NO2 Mixing Ratios Research Articles

New Observations of Upper Tropospheric NO 2 from TROPOMI

Abstract. Nitrogen oxides (NOx ≡ NO + NO2) in the NOx-limited upper troposphere (UT) are long-lived and so have a large influence on the oxidizing capacity of the troposphere and formation of the greenhouse gas ozone. Models misrepresent NOx in the UT and observations to address deficiencies in models are sparse. Here we obtain a year of near-global seasonal mean mixing ratios of NO2 in the UT (450–180 hPa) at 1 ° x 1° by applying cloud-slicing to partial columns of NO2 from TROPOMI. This follows refinement of the cloud-slicing algorithm with synthetic partial columns from the GEOS-Chem chemical transport model. We find that synthetic cloud-sliced UT NO2 are spatially consistent (R = 0.64) with UT NO2 calculated across the same cloud pressure range and scenes as are cloud-sliced (“true” UT NO2), but the cloud-sliced UT NO2 is 11–22 % more than the true all-sky seasonal mean. The largest contributors to differences between synthetic cloud-sliced and “true” UT NO2 are target resolution of the cloud-sliced product and uniformity of overlying stratospheric NO2. TROPOMI, prior to cloud-slicing, is corrected for a 13 % underestimate in stratospheric NO2 variance and a 50 % overestimate in free tropospheric NO2 determined by comparison to Pandora total columns at high-altitude sites in Mauna Loa, Izana and Altzomoni, and MAX-DOAS and Pandora tropospheric columns at Izana. Two cloud-sliced seasonal mean UT NO2 products for June 2019 to May 2020 are retrieved from corrected TROPOMI total columns using distinct TROPOMI cloud products that assume clouds are reflective boundaries (FRESCO-S) or water droplet layers (ROCINN-CAL). TROPOMI UT NO2 typically ranges from 20-30 pptv over remote oceans to > 80 pptv over locations with intense seasonal lightning. Spatial coverage is mostly in the tropics and subtropics with FRESCO-S and extends to the midlatitudes and polar regions with ROCINN-CAL, due to its greater abundance of optically thick clouds and wider cloud top altitude range. TROPOMI UT NO2 seasonal means are spatially consistent (R = 0.6–0.8) with an existing coarser spatial resolution (5° latitude x 8° longitude) UT NO2 product from the Ozone Monitoring Instrument (OMI). UT NO2 from TROPOMI is 12–26 pptv more than that from OMI due to increase in NO2 with altitude from the OMI pressure ceiling (280 hPa) to that for TROPOMI (180 hPa), but possibly also systematic altitude differences between the TROPOMI and OMI cloud products. The TROPOMI UT NO2 product offers potential to evaluate and improve representation of UT NOx in models and supplement aircraft observations that are sporadic and susceptible to large biases in the UT.

Meteorology-normalized impact of the COVID-19 lockdown upon NO<sub>2</sub> pollution in Spain

Abstract. The spread of the new coronavirus SARS-CoV-2 that causes COVID-19 forced the Spanish Government to implement extensive lockdown measures to reduce the number of hospital admissions, starting on 14 March 2020. Over the following days and weeks, strong reductions in nitrogen dioxide (NO2) pollution were reported in many regions of Spain. A substantial part of these reductions was obviously due to decreased local and regional anthropogenic emissions. Yet, the confounding effect of meteorological variability hinders a reliable quantification of the lockdown's impact upon the observed pollution levels. Our study uses machine-learning (ML) models fed by meteorological data along with other time features to estimate the “business-as-usual” NO2 mixing ratios that would have been observed in the absence of the lockdown. We then quantify the so-called meteorology-normalized NO2 reductions induced by the lockdown measures by comparing the estimated business-as-usual values with the observed NO2 mixing ratios. We applied this analysis for a selection of urban background and traffic stations covering the more than 50 Spanish provinces and islands. The ML predictive models were found to perform remarkably well in most locations, with an overall bias, root mean square error and correlation of +4 %, 29 % and 0.86, respectively. During the period of study, from the enforcement of the state of alarm in Spain on 14 March to 23 April, we found the lockdown measures to be responsible for a 50 % reduction in NO2 levels on average over all provinces and islands. The lockdown in Spain has gone through several phases with different levels of severity with respect to mobility restrictions. As expected, the meteorology-normalized change in NO2 was found to be stronger during phase II (the most stringent phase) and phase III of the lockdown than during phase I. In the largest agglomerations, where both urban background and traffic stations were available, a stronger meteorology-normalized NO2 change is highlighted at traffic stations compared with urban background sites. Our results are consistent with foreseen (although still uncertain) changes in anthropogenic emissions induced by the lockdown. We also show the importance of taking the meteorological variability into account for accurately assessing the impact of the lockdown on NO2 levels, in particular at fine spatial and temporal scales. Meteorology-normalized estimates such as those presented here are crucial to reliably quantify the health implications of the lockdown due to reduced air pollution.

