Weekday/Weekend Differences in OH Reactivity with VOCs and CO in Baltimore,Maryland

ABSTRACT

This study compared the first-order frequencies for OH associated with volatile organic compounds (VOCs) and CO (hereafter called OH reactivity with VOCs or CO), the product of the VOC or CO concentration, and their respective k_OH value, on an average weekday with that on an average weekend day at a core urban site in Baltimore, MD. The average daytime concentrations were calculated for each of the 55 available Photochemical Assessment Monitoring Station (PAMS) VOCs using data from the Baltimore site. The data were sorted in descending order to highlight the important species based on concentration. The OH reactivity with VOCs was sorted in descending order to identify the important species based on the magnitude of the OH reactivity. A similar process was followed for the OH reactivity with CO. The contribution of the significant species to the weekday/weekend difference in OH reactivity was examined.

The OH reactivity with C₅H₈ was the largest among the OH reactivity with the PAMS' VOCs and was the same on the weekday and weekend. The weekday/weekend difference in OH reactivity with VOCs was entirely due to differences in concentrations of the anthropogenic VOCs. The OH reactivity with VOCs was 11% larger on the weekday. When OH reactivity with CO was included, the OH reactivity was 13% larger on the weekday.     

Oxygenated volatile organic compounds (OVOCs) at an urban background site in Zürich (Europe): Seasonal variation and source allocation

Twenty-one oxygenated volatile organic compounds (OVOCs) were measured in four seasonal campaigns at an urban background site in Zürich (Switzerland) with a newly developed double adsorbent sampling unit coupled to a gas chromatograph–mass spectrometer (GC–MS). In addition, selected non-methane hydrocarbons (NMHCs) were measured, as well as formaldehyde in the summer and winter campaign. The most abundant compound measured in all seasons was ethanol, with peak values of more than 60 ppb. Its seasonal variation with a lower mean value in summer compared to that in winter implied mostly anthropogenic sources. In contrast, compounds with additional biogenic sources, or compounds known to be produced in the troposphere by oxidation processes, had seasonal maxima in summer (e.g. methanol, acetone, formaldehyde, methacrolein and 2-butenone (methyl vinyl ketone, MVK)).For the OVOCs it was estimated that local sources contributed 40% and 49% to the mixing ratios of the measured compounds in summer and in winter, respectively. Combustion was estimated to contribute 75% to these local sources independent of the season. About 50% of both the OVOC and NMHC levels in Zürich could be explained by the regional background, which included regional biogenic and anthropogenic sources in addition to secondary production. Industrial sources were identified for acetone, butanone (methyl ethyl ketone, MEK), n-propanol, iso-propanol, n-butanol, ethyl acetate and butyl acetate.     

Benzene, toluene, ethyl benzene and xylenes in ambient air and Pinus sylvestris L. needles: a comparative study between Belgium, Hungary and Latvia

Concentrations of benzene, toluene, ethyl benzene and xylenes (BTEX) in ambient air and in 1 yr old Pinus sylvestris pine needles were monitored along a busy road, petrol station and rural area of Belgium, Hungary and Latvia in a 1 yr period. To test P. sylvestris as a possible biomonitor for the BTEX concentrations, samples were taken in the four seasons. As the distribution of data was not normal, the level of pollution on different sites and seasons was compared and evaluated by non-parametric tests. The measured air concentrations did not differ significantly from one season to another throughout the year. There were, however, differences between sampling places. The C₂-alkylbenzene and toluene concentrations in the needles were similar in the autumn/winter and spring/summer periods but a significant decrease in their concentration was observed in every place between winter and spring. This effect was less obvious for toluene.     

