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The Copenhagen Metro comprises four lines, the M1, M2, M3 and M4, with 25 subterranean stations and an additional 14 stations above ground, serving ca. 80 million passengers annually. In this study we measure fine particulate matter (PM 2.5 ) and carbon dioxide (CO 2 ) concentrations in stations and in trains across the entire system. In partially underground lines, high PM 2.5 concentrations with an average of 109 μg m -3 are found in below-ground stations. The observed correlation between PM 2.5 concentration and distance between a station and a tunnel exit is attributed to ventilation via the piston effect. The piston effect via tunnel draught relief shafts was therefore found to be relatively limited. Filter samples of particulate matter are analysed using particle-induced X-ray emission and show an iron content of 88.6 % by mass which is quite different from above-ground particulate matter and consistent with particle production by train wheels, rails and brakes. The average concentration measured at the stations of a recently opened (2019) fully underground M3 closed loop line is 168 μg m -3 , further demonstrating that while piston effect-driven ventilation is effective in close proximity to tunnel openings, it is relatively limited via tunnel draught relief shafts. Measurements onboard trains show even higher PM 2.5 concentrations and the patterns in CO 2 concentrations suggest carriage ventilation by tunnel air. Ventilation via doors during platform stops caused a drop in observed PM (and CO 2 ) at stations, but the system is surprisingly polluted despite its recent construction. CO 2 mixing ratios ranged from ambient to around 600 ppm. Measures should be taken to control PM levels using a combination of source control and increased clean air supply of the Copenhagen and other similar metro systems., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Biotrickling filters (BTFs) applied to hydrophobic volatile organic compounds (VOCs) suffer from limited mass transfer. Phase transfer kinetic and equilibrium effects limit the biodegradation of hydrophobic VOCs especially at high concentrations. This study evaluates two strategies for overcoming the problem. First, a natural process was used to enhance the aqueous availability of styrene, a hydrophobic VOC model, by inoculating the BTF with a mixture of biosurfactant-generating bacteria. This method achieved a maximum elimination capacity (ECmax) of 139 g m -3 h -1 in the BTF at an empty bed residence time (EBRT) of 60s. The highest concentrations of the biosurfactants surfactin and rhamnolipid were 205 and 86 mg L -1 , respectively, in this step. Sequencing 16S rRNA confirmed the presence of biosurfactant-producing bacteria capable of biodegrading styrene in the BTF including Bacillus sonorensis, Bacillus subtilis, Lysinibacillus sphaericus, Lysinibacillus fusiformis, Alcaligenes feacalis, Arthrobacter creatinolyticus, and Kocuria rosea. Second, the effect of adding H 2 O 2 to the recycle liquid on the BTF performance was determined. The biodegradation and mineralization of styrene in the BTF operated at a loading rate of 266 g m -3 h -1 and H 2 O 2 /styrene molar ratio of 0.05 with EBRT as short as 15 s were 94% and 53%, respectively, with the EC of 250 g m -3 h -1 . High concentrations of antioxidant enzymes (peroxidase and catalase: 56 and 7 U g biomass -1 , respectively) were produced and biosurfactant generation was increased in this step, contributing to enhanced styrene biodegradation and mineralization. The styrene biodegradation and mineralization values in the BTF in the last day operated under similar conditions but without H 2 O 2 were 11.4% and 5.3%, respectively. The bacterial population had no considerable change in the BTF after adding H 2 O 2 . Accordingly, stimulating the BTF inoculated with biosurfactant-generating bacteria with H 2 O 2 is a promising strategy for improving the biodegradation of hydrophobic VOCs., (Copyright © 2019 Elsevier B.V. All rights reserved.)

