Measurement of NO(x) fluxes from a tall tower in Central London, UK and comparison with emissions inventories

Portable analytical system integrating purge and trap and microplasma optical emission spectrometry for online discriminative detection of NOx

Pollution caused by nitrogen oxides (NOx, mainly NO and NO2) has attracted considerable attention due to its negative implications on the environment and human health. Herein, a novel, portable, and battery-operated system integrating a purge and trap (P&T) system with a miniaturized point discharge optical emission spectrometer (μPD-OES) was developed for monitoring atmospheric NOx. Among NOx, NO2 can be selectively and directly absorbed by sodium hydroxide (NaOH) solution and converted to nitrite (NO2-). In contrast to NO2, NO cannot be absorbed by NaOH and must be oxidized to NO2 by KMnO4 prior to its absorption. When NO2 and NOx samplers were exposed simultaneously, NO can be calculated as the difference. The concentration of NOx in the air was determined by analyzing the NO2-. The obtained NO2- solution was purged with an argon carrier gas at a rate of 400 mL min-1 for 5 min to remove all the volatile organic compounds contained in the solution before reacting with cyclamate to generate volatile cyclohexene, which was efficiently isolated and preconcentrated by P&T. Subsequently, the cyclohexene was desorbed from the P&T unit and swept into the μPD-OES for its detection under optimized conditions. The system demonstrated a detection limit of 0.02 ppbv (0.04 μg/m3) for NO2, surpassing conventional methods. Precision expressed as the relative standard deviation (RSD, n = 11) was better than 3.5 %. The system was validated by analyzing two NO2 standard gases and fourteen ambient air samples, and the obtained analytical results demonstrated its promising potential for rapid, sensitive, and precise NOx evaluation and monitoring.

Evidence of Heating-Dominated Urban NOx Emissions

Road transport NOx emissions in many high-income countries have steadily reduced due to improved exhaust after-treatment technology. However, ambient concentrations of NO2, O3 and PM2.5 continue to exceed World Health Organization guidelines in many cities globally. The megacity of London has taken an international lead in mobility interventions through the use of low-emission zones. Using long-term air pollution flux measurements made from a communications tower, we show that the largest source of NOx emissions in central London has transitioned from road transport to space heating. Observations and supporting consumption/mobility data indicated that natural gas combustion in boilers was responsible for 72 ± 17% of NOx emissions in the measurement footprint (average years 2021-2023). Road transport has dominated air quality thinking on NO2 for many decades. However, in urban environments that are reliant on natural gas, building heating may now be an effective sector to prioritize for further NOx emissions intervention. With system-wide changes in the heat and power sector expected in the coming decades to achieve decarbonisation pledges, we project that very low urban emissions of NOx are achievable. The trajectory will, however, depend on choices made around urban buildings and their associated infrastructure and whether low-carbon fuel combustion or electrification pathways are chosen. We estimate a damage cost penalty of up to £600 M in the U.K. should hydrogen combustion replace natural gas for heating rather than technologies such as heat pumps.

Vertical distribution and transport of microplastics in the urban atmosphere: New insights from field observations

The distribution and transport of atmospheric microplastics (AMPs) have raised concerns regarding their potential effects on the environment and human health. Although previous studies have reported the presence of AMPs at ground level, there is a lack of comprehensive understanding of their vertical distribution in urban environments. To gain insight into the vertical profile of AMPs, field observations were conducted at four different heights (ground level, 118 m, 168 m and 488 m) of the Canton Tower in Guangzhou, China. Results showed that the profiles of AMPs and other air pollutants had similar layer distribution patterns, although their concentrations differed. The majority of AMPs were composed of polyethylene terephthalate and rayon fibers ranging from 30 to 50 μm. As a result of atmospheric thermodynamics, AMPs generated at ground level were only partially transported upward, leading to a decrease in their abundance with increasing altitude. The study found that the stable atmospheric stability and lower wind speed between 118 m and 168 m resulted in the formation of a fine layer where AMPs tended to accumulate instead of being transported upward. This study for the first time delineated the vertical profile of AMPs within the atmospheric boundary layer, providing valuable data for understanding the environmental fate of AMPs.

