Lifetime Of NOx Research Articles

The contribution of soil biogenic NO and HONO emissions from a managed hyperarid ecosystem to the regional NO<sub><i>x</i></sub> emissions during growing season

Abstract. A study was carried out to understand the contributions of soil biogenic NO emissions from managed (fertilized and irrigated) hyperarid ecosystems in NW China to the regional NOx emissions during the growing season. Soil biogenic net potential NO fluxes were quantified by laboratory incubation of soil samples from the three dominating ecosystems (desert, cotton, and grape fields). Regional biogenic NO emissions were calculated bottom-up hourly for the entire growing season (April–September 2010) by considering corresponding land use, hourly data of soil temperature, gravimetric soil moisture, and fertilizer enhancement factors. The regional HONO emissions were estimated using the ratio of the optimum condition ((FN,opt(HONO) to FN,opt (NO)). Regional anthropogenic NOx emissions were calculated bottom-up from annual statistical data provided by regional and local government bureaus which have been downscaled to monthly value. Regional top-down emission estimates of NOx were derived on the monthly basis from satellite observations (OMI) of tropospheric vertical NO2 column densities and prescribed values of the tropospheric NOx lifetime. In order to compare the top-down and bottom-up emission estimates, all emission estimates were expressed in terms of mass of atomic nitrogen. Consequently, monthly top-down NOx emissions (total) were compared with monthly bottom-up NOx emissions (biogenic + anthropogenic) for the time of the satellite overpass (around 13:00 LT) with the consideration of the diurnal cycle of bottom-up estimates. Annual variation in total Tohsun Oasis NOx emissions is characterized by a strong peak in winter (December–February) and a secondary peak in summer (June–August). During summer, soil biogenic emissions were from equal to double that of related anthropogenic emissions, and grape soils were the main contributor to soil biogenic emissions, followed by cotton soils, while emissions from the desert were negligible. The top-down and bottom-up emission estimates were shown to be useful methods to estimate the monthly/seasonal cycle of the total regional NOx emissions. The resulting total NOx emissions show a strong peak in winter and a secondary peak in summer, and the second maximum in summer was only found if the soil emissions were taken into account, which provides confidence in both completely independent methods. Despite the regional character of these findings, particularly the second maximum in summer provides substantial evidence to hypothesize that biogenic emissions from soils of managed drylands (irrigated and fertilized) in the growing period may be much more important contributors to regional NOx budgets of dryland regions than thought before.

Multi-satellite sensor study on precipitation-induced emission pulses of NO<sub><i>x</i></sub> from soils in semi-arid ecosystems

Abstract. We present a top-down approach to infer and quantify rain-induced emission pulses of NOx ( ≡ NO + NO2), stemming from biotic emissions of NO from soils, from satellite-borne measurements of NO2. This is achieved by synchronizing time series at single grid pixels according to the first day of rain after a dry spell of prescribed duration. The full track of the temporal evolution several weeks before and after a rain pulse is retained with daily resolution. These are needed for a sophisticated background correction, which accounts for seasonal variations in the time series and allows for improved quantification of rain-induced soil emissions. The method is applied globally and provides constraints on pulsed soil emissions of NOx in regions where the NOx budget is seasonally dominated by soil emissions. We find strong peaks of enhanced NO2 vertical column densities (VCDs) induced by the first intense precipitation after prolonged droughts in many semi-arid regions of the world, in particular in the Sahel. Detailed investigations show that the rain-induced NO2 pulse detected by the OMI (Ozone Monitoring Instrument), GOME-2 and SCIAMACHY satellite instruments could not be explained by other sources, such as biomass burning or lightning, or by retrieval artefacts (e.g. due to clouds). For the Sahel region, absolute enhancements of the NO2 VCDs on the first day of rain based on OMI measurements 2007–2010 are on average 4 × 1014 molec cm−2 and exceed 1 × 1015 molec cm−2 for individual grid cells. Assuming a NOx lifetime of 4 h, this corresponds to soil NOx emissions in the range of 6 up to 65 ng N m−2 s−1, which is in good agreement with literature values. Apart from the clear first-day peak, NO2 VCDs are moderately enhanced (2 × 1014 molec cm−2) compared to the background over the following 2 weeks, suggesting potential further emissions during that period of about 3.3 ng N m−2 s−1. The pulsed emissions contribute about 21–44 % to total soil NOx emissions over the Sahel.

