Gregory R. Carmichael

Role of mineral aerosol as a reactive surface in the global troposphere

A global three-dimensional model of the troposphere is used to simulate the sources, abundances, and sinks of mineral aerosol and the species involved in the photochemical oxidant, nitrogen, and sulfur cycles. Although the calculated heterogeneous removal rates on mineral aerosol are highly uncertain, mainly due to poorly known heterogeneous reaction rates, the reaction of SO2 on calcium-rich mineral aerosol is likely to play an important role downwind of arid source regions. This is especially important for regions in Asia, which are important and increasing emitters of sulfur compounds. Our results indicate that the assumption that sulfate aerosol follows an accumulation mode size distribution, is particularly in Asia likely to overestimate the sulfate aerosol climate-cooling effect. An even larger fraction of gas phase nitric acid may be associated with and neutralized by mineral aerosol. Interactions of N2O5, O3, and HO2-radicals with dust are calculated to affect the photochemical oxidant cycle, causing ozone decreases up to 10% in and nearby the dust source areas. Comparison of these results with limited available measurements indicates that the proposed reactions can indeed take place, although due to a lack of measurements a rigorous evaluation is not possible at this time.

ACE-ASIA Regional Climatic and Atmospheric Chemical Effects of Asian Dust and Pollution

Although continental-scale plumes of Asian dust and pollution reduce the amount of solar radiation reaching the earths surface and perturb the chemistry of the atmosphere, our ability to quantify these effects has been limited by a lack of critical observations, particularly of layers above the surface. Comprehensive surface, airborne, shipboard, and satellite measurements of Asian aerosol chemical composition, size, optical properties, and radiative impacts were performed during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) study. Measurements within a massive Chinese dust storm at numerous widely spaced sampling locations revealed the highly complex structure of the atmosphere, in which layers of dust, urban pollution, and biomass-burning smoke may be transported long distances as distinct entities or mixed together. The data allow a first-time assessment of the regional climatic and atmospheric chemical effects of a continental-scale mixture of dust and pollution. Our results show that radiative flux reductions during such episodes are sufficient to cause regional climate change.

A second generation model for regional-scale transport/chemistry/deposition

The regional-scale transport, chemistry and deposition of acidifying compounds, photochemical oxidants, and their precursors are analyzed using a second-generation Eulerian model. The important atmospheric processes are incorporated using chemical, dynamical and thermodynamical parameterizations having sufficient detail to accommodate boundary layer-free troposphere exchange in cloudy and cloud-free environments, and in-cloud and below-cloud wet removal and chemistry. Forty-one species are considered, many of which are also present in the liquid-drop phases. In the regional scale transport, the advected species are NO, NO2, SO2, SO−24, O3, HNO3, NH3, PAN, H2O2, HCHO, alkanes, C2H4, other olefins, aromatics, RCHO, ROOH, HNO2, RONO2 and RO2NO2. The model capabilities are illustrated by showing simulations in which non-precipitating clouds are present to absorb gas-phase species, chemically alter these, and then release them to the atmosphere.

Anthropogenic NOx emissions in Asia in the period 1990-2020

Nitrogen oxides emissions in Asia during the period 1990–2020 due to anthropogenic activity are presented. These estimates are based on the RAINS-ASIA methodology (Foell et al., 1995, Acid Rain and Emission Reduction in Asia, World Bank), which includes a dynamic model for energy forecasts, and information on 6 energy sectors and 9 fuel types. The energy forecasts are combined with process emission factors to yield NOx emission estimates at the country level, the regional level, and on a 1 degree by 1 degree grid. In 1990 the total NOx emissions are estimated to be ∼19 Tg NO2, with China (43%), India (18%) and Japan (13%) accounting for 75% of the total. Emissions by fuel are dominated by burning of hard coal and emissions by economic activity are dominated by the power, transport, and industrial sectors. These new estimates of NOx emissions are compared with those published by Hameed and Dignon (1988, Atmospheric Environment 22, 441–449) and Akimoto and Narita (1994, Atmospheric Environment 28, 213–225). Future emissions under a no-further-control scenario are also presented. During the period 1990–2020 the NOx emissions increase by 350%, to ∼86 Tg NO2. The increase in NOx emissions by sector and end-use varies between countries, but in all countries this increase is strongest in the power and transport sectors. These results highlight the dynamic nature of energy use in Asia, and the need to take the rapid growth in NOx emissions in Asia into account in studies of air pollution and atmospheric chemistry.