Vertical distributions of atmospheric black carbon in dry and wet seasons observed at a 356-m meteorological tower in Shenzhen, South China

Black carbon (BC) is a vital climate forcer in the atmosphere, but measurements of BC vertical profiles near the surface remain limited. This study investigates time-resolved vertical profiling of BC in both dry (December 2017) and wet (August 2018) seasons in Shenzhen, China, at a 356-m meteorological tower. In the dry season, five micro-aethalometers were deployed at different heights (2, 50, 100, 200, and 350 m), while four heights (2, 100, 200, and 350 m) were measured in the wet season. The concentrations of equivalent BC (eBC) showed a decreasing trend with altitude in the dry season, while a weaker vertical gradient was observed in the wet season. The diurnal variability of eBC in the dry season is also more significant than in the wet season. Correlation analysis between eBC concentrations at the ground and those at the upper levels suggest a better vertical mixing of eBC in the wet season than in the dry season. In the wet season when south wind prevailed, eBC concentration at ground level was likely reduced by the large amount of vegetation cover south to the sampling site. In the dry season, eBC concentrations at 350 m show little dependence on wind speed, implying that local emissions have a limited effect on eBC concentrations at 350 m. In the wet season when brown carbon influence was weak, higher wind speed leads to a higher Ångström exponent (AAE) at 350 m, likely associated with more aged BC particles. Cluster analysis of backward trajectories suggests that high eBC concentration was associated with air masses from Central China in both seasons. This study provides a better understanding on the influencing factors that affect the vertical distributions of BC in the lower part of the boundary layer.

Learning from the COVID-19 lockdown in berlin: Observations and modelling to support understanding policies to reduce NO2

Urban air pollution is a substantial threat to human health. Traffic emissions remain a large contributor to air pollution in urban areas. The mobility restrictions put in place in response to the COVID-19 pandemic provided a large-scale real-world experiment that allows for the evaluation of changes in traffic emissions and the corresponding changes in air quality. Here we use observational data, as well as modelling, to analyse changes in nitrogen dioxide, ozone, and particulate matter resulting from the COVID-19 restrictions at the height of the lockdown period in Spring of 2020. Accounting for the influence of meteorology on air quality, we found that reduction of ca. 30-50 % in traffic counts, dominated by changes in passenger cars, corresponded to reductions in median observed nitrogen dioxide concentrations of ca. 40 % (traffic and urban background locations) and a ca. 22 % increase in ozone (urban background locations) during weekdays. Lesser reductions in nitrogen dioxide concentrations were observed at urban background stations at weekends, and no change in ozone was observed. The modelled reductions in median nitrogen dioxide at urban background locations were smaller than the observed reductions and the change was not significant. The model results showed no significant change in ozone on weekdays or weekends. The lack of a simulated weekday/weekend effect is consistent with previous work suggesting that NOx emissions from traffic could be significantly underestimated in European cities by models. These results indicate the potential for improvements in air quality due to policies for reducing traffic, along with the scale of reductions that would be needed to result in meaningful changes in air quality if a transition to sustainable mobility is to be seriously considered. They also confirm once more the highly relevant role of traffic for air quality in urban areas.

Fluxes of nitrogen oxides above a subtropical forest canopy in China

Dry deposition of Nitrogen (N) in forests is commonly estimated from inferential method and/or throughfall measurements, with inevitable uncertainty. In this study, we applied an aerodynamic gradient method to directly measure the nitrogen oxides (NO_x) flux above the canopy of a subtropical forest in southeastern China for two consecutive years. The flux and transfer velocity generally reached the maximum absolute values in the midday, with the largest diurnal maximum of absolute flux values observed in the winter of 2015 and that of transfer velocity in the autumn of 2015. The annual average transfer velocity was -0.79 and -0.38 cm s^-1 in 2015 and 2016, respectively. Although the net downward NO_x fluxes predominated for both years, upward flux (net emission) of NO_x was observed during spring months, which reflected the possible bi-directional exchange balanced by soil-atmosphere and foliage-atmosphere exchanges. The NO_x concentration seemed to be the most important factor controlling the NO_x exchange above canopy, and could mainly explain the seasonal variation of N deposition. The linear regression between the NO_x flux and concertation was explored, and it was observed that the deposition of NO_x was offset by possible underlayer emission of NO_x when the ambient NO_x concentration below1.7 ppbv and 1.9 ppbv at night and in the day, respectively. The average dry deposition of NO_x for the two years was 6.28 ± 0.06 kg N ha^-1 a^-1, >40% of which might be uptake by the canopy, estimated by comparing the wet/throughfall deposition measurement of nitrate with the observation of NO_x flux. This indicated the importance of stomatal uptake of NO_x in nitrogen budget in subtropical forests.

