G. R. Carmichael

Transport of Asian air pollution to North America

Using observations from the Cheeka Peak Observatory in northwestern Washington State during March-April, 1997, we show that Asian anthropogenic emissions significantly impact the concentrations of a large number of atmospheric species in the air arriving to North America during spring. Isentropic back-trajectories can be used to identify possible times when this impact will be felt, however trajectories alone are not sufficient to indicate the presence of Asian pollutants. Detailed chemical and meteorological data from one of these periods (March 29th, 1997) indicates that the surface emissions were lifted into the free troposphere over Asia and then transported to North America in ∼6 days.

Size distributions and mixtures of dust and black carbon aerosol in Asian outflow: Physiochemistry and optical properties

[1] During Transport and Chemical Evolution over the Pacific (TRACE-P) and Asian Aerosol Characterization Experiment (ACE-Asia) we measured the dry size distribution of Asian aerosols, their state of mixing, and the optical properties of dust, black carbon (BC) and other aerosol constituents in combustion and/or dust plumes. Optical particle sizing in association with thermal heating extracted volatile components and resolved sizes for dust and refractory soot that usually dominated light absorption. BC was internally mixed with volatile aerosol in � 85% of accumulation mode particles and constituted � 5–15% of their mass. These optically effective sizes constrained the soot and dust size distributions and the imaginary part of the dust refractive index, k, to 0.0006 ± 0.0001. This implies a single-scatter albedo, v (550 nm), for dust ranging from 0.99+ for Dp <1 m mt o� 0.90 at Dp =1 0mm and a size-integrated campaign average near 0.97 ± 0.01. The typical mass scattering efficiency for the dust was � 0.3 m 2 g � 1 , and the mass absorption efficiency (MAE) was 0.009 m 2 g � 1 . Less dust south of 25� N and stronger biomass burning signatures resulted in lower values for v of � 0.82 in plumes aloft. Chemically inferred elemental carbon was moderately correlated with BC light absorption (R 2 = 0.40), while refractory soot volume between 0.1 and 0.5 mm was highly correlated (R 2 = 0.79) with absorption. However, both approaches yield an MAE for BC mixtures of � 7±2m 2 g � 1 and higher than calculated MAE values for BC of 5 m 2 g � 1 . The increase in the mass fraction of soot and BC in pollution aerosol in the presence of elevated dust appears to be due to uptake of the volatile components onto the coarse dust. This predictably lowered v for the accumulation mode from 0.84 in typical pollution to � 0.74 in high-dust events. A chemical transport model revealed good agreement between model and observed BC absorption for most of SE Asia and in biomass plumes but underestimated BC for combustion sources north of 25� N by a factor of � 3. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0350 Atmospheric Composition and Structure: Pressure, density, and temperature; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere— constituent transport and chemistry; KEYWORDS: dust, black carbon, absorption, single scatter albedo

Atmospheric brown clouds: Hemispherical and regional variations in long-range transport, absorption, and radiative forcing

polluted oceanic regions, the EC mass exceeds 0.5 m gm 3 , the OC mass exceeds 2 m gm 3 and sulfate mass exceeds 10 m gm 3 from the surface to 3 km. The brown clouds also have strong seasonal dependence. In the tropics the seasonal dependence is driven by pollution accumulating during the dry seasons, December to February in Northern Hemisphere tropics and June to August in Southern Hemisphere tropics. In the extratropics the pollution peaks during the summer. The brown cloud problem is not restricted to the tropical regions. Over the eastern half of US and western Europe the AODs exceeds 0.2 and absorption AODs exceed 0.02. Brown clouds also extend well into the western Pacific Ocean, the Indian Ocean reaching as far south as 60S and the eastern Atlantic Ocean. The largest total SO2 emission occurs over China and US, while SO2 emission per unit surface area is maximum over Germany and England. The largest total EC and OC emissions occur over China, but the largest OC emission per unit surface area occur over India. As a result, the maximum negative annual mean TOA direct forcing is over India and Germany. The surface annual-diurnal mean dimming over the regional hot spots is of the order of 10 W m 2 and 20 W m 2 over megacity hotpots.