Characteristics of water-soluble inorganic ions in PM2.5 and PM2.5–10 in the coastal urban agglomeration along the Western Taiwan Strait Region,China

PM_2.5 and PM_2.5–10 aerosol samples were collected in four seasons during November 2010, January, April, and August 2011 at 13 urban/suburban sites and one background site in Western Taiwan Straits Region (WTSR), which is the coastal area with rapid urbanization, high population density, and deteriorating air quality. The 10 days average PM_2.5 concentrations were 92.92, 51.96, 74.48, and 89.69 μg/m³ in spring, summer, autumn, and winter, respectively, exceeding the Chinese ambient air quality standard for annual average value of PM_2.5 (grade II, 35 μg/m³). Temporal distribution of water-soluble inorganic ions (WSIIs) in PM_2.5 was coincident with PM_2.5 mass concentrations, showing highest in spring, lowest in summer, and middle in autumn and winter. WSIIs took considerable proportion (42.2～50.1 %) in PM_2.5 and PM_2.5–10. Generally, urban/suburban sites had obviously suffered severer pollution of fine particles compared with the background site. The WSIIs concentrations and characteristics were closely related to the local anthropogenic activities and natural environment, urban sites in cities with higher urbanization level, or sites with weaker diffuse condition suffered severer WSIIs pollution. Fossil fuel combustion, traffic emissions, crustal/soil dust, municipal constructions, and sea salt and biomass burnings were the major potential sources of WSIIs in PM_2.5 in WTSR according to the result of principal component analysis.     

Seasonal dependence of the oxidation capacity of the city of Santiago de Chile

The oxidation capacity of the highly polluted urban area of Santiago de Chile has been evaluated during a winter measurement campaign from May 25 to June 07, 2005, with the results compared and contrasted with those previously evaluated during a summer campaign from March 8 to 20, 2005. The OH radical budget was evaluated in both campaigns employing a simple quasi-photostationary state model (PSS) constrained with simultaneous measurements of HONO, HCHO, O₃, NO, NO₂, j(O¹D), j(NO₂), 13 alkenes and meteorological parameters. In addition, a zero dimensional photochemical box model based on the Master Chemical Mechanism (MCMv3.1) has been used for the analysis of the radical budgets and concentrations of OH, HO₂ and RO₂. Besides the above parameters, the MCM model has been constrained by the measured CO and other volatile organic compounds (VOCs) including alkanes and aromatics. Total production and destruction rates of OH and HO₂ in winter are about two times lower than that during summer. Simulated OH levels by both PSS and MCM models are similar during the daytime for both winter and summer indicating that the primary OH sources and sinks included in the simple PSS model are predominant. On a 24 h basis, HONO photolysis was shown to be the most important primary OH radical source comprising 81% and 52% of the OH initiation rate during winter and summer, respectively followed by alkene ozonolysis (12.5% and 29%), photolysis of HCHO (6.1% and 15%), and photolysis of O₃ (<1% and 4%), respectively. During both winter and summer, there was a balance between the OH secondary production (HO₂ + NO) and destruction (OH + VOCs) showing that initiation sources of RO₂ and HO₂ are no net OH initiation sources. This result was found to be fulfilled also for all other studies investigated. Seasonal impacts on the radical budgets are also discussed.     

Seasonal changes in the chemical composition and reactivity of dissolved organic matter at the land-ocean interface of a subtropical river

Dissolved organic matter (DOM) is a critical component in aquatic ecosystems, yet its seasonal variability and reactivity remain not well constrained. These were investigated at the land-ocean interface of a subtropical river (Minjiang River, SE China), using absorption and fluorescence spectroscopy. The annual export flux of dissolved organic carbon (DOC) from the Minjiang River (5.48 × 10¹⁰ g year^?1) was highest among the rivers adjacent to the Taiwan Strait, with 72% occurring in spring and summer. The freshwater absorption coefficient a₂₈₀, DOC-specific UV absorbance SUVA₂₅₄ and humification index HIX were higher, while the spectral slope S_275–295 and biological index BIX were lower in summer than in winter. This suggests intensified export of terrestrial aromatic and high molecular weight constituents in the rainy summer season. Six fluorescent components were identified from 428 samples, including humic-like C1–C3, tryptophan-like C4 and C6, and tyrosine-like C5. The freshwater levels of four components (C1, C2, C4, and C6) were lower while that of C5 was higher in the wet season than in the dry season, suggesting contrasting seasonal variations of different constituents. Laboratory experiments were performed to assess the effects of photochemical and microbial degradation on DOM. Photo-degradation removed chromophoric and fluorescent DOM (CDOM and FDOM) effectively, which was stronger (i) for high molecular weight/humic constituents and (ii) during summer under higher solar radiation. Microbial degradation under laboratory controlled conditions generally showed little effect on DOC, and had smaller impact on CDOM and FDOM in winter than in summer. Overall, this study showed notable seasonal changes in the chemical composition and reactivity of DOM at the land-ocean interface, and demonstrated the significant effects of photo-degradation.

