142 research outputs found
The application of carbon-14 analyses to the source apportionment of atmospheric carbonaceous particulate matter:a review
Get PDFThe Henry's law coefficient of 2-nitrophenol over the temperature range 278â303 K
Get PDFAlthough 2-nitrophenol has been identified as an important environmental chemical
there is scarcity in the literature regarding the temperature dependence of its Henry's
law coefficient, H. Here a bubble purge method was used to measure H for 2-
nitrophenol over the temperature range 278â303 K. A novel approach in the data
treatment allowed correction of the data for non-equilibrium partitioning in the
apparatus to obtain the true equilibrium H value. The experimentally derived
temperature-dependent expression for H of 2-nitrophenol is lnH (M atmâ1)=(6290/T
(K))â16.6. The standard enthalpy and entropy of gas-to-liquid transfer for 2-
nitrophenol in aqueous solution are â52.3±8.1 kJmolâ1 and â138±28 Jmolâ1 Kâ1,
respectively. (Errors are 95% confidence intervals.
The effect of absorbent grid preparation method on precision and accuracy of ambient nitrogen dioxide measurements using Palmes passive diffusion tubes
Get PDFA few studies have suggested that the precision and accuracy of measurement of NO2 by Palmes-type passive diffusion tube (PDT) are affected by the method of preparation of the triethanolamine (TEA) absorbent coating on the grids. Theses studies have been quite limited in extent and have tended to evaluate PDT accuracy as zero bias between PDT NO2 value and the exposure-averaged NO2 determined by co-located chemiluminescence analyser. This ignores the well-documented intrinsic systematic biases on PDT-derived NO2, such as within-tube chemistry and exposure-duration nitrite loss, which may lead to non-zero bias values irrespective of effects of TEA absorbent preparation method on PDT accuracy. This paper reports on a statistical analysis of a large dataset comprising 680 duplicated PDT exposures spanning 146 separate exposures periods, spread over 5 urban exposure locations and a number of years. In each exposure period, PDTs prepared by between four and six different grid preparation methods were simultaneously compared. The preparation methods used combinations of the following: acetone or water as the TEA solvent; 20% or 50% as %TEA in the solution; and application of TEA solution by dipping grids for several minutes in the solution before drying and tube assembly, or by pipetting 50 ”L of solution directly onto grids already placed in the PDT cap. These represent the range of preparation procedures typically used. Accuracy was evaluated as maximised nitrite capture within an exposure. Data were analysed by General Linear Modelling including examination of interaction between different aspects of grid preparation method. PDT precision and accuracy were both significantly better, on average, when the PDT grids were prepared by dipping in TEA solution, and neither solvent or %TEA used for the dipping solution were important. Where PDT preparation by pipetting TEA solution onto grids is to be used, better performance was obtained using 20% TEA in water. A systematic positive bias in PDT measure of NO2, consistent with within-tube oxidation of NO to NO2 and independent of preparation method, was again evident in this work
Land-Use Regression Modelling of Intra-Urban Air Pollution Variation in China: Current Status and Future Needs
Get PDFRapid urbanization in China is leading to substantial adverse air quality issues, particularly for NO2 and particulate matter (PM). Land-use regression (LUR) models are now being applied to simulate pollutant concentrations with high spatial resolution in Chinese urban areas. However, Chinese urban areas differ from those in Europe and North America, for example in respect of population density, urban morphology and pollutant emissions densities, so it is timely to assess current LUR studies in China to highlight current challenges and identify future needs. Details of twenty-four recent LUR models for NO2 and PM2.5/PM10 (particles with aerodynamic diameters <2.5 ”m and <10 ”m) are tabulated and reviewed as the basis for discussion in this paper. We highlight that LUR modelling in China is currently constrained by a scarcity of input data, especially air pollution monitoring data. There is an urgent need for accessible archives of quality-assured measurement data and for higher spatial resolution proxy data for urban emissions, particularly in respect of traffic-related variables. The rapidly evolving nature of the Chinese urban landscape makes maintaining up-to-date land-use and urban morphology datasets a challenge. We also highlight the importance for Chinese LUR models to be subject to appropriate validation statistics. Integration of LUR with portable monitor data, remote sensing, and dispersion modelling has the potential to enhance derivation of urban pollution maps
