Assessment of the effect of degradation by atmospheric gaseous oxidants on measured annual average benzo[a]pyrene mass concentrations

Comparative analysis of contributions of wet deposition and photodegradation to the removal of atmospheric BaP by MFDCCA

Benzo [a] pyrene (BaP) in the atmosphere possess great carcinogenic potential to human health, and the understanding of its scavenging mechanisms has attracted considerable attention. In this work, a new quantitative method is proposed to make a comparative analysis of the long-term contributions of wet deposition and photodegradation to BaP removal based on multi-fractal detrended cross-correlation analysis (MFDCCA). According to the precipitation and global solar radiation (GSR) observations from 1998 to 2016 for two urban sites (Central/Western District and TsuenWan) in Hong Kong, the wet deposition and photodegradation of BaP are analyzed. Using MFDCCA method, long-term cross-correlation between precipitation/GSR and BaP are investigated. Moreover, the differences of multifractal features in cross-correlations of precipitation-BaP and GSR-BaP system are analyzed. Strong long-term persistence is observed in the cross-correlations for precipitation-BaP system in a one-year cycle; while cross-correlations between GSR and BaP show weak persistence over the whole timescale. Based on the meteorology in Hong Kong, this difference has been discussed. Then, contributions of wet deposition and photodegradation to atmospheric BaP removal are quantified based on MFDCCA method, which are further compared between summer and winter. The comparative analysis suggests that wet deposition plays a more significant role in the removal of atmospheric BaP. Specifically, in summer, the contributions of wet deposition are twice as much as that of photodegradation for both two sites; while in winter, the contribution of photodegradation is a little higher than that of wet deposition to BaP removal. Meanwhile, for wet deposition, the contributions in summer are about ten times greater than that in winter; while for photodegradation, the difference in contributions between summer and winter are relatively smaller. Furthermore, based on sliding window technique, the temporal evolutions in the contributions of wet deposition/photodegradation to BaP removal have been presented for both two sites. On this basis, it is discovered that the comprehensive contributions of wet deposition and photodegradation peak in June, and reach their lowest levels in December for both two sites. Quantifying the contribution of meteorological factors to the removal of atmospheric BaP is help for understanding its geochemical cycle.

Volatilization and oxidative artifacts of PM bound PAHs collected at low volume sampling (1): Laboratory and field evaluation

Laboratory and field studies were carried out to assess the effects of oxidative degradation and volatilization on PM₁₀ bound polycyclic aromatic hydrocarbons (PAHs), collected at low volume condition according to the EU sampling reference method EN12341:2014 (flow rate 2.3 m³ h^-1), on 47 mm quartz filters. For the laboratory experiments, pairs of twin samples were collected in field and, after treatments favoring decomposition or/and volatilization of PAHs on one sample, the PAH amount was compared with that of the corresponding untreated sample. Ozone exposure caused a general PAHs decay with more marked effects on benzo [a]pyrene, perylene and benz [a]anthracene; these compounds showed, similarly to benzo [ghi]perylene, correlations between ozone dose and losses. Treatments with zero air exhibited losses due to volatilization even for 5-ring PAHs up to benzo [a]pyrene, whereas a linear dependence was observed between filter PAH load and losses for benzo [a]anthracene, chrysene and benzofluoranthenes. Concentrations on samples collected simultaneously over 48, 24, 12 and 6 h were compared. Results confirmed a lack of temporal auto-consistency in the PAHs sampling methodology here adopted. In particular higher atmospheric PAH concentrations were ascertained on samples constituted by cumulative filters exposed over shorter sampling times. When 24-h and 2 × 12-h samples were compared, comparable losses were evaluated in the hot and cold seasons. This finding shows that, although in summer meteorology conditions favor sampling artifacts, the effectiveness of these phenomena continue in the winter, probably due to the larger amount of PAH available on the sampling filter (total PAHs ∼ 10 vs 0.5 ng m^-3).