     

Elemental Uptake by Seaweed,Plocamium corallorhirza Along the KwaZulu-Natal Coast of Indian Ocean,South Africa

This study reports the elemental uptake by Plocamium corallorhiza, a Rhodophyta class of coralline alga grown richly along KwaZulu-Natal coastline. The uptake of seven important elements, namely Fe, Mn, As, B, Ti, Zn and Hg, selected based on their abundance in the samples, were investigated for a one-year cycle, from June 2002 to May 2003, at four chosen sites located along the KwaZulu-Natal coastline. The sites spread over 150 km from North to South Coast are Zinkwasi, Ballito, Treasure Beach and Park Rynie. P. corallorhiza possess good manganese and arsenic-accumulating ability and has potential to be an excellent indicator for most of the metals studied. A typical P. corallorhiza sample at Park Rynie (winter) recorded Mn (14 ppm), Fe (6.02 ppm), As (8.4 ppm), B (1580 ppb), Zn (234 ppb), Ti (751 ppb) and Hg (15.8 ppb). The general trend found at all sites was a large decrease in iron concentration in spring and summer and increase in winter. Mercury uptake was lowest in winter and autumn at all sites. The highest mercury levels in the seaweeds were recorded during spring or summer.     

Seasonal and diurnal variations of ambient PM2.5 concentration in urban and rural environments in Beijing

Three years of measurement of PM_2.5 with 5-min time resolution was conducted from 2005 to 2007 in urban and rural environments in Beijing to study the seasonal and diurnal variations in PM_2.5 concentration. Pronounced seasonal variation was observed in the urban area, with the highest concentrations typically observed in the winter and the lowest concentrations generally found in the summer. In the rural area, the maximum in PM_2.5 concentration usually appeared during the spring, followed by a second maximum in the summer, while the minimum generally occurred in the winter. Significant diurnal variations in PM_2.5 concentration were observed in both urban and rural areas. In the urban area, the PM_2.5 concentration displays a bimodal pattern, with peaks between 7:00 and 8:00 a.m. and between 7:00 and 11:00 p.m. The minimum generally appears around noon. The morning peak is attributed to enhanced anthropogenic activity during rush hours. The decreases of boundary layer height and wind speed in the afternoon companying with increased source activity during the afternoon rush hour result in the highest PM_2.5 concentration during evening hours. In the rural area, the PM_2.5 concentration shows a unimodal pattern with a significant peak between 5:00 and 11:00 p.m.The seasonal and diurnal variations in PM_2.5 concentration in the urban area are mostly dominated by the seasonal and diurnal variability of boundary layer and source emissions. The year-to-year variability of rainfall also has an important influence on the seasonal variation of PM_2.5 in the urban area. The seasonal and diurnal wind patterns are more important factors for PM_2.5 variation in the rural area. Southerly winds carry pollutants emitted in southern urban areas northward and significantly enhance the PM_2.5 concentration level in the rural area.     

Characterization and seasonal variations of levoglucosan in fine particulate matter in Xi’an,China

PM_2.5 (particulate matter with an aerodynamic diameter <2.5 μm) samples (n = 58) collected every sixth day in Xi’an, China, from 5 July 2008 to 27 June 2009 are analyzed for levoglucosan (1,6-anhydro-β-d-glucopyranose) to evaluate the impacts of biomass combustion on ambient concentrations. Twenty-four-hour levoglucosan concentrations displayed clear summer minima and winter maxima that ranged from 46 to 1889 ng m^?3, with an average of 428 ± 399 ng m^?3. Besides agricultural burning, biomass/biofuel combustion for household heating with straws and branches appears to be of regional importance during the heating season in northwestern China. Good correlations (0.70 < R < 0.91) were found between levoglucosan relative to water-soluble K⁺, Cl^?, organic carbon (OC), elemental carbon (EC), and glyoxal. The highest levoglucosan/OC ratio of 2.3% was found in winter, followed by autumn (1.5%). Biomass burning contributed to 5.1–43.8% of OC (with an average of 17.6 ± 8.4%).