NO<sub><i>x</i></sub> lifetimes and emissions of cities and power plants in polluted background estimated by satellite observations

Abstract. We present a new method to quantify NOx emissions and corresponding atmospheric lifetimes from OMI NO2 observations together with ECMWF wind fields without further model input for sources located in a polluted background. NO2 patterns under calm wind conditions are used as proxy for the spatial patterns of NOx emissions, and the effective atmospheric NOx lifetime is determined from the change of spatial patterns measured at larger wind speeds. Emissions are subsequently derived from the NO2 mass above the background, integrated around the source of interest. Lifetimes and emissions are estimated for 17 power plants and 53 cities located in non-mountainous regions across China and the USA. The derived lifetimes for the ozone season (May–September) are 3.8 ± 1.0 h (mean ± standard deviation) with a range of 1.8 to 7.5 h. The derived NOx emissions show generally good agreement with bottom-up inventories for power plants and cities. Regional inventory shows better agreement with top-down estimates for Chinese cities compared to global inventory, most likely due to different downscaling approaches adopted in the two inventories.

Estimates of power plant NOx emissions and lifetimes from OMI NO2 satellite retrievals

Isolated power plants with well characterized emissions serve as an ideal test case of methods to estimate emissions using satellite data. In this study we evaluate the Exponentially-Modified Gaussian (EMG) method and the box model method based on mass balance for estimating known NOx emissions from satellite retrievals made by the Ozone Monitoring Instrument (OMI). We consider 29 power plants in the USA which have large NOx plumes that do not overlap with other sources and which have emissions data from the Continuous Emission Monitoring System (CEMS). This enables us to identify constraints required by the methods, such as which wind data to use and how to calculate background values. We found that the lifetimes estimated by the methods are too short to be representative of the chemical lifetime. Instead, we introduce a separate lifetime parameter to account for the discrepancy between estimates using real data and those that theory would predict. In terms of emissions, the EMG method required averages from multiple years to give accurate results, whereas the box model method gave accurate results for individual ozone seasons.

Influence of satellite-derived photolysis rates and NO<sub>x</sub> emissions on Texas ozone modeling

Abstract. Uncertain photolysis rates and emission inventory impair the accuracy of state-level ozone (O3) regulatory modeling. Past studies have separately used satellite-observed clouds to correct the model-predicted photolysis rates, or satellite-constrained top-down NOx emissions to identify and reduce uncertainties in bottom-up NOx emissions. However, the joint application of multiple satellite-derived model inputs to improve O3 state implementation plan (SIP) modeling has rarely been explored. In this study, Geostationary Operational Environmental Satellite (GOES) observations of clouds are applied to derive the photolysis rates, replacing those used in Texas SIP modeling. This changes modeled O3 concentrations by up to 80 ppb and improves O3 simulations by reducing modeled normalized mean bias (NMB) and normalized mean error (NME) by up to 0.1. A sector-based discrete Kalman filter (DKF) inversion approach is incorporated with the Comprehensive Air Quality Model with extensions (CAMx)–decoupled direct method (DDM) model to adjust Texas NOx emissions using a high-resolution Ozone Monitoring Instrument (OMI) NO2 product. The discrepancy between OMI and CAMx NO2 vertical column densities (VCDs) is further reduced by increasing modeled NOx lifetime and adding an artificial amount of NO2 in the upper troposphere. The region-based DKF inversion suggests increasing NOx emissions by 10–50% in most regions, deteriorating the model performance in predicting ground NO2 and O3, while the sector-based DKF inversion tends to scale down area and nonroad NOx emissions by 50%, leading to a 2–5 ppb decrease in ground 8 h O3 predictions. Model performance in simulating ground NO2 and O3 are improved using sector-based inversion-constrained NOx emissions, with 0.25 and 0.04 reductions in NMBs and 0.13 and 0.04 reductions in NMEs, respectively. Using both GOES-derived photolysis rates and OMI-constrained NOx emissions together reduces modeled NMB and NME by 0.05, increases the model correlation with ground measurement in O3 simulations, and makes O3 more sensitive to NOx emissions in the O3 non-attainment areas.

