The influence of weather-type and long-range transport on airborne particle concentrations in Edinburgh, UK

This study investigated the influence of regional-scale synoptic weather type and geographical source regions of air masses on two-particle concentration metrics (Black Smoke (BS) and PM10) in the city of Edinburgh, UK, between 1981 and 1996. Twenty-seven classifications of Jenkinson Daily Weather Types (JWT) were subdivided into 9 directional categories and 3 vorticity categories, and the influence of JWT category on BS and PM10 determined. Four-day air mass back-trajectories for 1 July 1995–30 June 1996 were computed and grouped into 8 categories depending on the geographical route followed. Significantly elevated concentrations of BS (median values 2, 5 and 4 μg m−3 greater than median for 1981–1996) and PM10 (median values 3, 5.5 and 8 μg m−3 greater than median for 1992–1996) were observed for anticyclonic, southerly and south-easterly weather types, respectively. These differences were not identified at conventional levels of significance for BS in 1995– 1996. This may reflect a shift in more recent times to lower concentrations of predominantly locally emitted BS less affected by regional scale meteorology. Conversely, significant inter-trajectory category differences were observed for PM10 during 1995–1996, with highest concentrations associated with Eastern European trajectories and south-easterly weather type categories (11.4 and 10.7 μg m−3 greater than annual means, respectively). The variation in particle concentration across weather-type was a significant proportion of total median particle concentration, and of a magnitude associated with adverse health outcomes. Thus current PM10 concentrations (and associated health outcomes) in Edinburgh are likely to be significantly influenced by regional-scale meteorology independent of local air quality management areas. Furthermore, changes in long-term trends in distributions of synoptic weather types indicate that future climate change may influence exposure to PM10 and the PM10:BS ratio in Edinburgh. Further definition of the relationships between long-range transport and particle concentration will improve classification of human exposure in epidemiological studies

Characterization of PM10-bound polycyclic aromatic hydrocarbons and associated carcinogenic risk in Bangkok, Thailand

Concentrations of ambient particulate-bound polycyclic aromatic hydrocarbons (pPAHs) were measured in PM10 samples collected at roadside, industrial and urban background sites in Bangkok between May 2013 and May 2014. The annual average PM10 concentrations were not significantly different between the roadside (56.4 ± 27.3 µg m−3) and industrial (51.0 ± 31.1 µg m−3) sites. The lowest annual mean PM10 was observed at the urban background site (39.8 ± 22.2 µg m−3). Seasonal variations of pPAHs were observed at the three sampling sites. The total pPAHs ranged between 1.09 and 13.10 ng m−3 (mean 4.85 ± 2.51 ng m−3), 1.49 and 9.39 ng m−3 (mean 3.84 ± 2.01 ng m−3) and 0.77 and 5.20 ng m−3 (mean 2.28 ± 1.16 ng m−3) at the roadside, industrial and urban background sites, respectively. The observed annual average benzo[a]pyrene concentrations were 0.47 ± 0.39 ng m−3, 0.35 ± 0.27 ng m−3 and 0.24 ± 0.19 ng m−3 at the roadside, industrial and urban background sites. Long-term carcinogenic health risk of inhalation exposure expressed as the toxicity equivalent to benzo[a]pyrene concentrations were calculated as 0.83, 0.72 and 0.39 ng m−3 at the industrial, roadside and urban background sites, respectively. The composition of pPAHs plays an important role in the carcinogenicity of a PAHs mixture

Estimation of spatial patterns of urban air pollution over a 4-week period from repeated 5-min measurements

Determination of intra-urban spatial variations in air pollutant concentrations for exposure assessment requires substantial time and monitoring equipment. The objective of this study was to establish if short-duration measurements of air pollutants can be used to estimate longer-term pollutant concentrations. We compared 5-min measurements of black carbon (BC) and particle number (PN) concentrations made once per week on 5 occasions, with 4 consecutive 1-week average nitrogen dioxide (NO2) concentrations at 18 locations at a range of distances from busy roads in Glasgow, UK. 5-min BC and PN measurements (averaged over the two 5-min periods at the start and end of a week) explained 40 - 80%, and 7 - 64% respectively, of spatial variation in the intervening 1-week NO2 concentrations for individual weeks. Adjustment for variations in background concentrations increased the percentage of explained variation in the bivariate relationship between the full set of NO2 and BC measurements over the 4-week period from 28% to 50% prior to averaging of repeat measurements. The averages of five 5-min BC and PN measurements made over 5 weeks explained 75% and 33% respectively of the variation in average 1-week NO2 concentrations over the same period. The relatively high explained variation observed between BC and NO2 measured on different time scales suggests that, with appropriate steps to correct or average out temporal variations, repeated short-term measurements can be used to provide useful information on longer-term spatial patterns for these traffic-related pollutants

