Assessing the Ability of Instantaneous Aircraft and Sonde Measurements to Characterize Climatological Means and Long-Term Trends in Tropospheric Composition

Over four decades of measurements exist that sample the 3-D composition of reactive trace gases in the troposphere from approximately weekly ozone sondes, instrumentation on civil aircraft, and individual comprehensive aircraft field campaigns. An obstacle to using these data to evaluate coupled chemistry-climate models (CCMs)the models used to project future changes in atmospheric composition and climateis that exact space-time matching between model fields and observations cannot be done, as CCMs generate their own meteorology. Evaluation typically involves averaging over large spatiotemporal regions, which may not reflect a true average due to limited or biased sampling. This averaging approach generally loses information regarding specific processes. Here we aim to identify where discrete sampling may be indicative of long-term mean conditions, using the GEOS-Chem global chemical-transport model (CTM) driven by the MERRA reanalysis to reflect historical meteorology from 2003 to 2012 at 2o by 2.5o resolution. The model has been sampled at the time and location of every ozone sonde profile available from the Would Ozone and Ultraviolet Radiation Data Centre (WOUDC), along the flight tracks of the IAGOSMOZAICCARABIC civil aircraft campaigns, as well as those from over 20 individual field campaigns performed by NASA, NOAA, DOE, NSF, NERC (UK), and DLR (Germany) during the simulation period. Focusing on ozone, carbon monoxide and reactive nitrogen species, we assess where aggregates of the in situ data are representative of the decadal mean vertical, spatial and temporal distributions that would be appropriate for evaluating CCMs. Next, we identically sample a series of parallel sensitivity simulations in which individual emission sources (e.g., lightning, biogenic VOCs, wildfires, US anthropogenic) have been removed one by one, to assess where and when the aggregated observations may offer constraints on these processes within CCMs. Lastly, we show results of an additional 31-year simulation from 1980-2010 of GEOS-Chem driven by the MACCity emissions inventory and MERRA reanalysis at 4o by 5o. We sample the model at every WOUDC sonde and flight track from MOZAIC and NASA field campaigns to evaluate which aggregate observations are statistically reflective of long-term trends over the period

Using satellite observed formaldehyde (HCHO) and nitrogen dioxide (NO2) as an indicator of ozone sensitivity in a SIP

Although State Implementation Plans (SIPs) typically rely on observations from ground-based networks and regulatory models, satellite data is increasingly available to state agencies and can also inform and supplement state implementation plans to improve air quality. An advantage of satellite data is that it provides information for a broader area than sampled by ground-based networks. This document provides examples and guidance for using satellite products of formaldehyde (HCHO) and nitrogen dioxide (NO2) to inform ground-level ozone sensitivity to emissions of nitrogen oxides (NOx) versus volatile organic compounds (VOC) in state implementation plans. Analysis of changes in ozone sensitivity over periods where emission controls have been implemented can provide insights into the efficacy of those past strategies and the likely efficacy of proposed future emission control programs

Chemical nonlinearities in relating intercontinental ozone pollution to anthropogenic emissions

Model studies typically estimate intercontinental influence on surface ozone by perturbing emissions from a source continent and diagnosing the ozone response in the receptor continent. Since the response to perturbations is non-linear due to chemistry, conclusions drawn from different studies may depend on the magnitude of the applied perturbation. We investigate this issue for intercontinental transport between North America, Europe, and Asia with sensitivity simulations in three global chemical transport models. In each region, we decrease anthropogenic emissions of NOx and nonmethane volatile organic compounds (NMVOCs) by 20% and 100%. We find strong nonlinearity in the response to NOx perturbations outside summer, reflecting transitions in the chemical regime for ozone production. In contrast, we find no significant nonlinearity to NOx perturbations in summer or to NMVOC perturbations year-round. The relative benefit of decreasing NOx vs. NMVOC from current levels to abate intercontinental pollution increases with the magnitude of emission reductions

Quantifying pollution inflow and outflow over East Asia in spring with regional and global models

