Recent laboratory studies suggest that secondary organic aerosol (SOA) formation rates are higher than assumed in current models. There is also evidence that SOA removal by dry and wet deposition ...occurs more efficiently than some current models suggest and that photolysis and heterogeneous oxidation may be important (but currently ignored) SOA sinks. Here, we have updated the global GEOS-Chem model to include this new information on formation (i.e., wall-corrected yields and emissions of semi-volatile and intermediate volatility organic compounds) and on removal processes (photolysis and heterogeneous oxidation). We compare simulated SOA from various model configurations against ground, aircraft and satellite measurements to assess the extent to which these improved representations of SOA formation and removal processes are consistent with observed characteristics of the SOA distribution. The updated model presents a more dynamic picture of the life cycle of atmospheric SOA, with production rates 3.9 times higher and sinks a factor of 3.6 more efficient than in the base model. In particular, the updated model predicts larger SOA concentrations in the boundary layer and lower concentrations in the upper troposphere, leading to better agreement with surface and aircraft measurements of organic aerosol compared to the base model. Our analysis thus suggests that the long-standing discrepancy in model predictions of the vertical SOA distribution can now be resolved, at least in part, by a stronger source and stronger sinks leading to a shorter lifetime. The predicted global SOA burden in the updated model is 0.88 Tg and the corresponding direct radiative effect at top of the atmosphere is −0.33 W m−2, which is comparable to recent model estimates constrained by observations. The updated model predicts a population-weighed global mean surface SOA concentration that is a factor of 2 higher than in the base model, suggesting the need for a reanalysis of the contribution of SOA to PM pollution-related human health effects. The potential importance of our estimates highlights the need for more extensive field and laboratory studies focused on characterizing organic aerosol removal mechanisms and rates.
An approximate formula for the UV Index (UVI) under cloud‐free, unpolluted, low surface albedo conditions is:
where μo is the cosine of the solar zenith angle and Ω is the total vertical ozone column ...(in Dobson Units, DU). The dependence on μo and Ω is based on a simple physical model of biologically weighted atmospheric transmission in the UV‐B and UV‐A spectral bands, with coefficients tuned to a detailed radiative transfer model, and is accurate to 10% or better over 0–60° and 200–400 DU. Other factors (clouds, haze, ground, etc.) mostly conserve this dependence and scale simply.
Air quality progress in the North American megacities of Los Angeles, New York, and Mexico City is reviewed, compared, and contrasted. Enormous progress made in North America over the last 5 decades ...provides a template for other megacities of the world, especially in developing countries, attempting to achieve rapid economic growth without compromising air quality. While the progress to date has been impressive, many challenges remain including the need to improve air quality while simultaneously mitigating climate change. The impact of pollutant emissions from megacities is felt long distances away from the local sources but no policy mechanisms currently exist to mitigate air quality impacts resulting from such pollution transport.
► Air quality progress in three North American megacities is reviewed. ► Enormous progress has been made over the last 5 decades. ► Further progress is required to meet health-based standards. ► North American progress can provide a template for developing megacities. ► A remaining challenge is to improve air quality while mitigating climate change.
Trifluoroacetic acid (TFA) is a breakdown product of several hydrochlorofluorocarbons (HCFC), regulated under the Montreal Protocol (MP), and hydrofluorocarbons (HFC) used mainly as refrigerants. ...Trifluoroacetic acid is (1) produced naturally and synthetically, (2) used in the chemical industry, and (3) a potential environmental breakdown product of a large number (>1 million) chemicals, including pharmaceuticals, pesticides, and polymers. The contribution of these chemicals to global amounts of TFA is uncertain, in contrast to that from HCFC and HFC regulated under the MP. TFA salts are stable in the environment and accumulate in terminal sinks such as playas, salt lakes, and oceans, where the only process for loss of water is evaporation. Total contribution to existing amounts of TFA in the oceans as a result of the continued use of HCFCs, HFCs, and hydrofluoroolefines (HFOs) up to 2050 is estimated to be a small fraction (<7.5%) of the approximately 0.2 μg acid equivalents/L estimated to be present at the start of the millennium. As an acid or as a salt TFA is low to moderately toxic to a range of organisms. Based on current projections of future use of HCFCs and HFCs, the amount of TFA formed in the troposphere from substances regulated under the MP is too small to be a risk to the health of humans and environment. However, the formation of TFA derived from degradation of HCFC and HFC warrants continued attention, in part because of a long environmental lifetime and due many other potential but highly uncertain sources.