California methane (CH 4 ) emissions are quantified for three years from two tower networks and one aircraft campaign. We used backward trajectory simulations and a mesoscale Bayesian inverse model, initialized by three inventories, to achieve the emission quantification. Results show total statewide CH 4 emissions of 2.05 ± 0.26 (at 95% confidence) Tg/yr, which is 1.14 to 1.47 times greater than the anthropogenic emission estimates by California Air Resource Board (CARB). Some of differences could be biogenic emissions, superemitter point sources, and other episodic emissions which may not be completely included in the CARB inventory. San Joaquin Valley (SJV) has the largest CH 4 emissions (0.94 ± 0.18 Tg/yr), followed by the South Coast Air Basin, the Sacramento Valley, and the San Francisco Bay Area at 0.39 ± 0.18, 0.21 ± 0.04, and 0.16 ± 0.05 Tg/yr, respectively. The dairy and oil/gas production sources in the SJV contribute 0.44 ± 0.36 and 0.22 ± 0.23 Tg CH 4 /yr, respectively. This study has important policy implications for regulatory programs, as it provides a thorough multiyear evaluation of the emissions inventory using independent atmospheric measurements and investigates the utility of a complementary multiplatform approach in understanding the spatial and temporal patterns of CH 4 emissions in the state and identifies opportunities for the expansion and applications of the monitoring network.

In this study, we explore observational, experimental, methodological, and practical aspects of the flux quantification of greenhouse gases from local point sources by using in situ airborne observations, and suggest a series of conceptual changes to improve flux estimates. We address the major sources of uncertainty reported in previous studies by modifying (1) the shape of the typical flight path, (2) the modeling of covariance and anisotropy, and (3) the type of interpolation tools used. We show that a cylindrical flight profile offers considerable advantages compared to traditional profiles collected as curtains, although this new approach brings with it the need for a more comprehensive subsequent analysis. The proposed flight pattern design does not require prior knowledge of wind direction and allows for the derivation of an ad hoc empirical correction factor to partially alleviate errors resulting from interpolation and measurement inaccuracies. The modified approach is applied to a use-case for quantifying CH 4 emission from an oil field south of San Ardo, CA, and compared to a bottom-up CH 4 emission estimate.

Gaseous elemental mercury (GEM) and reactive gaseous mercury (RGM) were measured during an eight month circumnavigation to obtain knowledge of their worldwide distributions in the marine boundary layer (MBL). Background GEM concentrations were found to be 1.32 ± 0.2 ng/m(3) (summer) and 2.62 ± 0.4 ng/m(3) (spring) in the northern hemisphere and 1.27 ± 0.2 ng/m(3) (spring and summer) in the southern hemisphere. Radiation and relative humidity are shown to control diurnal cycles of RGM. During the cruise the ship passed areas of clean MBL air, air influenced by biomass burning (South Atlantic) and air with high concentrations of GEM and RGM of unknown origin (Antarctic). High GEM concentrations above the Atlantic indicate that emission from the ocean can be an important GEM source. Our data combined with data from earlier cruises provides adequate information to establish a seasonal cycle for the Atlantic. Results show a cycle similar to that found at Mace Head, Ireland but with larger amplitude. We have improved the basic knowledge of mean GEM and RGM concentrations in the MBL worldwide and shown how natural sources and reemissions can affect GEM concentrations in the MBL.

