China PARTICUOLOGY

Liquid and solid particles in polar stratospheric clouds (PSCs) have been known to play a crucial role in the chemical loss of stratospheric ozone over the Antarctic and Arctic regions in late winter and early spring. The stratospheric aerosol and cloud particles provide the sites where fast heterogeneous chemical reactions convert inactive halogen reservoir species into potential ozone destroying radicals. The sedimentation of nitric acid-containing PSC particles irreversibly removes HNO₃ gas (denitrification) from the lower stratosphere, which slows the return of chlorine to its inactive forms, resulting in more severe stratospheric ozone destruction. Although these clouds have been investigated extensively during the past decade using in situ field observation, laboratory experiment and modeling studies, the detailed microphysics processes under cold stratospheric conditions are still uncertain. This paper reviews the recent advances in our understanding of PSCs.

The status of current knowledge on size-dependent aerosol removal by dry and wet processes, including dry deposition and impaction and nucleation scavenging, is reviewed. The largest discrepancies between theoretical estimations and measurement data on dry deposition and below-cloud scavenging are for submicron particles. Early dry deposition models, which developed based on chamber and wind tunnel measurements, tended to underestimate dry deposition velocity (V_d) for submicron particles by around one order of magnitude compared to recent field measurements. Recently developed models are able to predict reasonable V_d values for submicron particles but shift unrealistically the predicted minimum V_d to larger particle sizes. Theoretical studies of impaction scavenging of aerosol particles by falling liquid drops also substantially underestimate the scavenging coefficients for submicron particles. Empirical formulas based on field measurements can serve as an alternative to the theoretical scavenging models. Future development of size-resolved impaction scavenging models needs to include more precipitation properties (e.g., droplet surface area) and to be evaluated by detailed cloud microphysical models and available measurements. Several recently developed nucleation scavenging parameterizations for in-cloud removal of interstitial aerosol give comparable results when evaluated against parcel models; however, they need to be verified once suitable field measurements are available. More theoretical and field studies are also needed in order to better understand the role of organic aerosols in the nucleation scavenging process.

A review is presented on the distribution and characterization of polycyclic aromatic hydrocarbons (PAHs) and their derivatives, including nitro-PAHs and hydro-PAHs, on atmospheric particulates of East Asia. Generally, PAH compounds with two or three aromatic rings are released mainly into the gas phase, while those containing three or more aromatic rings are associated with particulate matter (PM) emission. Particle-associated PAHs are primarily adsorbed on fine particles, and little associated with coarse particles. Investigation into the concentration level of PAHs in different areas can serve not only to reflect the pollutant status and sources but also to lead to the formulation of control strategies. The results of the present study show that China has more severe PAH pollution than such East Asian countries as Japan and Korea.

Two common surface-dust emission schemes using critical wind speed and friction velocity were compared with the regional climate model RegCM3 in East Asia. In the comparison, transport of mineral dust and its distribution were simulated from March to April, 2001. Simulation results were also compared with TOMS aerosol index, showing that obvious differences exist in dust emission quantity and its column burden simulated by the dust emission schemes of friction velocity and wind speed criteria. The results obtained by the wind speed criterion are higher than that by friction velocity, bringing forth the problem whether or not the dust emission scheme matches the model. The obvious difference in the two schemes also explains the uncertainty of simulating mineral dust aerosol by modeling.

Traffic-generated fugitive dust is a source of urban atmospheric particulate pollution in Beijing. This paper introduces the resuspension method, recommended by the US EPA in AP-42 documents, for collecting Beijing road-surface dust. Analysis shows a single-peak distribution in the number size distribution and a double-peak mode for mass size distribution of the road surface dust. The median diameter of the mass concentration distribution of the road dust on a high-grade road was higher than that on a low-grade road. The ratio of PM_2.5 to PM₁₀ was consistent with that obtained in a similar study for Hong Kong. For the two selected road samples, the average relative deviation of the size distribution was 10.9% and 11.9%. All results indicate that the method introduced in this paper can effectively determine the size distribution of fugitive dust from traffic.