Differences in satellite-derived NOx emission factors between Eurasian and North American boreal forest fires

Current fire emission inventories apply universal emission factors (EFs) for the calculation of NOx emissions over large biomes such as boreal forest. However, recent satellite-based studies over tropical and subtropical regions have indicated spatio-temporal variations in EFs within specific biomes. In this study, satellite measurements of tropospheric NO2 vertical columns (TVC NO2) from the GOME-2 instrument and fire radiative power (FRP) from MODIS are used for the estimation of fire emission rates (FERs) of NOx over Eurasian and North American boreal forests. The retrieval of TVC NO2 is based on a stratospheric correction using simulated stratospheric NO2 instead of applying the reference sector method, which was used in a previous study. The model approach is more suitable for boreal latitudes. TVC NO2 and FRP are spatially aggregated to a 1° × 1° horizontal resolution and temporally averaged to monthly values. The conversion of the satellite-derived tropospheric NO2 columns into production rates of NOx from fire (Pf) is based on the NO2/NOx ratio as obtained from the MACC reanalysis data set and an assumed lifetime of NOx. A global land cover map is used to define boreal forests across these two regions in order to evaluate the FERs of NOx for this biome. The FERs of NOx, which are derived from the gradients of the linear relationship between Pf and FRP, are more than 30% lower for North American than for Eurasian boreal forest fires. We speculate that these discrepancies are mainly related to the variable nitrogen content in plant tissues, which is higher in deciduous forests dominating large parts in Eurasia. In order to compare the obtained values with EFs found in the literature, the FERs are converted into EFs. The satellite-based EFs of NOx are estimated at 0.83 and 0.61 g kg−1 for Eurasian and North American boreal forests, respectively, which is in good agreement with the value found in a recent emission factor compilation. However, recent fire emission inventories are based on EFs of NOx that are 3–5 times larger, which indicates that there are still large uncertainties in estimates of NOx from biomass burning, especially on the regional scale.

Estimation of the lifetime of nitrogen oxides over India using SCIAMACHY observations

The NOx lifetime is needed to determine NOx source strength and allow for source apportionment studies. Satellite observations of tropospheric column NO2 may be used to estimate the NOx lifetime by analysing tropospheric NO2 decay in the outflow region. In this study, we used Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) observations of tropospheric NO2 to quantify the lifetime of NOx, ‘’, over the Indian region. We analysed a tropospheric NO2 plume that originated over the city of Mumbai and its downwind evolution over the Arabian Sea as a function of distance (decay curve). The observed pattern of decay curve is fitted with the exponential decay fit model, which gives an e-folding distance of 107 km. The fit result yields a mean lifetime of NOx of about = 10.2 h based on SCIAMACHY observations. In addition, we quantified using CHIMERE chemical mechanisms and compared the result with the satellite-based estimation. The estimated for India, based on CHIMERE simulation, is about 11.9 h, comparable to the calculated by satellite observations.

Variations of OH radical in an urban plume inferred from NO2 column measurements

AbstractThe evolution of atmospheric composition downwind of a city depends strongly on the concentration of OH within the plume. We use space‐based observations of NO2, a molecule that affects both the sources and sinks of OH, to examine the functional dependence of OH concentration on the speed of the wind over Riyadh, Saudi Arabia. These observations illustrate the nonlinear dependence of the OH concentration on NO2 and on the rate of atmospheric mixing. We derive a range of NOx lifetimes of 5.5–8.0 h, lifetimes that correspond to an effective plume‐averaged OH concentration of 7.6 × 106 molecules cm–3 at fast (26 km h–1) and 5.2 × 106 molecules cm–3 at slow (4 km h–1) wind speeds.

Observations of total RONO<sub>2</sub> over the boreal forest: NO<sub>x</sub> sinks and HNO<sub>3</sub> sources