Implications:?PM_2.5 levoglucosan concentrations and the correlation between levoglucosan relative to other compounds during four seasons in Xi’an showed that the influence of biomass burning is maximum during the residential heating season (winter), although some important influences may be detected in spring (field preparation burnings) and autumn (corn stalks and wheat straw burning, fallen dead leaves burning) at Xi’an and surrounding areas. Household heating with biomass during winter was quite widespread in Guanzhong Plain. Therefore, the control of biomass/biofuel combustion could be an effective method to reduce pollutant emission on a regional scale.     

Source Apportionment of Ambient Total Suspended Particulates and Coarse Particulate Matter in Urban Areas of Jiaozuo,China

Abstract

Approximately 750 total suspended particulates (TSPs) and coarse particulate matter (PM₁₀) filter samples from six urban sites and a background site and >210 source samples were collected in Jiaozuo City during January 2002 to April 2003. They were analyzed for mass and abundances of 25 chemical components. Seven contributive sources were identified, and their contributions to ambient TSP/PM₁₀ levels at the seven sites in three seasons (spring, summer, and winter days) and a “whole” year were estimated by a chemical mass balance (CMB) receptor model. The spatial TSP average was high in spring and winter days at a level of approximately 530 ～g/m³ and low in summer days at 456 ～g/m³; however, the spatial PM₁₀ average exhibited little variation at a level of approximately 325 ～g/m³, and PM_10-to-TSP ratios ranged from 0.58 to 0.81, which suggested heavy particulate matter pollution existing in the urban areas. Apportionment results indicated that geological material was the largest contributor to ambient TSP/PM₁₀ concentrations, followed by dust emissions from construction activities, coal combustion, secondary aerosols, vehicle movement, and other industrial sources. In addition, paved road dust and re-entrained dust were also apportioned to the seven source types and found soil, coal combustion, and construction dust to be the major contributors.     

夏秋季节焦化厂附近大气中臭氧及其前体物变化特征和臭氧生成潜势分析

2012年6—10月,在我国北方某焦化厂厂界附近开展了O3、NO x、CO体积分数在线监测及VOCs样品采集分析工作,获得了夏、秋两季焦化厂厂界O3及其前体物的体积分数及其日变化趋势。焦化厂厂界附近O3、NO、CO体积分数均呈单峰型日变化,O3体积分数的季节差异不明显,夏季仅略高于秋季,而NO、CO体积分数秋季高于夏季,作为二次反应产物的NO2,其变化幅度秋季比夏季强烈。夏季TVOCs在各监测时段的小时体积分数呈现先上升后下降的日变化趋势,而秋季则呈现逐渐下降的日变化趋势。由较小VOCs/NO x的比值可初步判断,该焦化厂所在区域的大气光化学臭氧生成潜势处于VOCs控制区。在焦化厂下风向厂界附近,夏、秋两季TVOCs平均体积分数分别为(43.8±45.0)×10-9和(26.7±29.6)×10-9,苯系物、烷烃、烯烃的平均体积分数分别为(34.3±28.1)×10-9和(14.4±14.8)×10-9、(5.3±11.8)×10-9和(7.0±7.7)×10-9、(4.3±5.0)×10-9和(5.3±7.1)×10-9。夏、秋两季焦化厂附近臭氧生成潜势贡献最大的是苯系物(47.6%~65.8%),其次是烯烃(28.0%~41.9%),再次是烷烃(6.3%~10.5%)。     