Fingerprints and source profiles of fine and coarse sands that originate from Central Inner Mongolia during Asian continental sandstorms (ACS) can be used to identify the origin of Asian sands and to trace them as they travel downwind. Soil samples collected at various land surfaces in Central Inner Mongolia were resuspended using a dry powder atomizer in an enclosure chamber. The resuspended sands were then sampled by two dichotomous samplers situated at the bottom of the enclosure chamber for fine (PM_2.5) and coarse (PM_2.5-10) sands, respectively. The chemical composition of sands, including water-soluble ionic species, metallic contents, and carbonaceous contents, were further analyzed. Results from resuspension tests indicated that the soils contained considerably more coarse particles than fine. Moreover, Mg, K, Al, and Fe in coarse sand had strong correlations with each other. The ratio of Mg, K, Fe (or Al) to Al (or Fe) and OC/EC in the coarse sands can be used as the fingerprints of Asian sands originating from Central Inner Mongolia.

PM_1.0 (fine particles, with diameter < 1 μm), PM_2.5 (fine particles, with diameter < 2.5 μm) and PM₁₀ (coarse particles, with diameter < 10 μm) were measured at 24-hour intervals near a high-traffic road in Hong Kong, from October 2004 to September 2005. Mass concentrations were determined for the three particle fractions, averaging for PM_1.0, PM_2.5 and PM₁₀, respectively, 44.5±18.4, 55.4±25.5 and 81.3±37.7 μg·m^-3. PM_2.5 was 3.7 times the U.S. EPA's annual NAAQS of 15 μg·m^-3. Overall, PM_1.0 accounted for 44 to 69% (average 57%) of PM₁₀, while PM_2.5 accounted for 58 to 82% (average 71%) in this study. The particulate masses showed obvious seasonal patterns with high concentrations in cold seasons and low in warm seasons, especially high concentrations of PM_2.5-10 during the cold seasons. Diurnal variations of mass concentrations of PM_2.5 were determined during July, showing two major peaks in the morning and afternoon rush hours.

The objective of this study was to characterize the elemental carbon and organic carbon (EC and OC, respectively) content of aerosol particles (PM_2.5) collected at Tongliao, a site in the Horqin Sandland of northeastern China. During spring 2005, the PM_2.5 mass concentration was 126±71 μg·m^-3, with higher dust concentrations during five dust storms than on non-dusty days (255±77 vs. 106±44 μg·m^-3).The average OC and EC concentrations in PM_2.5 determined by a thermal/optical reflectance method were 15.7±7.3 μg·m^-3 and 3.3±1.7 μg·m^-3, respectively, and carbonaceous aerosol accounted for 9.9% of the PM_2.5 mass during dust storms compared to 21.7% on normal days. The average ratios of OC to EC during dust storms were similar to those on non-dusty days, and the correlation coefficient between OC and EC was high, 0.86. The high OC/EC ratios, the distributions of eight carbon fractions, and the strong relationship between K with OC and EC indicate that rural biomass burning was the dominant contributor to the regional carbonaceous aerosol.

Different-sized aerosols were collected by an Andersen air sampler to observe the detailed morphology of the black carbon (BC) aerosols which were separated chemically from the other accompanying aerosols, using a Scanning Electron Microscope equipped with an Energy Dispersive X-ray Spectrometer (SEM-EDX). The results indicate that most BC aerosols are spherical particles of about 50 nm in diameter and with a homogeneous surface. Results also show that these particles aggregate with other aerosols or with themselves to form larger agglomerates in the micrometer range. The shape of these 50-nm BC spherical particles was found to be very similar to that of BC particles released from petroleum-powered vehicular internal combustion engines. These spherical BC particles were shown to be different from the previously reported fullerenes found using Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight Mass Spectrometry (MALDI-TOF-MS).