Abstract. In contrast with the textbook view of remote chemistry where HNO3 formation is the primary sink of nitrogen oxides, recent theoretical analyses show that formation of RONO2 (ΣANs) from isoprene and other terpene precursors is the primary net chemical loss of nitrogen oxides over the remote continents where the concentration of nitrogen oxides is low. This then increases the prominence of questions concerning the chemical lifetime and ultimate fate of ΣANs. We present observations of nitrogen oxides and organic molecules collected over the Canadian boreal forest during the summer which show that ΣANs account for ~20% of total oxidized nitrogen and that their instantaneous production rate is larger than that of HNO3. This confirms the primary role of reactions producing ΣANs as a control over the lifetime of NOx (NOx = NO + NO2) in remote, continental environments. However, HNO3 is generally present in larger concentrations than ΣANs indicating that the atmospheric lifetime of ΣANs is shorter than the HNO3 lifetime. We investigate a range of proposed loss mechanisms that would explain the inferred lifetime of ΣANs finding that in combination with deposition, two processes are consistent with the observations: (1) rapid ozonolysis of isoprene nitrates where at least ~40% of the ozonolysis products release NOx from the carbon backbone and/or (2) hydrolysis of particulate organic nitrates with HNO3 as a product. Implications of these ideas for our understanding of NOx and NOy budget in remote and rural locations are discussed.

The influence of snow grain size and impurities on the vertical profiles of actinic flux and associated NO<sub>x</sub> emissions on the Antarctic and Greenland ice sheets

Abstract. We use observations of the absorption properties of black carbon and non-black carbon impurities in near-surface snow collected near the research stations at South Pole and Dome C, Antarctica, and Summit, Greenland, combined with a snowpack actinic flux parameterization to estimate the vertical profile and e-folding depth of ultraviolet/near-visible (UV/near-vis) actinic flux in the snowpack at each location. We have developed a simple and broadly applicable parameterization to calculate depth and wavelength dependent snowpack actinic flux that can be easily integrated into large-scale (e.g., 3-D) models of the atmosphere. The calculated e-folding depths of actinic flux at 305 nm, the peak wavelength of nitrate photolysis in the snowpack, are 8–12 cm near the stations and 15–31 cm away (>11 km) from the stations. We find that the e-folding depth is strongly dependent on impurity content and wavelength in the UV/near-vis region, which explains the relatively shallow e-folding depths near stations where local activities lead to higher snow impurity levels. We calculate the lifetime of NOx in the snowpack interstitial air produced by photolysis of snowpack nitrate against wind pumping (τwind pumping) from the snowpack, and compare this to the calculated lifetime of NOx against chemical conversion to HNO3 (τchemical) to determine whether the NOx produced at a given depth can escape from the snowpack to the overlying atmosphere. Comparison of τwind pumping and τchemical suggests efficient escape of photoproduced NOx in the snowpack to the overlying atmosphere throughout most of the photochemically active zone. Calculated vertical actinic flux profiles and observed snowpack nitrate concentrations are used to estimate the potential flux of NOx from the snowpack. Calculated NOx fluxes of 4.4 × 108–3.8 × 109 molecules cm−2 s−1 in remote polar locations and 3.2–8.2 × 108 molecules cm−2 s−1 near polar stations for January at Dome C and South Pole and June at Summit suggest that NOx flux measurements near stations may be underestimating the amount of NOx emitted from the clean polar snowpack.

Effects of biogenic nitrate chemistry on the NO<sub>x</sub> lifetime in remote continental regions

Abstract. We present an analysis of the NOx budget in conditions of low NOx (NOx = NO + NO2) and high biogenic volatile organic compound (BVOC) concentrations that are characteristic of most continental boundary layers. Using a steady-state model, we show that below 500 pptv of NOx, the NOx lifetime is extremely sensitive to organic nitrate (RONO2) formation rates. We find that even for RONO2 formation values that are an order of magnitude smaller than is typical for continental conditions significant reductions in NOx lifetime, and consequently ozone production efficiency, are caused by nitrate forming reactions. Comparison of the steady-state box model to a 3-D chemical transport model (CTM) confirms that the concepts illustrated by the simpler model are a useful approximation of predictions provided by the full CTM. This implies that the regional and global budgets of NOx, OH, and ozone will be sensitive to assumptions regarding organic nitrate chemistry. Changes in the budgets of these species affect the representation of processes important to air quality and climate. Consequently, CTMs must include an accurate representation of organic nitrate chemistry in order to provide accurate assessments of past, present, and future air quality and climate. These findings suggest the need for further experimental constraints on the formation and fate of biogenic RONO2.