The formulations of tropospheric gas-phase chemistry ("mechanisms") used in the regional-scale chemistry-transport models participating in the Air Quality Modelling Evaluation International ...Initiative (AQMEII) Phase 2 are intercompared by the means of box model studies. Simulations were conducted under idealized meteorological conditions, and the results are representative of mean boundary layer concentrations. Three sets of meteorological conditions - winter, spring/autumn and summer - were used to capture the annual variability, similar to the 3-D model simulations in AQMEII Phase 2. We also employed the same emissions input data used in the 3-D model intercomparison, and sample from these datasets employing different strategies to evaluate mechanism performance under a realistic range of pollution conditions. Box model simulations using the different mechanisms are conducted with tight constraints on all relevant processes and boundary conditions (photolysis, temperature, entrainment, etc.) to ensure that differences in predicted concentrations of pollutants can be attributed to differences in the formulation of gas-phase chemistry. The results are then compared with each other (but not to measurements), leading to an understanding of mechanism-specific biases compared to the multi-model mean. Our results allow us to quantify the uncertainty in predictions of a given compound in the 3-D simulations introduced by the choice of gas-phase mechanisms, to determine mechanism-specific biases under certain pollution conditions, and to identify (or rule out) the gas-phase mechanism as the cause of an observed discrepancy in 3-D model predictions. We find that the predictions of the median diurnal cycle of O3 over a set of emission conditions representing a network of station observations is within 4 ppbv (5%) across the different mechanisms. This variability is found to be very similar on both continents. There are considerably larger differences in predicted concentrations of NOx (up to plus or minus 25%), key radicals like OH (40%), HO2 (25%) and especially NO3 (>100%). Secondary substances like H2O2 (25%) or HNO3 (10%), as well as key volatile organic compounds like isoprene (>100%) or CH2O (20%) differ substantially as well. Calculation of an indicator of the chemical regime leads to up to 20% of simulations being classified differently by different mechanism, which would lead to different predictions of the most efficient emission reduction strategies. All these differences are despite identical meteorological boundary conditions, photolysis rates, as well as identical biogenic and inorganic anthropogenic emissions. Anthropogenic VOC emissions only vary in the way they are translated in mechanism-specific compounds, but are identical in the total emitted carbon mass and its spatial distribution. Our findings highlight that the choice of gas-phase mechanism is crucial in simulations for regulatory purposes, emission scenarios, as well as process studies that investigate other components like secondary formed aerosol components. We find that biogenic VOCs create considerable variability in mechanism predictions and suggest that these, together with nighttime chemistry should be areas of further mechanism improvement.
More than 3 decades after the discovery of the ozone hole, the
processes involved in its formation are believed to be understood in
great detail. Current state-of-the-art models can reproduce
the ...observed chemical composition in the springtime polar stratosphere,
especially regarding the quantification of halogen-catalysed ozone
loss.
However, we report here on a discrepancy between simulations and
observations during the less-well-studied period of the onset of
chlorine activation. During this period, which in the Antarctic is
between May and July, model simulations significantly overestimate
HCl, one of the key chemical species, inside the polar vortex during
polar night. This HCl discrepancy is also observed in the Arctic.
The discrepancy exists in different models to varying extents; here,
we discuss three independent ones, the Chemical Lagrangian
Model of the Stratosphere (CLaMS) as well as the Eulerian
models SD-WACCM (the specified dynamics version of the Whole Atmosphere
Community Climate Model) and TOMCAT/SLIMCAT. The HCl discrepancy points to
some unknown process in the formulation of stratospheric chemistry
that is currently not represented in the models. We characterise the HCl discrepancy in space and time for the
Lagrangian chemistry–transport model CLaMS, in which HCl in the
polar vortex core stays about constant from June to August in the
Antarctic, while the observations indicate a continuous HCl decrease
over this period. The somewhat smaller discrepancies in the Eulerian
models SD-WACCM and TOMCAT/SLIMCAT are also presented. Numerical
diffusion in the transport scheme of the Eulerian models is
identified to be a likely cause for the inter-model differences.
Although the missing process has not yet been identified, we
investigate different hypotheses on the basis of the characteristics
of the discrepancy.
An underestimated HCl uptake into the polar stratospheric cloud (PSC) particles that consist
mainly of H2O and HNO3 cannot explain it due to
the temperature correlation of the discrepancy. Also, a direct
photolysis of particulate HNO3 does not resolve the discrepancy
since it would also cause changes in chlorine chemistry in late
winter which are not observed.
The ionisation caused by galactic cosmic rays provides an additional
NOx and HOx source that can explain only about 20 %
of the discrepancy.
However, the model simulations show that a hypothetical
decomposition of particulate HNO3 by some other process not
dependent on the solar elevation, e.g. involving galactic cosmic
rays, may be a possible mechanism to resolve the HCl discrepancy.
Since the discrepancy reported here occurs during the beginning of
the chlorine activation period, where the ozone loss rates are small,
there is only a minor impact of about 2 % on the overall ozone
column loss over the course of Antarctic winter and spring.