Understanding the exchange processes between the atmospheric boundary layer and the free troposphere is crucial for estimating hemispheric transport of air pollution. Most studies of hemispheric air pollution transport have taken a large-scale perspective using global chemical transport models with fairly coarse spatial and temporal resolutions. In support of United Nations Task Force on Hemispheric Transport of Air Pollution (TF HTAP; www.htap.org), this study employs two high-resolution atmospheric chemistry models (WRF-Chem and CMAQ; 36×36 km) driven with chemical boundary conditions from a global model (MOZART; 1.9×1.9°) to examine the role of fine-scale transport and chemistry processes in controlling pollution export and import over the Asian continent in spring (March 2001). Our analysis indicates the importance of rapid venting through deep convection that develops along the leading edge of frontal system convergence bands, which are not adequately resolved in either of two global models compared with TRACE-P aircraft observations during a frontal event. Both regional model simulations and observations show that frontal outflows of CO, O3 and PAN can extend to the upper troposphere (6–9 km). Pollution plumes in the global MOZART model are typically diluted and insufficiently lofted to higher altitudes where they can undergo more efficient transport in stronger winds. We use sensitivity simulations that perturb chemical boundary conditions in the CMAQ regional model to estimate that the O3 production over East Asia (EA) driven by PAN decomposition contributes 20% of the spatial averaged total O3 response to European (EU) emission perturbations in March, and occasionally contributes approximately 50% of the total O3 response in subsiding plumes at mountain observatories (at approximately 2 km altitude). The response to decomposing PAN of EU origin is strongly affected by the O3 formation chemical regimes, which vary with the model chemical mechanism and NOx/VOC emissions. Our high-resolution models demonstrate a large spatial variability (by up to a factor of 6) in the response of local O3 to 20% reductions in EU anthropogenic O3 precursor emissions. The response in the highly populated Asian megacities is 40–50% lower in our high-resolution models than the global model, suggesting that the source-receptor relationships inferred from the global coarse-resolution models likely overestimate health impacts associated with intercontinental O3 transport. Our results highlight the important roles of rapid convective transport, orographic forcing, urban photochemistry and heterogeneous boundary layer processes in controlling intercontinental transport; these processes may not be well resolved in the large-scale models

North American isoprene influence on intercontinental ozone pollution

Changing land-use and climate may alter emissions of biogenic isoprene, a key ozone (O3) precursor. Isoprene is also a precursor to peroxy acetyl nitrate (PAN) and thus affects partitioning among oxidized nitrogen (NOy) species, shifting the balance towards PAN, which more efficiently contributes to long-range transport relative to nitric acid (HNO3) which rapidly deposits. With a suite of sensitivity simulations in the MOZART-2 global tropospheric chemistry model, we gauge the relative importance of the intercontinental influence of a 20% increase in North American (NA) isoprene and a 20% decrease in NA anthropogenic emissions (nitrogen oxides (NOx), non-methane volatile organic compounds (NMVOC) and NOx + NMVOC + carbon monoxide + aerosols). The surface O3 response to NA isoprene emissions (ΔO3_ISOP) in surface air over NA is about one third of the response to all NA anthropogenic emissions (ΔO3_ANTH; although with different signs). Over intercontinental distances, ΔO3_ISOP is relatively larger; in summer and fall, ΔO3_ISOP in surface air over Europe and North Africa (EU region) is more than half of ΔO3_ANTH. Future increases in NA isoprene emissions could thus offset decreases in EU surface O3 resulting from controls on NA anthropogenic emissions. Over the EU region, ΔPAN_ISOP at 700 hPa is roughly the same magnitude as ΔPAN_ANTH (oppositely signed). Outside of the continental source region, the percentage changes in PAN are at least twice as large as for surface O3, implying that long-term PAN measurements at high altitude sites may help to detect O3 precursor emission changes. We find that neither the baseline level of isoprene emissions nor the fate of isoprene nitrates contributes to the large diversity in model estimates of the anthropogenic emission influence on intercontinental surface O3 or oxidized nitrogen deposition reported in the recent TF HTAP multi-model studies (TFHTAP, 2007)

Projecting policy – relevant metrics for high summertime ozone pollution events over the 1 Eastern United States due to climate and emission changes during the 21st century

Over the eastern United States (EUS), nitrogen oxides (NOx) emission controls have led to improved air quality over the past two decades, but concerns have been raised that climate warming may offset some of these gains. Here we analyze the effect of changing emissions and climate, in isolation and combination, on EUS summertime surface ozone (O3) over the recent past and the 21st century in an ensemble of simulations performed with the Geophysical Fluid Dynamics Laboratory CM3 chemistry-climate model. The simulated summertime EUS O3 is biased high but captures the structure of observed changes in regional O3 distributions following NOx emission reductions. We introduce a statistical bias correction, which allows derivation of policy-relevant statistics by assuming a stationary mean state bias in the model, but accurate simulation of changes at each quantile of the distribution. We contrast two different 21st century scenarios: (i) representative concentration pathway (RCP) 4.5 and (ii) simulations with well-mixed greenhouse gases (WMGG) following RCP4.5 but with emissions of air pollutants and precursors held fixed at 2005 levels (RCP4.5_WMGG). We find under RCP4.5 no exceedance of maximum daily 8 hour average ozone above 75 ppb by mid-21st century, reflecting the U.S. NOx emissions reductions projected in RCP4.5, while more than half of the EUS exceeds this level by the end of the 21st century under RCP4.5_WMGG. Further, we find a simple relationship between the changes in estimated 1 year return levels and regional NOx emission changes, implying that our results can be generalized to estimate changes in the frequency of EUS pollution events under different regional NOx emission scenarios