Spatially resolved flux measurements of NOx from London suggest significantly higher emissions than predicted by inventories

To date, direct validation of city-wide emissions inventories for air pollutants has been difficult or impossible. However, recent technological innovations now allow direct measurement of pollutant fluxes from cities, for comparison with emissions inventories, which are themselves commonly used for prediction of current and future air quality and to help guide abatement strategies. Fluxes of NOx were measured using the eddy-covariance technique from an aircraft flying at low altitude over London. The highest fluxes were observed over central London, with lower fluxes measured in suburban areas. A footprint model was used to estimate the spatial area from which the measured emissions occurred. This allowed comparison of the flux measurements to the UK's National Atmospheric Emissions Inventory (NAEI) for NOx, with scaling factors used to account for the actual time of day, day of week and month of year of the measurement. The comparison suggests significant underestimation of NOx emissions in London by the NAEI, mainly due to its under-representation of real world road traffic emissions. A comparison was also carried out with an enhanced version of the inventory using real world driving emission factors and road measurement data taken from the London Atmospheric Emissions Inventory (LAEI). The measurement to inventory agreement was substantially improved using the enhanced version, showing the importance of fully accounting for road traffic, which is the dominant NOx emission source in London. In central London there was still an underestimation by the inventory of 30-40% compared with flux measurements, suggesting significant improvements are still required in the NOx emissions inventory.

Urban eddy covariance measurements reveal significant missing NOx emissions in Central Europe

Nitrogen oxide (NO_x) pollution is emerging as a primary environmental concern across Europe. While some large European metropolitan areas are already in breach of EU safety limits for NO₂, this phenomenon does not seem to be only restricted to large industrialized areas anymore. Many smaller scale populated agglomerations including their surrounding rural areas are seeing frequent NO₂ concentration violations. The question of a quantitative understanding of different NO_x emission sources is therefore of immanent relevance for climate and air chemistry models as well as air pollution management and health. Here we report simultaneous eddy covariance flux measurements of NO_x, CO₂, CO and non methane volatile organic compound tracers in a city that might be considered representative for Central Europe and the greater Alpine region. Our data show that NO_x fluxes are largely at variance with modelled emission projections, suggesting an appreciable underestimation of the traffic related atmospheric NO_x input in Europe, comparable to the weekend-weekday effect, which locally changes ozone production rates by 40%.

Have vehicle emissions of primary NO2 peaked?

Reducing ambient concentrations of nitrogen dioxide (NO₂) remains a key challenge across many European urban areas, particularly close to roads. This challenge mostly relates to the lack of reduction in emissions of oxides of nitrogen (NO_x) from diesel road vehicles relative to the reductions expected through increasingly stringent vehicle emissions legislation. However, a key component of near-road concentrations of NO₂ derives from directly emitted (primary) NO₂ from diesel vehicles. It is well-established that the proportion of NO₂ (i.e. the NO₂/NO_x ratio) in vehicle exhaust has increased over the past decade as a result of vehicle after-treatment technologies that oxidise carbon monoxide and hydrocarbons and generate NO₂ to aid the emissions control of diesel particulate. In this work we bring together an analysis of ambient NO_x and NO₂ measurements with comprehensive vehicle emission remote sensing data obtained in London to better understand recent trends in the NO₂/NO_x ratio from road vehicles. We show that there is evidence that NO₂ concentrations have decreased since around 2010 despite less evidence of a reduction in total NO_x. The decrease is shown to be driven by relatively large reductions in the amount of NO₂ directly emitted by vehicles; from around 25 vol% in 2010 to 15 vol% in 2014 in inner London, for example. The analysis of NO_x and NO₂ vehicle emission remote sensing data shows that these reductions have been mostly driven by reduced NO₂/NO_x emission ratios from heavy duty vehicles and buses rather than light duty vehicles. However, there is also evidence from the analysis of Euro 4 and 5 diesel passenger cars that as vehicles age the NO₂/NO_x ratio decreases. For example the NO₂/NO_x ratio decreased from 29.5 ± 2.0% in Euro 5 diesel cars up to one year old to 22.7 ± 2.5% for four-year old vehicles. At some roadside locations the reductions in primary NO₂ have had a large effect on reducing both the annual mean and number of hourly exceedances of the European Limit Values of NO₂.