Quantifying the dominant sources influencing the 2016 particulate matter pollution episode over northern India
Get PDFIntense episodes of fine particulate matter (PM2.5) pollution often overwhelm large areas of the Indo-Gangetic Plain (IGP) in northern India during the post-monsoon season, a time when crop residue burning is at its peak. We conduct idealised emission sensitivity experiments using the WRF-Chem model to investigate the leading causes and spatiotemporal extent of one such extreme episode from 31 Oct to 8 Nov 2016, when hourly PM2.5 levels exceeded 500 ÎŒg mâ3 across much of the IGP on several days. We utilise the anthropogenic emissions from EDGARv5.0 and the latest FINNv2.5 for fire emissions and evaluate modelled and observed ambient PM2.5 and black carbon (BC) concentrations across the IGP. The model captured the PM2.5 and BC peaks during the latter half of the episode and underestimated on other days. We find that biomass burning (BB) emissions during this episode have the strongest effect across the source regions in the upper (NW) IGP, followed by Delhi (middle IGP), where it contributes 50â80% to 24 h mean PM2.5. Complete elimination of BB emissions decreases PM2.5 concentrations by 400 ÎŒg mâ3 (80â90%) in the upper IGP and by 280 ÎŒg mâ3 (40â80%) across the middle IGP during this episode. Contributions from the BB source to daily varying BC concentrations are 80â90%, 40â85% and 10â60% across upper, middle and lower IGP, respectively. BB emissions dominantly contribute to daily mean secondary organic aerosols (80%), primary organic aerosols (90%), dust (60%), and nitrate (50%) components of PM2.5 across the upper and middle IGP. In comparison, the anthropogenic share of these compounds was nearly one-third everywhere except across the lower IGP. The buildup of the episode across the middle IGP was facilitated by prolonged atmospheric stratification and stagnation, causing BB-derived BC and PM2.5 to be trapped in the lowest 1 km. Our work emphasises the need for rigorous policy interventions during post-monsoon to reduce agricultural crop burning, together with targeted anthropogenic emissions control across the IGP, to minimise such extreme episodes in the future
Evaluation of WRF-Chem simulated meteorology and aerosols over northern India during the severe pollution episode of 2016
Get PDFWe use a state-of-the-art regional chemistry transport model (WRF-Chem v4.2.1) to simulate particulate air pollution over northern India during SeptemberâNovember 2016. This period includes a severe air pollution episode marked by exceedingly high levels of hourly PM2.5 (particulate matter having an aerodynamic diameterââ€â2.5â”m) during 30 October to 7 November, particularly over the wider Indo-Gangetic Plain (IGP). We provide a comprehensive evaluation of simulated seasonal meteorology (nudged by ERA5 reanalysis products) and aerosol chemistry (PM2.5 and its black carbon (BC) component) using a range of ground-based, satellite and reanalysis products, with a focus on the November 2016 haze episode. We find the daily and diurnal features in simulated surface temperature show the best agreement followed by relative humidity, with the largest discrepancies being an overestimate of night-time wind speeds (up to 1.5âmâsâ1) confirmed by both ground and radiosonde observations. Upper-air meteorology comparisons with radiosonde observations show excellent model skill in reproducing the vertical temperature gradient (r>0.9). We evaluate modelled PM2.5 at 20 observation sites across the IGP including eight in Delhi and compare simulated aerosol optical depth (AOD) with data from four AERONET sites. We also compare our model aerosol results with MERRA-2 reanalysis aerosol fields and MODIS satellite AOD. We find that the model captures many features of the observed aerosol distributions but tends to overestimate PM2.5 during September (by a factor of 2) due to too much dust, and underestimate peak PM2.5 during the severe episode. Delhi experiences some of the highest daily mean PM2.5 concentrations within