We investigate the benefit of assimilating high spatial‐temporal resolution nitrogen dioxide (NO2) measurements from a geostationary (GEO) instrument such as Tropospheric Emissions: Monitoring of Pollution (TEMPO) versus a low‐earth orbit (LEO) platform like TROPOspheric Monitoring Instrument (TROPOMI) on the inverse modeling of nitrogen oxides (NOx) emissions. We generated synthetic TEMPO and TROPOMI NO2 measurements based on emissions from the COVID‐19 lockdown period. Starting with emissions levels prior to the lockdown, we use the Weather Research and Forecasting Model coupled with Chemistry/Data Assimilation Research Testbed (WRF‐Chem/DART) to assimilate these pseudo‐observations in Observing System Simulation Experiments to adjust NOx emissions and quantify how well the assimilation of TEMPO versus TROPOMI measurements recovers the lockdown‐induced emissions changes. We find that NOx emission biases can be ameliorated using half as many simulation days when assimilating GEO observations, and the estimated NOx emissions in 23 out of 29 major urban regions in the US are more accurate. The root mean square error and coefficient of determination of posterior NOx emissions are reduced by 12.5%–41.5% and 1.5%–17.1%, respectively, across different regions. We conduct sensitivity experiments that use different data assimilation (DA) configurations to assimilate synthetic GEO observations. Results demonstrate that the temporal width of the DA window introduces −10% to −20% biases in the emissions inversion and constraining both NOx concentrations and emissions simultaneously yields the most accurate NOx emissions estimates. Our work serves as a valuable reference on how to appropriately assimilate GEO observations for constraining NOx emissions in future studies. Plain Language Summary: Nitrogen oxides (NOx) are major air pollutants and precursors to tropospheric ozone and secondary inorganic aerosols. The diverse natural and anthropogenic sources of NOx pose a challenge for NOx emissions estimates. Inverse modeling techniques which use observations to infer emissions can be applied to improve our understanding of anthropogenic NOx emissions. This study aims to compare the ability of the new geostationary (GEO) instrument Tropospheric Emissions: Monitoring of Pollution (TEMPO) and the existing low‐earth orbit instrument TROPOspheric Monitoring Instrument (TROPOMI) to constrain NOx emissions. Synthetic TEMPO and TROPOMI NO2 measurements are generated and assimilated to constrain NOx emissions in an idealized experiment in which the "true" emissions are known. The results show the true NOx emissions can be retrieved using half as many simulation days when assimilating GEO NO2 observations. Moreover, the experiment that assimilates GEO NO2 observations improves the accuracy of estimated NOx emissions by 12.5%–41.5% and 1.5%–17.1% in terms of root mean square error and coefficient of determination, respectively, across different air quality regions. The NOx emissions in most urban regions are better constrained when assimilating GEO NO2 data. We also propose best practices for assimilating GEO NO2 observations, which can serve as reference for future research. Key Points: True NOx emissions can be recovered using half as many simulation days when assimilating synthetic Tropospheric Emissions: Monitoring of Pollution (TEMPO) observations rather than TROPOspheric Monitoring InstrumentAssimilating synthetic TEMPO observations improve emissions inversion accuracy by 13%–42% across different regions of USThe best estimates of NOx emissions are achieved by using short data assimilation window (e.g., 30 min) and updating concentrations/emissions jointly [ABSTRACT FROM AUTHOR]

We present a detailed analysis of measurements of ozone (O3) and aerosols collected using multiple platforms and instruments during a 5‐day mini‐campaign in Alabama, for improving our understanding of the chemistry and air quality in the Southeastern United States. Our study features the first on‐road O3 lidar profiling on highways and the coordinated airborne observations that captured O3 and aerosol structures associated with synoptic weather conditions, stratospheric intrusions, wildfire smoke transport, and enhancements in urban areas. The Doppler wind lidar data suggest that the vertical structures and variability of O3 and aerosols at Dauphin Island, a coastal site in the Gulf of Mexico, were related to both the synoptic flow and local‐scale land‐sea breeze circulation. The onshore gulf breeze, which typically started in the afternoon, helped to quickly dilute air pollutants near the surface, resulting in large gradients toward inland areas. In contrast, gradients were much smaller on days without a gulf breeze. O3 and humidity were highly anti‐correlated in the lower free troposphere, but weakly anti‐correlated in the planetary boundary layer (PBL) suggesting more complicated emissions and sink of O3 in the PBL. Our on‐road and airborne measurement strategies to quantify the vertical and horizontal chemical gradients will be useful for future geostationary satellite validation. Plain Language Summary: We conducted a study in Alabama to measure ozone, aerosol, and meteorological variables using ground‐based, mobile, and airborne systems. Our study included the first use of a mobile ozone lidar, a remote sensing instrument, collecting data while driving on highways. The lidar captured the ozone structures related to urban emissions, weather conditions, and fire smoke transported from remote locations. We found that the distributions and variations of ozone and aerosols at Dauphin Island (DI), a coastal site in the Gulf of Mexico, were influenced by both large‐scale weather patterns and local winds called sea breeze that blow from the land to the sea in the morning and reverse the direction in the afternoon. We found the sea breeze helped to improve the air quality at DI, different from other megacities with much higher pollution. However, the sea breeze resulted in significant horizontal ozone differences, similar to other locations. We found a strong negative relationship between ozone and humidity at higher altitudes, but a weaker negative relationship at lower altitudes due to more complex emissions and loss of ozone at the surface. Our techniques to measure ozone and aerosol gradients vertically and horizontally will be valuable for evaluating future satellite observations. Key Points: 1. The first on‐road ozone lidar profiling data for quantification of 3‐D ozone and aerosol structures2. Large horizontal ozone gradients due to gulf breeze suggested by collocated ozone lidar, Doppler wind lidar, and airborne measurements3. Air with the stratosphere‐to‐troposphere transport source observed beneath the wildfire smoke [ABSTRACT FROM AUTHOR]

