Person: Jacob, Daniel
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Jacob
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Jacob, Daniel
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Publication Formaldehyde (HCHO) As a Hazardous Air Pollutant: Mapping Surface Air Concentrations from Satellite and Inferring Cancer Risks in the United States(American Chemical Society (ACS), 2017-05-05) Zhu, Lei; Jacob, Daniel; Keutsch, Frank; Mickley, Loretta; Scheffe, Richard; Strum, Madeleine; González Abad, Gonzalo; Chance, Kelly; Yang, Kai; Rappenglück, Bernhard; Millet, Dylan; Baasandorj, Munkhbayar; Jaeglé, Lyatt; Shah, ViralFormaldehyde (HCHO) is the most important carcinogen in outdoor air among the 187 hazardous air pollutants (HAPs) identified by the U.S. Environmental Protection Agency (EPA), not including ozone and particulate matter. However, surface observations of HCHO are sparse and the EPA monitoring network could be prone to positive interferences. Here we use 2005–2016 summertime HCHO column data from the OMI satellite instrument, validated with high-quality aircraft data and oversampled on a 5 × 5 km2 grid, to map surface air HCHO concentrations across the contiguous U.S. OMI-derived summertime HCHO values are converted to annual averages using the GEOS-Chem chemical transport model. Results are in good agreement with high-quality summertime observations from urban sites (−2% bias, r = 0.95) but a factor of 1.9 lower than annual means from the EPA network. We thus estimate that up to 6600–12 500 people in the U.S. will develop cancer over their lifetimes by exposure to outdoor HCHO. The main HCHO source in the U.S. is atmospheric oxidation of biogenic isoprene, but the corresponding HCHO yield decreases as the concentration of nitrogen oxides (NOx ≡ NO + NO2) decreases. A GEOS-Chem sensitivity simulation indicates that HCHO levels would decrease by 20–30% in the absence of U.S. anthropogenic NOx emissions. Thus, NOx emission controls to improve ozone air quality have a significant cobenefit in reducing HCHO-related cancer risks.Publication A mass budget for mercury and methylmercury in the Arctic Ocean(Wiley-Blackwell, 2016) Soerensen, Anne; Jacob, Daniel; Schartup, Amina; Fisher, Jenny; Lehnherr, Igor; St. Louis, Vincent L.; Heimbürger, Lars-Eric; Sonke, Jeroen E.; Krabbenhoft, David P.; Sunderland, ElynorElevated biological concentrations of methylmercury (MeHg), a bioaccumulative neurotoxin, are observed throughout the Arctic Ocean, but major sources and degradation pathways in seawater are not well understood. We develop a mass budget for mercury species in the Arctic Ocean based on available data since 2004 and discuss implications and uncertainties. Our calculations show that high total mercury (Hg) in Arctic seawater relative to other basins reflect large freshwater inputs and sea ice cover that inhibits losses through evasion. We find that most net MeHg production (20 Mg a−1) occurs in the subsurface ocean (20–200 m). There it is converted to dimethylmercury (Me2Hg: 17 Mg a−1), which diffuses to the polar mixed layer and evades to the atmosphere (14 Mg a−1). Me2Hg has a short atmospheric lifetime and rapidly degrades back to MeHg. We postulate that most evaded Me2Hg is redeposited as MeHg and that atmospheric deposition is the largest net MeHg source (8 Mg a−1) to the biologically productive surface ocean. MeHg concentrations in Arctic Ocean seawater are elevated compared to lower latitudes. Riverine MeHg inputs account for approximately 15% of inputs to the surface ocean (2.5 Mg a−1) but greater importance in the future is likely given increasing freshwater discharges and permafrost melt. This may offset potential declines driven by increasing evasion from ice-free surface waters. Geochemical model simulations illustrate that for the most biologically relevant regions of the ocean, regulatory actions that decrease Hg inputs have the capacity to rapidly affect aquatic Hg concentrations.Publication Isoprene Emissions in Africa Inferred from OMI Observations of Formaldehyde Columns(European Geosciences Union, 2012) Marais, Elose; Jacob, Daniel; Kurosu, Thomas; Chance, Kelly; Murphy, J. G.; Reeves, C.; Mills, G.; Casadio, S.; Millet, D. B.; Barkley, M. P.; Paulot, F.; Mao, J.We use 2005–2009 satellite observations of formaldehyde (HCHO) columns from the OMI instrument to infer biogenic isoprene emissions at monthly 1 × 1° resolution over the African continent. Our work includes new approaches to remove biomass burning influences using OMI absorbing