MAX-DOAS measurements of tropospheric NO<sub>2</sub> and HCHO in Munich and the comparison to OMI and TROPOMI satellite observations

Abstract. We present two-dimensional scanning Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of nitrogen dioxide (NO2) and formaldehyde (HCHO) in Munich. Vertical columns and vertical distribution profiles of aerosol extinction coefficient, NO2 and HCHO are retrieved from the 2D MAX-DOAS observations. The measured surface aerosol extinction coefficients and NO2 mixing ratios derived from the retrieved profiles are compared to in situ monitoring data, and the surface NO2 mixing ratios show a good agreement with in situ monitoring data with a Pearson correlation coefficient (R) of 0.91. The aerosol optical depths (AODs) show good agreement as well (R = 0.80) when compared to sun photometer measurements. Tropospheric vertical column densities (VCDs) of NO2 and HCHO derived from the MAX-DOAS measurements are also used to validate Ozone Monitoring Instrument (OMI) and TROPOspheric Monitoring Instrument (TROPOMI) satellite observations. Monthly averaged data show a good correlation; however, satellite observations are on average 30 % lower than the MAX-DOAS measurements. Furthermore, the MAX-DOAS observations are used to investigate the spatiotemporal characteristic of NO2 and HCHO in Munich. Analysis of the relations between aerosol, NO2 and HCHO shows higher aerosol-to-HCHO ratios in winter, which reflects a longer atmospheric lifetime of secondary aerosol and HCHO during winter. The analysis also suggests that secondary aerosol formation is the major source of these aerosols in Munich.

Evaluation of nitrogen oxides (NO<sub><i>x</i></sub>) sources and sinks and ozone production in Colombia and surrounding areas

Abstract. In Colombia, industrialization and a shift towards intensified agriculture have led to increased emissions of air pollutants. However, the baseline state of air quality in Colombia is relatively unknown. In this study we aim to assess the baseline state of air quality in Colombia with a focus on the spatial and temporal variability in emissions and atmospheric burden of nitrogen oxides (NOx = NO + NO2) and evaluate surface NOx, ozone (O3) and carbon monoxide (CO) mixing ratios. We quantify the magnitude and spatial distribution of the four major NOx sources (lightning, anthropogenic activities, soil biogenic emissions and biomass burning) by integrating global NOx emission inventories into the mesoscale meteorology and atmospheric chemistry model, namely Weather Research and Forecasting (WRF) coupled with Chemistry (collectively WRF-Chem), at a similar resolution (∼25 km) to the Emission Database for Global Atmospheric Research (EDGAR) anthropogenic emission inventory and the Ozone Monitoring Instrument (OMI) remote sensing observations. The model indicates the largest contribution by lightning emissions (1258 Gg N yr−1), even after already significantly reducing the emissions, followed by anthropogenic (933 Gg N yr−1), soil biogenic (187 Gg N yr−1) and biomass burning emissions (104 Gg N yr−1). The comparison with OMI remote sensing observations indicated a mean bias of tropospheric NO2 columns over the whole domain (WRF-Chem minus OMI) of 0.02 (90 % CI: [−0.43, 0.70]) ×1015 molecules cm−2, which is <5 % of the mean column. However, the simulated NO2 columns are overestimated and underestimated in regions where lightning and biomass burning emissions dominate, respectively. WRF-Chem was unable to capture NOx and CO urban pollutant mixing ratios, neither in timing nor in magnitude. Yet, WRF-Chem was able to simulate the urban diurnal cycle of O3 satisfactorily but with a systematic overestimation of 10 parts per billion (ppb) due to the equally large underestimation of NO mixing ratios and, consequently, titration. This indicates that these city environments are in the NOx-saturated regime with frequent O3 titration. We conducted sensitivity experiments with an online meteorology–chemistry single-column model (SCM) to evaluate how WRF-Chem subgrid-scale-enhanced emissions could explain an improved representation of the observed O3, CO and NOx diurnal cycles. Interestingly, the SCM simulation, showing especially a shallower nocturnal inversion layer, results in a better representation of the observed diurnal cycle of urban pollutant mixing ratios without an enhancement in emissions. This stresses that, besides application of higher-resolution emission inventories and model experiments, the diurnal cycle in boundary layer dynamics (and advection) should be critically evaluated in models such as WRF-Chem to assess urban air quality. Overall, we present a concise method to quantify air quality in regions with limited surface measurements by integrating in situ and remote sensing observations. This study identifies four distinctly different source regions and shows their interannual and seasonal variability during the last 1.5 decades. It serves as a base to assess scenarios of future air quality in Colombia or similar regions with contrasting emission regimes, complex terrain and a limited air quality monitoring network.