Consistency of Urban Background Black Carbon Concentration Measurements by Portable AE51 and Reference AE22 Aethalometers: Effect of Corrections for Filter Loading

Monitoring exposure to black carbon with portable devices is an important part of researching the health impacts of combustion-related air pollutants. We collected 786 hourly averaged equivalent black carbon (eBC) measurements from co-located duplicate portable AE51 Aethalometers and a UK Government reference AE22 Aethalometer (the data for the latter were corrected for filter darkening effects using a standard procedure), at an urban background site in Glasgow, UK. The AE51 and the reference concentrations were highly correlated (R 2 ≥ 0.87) for the combined deployment periods. The application of a previously reported method for correcting the AE51’s underestimation of concentrations, associated with filter loading, generally led to an overestimation of values (specifically, the normalised mean bias values for the two AE51s increased from –2% and +3% to +14% and +25% across the full range of measurements after correction). We found only limited and inconsistent evidence that the AE51 Aethalometers (attenuation [AE51_ATN] ≤ ~52) underestimated the eBC concentrations compared to the reference measurements. Thus, our observations indicate that the AE51 can achieve close agreement with the reference AE22 monitor without applying corrections for filter loading at relatively low AE51_ATN values in environments with low eBC concentrations

Temporal changes in field calibration relationships for Aeroqual S500 O3 and NO2 sensor-based monitors

Sensor-based monitors are increasingly used to measure air pollutant concentrations, but require calibration under field conditions. We made intermittent comparisons (6 times over 6-month period) between ozone and nitrogen dioxide concentrations measured by Aeroqual gas-sensitive semiconductor (O3) and electrochemical (NO2) sensors (two of each) and reference analysers in the UK Automatic Urban and Rural Network. Each deployment period was split into equal (n = 48 x1-hour) training and test datasets, to derive and test calibration equations respectively. We observed significant bivariate linear relationships between Aeroqual O3 and Reference O3 concentrations, and significant multiple linear relationships between Aeroqual NO2 and both Reference NO2 and Aeroqual O3 concentrations. Changes in monitor responses over time (including apparent baseline drift in O3 sensor output, and discrepancies between the 2 Aeroqual NO2 sensors) resulted in relatively inaccurate concentrations estimates (cf. reference concentrations) from calibration equations derived in the first training period and applied to subsequent test deployments (e.g. NO2 RMSE = 47.2 μg m-3 (n = 286) for a dataset of all test periods combined, for one of the two monitor pairs). Substantial improvements in accuracy of estimated concentrations were achieved by combination of repeated intermittent training data into a single calibration dataset (NO2 RMSE = 8.5 μg m-3 for same test dataset described above). This latter approach to field calibration is recommended

The metal content of airborne particles in Edinburgh: application to epidemiological research

Metals are putative causative agents in the association between ill health and exposure to airborne particles. We present preliminary results from an epidemiological study using exposure metrics of metal contained in PM10, PM2.5 and black smoke. A 1 yr monitoring and analysis campaign has been completed for 11 metals at 24 h sampling resolution. Empirical models of environmental determinants of metal concentration have been used to retrospectively extrapolate the pollutant time series. We are currently evaluating whether the use of metal concentration explains more of the variance in the population exposure–response relationship compared with the use of particle mass concentration alone

Practical Field Calibration of Portable Monitors for Mobile Measurements of Multiple Air Pollutants