aerosol optical depth data (to account for transport of fire plumes) and anthropogenic influences using AATSR satellite data for persistent small-flame fires (gas flaring). The resulting biogenic HCHO columns (ΩHCHO) from OMI follow closely the distribution of vegetation patterns in Africa. We infer isoprene emission (EISOP) from the local sensitivity S = ΔΩHCHO / ΔEISOP derived with the GEOS-Chem chemical transport model using two alternate isoprene oxidation mechanisms, and verify the validity of this approach using AMMA aircraft observations over West Africa and a longitudinal transect across central Africa. Displacement error (smearing) is diagnosed by anomalously high values of S and the corresponding data are removed. We find significant sensitivity of S to NOx under low-NOx conditions that we fit to a linear function of tropospheric column NO2. We estimate a 40% error in our inferred isoprene emissions under high-NOx conditions and 40–90% under low-NOx conditions. Our results suggest that isoprene emission from the central African rainforest is much lower than estimated by the state-of-the-science MEGAN inventory.Publication Regional warming from aerosol removal over the United States: Results from a transient 2010–2050 climate simulation(Elsevier BV, 2012) Mickley, Loretta; Leibensperger, Eric Michael; Jacob, Daniel; Rind, D.We use a general circulation model (NASA Goddard Institute for Space Studies GCM 3) to investigate the regional climate response to removal of aerosols over the United States. We perform a pair of transient 2010–2050 climate simulations following a scenario of increasing greenhouse gas concentrations, with and without aerosols over the United States and with present-day aerosols elsewhere. We find that removing U.S. aerosol significantly enhances the warming from greenhouse gases in a spatial pattern that strongly correlates with that of the aerosol. Warming is nearly negligible outside the United States, but annual mean surface temperatures increase by 0.4–0.6 K in the eastern United States. Temperatures during summer heat waves in the Northeast rise by as much as 1–2 K due to aerosol removal, driven in part by positive feedbacks involving soil moisture and low cloud cover. Reducing U.S. aerosol sources to achieve air quality objectives could thus have significant unintended regional warming consequences.Publication Chemistry of Hydrogen Oxide Radicals \((HO_x)\) in the Arctic Troposphere in Spring(European Geosciences Union, 2010) Mao, Jialin; Jacob, Daniel; Evans, M. J.; Olson, J. R.; Ren, X.; Brune, W. H.; St. Clair, J. M.; Crounse, J. D.; Spencer, K. M.; Beaver, M. R.; Wennberg, P. O.; Cubison, M. J.; Jimenez, J. L.; Fried, A.; Weibring, P.; Walega, J. G.; Hall, S. R.; Weinheimer, A. J.; Cohen, R. C.; Chen, G.; Crawford, J. H.; McNaughton, C.; Clarke, A. D.; Jaeglé, L.; Fisher, J. A.; Yantosca, R. M.; Le Sager, P; Carouge, C.We use observations from the April 2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals \((HO_x≡H+OH+\)peroxy radicals) and their reservoirs \((HO_y≡HO_x+\)peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed \(HO_2\) and \(H_2O_2\) concentrations. Computation of \(HO_x\) and \(HO_y\) gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect \(HO_2\) uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H2O2. We implemented such an uptake of \(HO_2\) by aerosol in the model using a standard reactive uptake coefficient parameterization with \(\gamma(HO_2)\) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different \(HO_x\) species and \(HO_y\) reservoirs. \(HO_2\) uptake by aerosol is then a major \(HO_x\) and \(HO_y\) sink, decreasing mean OH and \(HO_2\) concentrations in the Arctic troposphere by 32% and 31% respectively. Better rate and product data for \(HO_2\) uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.Publication Validation of Urban \(NO_2\) Concentrations and Their Diurnal and Seasonal Variations Observed from the SCIAMACHY and OMI Sensors Using In Situ Surface Measurements in Israeli Cities(European Geosciences Union, 2009) Boersma, K. F.; Jacob, Daniel; Trainic, M.; Rudich, Y.; DeSmedt, I.; Dirksen, R.; Eskes, H. J.We compare a full-year (2006) record of surface air \(NO_2\) concentrations measured in Israeli cities to coinciding retrievals of tropospheric \(NO_2\) columns from satellite sensors (SCIAMACHY aboard ENVISAT and OMI aboard Aura). This provides a large statistical data set for validation of \(NO_2\) satellite measurements in urban air, where validation is difficult yet crucial