OH and HO<sub>2</sub> radical chemistry in a midlatitude forest: measurements and model comparisons

Abstract. Reactions of the hydroxyl (OH) and peroxy (HO2 and RO2) radicals play a central role in the chemistry of the atmosphere. In addition to controlling the lifetimes of many trace gases important to issues of global climate change, OH radical reactions initiate the oxidation of volatile organic compounds (VOCs) which can lead to the production of ozone and secondary organic aerosols in the atmosphere. Previous measurements of these radicals in forest environments characterized by high mixing ratios of isoprene and low mixing ratios of nitrogen oxides (NOx) (typically less than 1–2 ppb) have shown serious discrepancies with modeled concentrations. These results bring into question our understanding of the atmospheric chemistry of isoprene and other biogenic VOCs under low NOx conditions. During the summer of 2015, OH and HO2 radical concentrations, as well as total OH reactivity, were measured using laser-induced fluorescence–fluorescence assay by gas expansion (LIF-FAGE) techniques as part of the Indiana Radical Reactivity and Ozone productioN InterComparison (IRRONIC). This campaign took place in a forested area near Indiana University's Bloomington campus which is characterized by high mixing ratios of isoprene (average daily maximum of approximately 4 ppb at 28 ∘C) and low mixing ratios of NO (diurnal average of approximately 170 ppt). Supporting measurements of photolysis rates, VOCs, NOx, and other species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism (RACM2) and the Master Chemical Mechanism (MCM 3.2), including versions of the Leuven isoprene mechanism (LIM1) for HOx regeneration (RACM2-LIM1 and MCM 3.3.1). Using an OH chemical scavenger technique, the study revealed the presence of an interference with the LIF-FAGE measurements of OH that increased with both ambient concentrations of ozone and temperature with an average daytime maximum equivalent OH concentration of approximately 5×106 cm−3. Subtraction of the interference resulted in measured OH concentrations of approximately 4×106 cm−3 (average daytime maximum) that were in better agreement with model predictions although the models underestimated the measurements in the evening. The addition of versions of the LIM1 mechanism increased the base RACM2 and MCM 3.2 modeled OH concentrations by approximately 20 % and 13 %, respectively, with the RACM2-LIM1 mechanism providing the best agreement with the measured concentrations, predicting maximum daily OH concentrations to within 30 % of the measured concentrations. Measurements of HO2 concentrations during the campaign (approximately a 1×109 cm−3 average daytime maximum) included a fraction of isoprene-based peroxy radicals (HO2*=HO2+αRO2) and were found to agree with model predictions to within 10 %–30 %. On average, the measured reactivity was consistent with that calculated from measured OH sinks to within 20 %, with modeled oxidation products accounting for the missing reactivity, however significant missing reactivity (approximately 40 % of the total measured reactivity) was observed on some days.

A compact, high-purity source of HONO validated by Fourier transform infrared and thermal-dissociation cavity ring-down spectroscopy

Abstract. A well-characterized source of nitrous acid vapour (HONO) is essential for accurate ambient air measurements by instruments requiring external calibration. In this work, a compact HONO source is described in which gas streams containing dilute concentrations of HONO are generated by flowing hydrochloric acid (HCl) vapour emanating from a permeation tube over continuously agitated dry sodium nitrite (NaNO2) heated to 50 ∘C. Mixing ratios of HONO and potential by-products including NO, NO2, and nitrosyl chloride (ClNO) were quantified by Fourier transform infrared (FTIR) and thermal-dissociation cavity ring-down spectroscopy (TD-CRDS). A key parameter is the concentration of HCl, which needs to be kept small (<4 ppmv) to avoid ClNO formation. The source produces gas streams containing HONO in air in >95 % purity relative to other nitrogen oxides. The source output is rapidly tuneable and stabilizes within 90 min. Combined with its small size and portability, this source is highly suitable for calibration of HONO instruments in the field.

