The rapid expansion of the Chinese economy is making its mark on the environment. Atmospheric pollution due to the release of nitrogen oxides from fossil fuel and biomass burning is expected to decrease in most industrialized countries but in some parts of the world rapid economic development could have the opposite effect. Satellite observations over the period 1996–2004 now provide confirmation of these predictions. Across parts of Europe and North America there have been dramatic reductions in nitrogen oxide concentrations in the lower atmosphere (the troposphere). But there was a significant increase of about 50% — with an accelerating trend in annual growth rate — over the industrial areas of China; this is much larger than predictions made based on emission inventories. Emissions from fossil fuel combustion and biomass burning reduce local air quality and affect global tropospheric chemistry. Nitrogen oxides are emitted by all combustion processes and play a key part in the photochemically induced catalytic production of ozone, which results in summer smog and has increased levels of tropospheric ozone globally1. Release of nitrogen oxide also results in nitric acid deposition, and—at least locally—increases radiative forcing effects due to the absorption of downward propagating visible light2. Nitrogen oxide concentrations in many industrialized countries are expected to decrease3, but rapid economic development has the potential to increase significantly the emissions of nitrogen oxides4,5,6,7 in parts of Asia. Here we present the tropospheric column amounts of nitrogen dioxide retrieved from two satellite instruments GOME8,9 and SCIAMACHY10 over the years 1996–2004. We find substantial reductions in nitrogen dioxide concentrations over some areas of Europe and the USA, but a highly significant increase of about 50 per cent—with an accelerating trend in annual growth rate—over the industrial areas of China, more than recent bottom-up inventories suggest6.

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Increase in tropospheric nitrogen dioxide over China observed from space

We present a new algorithm for the near-real time retrieval ? within 3 h of the actual satellite measurement ? of tropospheric NO2 columns from the Ozone Monitoring Instrument (OMI). The retrieval system is based on the combined retrieval-assimilation-modelling approach developed at KNMI for off-line tropospheric NO2 from the GOME and SCIAMACHY satellite instruments. We have adapted the off-line system such that the required a priori information ndash; profile shapes and stratospheric background NO2 ndash; is now immediately available upon arrival of the OMI NO2 slant columns and cloud data at KNMI. Slant column NO2 and cloud information arrives at KNMI typically within 80 min of actual OMI observations. Slant columns for NO2 are retrieved using differential optical absorption spectroscopy (DOAS) in the 405?465 nm range. Cloud fraction and cloud pressure are provided by a new cloud retrieval algorithm that uses the absorption of the O2?O2 collision complex near 477 nm. On-line availability of stratospheric slant columns and NO2 profiles is achieved by running the TM4 chemistry transport model (CTM) forward in time based on forecast ECMWF meteo and assimilated NO2 information from all previously observed orbits. OMI NO2 slant columns, after correction for spurious across-track variability, show a random error for individual pixels of approximately 0.7×1015molec.cm?2. As NO2 retrievals are very sensitive to clouds, we evaluated the consistency of cloud fraction and cloud pressure from the new O2?O2 (OMI) algorithm and from the Fast Retrieval Scheme for Cloud Observables (FRESCO). Cloud parameters from the O2?O2 (OMI) algorithm have similar frequency distributions as cloud parameters retrieved from FRESCO (SCIAMACHY) for August 2006. On average, OMI cloud fractions are higher by 0.011, and OMI cloud pressures exceed FRESCO cloud pressures by 60 hPa. As a consistency check, we intercompared OMI near-real time NO2 columns measured at 13:45 h local time to SCIAMACHY off-line NO2 columns measured at 10:00 h local time. In August 2006, both instruments observe very similar spatial patterns of tropospheric NO2 columns, and small differences for most locations on Earth where tropospheric NO2 columns are small. For regions that are strongly polluted, SCIAMACHY observes higher tropospheric NO2 columns than OMI.

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Near-real time retrieval of tropospheric NO 2 from OMI

Satellite remote sensing of surface air quality

[1] For the period 1996–2006, global distributions of tropospheric nitrogen dioxide (NO2) have been derived from radiances measured with the satellite instruments GOME (Global Ozone Monitoring Experiment) and SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY). A statistical analysis is applied to derive trends and seasonal variability for this period on a global scale. The time series of the monthly NO2 columns for these ten years have been fitted with a linear function superposed on an annual seasonal cycle on a grid with a spatial resolution of 1 by 1 .W e see significant reductions (up to 7% per year) in NO2 in Europe and parts of the eastern United States, and a strong increase in Asia, most particularly in China (up to 29% per year) but also in Iran and Russia. By comparing the data with the cloud information derived from the same satellite observations, the contribution of lightning to the total column of NO2 is estimated. The estimated NO2 from lightning is, especially in the tropics, in good agreement with lightning flash rate observations from space. The satellite observed seasonal variability of NO2 generally correlates well with independent observations and estimates of the seasonal cycle of specific NOx sources. Source categories considered are anthropogenic (fossil fuel and biofuel), biomass burning, soil emissions and lightning. Using the characteristics of the seasonal variability of these source categories, the dominant source of NOx emissions has been identified on a global scale and on a 1 by 1 grid.

https://aura.gsfc.nasa.gov/science/publications/VanderA_Eskes_Boersma_JGR2008.pdf

Trends, seasonal variability and dominant NOx source derived from a ten year record of NO2 measured from space