Impacts of biomass burning on tropospheric CO, NOx, and O3

This study utilizes the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory three-dimensional global chemical transport model to quantify the impacts of biomass burning on tropospheric concentrations of carbon monoxide (CO), nitrogen oxides (NOx), and ozone (O3). We construct updated global sources that emit 748 Tg CO/yr and 7.8 Tg N/yr in the surface layer. Both sources include six types of biomass: forest, savanna, fuelwood, agricultural residues, domestic crop residues (burned in the home for cooking and/or heating), and dried animal waste. Timing for the burning of forest, savanna, and agricultural residues is based upon regional cultural use of fire, vegetation type, local climate, and information gathered from satellite observations, while emissions from the burning of fuelwood, domestic crop residues, and dried animal waste are constant throughout the year. Based on agreement with observations, particularly of CO, we conclude that the collective uncertainty in our biomass burning sources is much less than the factor of two suggested by previous estimates of biomass burned in the tropics annually. Overall, biomass burning is a major source of CO and NOx in the northern high latitudes during the summer and fall and in the tropics throughout most of the year. While it contributes more than 50% of both the NOx and CO in the boundary layer over major source regions, it has a much larger global impact on the CO distribution in comparison to either NOx or O3, contributing 15 to 30% of the entire tropospheric CO background. The only significant biomass burning contribution to NOx at 500 mbar, due to the short lifetime of NOx in the lower troposphere, is a plume occurring July through October in the Southern Hemisphere subtropical free troposphere, stretching from South America to the western Pacific. The largest impacts on O3 are limited to those regions where NOx impacts are large as well. Near the surface, biomass burning indirectly contributes less than half of the total O3 concentrations over major tropical source regions, up to 15% throughout the year in the tropics, and 10 to 20% throughout the Southern Hemisphere during September through November. At 500 mbar, the largest contribution to O3 (20–30%) is correlated with the NOx plume during July through November. Biomass burning contributes less than 15% of either NOx or O3 in the upper troposphere.

The kinetic preprocessor KPP*/a software environment for solving chemical kinetics

Abstract The kinetic preprocessor (KPP) is a software tool that assists the computer simulation of chemical kinetic systems. The concentrations of a chemical system evolve in time according to the differential law of mass action kinetics. A computer simulation requires the implementation of the differential system and its numerical integration in time. KPP translates a specification of the chemical mechanism into fortran or c simulation code that implement the concentration time derivative function and its Jacobian, together with a suitable numerical integration scheme. Sparsity in Jacobian is carefully exploited in order to obtain computational efficiency. KPP incorporates a library with several widely used atmospheric chemistry mechanisms and users can add their own chemical mechanisms to the library. KPP also includes a comprehensive suite of stiff numerical integrators. The KPP development environment is designed in a modular fashion and allows for rapid prototyping of new chemical kinetic schemes as well as new numerical integration methods.

The episodic nature of air pollution transport from Asia to North America

We employ the Geophysical Fluid Dynamics Laboratory (GFDL) global chemistry transport model (GCTM) to address the episodic nature of trans-Pacific pollution. The strongest Asian CO episodes over North America (NA), occurring most frequently between February and May, are often associated with disturbances that entrain pollution over eastern Asia and amplify over the western Pacific Ocean. Using 55 ppb of Asian CO as a criterion for major events, we find that during a typical year three to five Asian pollution events analogous to those observed by Jaffe et al. [1999] are expected in the boundary layer all along the U.S. West Coast between February and May. In contrast to CO, Asia currently has a small impact on the magnitude and variability of background ozone arriving over NA from the west. Direct and indirect Asian contributions to episodic O3 events over the western United States are generally in the 3–10 ppbv range. The two largest total O3 events (>60 ppbv), while having trajectories which pass over Asia, show negligible impact from Asian emissions. However, this may change. A future emission scenario in which Asian NOx emissions increase by a factor of 4 from those in 1990 produces late spring ozone episodes at the surface of California with Asian contributions reaching 40 ppb. Such episodic contributions are certain to exacerbate local NA pollution events, especially in elevated areas more frequently exposed to free tropospheric and more heavily Asian-influenced air.