The isotopic composition of atmospheric nitrous oxide observed at the high-altitude research station Jungfraujoch, Switzerland

Abstract. Atmospheric nitrous oxide (N2O) levels have been continuously growing since preindustrial times. Mitigation requires information about sources and sinks on the regional and global scales. Isotopic composition of N2O in the atmosphere could contribute valuable constraints. However, isotopic records of N2O in the unpolluted atmosphere remain too scarce for large-scale N2O models. Here, we report the results of discrete air samples collected weekly to biweekly over a 5-year period at the high-altitude research station Jungfraujoch, located in central Switzerland. High-precision N2O isotopic measurements were made using a recently developed preconcentration and laser spectroscopy technique. The measurements of discrete samples were accompanied by in situ continuous measurements of N2O mixing ratios. Our results indicate a pronounced seasonal pattern with minimum N2O mixing ratios in late summer, associated with a maximum in δ15Nbulk and a minimum in intramolecular 15N site preference (δ15NSP). This pattern is most likely due to stratosphere–troposphere exchange (STE), which delivers N2O-depleted but 15N-enriched air from the stratosphere into the troposphere. Variability in δ15NSP induced by changes in STE may be masked by biogeochemical N2O production processes in late summer, which are possibly dominated by a low-δ15NSP pathway of N2O production (denitrification), providing an explanation for the observed seasonality of δ15NSP. Footprint analyses and atmospheric transport simulations of N2O for Jungfraujoch suggest that regional emissions from the planetary boundary layer contribute to seasonal variations of atmospheric N2O isotopic composition at Jungfraujoch, albeit more clearly for δ15NSP and δ18O than for δ15Nbulk. With the time series of 5 years, we obtained a significant interannual trend for δ15Nbulk after deseasonalization (-0.052±0.012 ‰ a−1), indicating that the atmospheric N2O increase is due to isotopically depleted N2O sources. We estimated the average isotopic signature of anthropogenic N2O sources with a two-box model to be -8.6±0.6 ‰ for δ15Nbulk, 34.8±3 ‰ for δ18O and 10.7±4 ‰ for δ15NSP. Our study demonstrates that seasonal variation of N2O isotopic composition in the background atmosphere is important when determining interannual trends. More frequent, high-precision and interlaboratory-compatible measurements of atmospheric N2O isotopocules, especially for δ15NSP, are needed to better constrain anthropogenic N2O sources and thus the contribution of biogeochemical processes to N2O growth on the global scale.

Brief Communication: The reliability of gas extraction techniques for analysing CH<sub>4</sub> and N<sub>2</sub>O compositions in gas trapped in permafrost ice wedges

Abstract. Methane (CH4) and nitrous oxide (N2O) compositions in ground ice may provide information on their production mechanisms in permafrost. However, existing gas extraction methods have not been well tested. We tested conventional wet and dry gas extraction methods using ice wedges from Alaska and Siberia, finding that both methods can extract gas from the easily extractable parts of the ice (e.g. gas bubbles) and yield similar results for CH4 and N2O mixing ratios. We also found insignificant effects of microbial activity during wet extraction. However, both techniques were unable to fully extract gas from the ground ice, presumably because gas molecules adsorbed onto or enclosed in soil aggregates are not easily extractable. Estimation of gas production in a subfreezing environment of permafrost should consider such incomplete gas extraction.

Development of Korean Air Quality Prediction System version 1 (KAQPS v1) with focuses on practical issues

Abstract. For the purpose of providing reliable and robust air quality predictions, an air quality prediction system was developed for the main air quality criteria species in South Korea (PM10, PM2.5, CO, O3 and SO2). The main caveat of the system is to prepare the initial conditions (ICs) of the Community Multiscale Air Quality (CMAQ) model simulations using observations from the Geostationary Ocean Color Imager (GOCI) and ground-based monitoring networks in northeast Asia. The performance of the air quality prediction system was evaluated during the Korea-United States Air Quality Study (KORUS-AQ) campaign period (1 May–12 June 2016). Data assimilation (DA) of optimal interpolation (OI) with Kalman filter was used in this study. One major advantage of the system is that it can predict not only particulate matter (PM) concentrations but also PM chemical composition including five main constituents: sulfate (SO42-), nitrate (NO3-), ammonium (NH4+), organic aerosols (OAs) and elemental carbon (EC). In addition, it is also capable of predicting the concentrations of gaseous pollutants (CO, O3 and SO2). In this sense, this new air quality prediction system is comprehensive. The results with the ICs (DA RUN) were compared with those of the CMAQ simulations without ICs (BASE RUN). For almost all of the species, the application of ICs led to improved performance in terms of correlation, errors and biases over the entire campaign period. The DA RUN agreed reasonably well with the observations for PM10 (index of agreement IOA =0.60; mean bias MB =-13.54) and PM2.5 (IOA =0.71; MB =-2.43) as compared to the BASE RUN for PM10 (IOA =0.51; MB =-27.18) and PM2.5 (IOA =0.67; MB =-9.9). A significant improvement was also found with the DA RUN in terms of bias. For example, for CO, the MB of −0.27 (BASE RUN) was greatly enhanced to −0.036 (DA RUN). In the cases of O3 and SO2, the DA RUN also showed better performance than the BASE RUN. Further, several more practical issues frequently encountered in the air quality prediction system were also discussed. In order to attain more accurate ozone predictions, the DA of NO2 mixing ratios should be implemented with careful consideration of the measurement artifacts (i.e., inclusion of alkyl nitrates, HNO3 and peroxyacetyl nitrates – PANs – in the ground-observed NO2 mixing ratios). It was also discussed that, in order to ensure accurate nocturnal predictions of the concentrations of the ambient species, accurate predictions of the mixing layer heights (MLHs) should be achieved from the meteorological modeling. Several advantages of the current air quality prediction system, such as its non-static free-parameter scheme, dust episode prediction and possible multiple implementations of DA prior to actual predictions, were also discussed. These configurations are all possible because the current DA system is not computationally expensive. In the ongoing and future works, more advanced DA techniques such as the 3D variational (3DVAR) method and ensemble Kalman filter (EnK) are being tested and will be introduced to the Korean air quality prediction system (KAQPS).