Remote sensing of NO2 emission from the central urban area of Shanghai (China) using the mobile DOAS technique

The mobile DOAS technique was used to quantify the NOxvehicle emission of the central urban area, enclosed by the Inner Ring Viaduct Road (IRVR), of Shanghai, China. Three field measurement campaigns were performed during the pre‐EXPO period in October 2009, the EXPO opening ceremony, and the closing ceremony period in 2010. The spatial and temporal distributions of NO2 Vertical Column Densities were derived, and the effects of traffic and wind conditions were studied. The averaged NO2 emissions rates from the IRVR area were determined to be 2.1 ± 0.9 ton/h, 2.8 ± 1.4 ton/h and 2.7 ± 1.4 ton/h for these three campaigns. The annual vehicle NOx emissions for the central urban area were estimated to be (2.3 ± 1.0) × 104 ton in 2009 and (3.0 ± 1.5) × 104 ton in 2010, and those for the whole city were (13.4 ± 5.9) × 104 ton in 2009 and (17.6 ± 8.4) × 104 ton in 2010, by considering a typical NO/NO2 ratio, NOx lifetime, as well as the traffic emission share of central urban area with respect to the entire city. The vehicle NOx emissions quantified by mobile DOAS were found to exhibit an increasing trend in the central urban area of Shanghai, by 1.6 times and 2.1 times from 2006 to 2009 and 2010. Due to wind field variations, unknown NOx chemical transformation, and uncertainties in AMFs, the systematic derivation of the final NOx emission rates obtained was about 55% for averaged several consecutive days.

The effect of the novel HO2 + NO → HNO3 reaction channel at South Pole, Antarctica

AbstractIt is well established that the reaction of HO2 with NO plays a central role in atmospheric chemistry, by way of OH/HO2 recycling and reduction of ozone depletion by HOx cycles in the stratosphere and through ozone production in the troposphere. Utilizing a photochemical box model, we investigate the impact of the recently observed HNO3 production channel (HO2+NO → HNO3) on NOx (NO + NO2), HOx (OH + HO2), HNO3, and O3 concentrations in the boundary layer at the South Pole, Antarctica. Our simulations exemplify decreases in peak O3, NO, NO2, and OH and an increase in HNO3. Also, mean OH is in better agreement with observations, while worsening the agreement with O3, HO2, and HNO3 concentrations observed at the South Pole. The reduced concentrations of NOx are consistent with expected decreases in atmospheric NOx lifetime as a result of increased sequestration of NOx into HNO3. Although we show that the inclusion of the novel HNO3 production channel brings better agreement of OH with field measurements, the modelled ozone and HNO3 are worsened, and the changes in NOx lifetime imply that snowpack NOx emissions and snowpack nitrate recycling must be re-evaluated.

Summertime NO<sub>x</sub> measurements during the CHABLIS campaign: can source and sink estimates unravel observed diurnal cycles?

Abstract. NOx measurements were conducted at the Halley Research Station, coastal Antarctica, during the austral summer period 1 January–10 February 2005. A clear NOx diurnal cycle was observed with minimum concentrations close to instrumental detection limit (5 pptv) measured between 04:00–05:00 GMT. NOx concentrations peaked (24 pptv) between 19:00–20:00 GMT, approximately 5 h after local solar noon. An optimised box model of NOx concentrations based on production from in-snow nitrate photolysis and chemical loss derives a mean noon emission rate of 3.48 × 108 molec cm−2 s−1, assuming a 100 m boundary layer mixing height, and a relatively short NOx lifetime of ~6.4 h. This emission rate compares to directly measured values ranging from 2.1 to 12.6 × 108 molec cm−2 s−1 made on 3 days at the end of the study period. Calculations of the maximum rate of NO2 loss via a variety of conventional HOx and halogen oxidation processes show that the lifetime of NOx is predominantly controlled by halogen processing, namely BrNO3 and INO3 gas-phase formation and their subsequent heterogeneous uptake. Furthermore the presence of halogen oxides is shown to significantly perturb NOx concentrations by decreasing the NO/NO2 ratio. We conclude that in coastal Antarctica, the potential ozone production efficiency of NOx emitted from the snowpack is mitigated by the more rapid NOx loss due to halogen nitrate hydrolysis.