The role of aqueous multiphase chemistry in the formation of secondary organic aerosol (SOA) remains difficult to quantify. We investigate it here by testing the rapid formation of moderate ...oxygen‐to‐carbon (O/C) SOA during a case study in Mexico City. A novel laboratory‐based glyoxal‐SOA mechanism is applied to the field data, and explains why less gas‐phase glyoxal mass is observed than predicted. Furthermore, we compare an explicit gas‐phase chemical mechanism for SOA formation from semi‐ and intermediate‐volatility organic compounds (S/IVOCs) with empirical parameterizations of S/IVOC aging. The mechanism representing our current understanding of chemical kinetics of S/IVOC oxidation combined with traditional SOA sources and mixing of background SOA underestimates the observed O/C by a factor of two at noon. Inclusion of glyoxal‐SOA with O/C of 1.5 brings O/C predictions within measurement uncertainty, suggesting that field observations can be reconciled on reasonable time scales using laboratory‐based empirical relationships for aqueous chemistry.
Key Points
S/I VOC oxidation forms low O/C SOA
A high O/C SOA source is needed
Aqueous aerosol multiphase chemistry helps improve O/C agreement
The formation of chemical oxidants, particularly ozone, in Mexico City were studied using a newly developed regional chemical/dynamical model (WRF-Chem). The magnitude and timing of simulated diurnal ...cycles of ozone (O
3), carbon monoxide (CO) and nitrogen oxides (NO
x
)
, and the maximum and minimum O
3 concentrations are generally consistent with surface measurements. Our analysis shows that the strong diurnal cycle in O
3 is mainly attributable to photochemical variations, while diurnal cycles of CO and NO
x
mainly result from variations of emissions and boundary layer height. In a sensitivity study, oxidation reactions of aromatic hydrocarbons (HCs) and alkenes yield highest peak O
3 production rates (20 and 18
ppbv
h
−1, respectively). Alkene oxidations, which are generally faster, dominate in early morning. By late morning, alkene concentrations drop, and oxidations of aromatics dominate, with lesser contributions from alkanes and CO. The sensitivity of O
3 concentrations to NO
x
and HC emissions was assessed. Our results show that daytime O
3 production is HC-limited in the Mexico City metropolitan area, so that increases in HC emissions increase O
3 chemical production, while increases in NO
x
emissions decrease O
3 concentrations. However, increases in both NO
x
and HC emissions yield even greater O
3 increases than increases in HCs alone. Uncertainties in HC emissions estimates give large uncertainties in calculated daytime O
3, while NO
x
emissions uncertainties are less influential. However, NO
x
emissions are important in controlling O
3 at night.
We present here a fully coupled global aerosol and chemistry model for the troposphere. The model is used to assess the interactions between aerosols and chemical oxidants in the troposphere, ...including (1) the conversion from gas‐phase oxidants into the condensed phase during the formation of aerosols, (2) the heterogeneous reactions occurring on the surface of aerosols, and (3) the effect of aerosols on ultraviolet radiation and photolysis rates. The present study uses the global three‐dimensional chemical/transport model, Model for Ozone and Related Chemical Tracers, version 2 (MOZART‐2), in which aerosols are coupled with the model. The model accounts for the presence of sulfate, soot, primary organic carbon, ammonium nitrate, secondary organic carbon, sea salt, and mineral dust particles. The simulated global distributions of the aerosols are analyzed and evaluated using satellite measurements (Moderate‐Resolution Imaging Spectroradiometer (MODIS)) and surface measurements. The results suggest that in northern continental regions the tropospheric aerosol loading is highest in Europe, North America, and east Asia. Sulfate, organic carbon, black carbon, and ammonium nitrate are major contributions for the high aerosol loading in these regions. Aerosol loading is also high in the Amazon and in Africa. In these areas the aerosols consist primarily of organic carbon and black carbon. Over the southern high‐latitude ocean (around 60°S), high concentrations of sea‐salt aerosol are predicted. The concentration of mineral dust is highest over the Sahara and, as a result of transport, spread out into adjacent regions. The model and MODIS show similar geographical distributions of aerosol particles. However, the model overestimates the sulfate and carbonaceous aerosol in the eastern United States, Europe, and east Asia. In the region where aerosol loading is high, aerosols have important impacts on tropospheric ozone and other oxidants. The model suggests that heterogeneous reactions of HO2 and CH2O on sulfate have an important impact on HOx (OH + HO2) concentrations, while the heterogeneous reaction of O3 on soot has a minor effect on O3 concentrations in the lower troposphere. The heterogeneous reactions on dust have very important impacts on HOx and O3 in the region of dust mobilization, where the reduction of HOx and O3 concentrations can reach a maximum of 30% and 20%, respectively, over the Sahara desert. Dust, organic carbon, black carbon, and sulfate aerosols have important impacts on photolysis rates. For example, the photodissociation frequencies of ozone and nitrogen dioxide are reduced by 20% at the surface in the Sahara, in the Amazon, and in eastern Asia, leading to 5–20% reduction in the concentration of HOx and to a few percent change in the O3 abundance in these regions.
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