for using these measurements to infer \(NO_x\) emissions by inverse modeling. Assuming that \(NO_2\) is well-mixed throughout the boundary layer (BL), and using observed average seasonal boundary layer heights, near-surface \(NO_2\) concentrations are converted into BL \(NO_2\) columns. The agreement between OMI and (13:45) BL \(NO_2\) columns (slope=0.93, n=542), and the comparable results at 10:00 h for SCIAMACHY, allow a validation of the seasonal, weekly, and diurnal cycles in satellite-derived \(NO_2\). OMI and BL \(NO_2\) columns show consistent seasonal cycles (winter \(NO_2\) 1.6–2.7× higher than summer). BL and coinciding OMI columns both show a strong weekly cycle with 45–50% smaller \(NO_2\) columns on Saturday relative to the weekday mean, reflecting the reduced weekend activity, and validating the weekly cycle observed from space. The diurnal difference between SCIAMACHY (10:00) and OMI (13:45) \(NO_2\) is maximum in summer when SCIAMACHY is up to 40% higher than OMI, and minimum in winter when OMI slightly exceeds SCIAMACHY. A similar seasonal variation in the diurnal difference is found in the source region of Cairo. The surface measurements in Israel cities confirm this seasonal variation in the diurnal cycle. Using simulations from a global 3-D chemical transport model (GEOS-Chem), we show that this seasonal cycle can be explained by a much stronger photochemical loss of \(NO_2\) in summer than in winter.Publication Optimized regional and interannual variability of lightning in a global chemical transport model constrained by LIS/OTD satellite data(Wiley-Blackwell, 2012) Murray, Lee Thomas; Jacob, Daniel; Logan, Jennifer; Hudman, Rynda C.; Koshak, William J.Nitrogen oxides (NOx≡ NO + NO2) produced by lightning make a major contribution to the global production of tropospheric ozone and OH. Lightning distributions inferred from standard convective parameterizations in global chemical transport models (CTMs) fail to reproduce observations from the Lightning Imaging Sensor (LIS) and the Optical Transient Detector (OTD) satellite instruments. We present an optimal regional scaling algorithm for CTMs to fit the lightning NOx source to the satellite lightning data in a way that preserves the coupling to deep convective transport. We show that monthly scaling using ~35 global regions significantly improves the tropical ozone simulation in the GEOS-Chem CTM as compared to a simulation unconstrained by the satellite data, and performs equally well to a simulation with local scaling. The coarse regional scaling preserves sufficient statistics in the satellite data to constrain the interannual variability (IAV) of lightning. After processing the LIS data to remove its diurnal sampling bias, we construct a monthly time series of lightning flash rates for 1998-2010 and 35ºS-35ºN. We find a correlation of IAV in tropical lightning with El Niño but not with the solar cycle or the quasi-biennial oscillation. The resulting global lightning NOx source in GEOS-Chem is 6.0 ± 0.5 Tg N a-1, compared to 5.5 ± 0.8 Tg N a-1 for the biomass burning source. Lightning NOx could have a large influence on the IAV of tropospheric ozone because it is released in the upper troposphere where ozone production is most efficient.Publication Meteorological Modes of Variability for Fine Particulate Matter (PM2.5) Air Quality in the United States: Implications for PM2.5 Sensitivity to Climate Change(European Geosciences Union, 2012) Tai, A. P. K.; Mickley, Loretta; Jacob, Daniel; Leibensperger, Eric Michael; Zhang, L.; Fisher, Jenny; Pye, H. O. T.We applied a multiple linear regression model to understand the relationships of PM2.5 with meteorological variables in the contiguous US and from there to infer the sensitivity of PM2.5 to climate change. We used 2004–2008 PM2.5 observations from ~1000 sites (~200 sites for PM2.5 components) and compared to results from the GEOS-Chem chemical transport model (CTM). All data were deseasonalized to focus on synoptic-scale correlations. We find strong positive correlations of PM2.5 components with temperature in most of the US, except for nitrate in the Southeast where the correlation is negative. Relative humidity (RH) is generally positively correlated with sulfate and nitrate but negatively correlated with organic carbon. GEOS-Chem results indicate that most of the correlations of PM2.5 with temperature and RH do not arise from direct dependence but from covariation with synoptic transport. We applied principal component analysis and regression to