Abstract. We present a new algorithm for the near-real time retrieval -within 3 h of the actual satellite measurement -of tropospheric NO 2 columns from the Ozone Monitoring Instrument (OMI). The retrieval is based on the combined retrieval-assimilation-modelling approach developed at KNMI for off-line tropospheric NO 2 from the GOME and SCIAMACHY satellite instruments. We have adapted the off-line system such that the required a priori informationprofile shapes and stratospheric background NO 2 -is now immediately available upon arrival (within 80 min of observation) of the OMI NO 2 slant columns and cloud data at KNMI. Slant columns for NO 2 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 O 2 -O 2 collision complex near 477 nm. Online availability of stratospheric slant columns and NO 2 profiles is achieved by running the TM4 chemistry transport model (CTM) forward in time based on forecast ECMWF meteo and assimilated NO 2 information from all previously observed orbits. OMI NO 2 slant columns, after correction for spurious across-track variability, show a random error for individual pixels of approximately 0.7×10 15

Abstract. We describe a new algorithm for the retrieval of nitrogen dioxide (NO2) vertical columns from nadir-viewing satellite instruments. This algorithm (SP2) is the basis for the Version 2.1 OMI This algorithm (SP2) is the basis for the Version 2.1 Ozone Monitoring Instrument (OMI) NO2 Standard Product and features a novel method for separating the stratospheric and tropospheric columns. NO2 Standard Product and features a novel method for separating the stratospheric and tropospheric columns. The approach estimates the stratospheric NO2 directly from satellite data without using stratospheric chemical transport models or assuming any global zonal wave pattern. Tropospheric NO2 columns are retrieved using air mass factors derived from high-resolution radiative transfer calculations and a monthly climatology of NO2 profile shapes. We also present details of how uncertainties in the retrieved columns are estimated. The sensitivity of the retrieval to assumptions made in the stratosphere–troposphere separation is discussed and shown to be small, in an absolute sense, for most regions. We compare daily and monthly mean global OMI NO2 retrievals using the SP2 algorithm with those of the original Version 1 Standard Product (SP1) and the Dutch DOMINO product. The SP2 retrievals yield significantly smaller summertime tropospheric columns than SP1, particularly in polluted regions, and are more consistent with validation studies. SP2 retrievals are also relatively free of modeling artifacts and negative tropospheric NO2 values. In a reanalysis of an INTEX-B validation study, we show that SP2 largely eliminates an ~20% discrepancy that existed between OMI and independent in situ springtime NO2 SP1 measurements.

Abstract. The Ozone Monitoring Instrument (OMI) onboard NASA's Aura satellite has been providing global observations of the ozone layer and key atmospheric pollutant gases, such as nitrogen dioxide (NO2) and sulfur dioxide (SO2), since October 2004. The data products from the same instrument provide consistent spatial and temporal coverage and permit the study of anthropogenic and natural emissions on local-to-global scales. In this paper, we examine changes in SO2 and NO2 over some of the world's most polluted industrialized regions during the first decade of OMI observations. In terms of regional pollution changes, we see both upward and downward trends, sometimes in opposite directions for NO2 and SO2, for different study areas. The trends are, for the most part, associated with economic and/or technological changes in energy use, as well as regional regulatory policies. Over the eastern US, both NO2 and SO2 levels decreased dramatically from 2005 to 2015, by more than 40 and 80 %, respectively, as a result of both technological improvements and stricter regulations of emissions. OMI confirmed large reductions in SO2 over eastern Europe's largest coal-fired power plants after installation of flue gas desulfurization devices. The North China Plain has the world's most severe SO2 pollution, but a decreasing trend has been observed since 2011, with about a 50 % reduction in 2012–2015, due to an economic slowdown and government efforts to restrain emissions from the power and industrial sectors. In contrast, India's SO2 and NO2 levels from coal power plants and smelters are growing at a fast pace, increasing by more than 100 and 50 %, respectively, from 2005 to 2015. Several SO2 hot spots observed over the Persian Gulf are probably related to oil and gas operations and indicate a possible underestimation of emissions from these sources in bottom-up emission inventories. Overall, OMI observations have proved valuable in documenting rapid changes in air quality over different parts of the world during last decade. The baseline established during the first 11 years of OMI is indispensable for the interpretation of air quality measurements from current and future satellite atmospheric composition missions.