[1] We present an approach to infer ground-level nitrogen dioxide (NO2) concentrations by applying local scaling factors from a global three-dimensional model (GEOS-Chem) to tropospheric NO2 columns retrieved from the Ozone Monitoring Instrument (OMI) onboard the Aura satellite. Seasonal mean OMI surface NO2 derived from the standard tropospheric NO2 data product (Version 1.0.5, Collection 3) varies by more than two orders of magnitude ( 10 ppbv) over North America. Two ground-based data sets are used to validate the surface NO2 estimate and indirectly validate the OMI tropospheric NO2 retrieval: photochemical steady-state (PSS) calculations of NO2 based on in situ NO and O3 measurements, and measurements from a commercial chemiluminescent NO2 analyzer equipped with a molybdenum converter. An interference correction algorithm for the latter is developed using laboratory and field measurements and applied using modeled concentrations of the interfering species. The OMI-derived surface NO2 mixing ratios are compared with an in situ surface NO2 data obtained from the U.S. Environmental Protection Agency's Air Quality System (AQS) and Environment Canada's National Air Pollution Surveillance (NAPS) network for 2005 after correcting for the interference in the in situ data. The overall agreement of the OMI-derived surface NO2 with the corrected in situ measurements and PSS-NO2 is −11–36%. A larger difference in winter/spring than in summer/fall implies a seasonal bias in the OMI NO2 retrieval. The correlation between the OMI-derived surface NO2 and the ground-based measurements is significant (correlation coefficient up to 0.86) with a tendency for higher correlations in polluted areas. The satellite-derived data base of ground level NO2 concentrations could be valuable for assessing exposures of humans and vegetation to NO2, supplementing the capabilities of the ground-based networks, and evaluating air quality models and the effectiveness of air quality control strategies.

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Ground-level nitrogen dioxide concentrations inferred from the satellite-borne Ozone Monitoring Instrument

in the Mount Cimone area is good (R 2 = 0.9) with the mixing properties of the atmosphere being the most important parameter for a valid comparison of the measurements. However, even when the atmospheric mixing properties are optimal for comparison, the ratio between GOME and ground-based tropospheric column data may not be unity. It is demonstrated that the values obtained (less than 1) are related to the fraction of the satellite ground pixel occupied by the NO2 hot spot. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; KEYWORDS: tropospheric NO2, satellite validation, Po basin

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First comparison between ground-based and satellite-borne measurements of tropospheric nitrogen dioxide in the Po basin

Transport of photochemical oxidants along the northwestern adriatic coast

Recent theoretical and experimental studies have shown how local wind fields, such as the land-sea breeze circulation in coastal areas, influence the transport of oxidant precursors and the residence time and re-entry of photochemical compounds. Simple models usually do not take into account the vertical layering of pollutants, assuming a uniform concentration distribution within the boundary layer during transport processes. The present study reports the results of a monitoring survey conducted in an industrial coastal area to assess the time evolution of the vertical layering of ozone subject to land-sea breeze circulation in the boundary layer.

Vertical layering of photochemical ozone during land-sea breeze transport

During the warm months, the whole area surrounding Ravenna's petrochemical plant, which is about 5 km inshore of the northern Adriatic coast (Po Valley region), is subject to photochemical reactions producing high values of ozone concentration. The transport offshore of oxidants' precursors by the land breeze, photochemical reactions and subsequent transport inshore of oxidants by the sea breeze, may cause high concentrations of ozone along the coast up until the late evening. Similar phenomena have already been observed in other coastal regions. Classifying the ozone data into days with different circulations, the following can be seen: on days with W or NW circulation, a regular diurnal variation of ozone correlated with solar radiation is found; with winds from E or SE, the ozone variations are irregular and ozone concentrations can maintain high levels ((40÷70) p.p.b.) even during the nightime. This is a report of the results of several field measurements aimed at showing the ozone production and destruction processes, as well as its recirculation mechanisms under breeze conditions.

Transport of photochemical ozone along the western Adriatic shore near a petrochemical plant

The time-space distribution of photochemical ozone during the summer along the western coast of the upper Adriatic Sea is strongly influenced by small-scale transport phenomena, especially land and sea breezes (Giovanelli et al., 1985). When these events involve the movement of air masses rich in primary and secondary pollutants across one or more lines of physical discontinuities, such as that between land and sea or across the inshore strip (flat, predominantly farmland) lying between seaboard and city, they can determine irregular, varying ozone distribution patterns at ground level in areas only a few kilometers apart. An empirical model was developed to characterize these local differences in ozone distribution over the entire area. The method makes it possible to compare the data recorded at the various sites and times by “standardizing” parameters for meteorological and local-diffusion conditions as well as for source emission values by processing the data from a series of measurements taken at several coastal sites, both inshore and offshore, during the summers 1980–1985. The resulting space-time models show, despite the limited number of monitor stations, “local” differences in the processes of production, transport, and spread of photochemical ozone in the area studied. The final distribution of the mean daily evolution (MDE) of ozone throughout the greater Ravenna area is reported and discussed.

F. Fortezza

Papers

First comparison between ground-based and satellite-borne measurements of tropospheric nitrogen dioxide in the Po basin

Transport of photochemical oxidants along the northwestern adriatic coast

Vertical layering of photochemical ozone during land-sea breeze transport

Transport of photochemical ozone along the western Adriatic shore near a petrochemical plant

Time-space model of summer ozone distribution around Ravenna