Effects of model resolution on the interpretation of satellite NO<sub>2</sub> observations

Abstract. Inference of NOx emissions (NO+NO2) from satellite observations of tropospheric NO2 column requires knowledge of NOx lifetime, usually provided by chemical transport models (CTMs). However, it is known that species subject to non-linear sources or sinks, such as ozone, are susceptible to biases in coarse-resolution CTMs. Here we compute the resolution-dependent bias in predicted NO2 column, a quantity relevant to the interpretation of space-based observations. We use 1-D and 2-D models to illustrate the mechanisms responsible for these biases over a range of NO2 concentrations and model resolutions. We find that predicted biases are largest at coarsest model resolutions with negative biases predicted over large sources and positive biases predicted over small sources. As an example, we use WRF-CHEM to illustrate the resolution necessary to predict 10 AM and 1 PM NO2 column to 10 and 25% accuracy over three large sources, the Four Corners power plants in NW New Mexico, Los Angeles, and the San Joaquin Valley in California for a week-long simulation in July 2006. We find that resolution in the range of 4–12 km is sufficient to accurately model nonlinear effects in the NO2 loss rate.

Characterization of an eastern U.S. severe air pollution episode using WRF/Chem

[1] On 8–11 July 2007 the eastern United States experienced a severe heat wave and smog event with maximum temperatures approaching 38°C and maximum 8 h average ozone mixing ratios of 125 ppbv. We examine this episode with observations and numerical simulations using the Weather Research and Forecasting model with online chemistry (WRF/Chem with RADM2). The general features of this severe smog event–a broad area of high pressure, weak winds and heavy pollution, terminated by the passage of a cold front–were well simulated by the model. WRF/Chem underpredicted O3 maxima by 5–8 ppbv where air quality was poor, usually in the northeast, but overpredicted maxima by up to 16 ppbv where ozone amounts were low, usually in the southeast. Simulated O3 vertical profiles over Beltsville, Maryland, showed good agreement with ozonesonde measurements, but the model boundary layer was too deep on 9 July, contributing to the low bias over this region. The representation of NOx chemistry in RADM2 may lead to an underestimation of NOx lifetime and is likely partially responsible for low O3 biases in the most polluted area in the northeast. To simulate the maximum effect of nighttime multiphase NOy loss, we set the N2O5 heterogeneous hydrolysis reaction rate constant to zero. This increased the mean bias outside the area of highest ozone concentration but substantially improved O3 and NOy over most of the domain, especially in smoggy areas such as the rural, Pinnacles site.

Ozone, oxides of nitrogen, and carbon monoxide during pollution events over the eastern United States: An evaluation of emissions and vertical mixing

[1] Chemical transport models such as the Community Multiscale Air Quality (CMAQ) model provide useful guidance on air pollution control strategies. We evaluate the performance of a 12 km resolution CMAQ simulation with surface and aircraft observations of CO, O3, and NOx during the summer of 2002. When all data are considered, on average, modeled and observed CO total column contents (surface to 3,000 m) agreed to within 14% in the morning and 22% in the afternoon. Reducing the deposition velocity for CO improves model-measurement agreement but did not eliminate the model bias. The majority of observed vertical profiles have a maximum near the surface. Although many observed spirals had a secondary maximum at the top of the boundary layer, indicating subgrid-scale shallow convection. The model was not able to replicate these vertical structures. Water vapor profiles likewise showed greater vertical variability in the observations than in the model. General conclusions from these model-measurement comparisons: total CO emissions estimates are either adequate or underestimated, but there is no evidence of gross error; NOx emissions from mobile sources may be overestimated while the lifetime of NOx may be underestimated in CMAQ 4.5.1 with CBIV, and vertical mixing in the model boundary layer may be too fast, but venting out of the boundary layer into the lower free troposphere may be too slow.

Simulation of the diurnal variations of the oxygen isotope anomaly (Δ<sup>17</sup>O) of reactive atmospheric species

