Nitrogen oxides in the free troposphere: Implications for tropospheric oxidants and the interpretation of satellite NO2 measurements
Viral Shah1,a,Daniel J. Jacob1,2,Ruijun Dang1,Lok N. Lamsal3,4,Sarah A. Strode3,5,Stephen D. Steenrod3,4,K. Folkert Boersma6,7,Sebastian D. Eastham8,9,Thibaud M. Fritz8,Chelsea Thompson10,11,Jeff Peischl10,11,Ilann Bourgeois10,11,b,Ilana B. Pollack12,Benjamin A. Nault13,Ronald C. Cohen14,15,Pedro Campuzano-Jost16,17,Jose L. Jimenez16,17,Simone T. Andersen18,Lucy J. Carpenter18,Tomás Sherwen18,19,and Mat J. Evans18,19Viral Shah et al.Viral Shah1,a,Daniel J. Jacob1,2,Ruijun Dang1,Lok N. Lamsal3,4,Sarah A. Strode3,5,Stephen D. Steenrod3,4,K. Folkert Boersma6,7,Sebastian D. Eastham8,9,Thibaud M. Fritz8,Chelsea Thompson10,11,Jeff Peischl10,11,Ilann Bourgeois10,11,b,Ilana B. Pollack12,Benjamin A. Nault13,Ronald C. Cohen14,15,Pedro Campuzano-Jost16,17,Jose L. Jimenez16,17,Simone T. Andersen18,Lucy J. Carpenter18,Tomás Sherwen18,19,and Mat J. Evans18,19
1Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 01238, USA
2Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
3Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
4University of Maryland Baltimore County, Baltimore, MD 21250, USA
5GESTAR II, Morgan State University, Baltimore, MD 21251, USA
6Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands
7Wageningen University, Wageningen, the Netherlands
8Laboratory for Aviation and the Environment, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
9Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
10NOAA Chemical Sciences Laboratory, Boulder, CO 80305, USA
11Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309, USA
12Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO 80523, USA
13Center for Aerosols and Cloud Chemistry, Aerodyne Research, Inc., Billerica, MA 01821, USA
14Department of Earth and Planetary Science, University of California Berkeley, Berkeley, CA 94720, USA
15Department of Chemistry, University of California Berkeley, Berkeley, CA 94720, USA
16Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
17Department of Chemistry, University of Colorado, Boulder, CO 80309, USA
18Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, YO10 5DD, UK
19National Centre for Atmospheric Science, University of York, York YO10 5DD, UK
anow at: Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA, and Science Systems and Applications, Inc., Lanham, MD 20706, USA
bnow at: Extreme Environments Research Laboratory, École Polytechnique Fédérale de Lausanne Valais Wallis, Sion, Switzerland, and Plant Ecology Research Laboratory, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
1Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 01238, USA
2Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
3Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
4University of Maryland Baltimore County, Baltimore, MD 21250, USA
5GESTAR II, Morgan State University, Baltimore, MD 21251, USA
6Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands
7Wageningen University, Wageningen, the Netherlands
8Laboratory for Aviation and the Environment, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
9Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
10NOAA Chemical Sciences Laboratory, Boulder, CO 80305, USA
11Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309, USA
12Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO 80523, USA
13Center for Aerosols and Cloud Chemistry, Aerodyne Research, Inc., Billerica, MA 01821, USA
14Department of Earth and Planetary Science, University of California Berkeley, Berkeley, CA 94720, USA
15Department of Chemistry, University of California Berkeley, Berkeley, CA 94720, USA
16Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
17Department of Chemistry, University of Colorado, Boulder, CO 80309, USA
18Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, YO10 5DD, UK
19National Centre for Atmospheric Science, University of York, York YO10 5DD, UK
anow at: Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA, and Science Systems and Applications, Inc., Lanham, MD 20706, USA
bnow at: Extreme Environments Research Laboratory, École Polytechnique Fédérale de Lausanne Valais Wallis, Sion, Switzerland, and Plant Ecology Research Laboratory, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
Abstract. Satellite-based retrievals of tropospheric NO2 columns are used to infer NOx (NO+NO2) emissions at the surface. These retrievals rely on model information for the vertical distribution of NO2. The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NOx also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC4RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating background NOx and attribute this background to sources. NO2 measurements over the southeast US during SEAC4RS and DC3 show increasing concentrations in the upper troposphere above 10 km, which is not replicated by GEOS-Chem although the model is consistent with the NO measurements. Using concurrent NO, NO2 and ozone observations from a DC3 flight in a thunderstorm outflow, we show that NO2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO2 reservoirs, such as peroxynitric acid (HNO4) and methyl peroxy nitrate (MPN). We find that NO2 concentrations calculated from the NO measurements and NO-NO2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO2 profiles throughout the troposphere for SEAC4RS and DC3 but overestimates NO2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NOx chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the ATom campaigns over the Pacific and Atlantic Oceans, indicating a missing NOx source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The average NO2 column density for the ATom campaign in the GEOS-Chem simulation is 2.4×1014 molec cm-2 with particulate nitrate photolysis and 1.5×1014 molec cm-2 without, compared to 1.9×1014 molec cm-2 in the observations (using PSS NO2) and 1.4–2.4×1014 molec cm-2 in the GMI, TM5 and CAMS models. We find from GEOS-Chem that lightning is the main primary NOx source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5 ppbv in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19 %. The contribution of the free tropospheric background to the tropospheric NO2 columns observed by satellites over the contiguous US increases from 25 % in winter to 65 % in summer according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NOx emissions from satellite NO2 column measurements.
This manuscript contains important analyses with regard to observations and model calculations of NOx in the free troposphere. The paper convincingly proves that NO2 from both laser-induced fluoresence and pholoysis/chemiluminescence instruments have significant high biaes due to interferences from other NOy species. NO2 calculated from photostationary state (PSS) assumptions likely yield a better estimate. However, the GEOS-Chem model overestimates the PSS-NO2 in the free troposphere in the southeast US during the SEAC4RS and DC3 experiments. In the remote free troposphere the model underestimates NO during ATom, but inclusion of of photolysis of particulate nitrate greatly improves the simulations. The implications of these findings for NO2 satellite retrievals are discussed. As found in previous studies, lightning is noted as the primary NOx source to the free troposphere over the tropics and southern midlatitudes in all seasons and over the US in summer. The free tropospheric component of the NO2 column over the US in summer (65%) is sufficiently large to make surface emissions estimates in this season difficult. This is an important conclusion of the manuscript. The paper is very well written and should be published with just a few minor revisions as noted below:
line 131: ....column retrievals, if the airborne measurements are assumed to be correct.
line 167: NOy/NO > 3 seems like this would be aged emssions, not fresh. Maybe this should be < 3 ?
line 563: "....errors in modeled tropospheric NO2 columns over clean areas in relatively small." This doesn't seem correct based on the model results shown in Figure 6. The difference between models is ~1 x 10^14 and the PSS-based NO2 column is ~1.9 x 10^14. Wouldn't this imply an uncertainty greater than 50%?
Background NOx affects global tropospheric chemistry and the retrieval and interpretation of satellite NO2 measurements. We use aircraft measurements to evaluate the simulation of NOx in global atmospheric chemistry models. We find that recycling of NOx from its reservoirs over the oceans is faster than that simulated in the models, resulting in large increases in simulated tropospheric ozone and OH. Over the US, background NO2 contributes the majority of the tropospheric NO2 column in summer.
Background NOx affects global tropospheric chemistry and the retrieval and interpretation of...