Benchmarking stiff ODE solvers for atmospheric chemistry problems II: Rosenbrock solvers

Abstract In the numerical simulation of atmospheric transport-chemistry processes, a major task is the integration of the stiff systems of ordinary differential equations describing the chemical transformations. It is therefore of interest to systematically search for stiff solvers which can be identified as close to optimal for atmospheric applications. In this paper we continue our investigation from Sandu et al. (1996, CWI Report NM-R9603 and Report in Comput. Math., No. 85) and compare eight solvers on a set of seven box-models used in present day models. The focus is on Rosenbrock solvers. These turn out to be very well suited for our application when they are provided with highly efficient sparse matrix techniques to economize on the linear algebra. Two of the Rosenbrock solvers tested are from the literature, viz. rodas and Ros 4, and two are new and specially developed for air quality applications, viz. rodas 3 and ros 3.

Benchmarking stiff ode solvers for atmospheric chemistry problems-I. implicit vs explicit

Abstract In many applications of atmospheric transport-chemistry problems, a major task is the numerical integration of the stiff systems of ordinary differential equations describing the chemical transformations. This paper presents a comprehensive numerical comparison between five dedicated explicit and four implicit solvers for a set of seven benchmark problems from actual applications. The implicit solvers use sparse matrix techniques to economize on the numerical linear algebra overhead. As a result they are often more efficient than the dedicated explicit ones, particularly when approximately two or more figures of accuracy are required. In most test cases, sparse RODAs, a Rosenbrock solver, came out as most competitive in the 1% error region. Of the dedicated explicit solvers, TWOSTEP came out as best. When less than 1% accuracy is aimed at, this solver performs very efficiently for tropospheric gas-phase problems. However, like all other dedicated explicit solvers, it cannot efficiently deal with gas-liquid phase chemistry. The results presented may constitute a guide for atmospheric modelers to select a suitable integrator based on the type and dimension of their chemical mechanism and on the desired level of accuracy. Furthermore, we would like to consider this paper an open invitation for other groups to add new representative test problems to those described here and to benchmark their numerical algorithms in our standard computational environment.

Inorganic composition of fine particles in mixed mineral dust–pollution plumes observed from airborne measurements during ACE‐Asia

Chemical characteristics of inorganic water-soluble aerosol particles measured in large Asian springtime dust events during the Asian Pacific Regional Characterization Experiment (ACE-Asia) were investigated. Three specific flights (flights 6, 7, and 10) in the Yellow Sea boundary layer with high mineral dust concentrations mixed with pollutants from Asian urban centers are presented. Measurements during a similar campaign, Transport and Chemical Evolution over the Pacific (TRACE-P), in the same region suggested that fine-particle ammonium sulfate and nitrate salts, and potassium, apparently from biomass burning, are common particle ionic constituents in polluted air. Observations from the ACE campaign show similar characteristics and found that the main component of water-soluble mineral dust was Mg 2+ and Ca 2+ . Ion charge balances of measured fine and total aerosol suggest that a significant fraction of the Mg 2+ and Ca 2+ observed were in the form of carbonates. In polluted air mixed with dust that advected directly from large urban regions in roughly half a day to 1 day (flights 6 and 7), much of the fine-particle nitrate and sulfate (approximately 80%) was apparently associated with ammonium or potassium, the rest likely associated with mineral dust. Only air masses that spent 2-5 days over the Yellow Sea (flight 10) had clear evidence of Cl - depletion. Initial mass accommodation coefficients much less than 0.1 for uptake of SO 2 or HNO 3 by mineral dust in urban plumes containing fossil fuel and biomass-burning emissions could explain the observations. The data suggest an accommodation coefficient dependence on relative humidity.

Central American biomass burning smoke can increase tornado severity in the U.S.