The conventional two-wavelength differential absorption lidar (DIAL) has measured air pollutants such as nitrogen dioxide (NO 2). However, high concentrations of aerosol within the planetary boundary layer (PBL) can cause significant retrieval errors using only a two-wavelength DIAL technique to measure NO 2. We proposed a new technique to obtain more accurate measurements of NO 2 using a three-wavelength DIAL technique based on an optical parametric oscillator (OPO) laser. This study derives the three-wavelength DIAL retrieval equations necessary to retrieve vertical profiles of NO 2 in the troposphere. Additionally, two rules to obtain the optimum choice of the three wavelengths applied in the retrieval are designed to help increase the differences in the NO 2 absorption cross-sections and reduce aerosol interference. NO 2 retrieval relative uncertainties caused by aerosol extinction, molecular extinction, absorption of gases other than the gas of interest and backscattering are calculated using two-wavelength DIAL (438 and 439.5 nm) and three-wavelength DIAL (438, 439.5 and 441 nm) techniques. The retrieval uncertainties in aerosol extinction using the three-wavelength DIAL technique are reduced to less than 2 % of those when using the two-wavelength DIAL technique. Moreover, the retrieval uncertainty analysis indicates that the three-wavelength DIAL technique can reduce more fluctuation caused by aerosol backscattering than the two-wavelength DIAL technique. This study presents NO 2 concentration profiles which were obtained using the HU (Hampton University) three-wavelength OPO DIAL. As a first step to assess the accuracy of the HU lidar NO 2 profiles, we compared the NO 2 profiles to simulated data from the Weather Research and Forecasting Chemistry (WRF-Chem) model. This comparison suggests that the NO 2 profiles retrieved with the three-wavelength DIAL technique have similar vertical structure and magnitudes typically within ± 0.1 ppb compared to modeled profiles. [ABSTRACT FROM AUTHOR]

The conventional two-wavelength Differential Absorption Lidar (DIAL) has measured air pollutants such as nitrogen dioxide (NO2). However, high concentrations of aerosol within the planetary boundary layer (PBL) can cause significant retrieval errors using only a two wavelength DIAL technique to measure NO2. We proposed a new technique to obtain more accurate measurements of NO2 using a three-wavelength DIAL technique based on an Optical Parametric Oscillator (OPO) laser. This study derives the three-wavelength DIAL retrieval equations necessary to retrieve vertical profiles of NO2 in the troposphere. Additionally, two rules to obtain the optimum choice of the three wavelengths applied in the retrieval are designed to help increase the differences of the NO2 absorption cross sections and reduce aerosol interference. NO2 retrieval relative uncertainties caused by aerosol extinction, molecular extinction, absorption of gases other than the gas of interest and backscattering are calculated using two-wavelength DIAL (438 nm and 439.5 nm) and three-wavelength DIAL (438 nm, 439.5 nm and 441 nm) techniques. The retrieval uncertainties of aerosol extinction using the three-wavelength DIAL technique are reduced to less than 2% of using the two-wavelength DIAL technique. Moreover, the retrieval uncertainty analysis indicates that the three-wavelength DIAL technique can reduce more fluctuation caused by aerosol backscattering than two wavelength DIAL technique. This study presents NO2 concentration profiles which were obtained using the HU (Hampton University) three-wavelength OPO DIAL. As a first step to assess the accuracy of the HU lidar NO2 profiles we compared the retrievals to simulated data from WRF-Chem model. This comparison suggests that the NO2 profiles retrieved with the three-wavelength DIAL technique have similar vertical structure, and magnitudes typically within ±0.1 ppb, of modeled profiles. [ABSTRACT FROM AUTHOR]