Portable calibrator for NO based on the photolysis of N<sub>2</sub>O and a combined NO<sub>2</sub>∕NO∕O<sub>3</sub> source for field calibrations of air pollution monitors

Abstract. A highly portable calibration source of nitric oxide (NO) based on the photolysis of nitrous oxide (N2O) supplied by 8 or 16 g disposable cartridges is demonstrated to serve as an accurate and reliable transfer standard for the calibration of NO monitors in the field. The instrument provides output mixing ratios in the range 0–1000 ppb with a precision and accuracy better than the greater of 3 ppb or 3 % of the target NO mixing ratio over a wide range of environmental conditions of ambient temperature (8.5–35.0 ∘C), pressure (745–1015 mbar corresponding to 2.7–0.0 km of elevation), and relative humidity (0 %–100 % RH). The combination of the NO calibration source with a previously described ozone calibration source based on the photolysis of oxygen in air provides a new instrument capable of outputting calibrated mixing ratios of NO, ozone (O3), and nitrogen dioxide (NO2), where the NO2 is produced by the stoichiometric gas-phase reaction of NO with O3. The portable NO2/NO/O3 calibration source requires no external gas cylinders and can be used for calibrations of NO, NO2, and O3 instruments for mixing ratios up to 1000, 500, and 1000 ppb, respectively. This portable calibrator may serve as a convenient transfer standard for field calibrations of ozone and NOx air pollution monitors.

Quantification of nitrous acid (HONO) and nitrogen dioxide (NO2) in ambient air by broadband cavity-enhanced absorption spectroscopy (IBBCEAS) between 361 and 388 nm

Abstract. This work describes an incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) instrument for quantification of HONO and NO2 mixing ratios in ambient air. The instrument is operated in the near-ultraviolet spectral region between 361 and 388 nm. The mirror reflectivity and optical cavity transmission function were determined from the optical extinction observed when sampling air and helium. To verify the accuracy of this approach, Rayleigh scattering cross sections of nitrogen and argon were measured and found to be in quantitative agreement with literature values. The mirror reflectivity exceeded 99.98 %, at its maximum near 373 nm, resulting in an absorption path length of 6 km from a 1 m long optical cavity. The instrument precision was assessed through Allan variance analyses and showed minimum deviations of ±58 and ±210 pptv (1σ) for HONO and NO2, respectively, at an optimum acquisition time of 5 min. Measurements of HONO and NO2 mixing ratios in laboratory-generated mixtures by IBBCEAS were compared to thermal dissociation cavity ring-down spectroscopy (TD-CRDS) data and agreed within combined experimental uncertainties. Sample ambient air data collected in Calgary are presented.

Estimation of NO<sub><i>x</i></sub> and SO<sub>2</sub> emissions from Sarnia, Ontario, using a mobile MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy) and a NO<sub><i>x</i></sub> analyzer

Abstract. Sarnia, Ontario, experiences pollutant emissions disproportionate to its relatively small size. The small size of the city limits traditional top-down emission estimate techniques (e.g., satellite) but a low-cost solution for emission monitoring is the mobile MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy). Measurements were made using this technique from 21 March 2017 to 23 March 2017 along various driving routes to retrieve vertical column densities (VCDs) of NO2 and SO2 and to estimate emissions of NOx and SO2 from the Sarnia region. A novel aspect of the current study was the installation of a NOx analyzer in the vehicle to allow real time measurement and characterization of near-surface NOx∕NO2 ratios across the urban plumes, allowing improved accuracy of NOx emission estimates. Confidence in the use of near-surface-measured NOx∕NO2 ratios for estimation of NOx emissions was increased by relatively well-mixed boundary layer conditions. These conditions were indicated by similar temporal trends in NO2 VCDs and mixing ratios when measurements were sufficiently distant from the sources. Leighton ratios within transported plumes indicated peroxy radicals were likely disturbing the NO–NO2–O3 photostationary state through VOC (volatile organic compound) oxidation. The average lower-limit emission estimate of NOx from Sarnia was 1.60±0.34 t h−1 using local 10 m elevation wind-speed measurements. Our estimates were larger than the downscaled annual 2017 NPRI-reported (National Pollution Release Inventory) industrial emissions of 0.9 t NOx h−1. Our lower-limit estimate of SO2 emissions from Sarnia was 1.81±0.83 t SO2 h−1, equal within uncertainty to the 2017 NPRI downscaled value of 1.85 t SO2 h−1. Satellite-derived NO2 VCDs over Sarnia from the ozone monitoring instrument (OMI) were lower than mobile MAX-DOAS VCDs, likely due to the large pixel size relative to the city's size. The results of this study support the utility of the mobile MAX-DOAS method for estimating NOx and SO2 emissions in relatively small, highly industrialized regions, especially when supplemented with mobile NOx measurements.