Seasonal variability of aerosol optical properties over Beijing

The knowledge of aerosol properties at local and regional scale is important in understanding of the global climate change. In this study, the aerosol optical properties over Beijing have been presented from the Aerosol Robotic Network (AERONET) measurements during 2002–2007. The aerosol optical depth (AOD) showed a distinct seasonal variation with high values in spring (March–May) and summer (June–August). The magnitude of Ångström exponent (α) was found to be relatively high throughout the year and the highest values (1.27) occurred in summer and the lowest (1.0) in spring. The water vapor retrieved from AERONET was found to be highest (2.60 cm) in summer. The fine modes of aerosol volume size distributions showed the highest peak around radius 0.15 μm in spring, autumn (September–November) and winter (December–February), and radius 0.19 μm in summer. The coarse modes showed the maxima peak at radius 3.0 μm in all seasons. The asymmetry factor (g) has considered as 0.65 at 440, 675, 870 and 1020 nm over Beijing in climate and radiation models. The average values of the single scattering albedo (SSA) at the four wavelengths were taken as 0.89, 0.91, 0.87 and 0.86 in spring, summer, autumn and winter, respectively. Both real and imaginary parts of the refractive index showed low wavelength dependence. The highest averages of real (1.52) and imaginary parts (0.0165) were found in spring and winter respectively in the wavelength range of 440–1020 nm. The aerosol properties over Beijing were found to highly dependent on season, and changes in aerosol properties were mainly attributed to the presence of dust as the main component during the spring season and the dominance of anthropogenic pollutants during the winter season.     

Rate Constants for the Reaction of OH with Halocarbons in the Presence of O2 + N2

Rates of CO₂ production in the reaction CO + OH and CO + OH + halocarbon have been used to determine rate constants for some OH + halocarbon reactions at 29.5°C relative to that of k(CO + OH) = 2.69 × 10^?13 cm³ molecule^?1 sec^?1. The following rate constants were obtained: k(OH + CH₃Cl) = 3.1 ± 0.8, k(OH + CH₂Cl₂) = 2.7 ± 1.0, k(OH + C₂H₅Cl) = 44.0 ± 25, k(OH + CICH₂CH₂CI) = 6.5, (<29) and k(OH + CH₃CCl₃) = 2.1 (<5.7) cm³ molecule^?1 sec^?1 × 10^?14. The k values, CH₂Cl₂ excepted, are in substantial agreement with determinations made in nonoxygen environments. The present results for CH₂Cl₂ are almost certainly in error due to difficulties with the competitive approach used.     

Spatial distribution of source locations for particulate nitrate and sulfate in the upper-midwestern United States

Two back-trajectory analysis methods designed to be used with multiple site data, simplified quantitative transport bias analysis (SQTBA) and residence time weighted concentration (RTWC), were applied to nitrate and sulfate concentration data from two rural sites (the Mammoth Cave National Park and the Great Smoky Mountain National Park) and five urban sites (Chicago, Cleveland, Detroit, Indianapolis, and St. Louis) for an intensive investigation on the spatial patterns of origins for these two species in the upper-midwestern area. The study was made by dividing the data into five categories: all sites and all seasons, rural sites in summer, rural sites in winter, urban sites in summer, and urban sites in winter. A general conclusion was that the origins of the nitrate in these seven sites were mainly in the upper-midwestern areas, while the sulfate in these seven sites were mainly from the Ohio and Tennessee River Valley areas. The upper-midwestern areas are regions of high ammonia emissions rather than high NO_x emissions. In the winter, metropolitan areas showed the highest nitrate emission potential suggesting the importance of local NO_x emissions. In the summer, ammonia emissions from fertilizer application in the lower midwestern area made a significant contribution to nitrate in the rural sites of this study. The impact of the wind direction prevalence on the source spatial patterns was observed by comparing the urban and rural patterns of the summer. The differences between the results of two methods are discussed and suggestions for applying these methods are also provided.     

西安市及周边地区MODIS气溶胶光学厚度与PM10浓度关系模型研究简

西安是空气污染监控和防治有代表性的西部大型城市。研究了西安市及周边地区上空气溶胶光学厚度与PM₁₀浓度的关系模型。利用2011—2012年MODIS卫星气溶胶光学厚度（AOD）遥感产品,通过数据匹配,利用地面气象观测站点的能见度数据和相对湿度数据对AOD产品进行垂直标高订正和湿度订正,2项订正显著提高了AOD和地面PM₁₀浓度的相关性,相关系数从0.36提高到0.65,按季节分类统计和订正春至冬四季的相关系数分别为0.57、0.71、0.62和0.87,夏季和冬季的订正更为有效,可用性更高,这可能由于受到不同季节气溶胶来源和特征的影响。为研究中国西部大型城市,特别是西安市空气环境监测和区域联防联控提供了一种有效方法。     