To reduce inaccuracies in the measurement of air pollutants by portable monitors it is necessary to establish quantitative calibration relationships against their respective reference analyser. This is usually done under controlled laboratory conditions or one-off static co-location alongside a reference analyser in the field, neither of which may adequately represent the extended use of portable monitors in exposure assessment research. To address this, we investigated ways of establishing and evaluating portable monitor calibration relationships from repeated intermittent deployment cycles over an extended period involving stationary deployment at a reference site, mobile monitoring, and completely switched off. We evaluated four types of portable monitors: Aeroqual Ltd. (Auckland, New Zealand) S500 O3 metal oxide and S500 NO2 electrochemical; RTI (Berkeley, CA, USA) MicroPEM PM2.5; and, AethLabs (San Francisco, CA, USA) AE51 black carbon (BC). Innovations in our study included: (i) comparison of calibrations derived from the individual co-locations of a portable monitor against its reference analyser or from all the co-location periods combined into a single dataset; and, (ii) evaluation of calibrated monitor estimates during transient measurements with the portable monitor close to its reference analyser at separate times from the stationary co-location calibration periods. Within the ~7 month duration of the study, ‘combined’ calibration relationships for O3, PM2.5, and BC monitors from all co-locations agreed more closely on average with reference measurements than ‘individual’ calibration relationships from co-location deployment nearest in time to transient deployment periods. ‘Individual’ calibrations relationships were sometimes substantially unrepresentative of the ‘combined’ relationships. Reduced quantitative consistency in field calibration relationships for the PM2.5 monitors may have resulted from generally low PM2.5 concentrations that were encountered in this study. Aeroqual NO2 monitors were sensitive to both NO2 and O3 and unresolved biases. Overall, however, we observed that with the ‘combined’ approach, ‘indicative’ measurement accuracy (±30% for O3, and ±50% for BC and PM2.5) for 1 h time averaging could be maintained over the 7-month period for the monitors evaluated here

Influence of wind-speed on short-duration NO2 measurements using Palmes and Ogawa passive diffusion samplers

We assessed the precision and accuracy of nitrogen dioxide (NO2) concentrations over 2-day, 3-day and 7-day exposure periods measured with the following types of passive diffusion samplers: standard (open) Palmes tubes; standard Ogawa samplers with commercially-prepared Ogawa absorbent pads (Ogawa[S]); and modified Ogawa samplers with absorbent-impregnated stainless steel meshes normally used in Palmes tubes (Ogawa[P]). We deployed these passive samplers close to the inlet of a chemiluminescence NO2 analyser at an urban background site in Glasgow, UK over 32 discrete measurement periods. Duplicate relative standard deviation was < 7% for all passive samplers. The Ogawa[P], Ogawa[S] and Palmes samplers explained 93%, 87% and 58% of temporal variation in analyser concentrations respectively. Uptake rates for Palmes and Ogawa[S] samplers were positively and linearly associated with wind-speed (P < 0.01 and P < 0.05 respectively). Computation of adjusted uptake rates using average wind-speed observed during each sampling period increased the variation in analyser concentrations explained by Palmes and Ogawa[S] estimates to 90% and 92% respectively, suggesting that measurements can be corrected for shortening of diffusion path lengths due to wind-speed to improve the accuracy of estimates of short-term NO2 exposure. Monitoring situations where it is difficult to reliably estimate wind-speed variations, e.g. across multiple sites with different unknown exposures to local winds, and personal exposure monitoring, are likely to benefit from protection of these sampling devices from the effects of wind, for example by use of a mesh or membrane across the open end. The uptake rate of Ogawa[P] samplers was not associated with wind-speed resulting in a high correlation between estimated concentrations and observed analyser concentrations. The use of Palmes meshes in Ogawa[P] samplers reduced the cost of sampler preparation and removed uncertainty associated with the unknown manufacturing process for the commercially-prepared collection pads

Intercomparison study of NOx passive diffusion tubes with chemiluminescence analysers and evaluation of bias factors

Passive diffusion tubes (PDTs) are an inexpensive and simple method to monitor air pollutants. Numerous studies have investigated the performance of PDTs for NO2 but little attention has been paid to PDTs for NOx. The aim of this study was to evaluate the performance of NOx PDTs in three different urban environments. Duplicate NOx and NO2 PDTs were co-located with chemiluminescence analysers at kerbside, urban centre and background sites in the city of Glasgow for twelve 1-week exposures. PDT measurements generally showed good temporal correlations with NOx and NO2 determined by the continuous analysers. However detailed evaluation showed PDT measurements were variously influenced by factors causing bias, according to individual site characteristics: positive bias in both NOx and NO2 PDTs due to wind-associated shortening of diffusion path; positive bias in NO2 PDTs due to within tube chemical reaction between NO and O3; and, where NO concentrations were high, negative bias in NOx PDTs assumed due to incomplete oxidation of NO by the in-cap oxidising granules. In conclusion, where ambient NOx is low (less than a few tens of mgm3), and PDTs are in sheltered locations, NOx PDTs should perform well over 1-week exposures; however substantial negative bias for NOx PDTs is expected in polluted roadside environments for exposures of several weeks as is usually the case in ambient air quality deployment. Observations from this study suggest that sheltering PDTs from high wind is important to minimise positive bias due to wind-associated shortening of the diffusion path