Simulation and experimental study of the NOx reduction by unburned H2 in TWC for a hydrogen engine

The unburned H2 can be used to reduce NO emission in conventional TWC (three-way catalyst) for a hydrogen internal combustion engine when it works at equivalence ratio marginally higher than the stoichiometric ratio. To explore the effects and feasibility of this reaction, a Perfectly Stirred Reactor simulation model of TWC has been built with simplified mechanisms. Experiments on a 2.3 L turbocharged hydrogen engine are used to verify the conclusion. It shows that rising initial temperature accelerates the reduction of NO and the maximum reaction rate occurs at 400 °C temperature. The conversion efficiency of NO remains approximately 0 when temperatures below 300 °C. The efficiency reaches a peak value of approximately 98% with 400 °C and declines gradually. The unburned H2 to NO mixing ratio greater than 1.5 in TWC guarantees 100% NO conversion efficiency. The experiments indicate that the NOx concentration decreases from 2056 ppm to 41 ppm at the stoichiometric ratio after the treatment of TWC and NOx reaches 0 ppm with a rich ratio. Results also demonstrate that the suitable reaction temperatures for TWC locate in the range of 400 °C–500 °C. Therefore, if the temperature and the mixing ratio are appropriate, it can achieve zero emissions with NOx reduction by unburned H2 in conventional TWC for a hydrogen engine.

Sensitivity analysis of the dependence of the Carbon Bond Mechanism IV (CBM-IV) on the initial air composition under an urban condition

CBM-IV mechanism is one of the most-commonly used atmospheric reaction mechanisms in the model studies of air pollution. In this study, by using a concentration sensitivity analysis, we investigated the dependence of NO, NO2 and O3 on the initial air composition in the CBM-IV mechanism under a typical urban condition. The initial values of the components that are identified important in the sensitivity analysis were also modified to see the relevant changes in the reaction system. The results show that at the beginning of the simulation, NO, NO2 and O3 are mostly affected by the titration reaction. During this period, the NO mixing ratio depends negatively on the initial O3 and positively on the initial nitrogen oxides. Moreover, the NO2 mixing ratio was found positively correlated with the initial NO, while its dependence on the initial O3 transits between positive and negative. In contrast to that, O3 at this stage has the strongest positive dependence on its own initial concentration, but a negative association with the initial NO. However, as the reaction proceeds, the dependence of O3 and the nitrogen oxides on the initial air composition changes. During the end of the simulation, the change of NO and NO2 was found moderately associated with their initial values, but the influence caused by the modification of the initial O3 is negligible. Similarly, ozone at this time was also found less impacted by its initial value. Instead, the ozone level under equilibrium relies mostly on the initial NO and NO2.

Trends in N<sub>2</sub>O and SF<sub>6</sub> mole fractions in archived air samples from Cape Meares, Oregon (USA), 1978–1996

