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.

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[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.

Nitrogen oxides (NOx = NO + NO2) are produced during combustion processes and, thus may serve as a proxy for fossil fuel‐based energy usage and coemitted greenhouse gases and other pollutants. We use high‐resolution nitrogen dioxide (NO2) data from the Ozone Monitoring Instrument (OMI) to analyze changes in urban NO2 levels around the world from 2005 to 2014, finding complex heterogeneity in the changes. We discuss several potential factors that seem to determine these NOx changes. First, environmental regulations resulted in large decreases. The only large increases in the United States may be associated with three areas of intensive energy activity. Second, elevated NO2 levels were observed over many Asian, tropical, and subtropical cities that experienced rapid economic growth. Two of the largest increases occurred over recently expanded petrochemical complexes in Jamnagar (India) and Daesan (Korea). Third, pollution transport from China possibly influenced the Republic of Korea and Japan, diminishing the impact of local pollution controls. However, in China, there were large decreases over Beijing, Shanghai, and the Pearl River Delta, which were likely associated with local emission control efforts. Fourth, civil unrest and its effect on energy usage may have resulted in lower NO2 levels in Libya, Iraq, and Syria. Fifth, spatial heterogeneity within several megacities may reflect mixed efforts to cope with air quality degradation. We also show the potential of high‐resolution data for identifying NOx emission sources in regions with a complex mix of sources. Intensive monitoring of the world's tropical/subtropical megacities will remain a priority, as their populations and emissions of pollutants and greenhouse gases are expected to increase significantly.

Anthropogenic emissions of nitrogen oxides (NOx) can change rapidly due to economic growth or control measures. Bottom‐up emissions estimated using source‐specific emission factors and activity statistics require years to compile and can become quickly outdated. We present a method to use satellite observations of tropospheric NO2 columns to estimate changes in NOx emissions. We use tropospheric NO2 columns retrieved from the SCIAMACHY satellite instrument for 2003–2009, the response of tropospheric NO2 columns to changes in NOx emissions determined from a global chemical transport model (GEOS‐Chem), and the bottom‐up anthropogenic NOx emissions for 2006 to hindcast and forecast the inventories. We evaluate our approach by comparing bottom‐up and hindcast emissions for 2003. The two inventories agree within 6.0% globally and within 8.9% at the regional scale with consistent trends in western Europe, North America, and East Asia. We go on to forecast emissions for 2009. During 2006–2009, anthropogenic NOx emissions over land increase by 9.2% globally and by 18.8% from East Asia. North American emissions decrease by 5.7%.

[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,

China’s policy interventions to reduce the spread of the coronavirus disease 2019 have environmental and economic impacts. Tropospheric nitrogen dioxide indicates economic activities, as nitrogen dioxide is primarily emitted from fossil fuel consumption. Satellite measurements show a 48% drop in tropospheric nitrogen dioxide vertical column densities from the 20 days averaged before the 2020 Lunar New Year to the 20 days averaged after. This decline is 21 ± 5% larger than that from 2015 to 2019. We relate this reduction to two of the government’s actions: the announcement of the first report in each province and the date of a province’s lockdown. Both actions are associated with nearly the same magnitude of reductions. Our analysis offers insights into the unintended environmental and economic consequences through reduced economic activities.