Tornadoes in the Southeast and central U.S. are episodically accompanied by smoke from biomass burning in central America. Analysis of the 27 April 2011 historical tornado outbreak shows that adding smoke to an environment already conducive to severe thunderstorm development can increase the likelihood of significant tornado occurrence. Numerical experiments indicate that the presence of smoke during this event leads to optical thickening of shallow clouds while soot within the smoke enhances the capping inversion through radiation absorption. The smoke effects are consistent with measurements of clouds and radiation before and during the outbreak. These effects result in lower cloud bases and stronger low-level wind shear in the warm sector of the extratropical cyclone generating the outbreak, two indicators of higher probability of tornadogenesis and tornado intensity and longevity. These mechanisms may contribute to tornado modulation by aerosols, highlighting the need to consider aerosol feedbacks in numerical severe weather forecasting.

Inverse Modeling of Aerosol Dynamics Using Adjoints: Theoretical and Numerical Considerations

In this paper we develop the algorithmic tools needed for inverse modeling of aerosol dynamics. Continuous and discrete adjoints of the aerosol dynamic equation are derived, as well as sensitivity coefficients with respect to the coagulation kernel, the growth rate, and the emission and deposition coefficients. Numerical tests performed in the twin experiment framework for a single component model problem show that the initial distributions and the dynamic parameters can be recovered from time series of observations of particle size distributions.

Carbonaceous species in fine particles at the background sites in Korea between 1994 and 1999

Abstract Organic carbon (OC) and elemental carbon (EC) in fine particles (PM2.5) at two background sites, Kosan and Kangwha in Korea were measured during intensive field studies between 1994 and 1999. Fine particles were collected on pre-fired quartz filters in a low-volume sampler and analyzed using the selective thermal oxidation method with MnO2 catalyst. The OC and EC concentrations at Kosan located at western tip of Cheju Island in southern Korea are lower than those at Kangwha located at western coastal area in mid-Korean peninsula. Still, the OC concentrations at Kosan are generally higher than those at other background areas in Japan and USA. The EC concentrations at Kosan are lower than or comparable to those at other background areas. The total carbon (TC, sum of OC and EC) to EC ratio values at both sites were higher than those at other background areas in Japan and USA. At Kosan, the OC and EC concentrations when air parcels were from southern China were higher than those when air parcels were coming from northern China. However, at Kangwha, the differences were statistically not clear since most air parcels were from northern China. Except when air parcels were from the North Pacific during summer, the OC and EC concentrations are well correlated indicating that both OC and EC share the same emission/transport characteristics. From the gaseous hydrocarbon data and the OC and EC relationship, it was found that during summer local biogenic emissions of OC might be significant at Kosan.

Technical note: Coordination and harmonization of the multi-scale, multi-model activities HTAP2, AQMEII3, and MICS-Asia3: simulations, emission inventories, boundary conditions, and model output formats

We present an overview of the coordinated global numerical modelling experiments performed during 2012–2016 by the Task Force on Hemispheric Transport of Air Pollution (TF HTAP), the regional experiments by the Air Quality Model Evaluation International Initiative (AQMEII) over Europe and North America, and the Model Intercomparison Study for Asia (MICS-Asia). To improve model estimates of the impacts of intercontinental transport of air pollution on climate, ecosystems, and human health and to answer a set of policy-relevant questions, these three initiatives performed emission perturbation modelling experiments consistent across the global, hemispheric, and continental/regional scales. In all three initiatives, model results are extensively compared against monitoring data for a range of variables (meteorological, trace gas concentrations, and aerosol mass and composition) from different measurement platforms (ground measurements, vertical profiles, airborne measurements) collected from a number of sources. Approximately 10 to 25 modelling groups have contributed to each initiative, and model results have been managed centrally through three data hubs maintained by each initiative. Given the organizational complexity of bringing together these three initiatives to address a common set of policy-relevant questions, this publication provides the motivation for the modelling activity, the rationale for specific choices made in the model experiments, and an overview of the organizational structures for both the modelling and the measurements used and analysed in a number of modelling studies in this special issue.