Abstract. The isotope anomaly (Δ17O) of secondary atmospheric species such as nitrate (NO3−) or hydrogen peroxide (H2O2) has potential to provide useful constrains on their formation pathways. Indeed, the Δ17O of their precursors (NOx, HOx etc.) differs and depends on their interactions with ozone, which is the main source of non-zero Δ17O in the atmosphere. Interpreting variations of Δ17O in secondary species requires an in-depth understanding of the Δ17O of their precursors taking into account non-linear chemical regimes operating under various environmental settings. This article reviews and illustrates a series of basic concepts relevant to the propagation of the Δ17O of ozone to other reactive or secondary atmospheric species within a photochemical box model. We present results from numerical simulations carried out using the atmospheric chemistry box model CAABA/MECCA to explicitly compute the diurnal variations of the isotope anomaly of short-lived species such as NOx and HOx. Using a simplified but realistic tropospheric gas-phase chemistry mechanism, Δ17O was propagated from ozone to other species (NO, NO2, OH, HO2, RO2, NO3, N2O5, HONO, HNO3, HNO4, H2O2) according to the mass-balance equations, through the implementation of various sets of hypotheses pertaining to the transfer of Δ17O during chemical reactions. The model results confirm that diurnal variations in Δ17O of NOx predicted by the photochemical steady-state relationship during the day match those from the explicit treatment, but not at night. Indeed, the Δ17O of NOx is "frozen" at night as soon as the photolytical lifetime of NOx drops below ca. 10 min. We introduce and quantify the diurnally-integrated isotopic signature (DIIS) of sources of atmospheric nitrate and H2O2, which is of particular relevance to larger-scale simulations of Δ17O where high computational costs cannot be afforded.

Assimilation of OMI NO2 retrievals into a regional chemistry-transport model for improving air quality forecasts over Europe

This paper presents the assimilation of satellite NO2 observations into a chemistry-transport model (CTM). The NO2 columns from Ozone Monitoring Instrument (OMI) aboard NASA Aura satellite are used during November–December 2005. These satellite observations are assimilated in an air quality model from Polyphemus, in order to better forecast NO2 in Europe. The optimal-interpolation method is applied to produce analyzed columns, and these analyzed columns are mapped to model concentrations assuming that the model vertical profile is perfect. Good consistency is seen in the comparisons of model simulations, satellite data and ground observations before assimilation. The model results with and without assimilation are then compared with ground observations for evaluating the assimilation effects. It is found that the assimilation can improve the NO2 forecasts by reducing their discrepancies against ground observations, indicating a better NO2 forecast obtained with OMI observations. Such improvements are seen in a cold season rather than in a warm one probably due to the longer lifetime of NOx and the initial condition changes having more impacts in winter. This also suggests that the assimilation of the short-lived species like NO2 is a complicate problem.

Indirect validation of tropospheric nitrogen dioxide retrieved from the OMI satellite instrument: Insight into the seasonal variation of nitrogen oxides at northern midlatitudes

We examine the seasonal variation in lower tropospheric nitrogen oxides (NOx = NO + NO2) at northern midlatitudes by evaluating tropospheric NO2 columns observed from the Ozone Monitoring Instrument (OMI) satellite instrument with surface NO2 measurements (SouthEastern Aerosol Research and Characterization and Air Quality System) and current bottom‐up NOx emission inventories, using a global model of tropospheric chemistry (GEOS‐Chem). The standard (SP) and DOMINO (DP) tropospheric NO2 column products from OMI exhibit broadly similar spatial and seasonal variation, but differ substantially over continental source regions. A comparison of the two OMI tropospheric NO2 products with in situ surface NO2 concentrations and bottom‐up NOx emissions over the southeast United States indicates that annual mean NO2 columns from the DP are biased high by 21%–33% and those from the SP are biased high by 27%–43%. The bias in SP columns is highly seasonal, 67%–74% in summer compared with −6% to −1% in winter. Similar seasonal differences exist between top‐down and bottom‐up NOx emission inventories over North America, Europe, and East Asia. The air mass factor largely explains the observed seasonal difference between the DP and SP, and in turn the seasonal SP bias. We develop a third product (DP_GC) using averaging kernel information from the DP and NO2 vertical profiles from GEOS‐Chem. This product reduces to 5%–21% the annual mean bias over the southeast United States. We use the seasonal variation in the DP_GC to estimate the seasonal variation in the lifetime of lower tropospheric NOx against oxidation to HNO3 over the eastern United States. The effective NOx lifetime at OMI overpass time (early afternoon) ranges from 7.6 h in summer to 17.8 h in winter, consistent within 3 h of the simulated lifetime. GEOS‐Chem calculations reveal that the seasonal variation in OMI NO2 columns largely reflects gas‐phase oxidation of NO2 in summer with an increasing role for heterogenous chemistry in winter.