Atmospheric Sulfates from Biological Sources

Bacteriogenic production of H₂S occurs in fine-grained anoxic muds, is promoted by organic and nutrient pollution of water, peaks in the warm months of the year, and is the source of most of the estimated 100 to 200 million tons of biogenic sulfur annually contributed to the global atmosphere. We tested the hypothesis that biogenic sulfur contributes to the atmospheric load of sulfate in urban and nonurban sites by statistical analyses of the 24 hour sulfate levels measured in 4 coastal and 3 Inland nonurban sites where pollutant sulfur dioxide emissions are absent or negligible, and in 8 coastal and 10 inland urban sites, all located in New England or Middle Atlantic states.

Comparisons of annual and seasonal mean sulfate levels show that in nonurban groups summertime sulfate levels significantly exceed wintertime levels, and in summer, sulfate levels in urban sites are nearly the same as in nonurban sites. Comparisons of group sulfate means in 4 New York cities near extended bodies of polluted water with those in 10 inland upstate New York cities show significantly higher levels in the cities near polluted water in spring, summer, and fall and for the year as a whole, but not in winter, when the levels were similar. When the nonurban and urban sites are grouped for proximity to coasts (where bacterial sulfate reduction is active in sediments) paired groups of coastal and inland urban and nonurban sites show no significant differences in sulfate levels in summer and fall.

Studies of the summertime sulfate means in New York state show no evidence of an elevated anthropogenic background which could explain the high summertime sulfate level observed in one nonurban site in that state, while analyses of the day to day fluctuations in urban and nonurban sites support the conclusion that nonurban sites have large local (biogenic) sulfate sources in summer and fall, and that local sulfate sources also exist in spring and may exist in winter.

We conclude that biogenic sulfate sources contribute most of the sulfate observed in the cities studied during summer and fall, and in some cities also contribute in other seasons. These biogenic contributions vary with local conditions and are estimated to contribute up to 6 µg/m³ (50%) or more to the annual geometric mean sulfate levels observed in some cities located near extensive bodies of polluted water.     

Characterization and contribution to PM2.5 of semi-volatile aerosols in Paris (France)

Collocated PM_2.5 measurements using a conventional R&P TEOM (model 1400a) and a TEOM-FDMS were performed at a Paris urban background site during winter/summer field experiments. Results showed that conventional TEOM underestimates PM_2.5 mass concentrations by about 50% in winter and 35% in summer. They also confirmed that this negative sampling artifact, due to the volatilization of semi-volatile material (SVM) inside the instrument, cannot be accurately accommodated by a single correction factor because of SVM routine fluctuations. A basic filter-based investigation of the SVM chemical composition also indicated that SVM, measured by the TEOM–FDMS, is mainly formed by ammonium nitrate in winter while significant contributions of semi-volatile organic matter were observed in summer. The latter species was found to possibly account for more than 50% of secondary organic aerosol formed during summer afternoons. These findings call for more investigation of the SVM chemical composition, particularly during the summer season, in Paris and in Europe.     

Mutagenicity of Airborne Particulates Assessed by Salmonella Assay and the SOS Chromotest in Wrocław,Poland