the study region, with dominant components nitrate (âŒ25â%), dust (âŒ25â%), secondary organic aerosols (âŒ20â%) and ammonium (âŒ10â%). Modelled PM2.5 and BC spatially correlate well with MERRA-2 products across the whole domain. High AOD at 550nm across the IGP is also well predicted by the model relative to MODIS satellite (râ„0.8) and ground-based AERONET observations (râ„0.7), except during September. Overall, the model realistically captures the seasonal and spatial variations of meteorology and ambient pollution over northern India. However, the observed underestimations in pollutant concentrations likely come from a combination of underestimated emissions, too much night-time dispersion, and some missing or poorly represented aerosol chemistry processes. Nevertheless, we find the model is sufficiently accurate to be a useful tool for exploring the sources and processes that control PM2.5 levels during severe pollution episodes.</p
Long-term CH3Br and CH3Cl flux measurements in temperate salt marshes
Get PDFFluxes of CH3Br and CH3Cl and their relationship with potential drivers such as sunlight, temperature and soil moisture, were monitored at fortnightly to monthly intervals for more than two years at two contrasting temperate salt marsh sites in Scotland. Manipulation experiments were conducted to further investigate possible links between drivers and fluxes. Fluxes followed both seasonal and diurnal trends with highest fluxes during summer days and lowest (negative) fluxes during winter nights. Mean (± 1 sd) annually and diurnally-weighted net emissions from the two sites were found to be 300 ± 44 ng mâ2 hâ1 for CH3Br and 662 ± 266 ng mâ2 hâ1 for CH3Cl. The fluxes from this work are similar to findings from this and other research groups for salt marshes in cooler, higher latitude climates, but lower than values from salt marshes in the Mediterranean climate of southern California. Statistical analysis generally did not demonstrate a strong link between temperature or sunlight levels and methyl halide fluxes, although it is likely that temperatures have a weak direct influence on emissions, and both certainly have indirect influence via the annual and daily cycles of the vegetation. CH3Cl flux magnitudes from different measurement locations depended on the plant species enclosed whereas such dependency was not discernible for CH3Br fluxes. In 14 out of 18 collars with vegetation CH3Br and CH3Cl net fluxes were significantly positively correlated. The CH3Cl/CH3Br net-emission mass ratio was 2.2, a magnitude lower than mass ratios of global methyl halide budgets (~22) or emissions from tropical rainforests (~60). This is likely due to preference for CH3Br production by the relatively high bromine content in the salt marsh plant material. Extrapolation based solely on data from this study yields salt marsh contributions of 0.5â3.2% and 0.05â0.33%, respectively, of currently-estimated total global production of CH3Br and CH3Cl, but actual global contributions likely lie between these values and those derived from southern California
Rate constant for reaction of CH (X2-PI) with ketene
Get PDFA bimolecular rate constant for the reactive removal of CH (X 2Π) by ketene (CH2CO) has been measured under pseudo-first-order conditions using multiphoton dissociation of CH2CO at 308 nm to generate the radicals and laser induced fluorescence to detect them. A value of (2.4 ± 0.2) à 10-10 cm3 molecule-1 s-1 was obtained at room temperature (295 ± 2 K)
Methyl bromide and methyl chloride fluxes from temperate forest litter
Get PDFMethyl halide fluxes were measured from fine (nonwoody) litter samples in a
temperate deciduous forest site in Scotland on 16 occasions over more than a
year and from a coniferous forest site. The resulting mean (+/-1 sd) CH3Br and CH3Cl fluxes were 4.1 +/- 3.7 ng kg-1 h-1 and 0.98 +/- 0.62 ”g kg-1 h-1, respectively, for dry mass leaf litter and 5.7 +/- 6.3 ng kg-1 h-1 and 0.47 +/- 0.14 ”g kg-1 h-1 for dry mass needle litter. Temporal variations of net fluxes from leaf litter were significantly greater than spatial variations suggesting seasonality in the fluxes. The mean CH3Cl/CH3Br mass ratio of fluxes was ~200, an order of magnitude larger than the ratio of their estimated global turnovers. Temperate forest litter may be a moderate net source of CH3Cl globally but a negligible source of CH3Br. These statements refer to the nonwoody litter component only
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