Timing and seasonality of the United States ‘warming hole’

The United States ‘warming hole’ is a region in the southeast/central U.S. where observed long-term surface temperature trends are insignificant or negative. We investigate the roles of anthropogenic forcing and internal variability on these trends by systematically examining observed seasonal temperature trends over all time periods of at least 10 years during 1901–2015. Long-term summer cooling in the north central U.S. beginning in the 1930s reflects the recovery from the anomalously warm ‘Dust Bowl’ of that decade. In the northeast and southern U.S., significant summertime cooling occurs from the early 1950s to the mid 1970s, which we partially attribute to increasing anthropogenic aerosol emissions (median fraction of the observed temperature trends explained is 0.69 and 0.17, respectively). In winter, the northeast and southern U.S. cool significantly from the early 1950s to the early 1990s, but we do not find evidence for a significant aerosol influence. Instead, long-term phase changes in the North Atlantic Oscillation contribute significantly to this cooling in both regions, while the Pacific Decadal Oscillation also contributes significantly to southern U.S. cooling. Rather than stemming from a single cause, the U.S. warming hole reflects both anthropogenic aerosol forcing and internal climate variability, but the dominant drivers vary by season, region, and time period

Temperature and precipitation extremes in the United States: Quantifying the responses to anthropogenic aerosols and greenhouse gases

Changes in extreme temperatures, heat waves, heavy rainfall events, and precipitation frequency can have adverse impacts on human health, air quality, agricultural productivity, and water resources. Using the aerosol only (AER) and greenhouse gas only (GHG) "single forcing" simulations (3 ensemble members each) from the GFDL CM3 chemistry-climate model, we investigate aerosol- versus greenhouse gas-induced changes in high temperature and precipitation extremes over the United States. We identify changes in these events from 1860 to 2005 and the associated large-scale dynamical conditions. Small changes in these extremes in the "all forcing" simulations reflect cancellations between the individual, opposite-signed effects of increasing anthropogenic aerosols and greenhouse gases. In AER, aerosols lead to lower extreme high temperatures and fewer warm spells over the western US (-2.1 K regional average; -20 days/year) and over the central and northeast US (-1.5 K; -12 days/year). In GHG, a similar but opposite-signed response pattern occurs (+2.7 K and +14 days/year over the western US; +2.5 K and +10 days/year in the central and northeast US). The similar spatial response patterns in AER versus GHG suggest a preferred regional mode of response that is largely independent of the regional distribution of the forcing agent. The influence of both greenhouse gases and aerosols on extreme high temperature is weakest in the southeast US, collocated with the observed "warming hole". No statistically significant change occurs in AER, and a warming of only +1.8 K occurs in GHG. Warming in this region continues to be muted over the 21st century under the RCP 8.5 scenario, with increases in extreme temperatures more than 1 K smaller than elsewhere. Aerosols induce decreases in the number of days per year with at least 10mm of precipitation (R10mm) over the eastern US in summer and winter and over the southern US in spring of roughly 1 day/year. In contrast, greenhouse gases induce increases in R10mm over the eastern US in winter (+0.8 days/year), the northern and central US during spring (+1 day/year), and the southeast US during summer (+0.5 days/year), but decreases over the northeast US in summer (-0.2 days/year). In RCP 8.5, the patterns of extreme temperature and precipitation associated with greenhouse gas forcing dominate

Application of empirical orthogonal functions to evaluate ozone simulations with regional and global models

Empirical orthogonal functions are used together with standard statistical metrics to evaluate the ability of models with different spatial resolutions to reproduce observed patterns of surface ozone (O3) in the eastern United States in the summer of 1995. We examine simulations with the regional Multiscale Air Quality Simulation Platform model (horizontal resolution of 36 km2) and the global GEOS-CHEM model (2° × 2.5° and 4° × 5°). As the model resolution coarsens, the ability to resolve local O3 maxima (O3 ≥ 90 ppbv) is compromised, but the spatial correlation improves. This result shows that synoptic-scale processes modulating O3 concentrations are easier to capture in models than processes occurring on smaller scales. Empirical orthogonal functions (EOFs) derived from the observed O3 fields reveal similar modes of variability when averaged onto the three model horizontal resolutions. The EOFs appear to represent (1) an east-west pattern associated with frontal passages, (2) a midwest-northeast pattern associated with migratory high-pressure systems, and (3) a southeast stagnation pattern linked to westward extension of the Bermuda High. All models capture the east-west and southeast EOFs, but the midwest-northeast EOF is misplaced in GEOS-CHEM. GEOS-CHEM captures the principal components of the observational EOFs when the model fields are projected onto these EOFs, implying that it can resolve the contribution of the EOFs to the observed variance. We conclude that coarse-resolution global models can successfully simulate the synoptic conditions leading to high-O3 episodes in the eastern United States