Abstract

Ambient air particulate matter less than 2.5 μm in aerodynamic diameter (PM_2.5) samples were collected during summer and autumn using a Staplex high-volume air sampler. They were later extracted with dichloromethane in a Soxhlet apparatus. Polyaromatic hydrocarbon (PAH) content in extracts was determined by the high-performance liquid chromatography technique using fluorescence detection, whereas the nitro-PAH content was determined by gas chromatography using mass detection. Four Salmonella typhimurium strains (TA98, TA100, YG1041, and YG1042) were used in assays conducted with and without metabolic activation. The extracts were also tested with the SOS chromotest supplied by Environmental Biodetection Products Incorporated. The obtained results confirmed the Salmonella assay and the SOS chromotest usability for the purpose of atmospheric pollution monitoring within an urban agglomeration. The atmospheric pollution extracts under examination differed among each other regarding total content and percentage of individual compounds, depending on the season of sampling. The highest total PAH content and the highest nitro-PAH content in the tested samples as well as the most extensive range of detected compounds were found in the autumn season (heating season). The highest mutagenicity was noted for PM_2.5 samples collected in autumn. The high values of mutagenicity ratios and induction factors were obtained from assays carried out with and without metabolic activation, which is an argument for the presence of promutagens and direct mutagens. The YG1041 strain proved to be the most effective in detection of mutagenicity of the suspended dust extracts because of its notably high sensitivity to nitro-aromatic compounds. The SOS chromotest was very sensitive to a large spectrum of genotoxic air pollutants and showed a high degree of similarity with the results of the Salmonella assay. In comparison with the frequently used Ames test, the SOS chromotest enables quick analysis of the genotoxic effects of samples using only one tester strain. In addition, its miniaturized design decreases the consumption of tested samples.     

Seasonal variation on size distribution and concentration of PAHs in Guangzhou city, China

Size distribution aerosol samples were collected at an urban location of Guangzhou in four seasons of 2003-2004 by a MOUDI (Micro-orifice Uniform Deposit Impactor). The particle loading (PM10: 80-397 microg m(-3)) was comparable with some other Asia cities; however, much higher than that of Western Europe and North America. Polycyclic aromatic hydrocarbons (PAHs) were measured by gas chromatography with mass selective detector (GC-MS). Seasonal effects on the size distribution of PAHs are presented. Bimode (accumulation and coarse mode) and unimode (accumulation mode) distributions were observed for low-molecule-weight and high-molecule-weight PAHs. A slight shift to larger particles was found for the accumulation mode in autumn and winter, compared with that of spring and summer. One explanation is that the longer aging process of PAHs in autumn and winter would result in volatilization from finer particles followed by condensation onto coarser particles. Another is there was mixing process of local emission with long-range transported aerosol in autumn and winter. The relative higher value of IcdP/(BghiP+IcdP) and lower value of BghiP/BeP in winter also give evidences to the mixing process. The level of PAHs concentration has been much elevated in recent years. This can be attributed to the fast growth of motor vehicle and energy consumption.     

Influence of temperature on the chemical removal of 3-methylbutanal,trans-2-methyl-2-butenal,and 3-methyl-2-butenal by OH radicals in the troposphere

Absolute rate coefficients for the gas-phase reactions of OH radical with 3-methylbutanal (k₁), trans-2-methyl-2-butenal (k₂), and 3-methyl-2-butenal (k₃) have been obtained with the pulsed laser photolysis/laser-induced fluorescence technique. Gas-phase concentration of aldehydes was measured by UV absorption spectroscopy at 185 nm. Experiments were performed over the temperature range of 263–353 K at total pressures of helium between 46.2 and 100 Torr. No pressure dependence of all k_i (i = 1–3) was observed at all temperatures. In contrast, a negative temperature dependence of k_i (i.e., k_i increases when temperature decreases) was observed in that T range. The resulting Arrhenius expressions (±2σ) are: k₁(T) = (5.8 ± 1.7)×10^?12 exp{(499 ± 94)/T} cm³ molecule^?1 s^?1, k₂(T)=(6.9 ± 0.9)×10^?12 exp{(526 ± 42)/T} cm³ molecule^?1 s^?1, k₃(T)=(5.6 ± 1.2)×10^?12 exp{(666 ± 54)/T} cm³ molecule^?1 s^?1.The tropospheric lifetimes derived from the above OH-reactivity trend are estimated to be higher for 3-methylbutanal than those for the unsaturated aldehydes. A comparison of the tropospheric removal of these aldehydes by OH radicals with other homogeneous degradation routes leads to the conclusion that this reaction can be the main homogeneous removal pathway. However, photolysis of these aldehydes in the actinic region (λ > 290 nm) could play an important role along the troposphere, particularly for 3-methyl-2-butenal. This process could compete with the OH reaction for 3-methylbutanal or be negligible for trans-2-methyl-2-butenal in the troposphere.