Abstract. Quantifying historical trends in atmospheric greenhouse gases (GHGs) is important to understanding changes in their budgets and for climate modeling, which simulates historic and projects future climate. Archived samples analyzed using updated measurement techniques and calibration scales can reduce uncertainties in historic records of GHG mole fractions and their trends in time. Here, we present historical measurements of two important GHGs, nitrous oxide (N2O) and sulfur hexafluoride (SF6), collected at the midlatitude Northern Hemisphere station Cape Meares, Oregon (USA, 45.5∘ N, 124∘ W), between 1978 and 1996 in archived air samples from the Oregon Health and Science University – Portland State University (OHSU–PSU) air archive. N2O is the third most important anthropogenically forced GHG behind carbon dioxide (CO2) and methane (CH4). SF6 has a low abundance in the atmosphere, but is one of the most powerful GHGs known. Measurements of atmospheric N2O made during this period are available for select locations, but before mid-1990 they have larger uncertainties than more recent periods due to advancements made in gas chromatography (GC) methods. Few atmospheric SF6 measurements exist pre-1990, particularly in the Northern Hemisphere. The GC system used to measure N2O and SF6 mixing ratios in this work is designed to be fully automated, and is capable of running up to 15 samples per batch. Measurement precision (1σ) of N2O and SF6 is 0.16 % and 1.1 %, respectively (evaluated at 328.7 ppb and 8.8 ppt). Samples were corrected for detector response nonlinearity when measured against our reference standard, with the corrections determined to be 0.14 ppb ppb−1 in N2O and 0.03 ppt ppt−1 in SF6. The mixing ratio of N2O in archived samples is found to be 301.5±0.3 ppb in 1980 and rises to 313.5±0.3 ppb in 1996. The average growth rate over this period is 0.78±0.03 ppb yr−1 (95 % CI). The seasonal amplitude is statistically robust, with a maximum anomaly of 0.3 ppb near April and a minimum near November of −0.4 ppb. Measurements of N2O match well with previously reported values for Cape Meares and other comparable locations. The mixing ratio of SF6 in analyzed samples is found to be 0.85±0.03 ppt in 1980 and rises to 3.83±0.03 ppt in 1996. The average growth rate over this period is 0.17±0.01 ppt yr−1 (95 % CI). The seasonality is statistically robust and has an annual peak amplitude of 0.04 ppt near January and a minimum amplitude of −0.03 ppt near July. These are unique SF6 results from this site and represent a significant increase in the SF6 data available during the 1980s and early 1990s. The mixing ratio and growth rate of SF6 measured compares well to other Northern Hemisphere measurements over this period. From these N2O and SF6 measurements, we conclude that sample integrity is generally robust in the OHSU-PSU air archive for N2O and SF6.

Triple Oxygen and Clumped Isotope Compositions of CO2 in the Middle Troposphere

AbstractWe report the oxygen isotopic anomaly (Δ17O) and clumped isotopic composition (Δ47) of CO2 sampled from the middle troposphere (~10 km above ground) during two CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) flights in summer and winter seasons: one from Düsseldorf to Isla Margarita on 11 August 2001 and the other from Puerto Plata to Munich on 21 February 2002. The mid‐tropospheric Δ17O values were higher than the near‐surface values by 0.051 ± 0.010 ‰, indicating enhanced stratospheric influence. The Δ47 values were also higher by 0.160 ± 0.015 ‰ compared to that from the lower troposphere. Stratospheric influence is supported by the observed correlations of the Δ17O values with the mixing ratios of N2O, O3, and 14CO, which carry stratospheric signatures. The increase in the anomaly in the free troposphere represents a balance between the stratosphere‐troposphere exchange fluxes and biogeochemical/hydrospheric isotopic resetting rates at the Earth surface. Using a simple two box model with the free tropospheric Δ17O values, an average surface exchange rate of CO2 between 327 and 772 PgC/year is inferred, giving a CO2 turnover time of 1.4 to 2.8 years in the atmosphere. The present estimates are based on a rather small set of free tropospheric data and have relatively large uncertainties, but they agree with the existing model and proxy‐based estimates.

Near-surface and path-averaged mixing ratios of NO<sub>2</sub> derived from car DOAS zenith-sky and tower DOAS off-axis measurements in Vienna: a case study

Abstract. Nitrogen dioxide (NO2), produced as a result of fossil fuel combustion, biomass burning, lightning, and soil emissions, is a key urban and rural tropospheric pollutant. In this case study, ground-based remote sensing has been coupled with the in situ network in Vienna, Austria, to investigate NO2 distributions in the planetary boundary layer. Near-surface and path-averaged NO2 mixing ratios within the metropolitan area of Vienna are estimated from car DOAS (differential optical absorption spectroscopy) zenith-sky and tower DOAS horizon observations. The latter configuration is innovative in the sense that it obtains horizontal measurements at more than a hundred different azimuthal angles – within a 360∘ rotation taking less than half an hour. Spectral measurements were made with a DOAS instrument on nine days in April, September, October, and November 2015 in the zenith-sky mode and on five days in April and May 2016 in the off-axis mode. The analysis of tropospheric NO2 columns from the car measurements and O4 normalized NO2 path averages from the tower observations provide interesting insights into the spatial and temporal NO2 distribution over Vienna. Integrated column amounts of NO2 from both DOAS-type measurements are converted into mixing ratios by different methods. The estimation of near-surface NO2 mixing ratios from car DOAS tropospheric NO2 vertical columns is based on a linear regression analysis including mixing height and other meteorological parameters that affect the dilution and reactivity in the planetary boundary layer – a new approach for such conversion. Path-averaged NO2 mixing ratios are calculated from tower DOAS NO2 slant column densities by taking into account topography and geometry. Overall, lap averages of near-surface NO2 mixing ratios obtained from car DOAS zenith-sky measurements, around a circuit in Vienna, are in the range of 3.8 to 26.1 ppb and in good agreement with values obtained from in situ NO2 measurements for days with wind from the southeast. Path-averaged NO2 mixing ratios at 160 m above the ground as derived from the tower DOAS measurements are between 2.5 and 9 ppb on two selected days with different wind conditions and pollution levels and show similar spatial distribution as seen in the car DOAS zenith-sky observations. We conclude that the application of the two methods to obtain near-surface and path-averaged NO2 mixing ratios is promising for this case study.