[1] We present an approach to infer ground-level nitrogen dioxide (NO 2 ) concentrations by applying local scaling factors from a global three-dimensional model (GEOS-Chem) to tropospheric NO 2 columns retrieved from the Ozone Monitoring Instrument (OMI) onboard the Aura satellite. Seasonal mean OMI surface NO 2 derived from the standard tropospheric NO 2 data product (Version 1.0.5, Collection 3) varies by more than two orders of magnitude (<0.1->10 ppbv) over North America. Two ground-based data sets are used to validate the surface NO 2 estimate and indirectly validate the OMI tropospheric NO 2 retrieval: photochemical steady-state (PSS) calculations of NO 2 based on in situ NO and O 3 measurements, and measurements from a commercial chemiluminescent NO 2 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 NO 2 mixing ratios are compared with an in situ surface NO 2 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 NO 2 with the corrected in situ measurements and PSS-NO 2 is À11-36%. A larger difference in winter/spring than in summer/fall implies a seasonal bias in the OMI NO 2 retrieval. The correlation between the OMI-derived surface NO 2 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 NO 2 concentrations could be valuable for assessing exposures of humans and vegetation to NO 2 , supplementing the capabilities of the ground-based networks, and evaluating air quality models and the effectiveness of air quality control strategies.Citation: Lamsal, L. N., R.

[1] We examine the seasonal variation in lower tropospheric nitrogen oxides (NO x = NO + NO 2 ) at northern midlatitudes by evaluating tropospheric NO 2 columns observed from the Ozone Monitoring Instrument (OMI) satellite instrument with surface NO 2 measurements (SouthEastern Aerosol Research and Characterization and Air Quality System) and current bottom-up NO x emission inventories, using a global model of tropospheric chemistry (GEOS-Chem). The standard (SP) and DOMINO (DP) tropospheric NO 2 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 NO 2 products with in situ surface NO 2 concentrations and bottom-up NO x emissions over the southeast United States indicates that annual mean NO 2 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 NO x 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 NO 2 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 NO x against oxidation to HNO 3 over the eastern United States. The effective NO x 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 NO 2 columns largely reflects gas-phase oxidation of NO 2 in summer with an increasing role for heterogenous chemistry in winter.Citation: Lamsal, L. N., R. V. Martin, A. van Donkelaar, E. A. Celarier, E. J. Bucsela, K. F. Boersma, R. Dirksen, C. Luo, and Y. Wang (2010), Indirect validation of tropospheric nitrogen dioxide retrieved from the OMI satellite instrument: Insight into the seasonal variation of nitrogen oxides at northern midlatitudes,

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[1] We review the standard nitrogen dioxide (NO 2 ) data product (Version 1.0.), which is based on measurements made in the spectral region 415-465 nm by the Ozone Monitoring Instrument (OMI) on the NASA Earth Observing System-Aura satellite. A number of ground-and aircraft-based measurements have been used to validate the data product's three principal quantities: stratospheric, tropospheric, and total NO 2 column densities under nearly or completely cloud-free conditions. The validation of OMI NO 2 is complicated by a number of factors, the greatest of which is that the OMI observations effectively average the NO 2 over its field of view (minimum 340 km 2 ), while a ground-based instrument samples at a single point. The tropospheric NO 2 field is often very inhomogeneous, varying significantly over tens to hundreds of meters, and ranges from <10 15 cm À2 over remote, rural areas to >10 16 cm À2 over urban and industrial areas. Because of OMI's areal averaging, when validation measurements are made near NO 2 sources the OMI measurements are expected to underestimate the ground-based, and this is indeed seen. Further, we use several different instruments, both new and mature, which might give inconsistent NO 2 amounts; the correlations between nearby instruments is 0.8-0.9. Finally, many of the validation data sets are quite small and span a very short length of time; this limits the statistical conclusions that can be drawn from them. Despite these factors, good agreement is generally seen between the OMI and ground-based measurements, with OMI stratospheric NO 2 underestimated by about 14% and total and tropospheric columns underestimated by 15-30%. Typical correlations between OMI NO 2 and ground-based measurements are generally >0.6.

Abstract. We describe the new version 3.0 NASA Ozone Monitoring Instrument (OMI) standard nitrogen dioxide (NO 2 ) products (SPv3). The products and documentation are publicly available from the NASA Goddard Earth Sciences Data and Information Services Center (https://disc. gsfc.nasa.gov/datasets/OMNO2_V003/summary/). The major improvements include (1) a new spectral fitting algorithm for NO 2 slant column density (SCD) retrieval and (2) higherresolution (1 • latitude and 1.25 • longitude) a priori NO 2 and temperature profiles from the Global Modeling Initiative (GMI) chemistry-transport model with yearly varying emissions to calculate air mass factors (AMFs) required to convert SCDs into vertical column densities (VCDs). The new SCDs are systematically lower (by ∼ 10-40 %) than previous, version 2, estimates. Most of this reduction in SCDs is propagated into stratospheric VCDs. Tropospheric NO 2 VCDs are also reduced over polluted areas, especially over western Europe, the eastern US, and eastern China. Initial evaluation over unpolluted areas shows that the new SPv3 products agree better with independent satellite-and ground-based Fourier transform infrared (FTIR) measurements. However, further evaluation of tropospheric VCDs is needed over polluted areas, where the increased spatial resolution and more refined AMF estimates may lead to better characterization of pollution hot spots.