Secondary organic aerosol (SOA) plays a central role in air pollution and climate. However, the description of the sources and mechanisms leading to SOA is elusive despite decades of research. While stable isotope analysis is increasingly used to constrain sources of ambient aerosol, in many cases it is difficult to apply because neither the isotopic composition of aerosol precursors nor the fractionation of aerosol forming processes is well characterised. In this paper, SOA formation from ozonolysis of α-pinene - an important precursor and perhaps the best-known model system used in laboratory studies - was investigated using position-dependent and average determinations of 13C in α-pinene and advanced analysis of reaction products using thermal-desorption proton-transfer-reaction mass spectrometry (PTR-MS). The total carbon (TC) isotopic composition δ13C of the initial α-pinene was measured, and the δ13C of the specific carbon atom sites was determined using positionspecific isotope analysis (PSIA). The PSIA analysis showed variations at individual positions from -6.9 to C10:5‰relative to the bulk composition. SOA was formed from α-pinene and ozone in a constant-flow chamber under dark, dry, and low-NOx conditions, with OH scavengers and in the absence of seed particles. The excess of ozone and long residence time in the flow chamber ensured that virtually all α-pinene had reacted. Product SOA was collected on two sequential quartz filters. The filters were analysed offline by heating them stepwise from 100 to 400 °C to desorb organic compounds that were (i) detected using PTR-MS for chemical analysis and to determine the O: C ratio, and (ii) converted to CO2 for 13C analysis. More than 400 ions in the mass range 39-800 Da were detected from the desorbed material and quantified using a PTR-MS. The largest amount desorbed at 150 °C. The O: C ratio of material from the front filter increased from 0.18 to 0.25 as the desorption temperature was raised from 100 to 250 °C. At temperatures above 250 °C, the O: C ratio of thermally desorbed material, presumably from oligomeric precursors, was constant. The observation of a number of components that occurred across the full range of desorption temperatures suggests that they are generated by thermal decomposition of oligomers. The isotopic composition of SOA was more or less independent of desorption temperature above 100 °C. TC analysis showed that SOA was enriched in 13C by 0.6-1.2‰ relative to the initial α-pinene. According to mass balance, gas-phase products will be depleted relative to the initial α-pinene. Accordingly, organic material on the back filters, which contain adsorbed gas-phase compounds, is depleted in 13C in TC by 0.7‰ relative to the initial α-pinene, and by 1.3‰compared to SOA collected on the front filter. The observed difference in 13C between the gas and particle phases may arise from isotope-dependent changes in the branching ratios in the α-pinene + O3 reaction. Alternatively, some gasphase products involve carbon atoms from highly enriched and depleted sites, as shown in the PSIA analysis, giving a non-kinetic origin to the observed fractionations. In either case, the present study suggests that the site-specific distribution of 13C in the source material itself governs the abundance of 13C in SOA. [ABSTRACT FROM AUTHOR]

High ozone (O 3 ) concentrations at low altitudes (1.5–4 km) were detected from airborne Alpha Jet Atmospheric eXperiment (AJAX) measurements on 30 May 2012 off the coast of California (CA). We investigate the causes of those elevated O 3 concentrations using airborne measurements and various models. GEOS-Chem simulation shows that the contribution from local sources is likely small. A back-trajectory model was used to determine the air mass origins and how much they contributed to the O 3 over CA. Low-level potential vorticity (PV) from Modern Era Retrospective analysis for Research and Applications 2 (MERRA-2) reanalysis data appears to be a result of the diabatic heating and mixing of airs in the lower altitudes, rather than be a result of direct transport from stratospheric intrusion. The Q diagnostic, which is a measure of the mixing of the air masses, indicates that there is sufficient mixing along the trajectory to indicate that O 3 from the different origins is mixed and transported to the western U.S. The back-trajectory model simulation demonstrates the air masses of interest came mostly from the mid troposphere (MT, 76%), but the contribution of the lower troposphere (LT, 19%) is also significant compared to those from the upper troposphere/lower stratosphere (UT/LS, 5%). Air coming from the LT appears to be mostly originating over Asia. The possible surface impact of the high O 3 transported aloft on the surface O 3 concentration through vertical and horizontal transport within a few days is substantiated by the influence maps determined from the Weather Research and Forecasting–Stochastic Time Inverted Lagrangian Transport (WRF-STILT) model and the observed increases in surface ozone mixing ratios. Contrasting this complex case with a stratospheric-dominant event emphasizes the contribution of each source to the high O 3 concentration in the lower altitudes over CA. Integrated analyses using models, reanalysis, and diagnostic tools, allows high ozone values detected by in-situ measurements to be attributed to multiple source processes. [ABSTRACT FROM AUTHOR]