Fast heterogeneous loss of N2O5 leads to significant nighttime NOx removal and nitrate aerosol formation at a coastal background environment of southern China

Nitrate radical (NO3) and dinitrogen pentoxide (N2O5) play crucial roles in the nocturnal atmosphere. To quantify their impacts, we deployed a thermal-dissociation chemical ionization mass spectrometry (TD-CIMS), to measure their concentration, as well as ClNO2 at a coastal background site in the southern of China during the late autumn of 2012. Moderate levels of NO3, N2O5 and high concentration of ClNO2 were observed during the study period, indicating active NOx-O3 chemistry in the region. Distinct features of NO3, N2O5 and ClNO2 mixing ratios were observed in different airmasses. Further analysis revealed that the N2O5 heterogeneous reaction was the dominant loss of N2O5 and NO3, which showed higher loss rate compared to that in other coastal sites. Especially, the N2O5 loss rates could reach up to 0.0139 s−1 when airmasses went across the sea. The fast heterogeneous loss of N2O5 led to rapid NOx loss which could be comparable to the daytime process through NO2 oxidization by OH, and on the other hand, to rapid nitrate aerosol formation. In summary, our results revealed that the N2O5 hydrolysis could play significant roles in regulating the air quality by reducing NOx but forming nitrate aerosols.

Simulating the Weekly Cycle of NOx‐VOC‐HOx‐O3 Photochemical System in the South Coast of California During CalNex‐2010 Campaign

AbstractUnited States Environmental Protection Agency guidance on the use of photochemical models for assessing the efficacy of an emissions control strategy for ozone requires that modeling be used in a relative sense. Consequently, testing a modeling system's ability to predict changes in ozone resulting from emission changes is critical. We evaluate model simulations for precursor species (NOx, CO, and volatile organic compounds [VOCs]), radicals (OH and HO2), a secondary pollutant (O3), and the model response of these compounds to weekend/weekday emission changes during California Nexus study in 2010. The modeling system correctly simulated the broad spatial and temporal variation of NOx and O3 in California South Coast. Although the model generally underpredicted the daytime mixing ratios of NO2 at the surface and overpredicted the NO2 column, the simulated weekend to weekday ratios are consistent with each other and match the observed ratios well. The modeling system exhibited reasonable performance in simulating the VOC compounds with fossil fuel origins but has larger bias in simulating certain species associated with noncombustion sources. The modeling system successfully captured the weekend changes of the enhancement ratios for various VOC species to CO and the relative changes of HOx, which are indicators of faster chemical processing on weekends. This work demonstrates satisfactory model performances for O3 and most relevant chemical compounds with more robust performance in simulating weekend versus weekday changes. Improved planetary boundary layer height simulations, a better understanding of OH‐HO2 cycling, continued improvement of emissions, especially urban biogenic emissions and emissions of oxygenated VOCs, are important for future model improvement.

The Mainz profile algorithm (MAPA)

Abstract. The Mainz profile algorithm (MAPA) derives vertical profiles of aerosol extinction and trace gas concentrations from MAX-DOAS measurements of slant column densities under multiple elevation angles. This paper presents (a) a detailed description of the MAPA (v0.98), (b) results for the CINDI-2 campaign, and (c) sensitivity studies on the impact of a priori assumptions such as flag thresholds. Like previous profile retrieval schemes developed at MPIC, MAPA is based on a profile parameterization combining box profiles, which also might be lifted, and exponential profiles. But in contrast to previous inversion schemes based on least-square fits, MAPA follows a Monte Carlo approach for deriving those profile parameters yielding best match to the MAX-DOAS observations. This is much faster and directly provides physically meaningful distributions of profile parameters. In addition, MAPA includes an elaborated flagging scheme for the identification of questionable or dubious results. The AODs derived with MAPA for the CINDI-2 campaign show good agreement with AERONET if a scaling factor of 0.8 is applied for O4, and the respective NO2 and HCHO surface mixing ratios match those derived from coincident long-path DOAS measurements. MAPA results are robust with respect to modifications of the a priori MAPA settings within plausible limits.