identify the dominant meteorological modes controlling PM2.5 variability, and show that 20–40% of the observed PM2.5 day-to-day variability can be explained by a single dominant meteorological mode: cold frontal passages in the eastern US and maritime inflow in the West. These and other synoptic transport modes drive most of the overall correlations of PM2.5 with temperature and RH except in the Southeast. We show that interannual variability of PM2.5 in the US Midwest is strongly correlated with cyclone frequency as diagnosed from a spectral-autoregressive analysis of the dominant meteorological mode. An ensemble of five realizations of 1996–2050 climate change with the GISS general circulation model (GCM) using the same climate forcings shows inconsistent trends in cyclone frequency over the Midwest (including in sign), with a likely decrease in cyclone frequency implying an increase in PM2.5. Our results demonstrate the need for multiple GCM realizations (because of climate chaos) when diagnosing the effect of climate change on PM2.5, and suggest that analysis of meteorological modes of variability provides a computationally more affordable approach for this purpose than coupled GCM-CTM studies.Publication Multi-Decadal Decline of Mercury in the North Atlantic Atmosphere Explained by Changing Subsurface Seawater Concentrations(American Geophysical Union, 2012) Soerensen, Anne; Jacob, Daniel; Streets, David G.; Witt, Melanie L. I.; Ebinghaus, Ralf; Mason, Robert P.; Andersson, Maria; Sunderland, Elsie M.[1] We analyze 1977–2010 trends in atmospheric mercury (Hg) from 21 ship cruises over the North Atlantic (NA) and 15 over the South Atlantic (SA). We find a steep 1990–2009 decline of −0.046 ± 0.010 ng m−3 a−1 (−2.5% a−1) over the NA (steeper than at Northern Hemispheric land sites) but no significant decline over the SA. Surface water Hg0 measurements in the NA show a decline of −5.7% a−1since 1999, and limited subsurface ocean data show an ∼80% decline from 1980 to present. We use a coupled global atmosphere-ocean model to show that the decline in NA atmospheric concentrations can be explained by decreasing oceanic evasion from the NA driven by declining subsurface water Hg concentrations. We speculate that this large historical decline of Hg in the NA Ocean could have been caused by decreasing Hg inputs from rivers and wastewater and by changes in the oxidant chemistry of the atmospheric marine boundary layer.Publication Intercomparison Methods for Satellite Measurements of Atmospheric Composition: Application to Tropospheric Ozone from TES and OMI(European Geosciences Union, 2010) Zhang, L.; Jacob, Daniel; Liu, Xiong; Logan, Jennifer; Chance, Kelly; Eldering, A.; Bojkov, B. R.We analyze the theoretical basis of three different methods to validate and intercompare satellite measurements of atmospheric composition, and apply them to tropospheric ozone retrievals from the Tropospheric Emission Spectrometer (TES) and the Ozone Monitoring Instrument (OMI). The first method (in situ method) uses in situ vertical profiles for absolute instrument validation; it is limited by the sparseness of in situ data. The second method (CTM method) uses a chemical transport model (CTM) as an intercomparison platform; it provides a globally complete intercomparison with relatively small noise from model error. The third method (averaging kernel smoothing method) involves smoothing the retrieved profile from one instrument with the averaging kernel matrix of the other; it also provides a global intercomparison but dampens the actual difference between instruments and adds noise from the a priori. We apply the three methods to a full year (2006) of TES and OMI data. Comparison with in situ data from ozonesondes shows mean positive biases of 5.3 parts per billion volume (ppbv) (10%) for TES and 2.8 ppbv (5%) for OMI at 500 hPa. We show that the CTM method (using the GEOS-Chem CTM) closely approximates results from the in situ method while providing global coverage. It reveals that differences between TES and OMI are generally less than 10 ppbv (18%), except at northern mid-latitudes in summer and over tropical continents. The CTM method further allows for CTM evaluation using both satellite observations. We thus find that GEOS-Chem underestimates tropospheric ozone in the tropics due to possible underestimates of biomass burning, soil, and lightning emissions. It overestimates ozone in the northern subtropics and southern mid-latitudes, likely because of excessive stratospheric influx of ozone.