATom-1 measurements generally favor large cloud effects but are not precise or robust enough to point out the best cloud-modeling approach. The models here have resolutions of 50–200 km and thus reduce the occurrence of clear sky when averaging over grid cells. In situ measurements also average scattered sunlight over a mixed cloud field, but only out to scales of tens of kilometers. A primary uncertainty remains in the role of clouds in chemistry, in particular, how models average over cloud fields, and how such averages can simulate measurements.
The coupled chemistry of methane, carbon monoxide (CO), and hydroxyl radical (OH) can modulate methane's 9‐year lifetime. This is often ignored in methane flux inversions, and the impacts of ...neglecting interactive chemistry have not been quantified. Using a coupled‐chemistry box model, we show that neglecting the effect of methane source perturbation on OH can lead to a 25% bias in estimating abrupt changes in methane sources after only 10 years. Further, large CO emissions, such as from biomass burning, can increase methane concentrations by extending the methane lifetime through impacts on OH. Finally, we quantify the biases of including (or excluding) coupled chemistry in the context of recent methane and CO trends. Decreasing CO concentrations, beginning in the 2000's, have notable impacts on methane flux inversions. Given these nonnegligible errors, decadal methane emissions inversions should incorporate chemical feedbacks for more robust methane trend analyses and source attributions.
Plain Language Summary
Methane inversion studies commonly assume that atmospheric methane has a 9‐year lifetime, but the decay rate of methane perturbations can be extended by 40%. This effect is from interactions of other atmospheric compounds with methane's main sink, the hydroxyl radical. This is important for estimating global emissions over recent decades. We show that one of these compounds, carbon monoxide (CO), emitted from wildfires during El Niño, can lead to large increases in methane concentrations by extending the methane lifetime. Moreover, ignoring these effects can lead up to a 25% error in estimating methane emissions changes after a decade. Finally, we show that the effect of decreasing CO on methane has reduced the methane lifetime and has led to potential biases in calculating methane emissions. Thus, attributing causes of recent methane emissions trends are dependent on the consideration of compounds indirectly affecting the methane lifetime, which may have implications for future mitigation plans.
Key Points
Neglecting chemical feedbacks can bias estimates of methane emissions perturbations by up to 25% over 10 years
Strong biomass burning events, such as El Niño, can indirectly increase the methane growth rate through emissions of CO
Attributions of decadal trends in methane are dependent on the assumptions about both OH and CO
Fluctuations in atmospheric CO2 can be measured with great precision and are used to identify human-driven sources as well as natural cycles of ocean and land carbon. One source of variability is the ...stratosphere, where the influx of aged CO2-depleted air can produce fluctuations at the surface. This process has been speculated to be a potential source of interannual variability (IAV) in CO2 that might obscure the quantification of other sources of IAV. Given the recent success in demonstrating that the stratospheric influx of N2O- and chlorofluorocarbon-depleted air is a dominant source of their surface IAV in the Southern Hemisphere, I apply the same model and measurement analysis here to CO2. Using chemistry-transport modeling or scaling of the observed N2O variability, I find that the stratosphere-driven surface variability in CO2 is at most 10 % of the observed IAV and is not an important source. Diagnosing the amplitude of the CO2 annual cycle and its increase from 1985 to 2021 through the annual variance gives rates similar to traditional methods in the Northern Hemisphere (BRW, MLO) but can identify the emergence of small trends (0.08 ppm per decade) in the Southern Hemisphere (SMO, CGO).
PDF