Volatilization and oxidative artifacts of PM bound PAHs at low volume sampling (2): Evaluation and comparison of mitigation strategies effects

The artifacts induced by oxidative degradation and volatilization were assessed with regards to determination of particulate atmospheric PAHs collected at low volume conditions (2.3 m³ h^-1) according to the EU Reference Method EN 12341:2014. In order to evaluate the oxidative degradation, PAH measures carried out through collecting airborne particulate with and without ozone denuders were examined. Simultaneously, the influence of volatilization was investigated by comparing concentrations of PAHs in particulate samples collected over 24-h and 12-h using conventional instruments. Summer and winter/spring campaigns were carried out in order to assess the influence of environmental contour on the artifact processes. Oxidative degradation led to a general decrease of PAH concentrations in both periods; in particular, the highest losses were observed for benzo[a]pyrene and perylene reaching, in average, ca. 20%. In the summer, the effect of volatilization exceeded that of oxidative degradation for light PAHs up to benzo[e]pyrene. In the winter/spring time, the influence of both artifact typologies could be mitigated by splitting the normal 24-h collection interval starting at midnight into two 12-h intervals. A mitigation of the losses could even be obtained by fixing the start time sampling fixed at noon or in the first hours of the day. Finally, the feasibility of collecting PAHs through prolonged sampling (>1 month) at the flow rate of 1.1 L min^-1 was preliminarily investigated. Results indicated that this approach is unsuitable for minimizing the oxidative artifacts.

Profiling quinones in ambient air samples collected from the Athabasca region (Canada)

This paper presents new findings on polycyclic aromatic hydrocarbon oxidation products-quinones that were collected in ambient air samples in the proximity of oil sands exploration. Quinones were characterized for their diurnal concentration variability, phase partitioning, and molecular size distribution. Gas-phase (GP) and particle-phase (PM) ambient air samples were collected separately in the summer; a lower quinone content was observed in the PM samples from continuous 24-h sampling than from combined 12-h sampling (day and night). The daytime/nocturnal samples demonstrated that nighttime conditions led to lower concentrations and some quinones not being detected. The highest quinone levels were associated with wind directions originating from oil sands exploration sites. The statistical correlation with primary pollutants directly emitted from oil sands industrial activities indicated that the bulk of the detected quinones did not originate directly from primary emission sources and that quinone formation paralleled a reduction in primary source NO_x levels. This suggests a secondary chemical transformation of primary pollutants as the origin of the determined quinones. Measurements of 19 quinones included five that have not previously been reported in ambient air or in Standard Reference Material 1649a/1649b and seven that have not been previously measured in ambient air in the underivatized form. This is the first paper to report on quinone characterization in secondary organic aerosols originating from oil sands activities, to distinguish chrysenequinone and anthraquinone positional isomers in ambient air, and to report the requirement of daylight conditions for benzo[a]pyrenequinone and naphthacenequinone to be present in ambient air.

Mechanistic and kinetic studies on OH-initiated atmospheric oxidation degradation of benzo[α]pyrene in the presence of O2 and NO(x)

The degradation of polycyclic aromatic hydrocarbons (PAHs) in the atmosphere can lead to toxic derivatives which contribute to the carcinogenic potential of particulate organic matter. This paper aimed to investigate the mechanism of the OH-initiated oxidation degradation of benzo[α]pyrene (BaP), a cancer risk indicator. High-accuracy molecular orbital calculations were carried out, and all of the possible degradation pathways were discussed. The theoretical results were compared with the available experimental observation. The possible secondary reactions were also investigated. The rate constants of the crucial elementary steps were evaluated by using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The dominant degradation products involve benzo[α]pyren-ol, nitro-benzo[α]pyrene, benzo[α]pyrene-7,10-dione as well as several ring-opened products such as alkyl substituted benzanthraldehyde et al. In particular, water plays an important role in the degradation pathways leading to the formation of nitro-benzo[α]pyrene. This work provides a comprehensive investigation of the OH-initiated degradation of BaP and should help to clarify its potential risk.