Based on daily visibility data obtained from 1980–2002 and air pollution index data from 2001–2004 in Xi'an, long-term variations and relationships for daily horizontal extinction coefficient and mass concentration of PM₁₀ have been evaluated. A decreasing trend was found in horizontal extinction coefficient during the past 23 years, with higher values observed in 1980s relative to 1990s, and the highest and lowest values in winter and summer, respectively. Significant correlation and similar seasonal variations existed between horizontal extinction coefficient and PM₁₀ concentration, suggesting the high influence of PM₁₀ to the visibility drop at a site in the Guanzhong Plain of central China during the past two decades.

Aerosol Optical Thickness (AOT), water vapor content and derived Angstrom exponent acquired by a CIMEL sun photometer in Beijing are analyzed. Monthly means computed from quality-assured daily means, seasonal trends and inter-annual variations are presented and discussed. Summer has the highest seasonal average AOT at 440 nm (τ_a440), Angstrom exponent (α_440–870) and water vapor content with the values 0.93, 1.34 and 3.0 cm, respectively. The second highest seasonal average τ_a440 appears in spring with the largest variation of α_440–870 and minimum α_440–870 0.99 due to the impact of coarse particles. The minimum seasonal average τ_a440 (0.44) and water vapor content (0.4 cm) appear in winter. The annual average τ_a440, α_440–870 and water vapor content for about 4-year observation period are 0.70, 1.19 and 1.4 cm, respectively. All monthly average Angstrom exponents are within 0.8–1.4, which indicates aerosol in Beijing is a very complex mixture of both fine- and coarse-mode particles (from anthropogenic influence and natural mineral dust).

Factors affecting tropospheric nitrate aerosol formation were investigated through a number of sensitivity studies using the thermodynamic equilibrium model ISORROPIA and the condensed gas-phase chemistry model. Results indicate that the equilibrium concentration of nitrate aerosol varies with solar zenith angle, air temperature, relative humidity, initial SO₂ concentration, initial NO₂ concentration and initial NH₃ concentration. NH₃ concentration and solar zenith angle play the most important roles, followed by relative humidity and NO₂ concentration. It seems that the relations between nitrate aerosol and these four factors, as well as air temperature, are strong and highly nonlinear due to the coupling effect. The influence of SO₂ concentration on nitrate aerosol is near-linear and weak.

The time series of indoor and outdoor particle number concentrations in a naturally ventilated residential building in Xi'an were tested simultaneously for 7 days in summer. The relationships between indoor and outdoor concentrations were examined and discussed, and linear regression analysis was employed to correlate the indoor and outdoor concentrations. The diurnal cycles of indoor and outdoor particle concentrations of different particle size ranges all showed positive correlations between indoor and outdoor number concentrations. The I/O ratios of number concentrations varied with the increase of particle size in the range of 0.89 (±0.19) to 0.99 (±0.15).

Indoor dustfall samples were collected in Pit 1 and Pit 2 of Emperor Qin's Terra-cotta Warriors and Horses Museum in Xi'an, Shaanxi Province, China. The morphological and elemental analysis for long-term (>10 years) and short-term (~0.5 year) dustfall particles and lacquer pieces on the faces of partly assembled warriors were performed with scanning electron microscopy with energy dispersive X-ray spectrometer (SEM/EDX). The results showed that the majority of dustfall samples appeared to be composed of mineral dust particles, fly ash, and biological particles with different elemental compositions. Most of the mineral particles had irregular shapes with smooth edges. Small floccules of calcium sulfate were more frequently observed in long-term dustfall samples than in short-term samples, implying a progressive chemical reaction between continuous dustfall particles and atmospheric sulfur dioxide. Crystals of calcium sulfate were also found near pits on both inner and outer surfaces of lacquer pieces, revealing an acid chemical reaction between sulfur dioxide and lacquer material as the cause of the formation of erosion pits and cracks on surface of lacquer pieces.

The Monsoon Asia Integrated Regional Study (MAIRS) is a new Earth System Science Partnership (ESSP) program aimed at the integrated study of environmental changes over monsoon Asian region. This paper briefly introduces MAIRS, its background and concept, scientific themes and objectives, data requirements and its information system, intensive observation experiment, and its linkage with ongoing international projects.