The Role of Mineral Aerosol in Tropospheric Chemistry in East Asia—A Model Study

A detailed gas-phase chemistry mechanism is combined with dust surface uptake processes to explore possible impacts of mineral dust on tropospheric chemistry. The formations of sulfate and nitrate on dust are studied along with the dust effects on the photochemical oxidant cycle for the long-range-transported particles with a diameter of 0.1‐40 mm. The results show that mineral dust may influence tropospheric sulfate, nitrate, and O 3 formation by affecting trace gas concentrations and the tropospheric oxidation capacity through surface processes. The postulated heterogeneous mechanism provides a plausible interpretation for the observed high nitrate and sulfate on dust and the anticorrelation between O 3 and dust in East Asia. The presence of dust results in decreases in the concentrations of SO 2 (10%‐53%), (16%‐100%, defined as NO 3 1 N2O5 1 HNO3), HxOy (11%‐59%, p NOy defined as OH1 HO2 1 H2O2), and O3 (11%‐40%) under model conditions representative of spring dust storms in East Asia. The decrease in solar actinic flux and the surface uptake of O 3 and its precursors contribute to the total O 3 decrease for the conditions studied. Nitrate and sulfate, 0.9‐2.1 and 0.3‐10 m gm 23, respectively, are formed on dust particles, mostly in the size range of 1.5‐10 mm. The magnitude of the dust effect strongly depends on the preexisting dust surfaces, the initial conditions, and the selection of model parameters associated with surface uptake processes. The impact of dust reactions on O 3 reduction is highly sensitive to the uptake coefficient and to the possible renoxification from the surface reaction of HNO 3 on dust.

The STEM-II regional scale acid deposition and photochemical oxidant model—I. An overview of model development and applications

Abstract The STEM-II pollutant transport/transformation/removal model has been enhanced to include scavenging by clouds and precipitation. The Advanced Scavenging Model (ASM) and Reactive Scavenging Model (RSM) have been coupled to STEM-II to enable comprehensive simulation and diagnosis analysis of field experiments designed to study acid deposition. In this paper, the STEM-II/ASM/RSM model is described. The coupled model is being used to analyze the PRECP-I field studies on an urban/suburban scale (Philadelphia), mesoscale (lower Ohio River Valley), and regional scale (eastern U.S.). The time period selected for detailed simulations and analysis is 29 April–5 May 1985. Subsequent papers will detail the results of these simulations.

Reactive nitrogen chemistry in aerosol water as a source of sulfate during haze events in China.

Multiphase chemistry of NO2 and alkaline matter in aerosol water explains rapid sulfate formation and severe haze in China. Fine-particle pollution associated with winter haze threatens the health of more than 400 million people in the North China Plain. Sulfate is a major component of fine haze particles. Record sulfate concentrations of up to ~300 μg m−3 were observed during the January 2013 winter haze event in Beijing. State-of-the-art air quality models that rely on sulfate production mechanisms requiring photochemical oxidants cannot predict these high levels because of the weak photochemistry activity during haze events. We find that the missing source of sulfate and particulate matter can be explained by reactive nitrogen chemistry in aerosol water. The aerosol water serves as a reactor, where the alkaline aerosol components trap SO2, which is oxidized by NO2 to form sulfate, whereby high reaction rates are sustained by the high neutralizing capacity of the atmosphere in northern China. This mechanism is self-amplifying because higher aerosol mass concentration corresponds to higher aerosol water content, leading to faster sulfate production and more severe haze pollution.