PCDD and PCDF concentrations in a traffic tunnel environment

In an effort to understand the fundamental aspects of air quality in traffic tunnel environments, field campaigns were conducted to measure polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and other important pollutants within two traffic tunnels in Nam San (NS) and Hong Ji (HJ) in Korea in 2009 and 2010. The mean concentrations of ∑PCDD/Fs (in fg/m(3)) at the two tunnel sites were 1270 (± 880) and 1200 (± 810), respectively. These values were moderately lower than those measured at a non-tunnel urban background site (1350 (± 780) fg/m(3))--selected as a reference in this study. In addition, seasonal patterns of dioxin concentrations were clearly evident at the traffic tunnels like the urban reference site, showing higher levels during the winter (and spring) than the summer (and fall). The observed seasonal variations were driven by changes in the concentrations of ∑PCDF congeners, while ∑PCDD concentrations showed little seasonality. The results of our study suggest that there is no significant difference in source characteristics between the two investigated tunnel sites and urban location, although the role of gasoline and diesel fueled vehicles are considered as the major source in determining the PCDDs and PCDF levels in a tunnel environment. However, given the relative increase in other important ambient pollutant (e.g. PM10) concentrations over ∑PCDD/Fs in tunnel air (compared to urban background air), the balance of sources in tunnels is clearly different from those in urban air overall.

Twenty years of measurement of polycyclic aromatic hydrocarbons (PAHs) in UK ambient air by nationwide air quality networks

The impact of human activities on the health of the population and of the wider environment has prompted action to monitor the presence of toxic compounds in the atmosphere. Toxic organic micropollutants (TOMPs) are some of the most insidious and persistent of these pollutants. Since 1991 the United Kingdom has operated nationwide air quality networks to assess the presence of TOMPs, including polycyclic aromatic hydrocarbons (PAHs), in ambient air. The data produced in 2010 marked 20 years of nationwide PAH monitoring. This paper marks this milestone by providing a novel and critical review of the data produced since nationwide monitoring began up to the end of 2011 (the latest year for which published data is available), discussing how the networks performing this monitoring has evolved, and elucidating trends in the concentrations of the PAHs measured. The current challenges in the area and a forward look to the future of air quality monitoring for PAHs are also discussed briefly.

A temperature-based approach to predicting lost data from highly seasonal pollutant data sets

A new technique to predict concentrations of benzo[a]pyrene (BaP) in ambient air during periods of lost data has been developed and tested. This new technique is based on the relationship between ambient temperature and BaP concentration observed at individual monitoring stations over many years. The technique has been tested on monthly data of BaP concentrations in PM10 at individual monitoring stations on the UK PAH Monitoring Network. The annual average concentration values produced with and without the use of predicted data have been compared to the actual annual averages in the absence of any data loss. The use of predicted data is a significant improvement when compared with the averages produced in the absence of any data prediction and outperforms previous prediction strategies associated with intra-year trends. Furthermore the technique is suitable for the prediction of long periods of missing data, which other prediction techniques have not been able to deal with satisfactorily.

Data loss from time series of pollutants in ambient air exhibiting seasonality: consequences and strategies for data prediction

The effect of data loss on annual average concentrations of seasonal and non-seasonal pollutants in ambient air has been investigated. The bias caused to the true annual average has been shown to be significant for measurements of benzo[a]pyrene (BaP) in PM(10) (a highly seasonal pollutant) even when legislative requirements for data capture and time coverage are still met. In order to mitigate this bias, strategies to predict concentrations during periods of lost data have been tested. These have been based on fitting quadratic relationships to available data of BaP in PM(10) at individual monitoring stations on the UK PAH Monitoring Network. The annual average concentration values produced with and without the use of predicted data have been compared to the actual annual averages in the absence of data loss. The use of predicted data is a significant (but not universal) improvement at urban and rural monitoring stations where the data exhibit consistently good fits to the predicted quadratic model. At industrial stations, where the quadratic model fails, the use of predicted data shows no improvement, although the effect of lost data at these sites on the annual average is much less noticeable because of their lack of seasonality.