Suchergebnisse

Formaldehyde (CHO) is a tracer of the photochemical activity of the atmosphere. Linked to air quality, CHO is an ozone (O) precursor and serves as a proxy for natural and anthropogenic reactive organic emissions. As a product of the photooxidation of methane (CH) and other hydrocarbons (e.g., isoprene), CHO represents an important source of radicals in the remote free troposphere. This work aims at improving the characterization of this part of the troposphere where data are scarce. In particular, this study assesses the presence of CHO at two high-altitude remote sites: El Teide (TEI, 3570 m a.s.l., Tenerife, Canary Islands, Spain) and Pic du Midi (PDM, 2877 m a.s.l., French Pyrenees). Through ground-based remote sensing measurements performed during two field campaigns in July (TEI) and September (PDM) 2013, this study presents the vertical distribution of CHO at both locations. Results at PDM show that CHO mixing ratios follow a decreasing vertical profile with a mean maximum of 0.5 ± 0.2 nmol mol (i.e., ppbv) at the instruments' altitude. At TEI, observations indicate an uplifted layer of CHO with a mean maximum of 1.3 ± 0.3 nmol mol at 3.8 km a.s.l. (i.e., 300 m above the instrument's altitude). At both remote sites, the observed CHO levels are higher than expected for background methane oxidation (a threefold increase in the case of TEI). Air mass back trajectory analysis links CHO observations with abundant natural (e.g. forests) and/or anthropogenic isoprene emissions from the region nearby PDM, while the high CHO levels detected at TEI indicate in-plume formation of CHO resulting from its precursors emitted from west-African and Canadian fires. Finally, as a key trace gas for O and HO chemistries, we estimate the upper limit of bromine monoxide (BrO) in the free troposphere at TEI and PDM to be 0.8 and 1.5 pmol mol (i.e., pptv) respectively. ; The TEI campaign was funded through the AMISOC project (Atmospheric MInor Species relevant to the Ozone Chemistry, Spanish National RCD Funding Agency, CGL2011-24891). The PDM campaign was funded by the European Research Council Executive Agency under the European Union's Horizon 2020 Research and innovation program (ERC-2010-STG 258537) to JES. LGM would like to the MINECO support under grant VHODCA (CTM2017–83199P). The authors would like to thank the assistance of the personal working at the Izaña and at the Pic du Midi observatories. The authors acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and READY website (http://www.ready.noaa.gov) used in this publication. Also, we acknowledge the use of the aerosol product (AOD) from AERONET (http://aeronet.gsfc.nasa.gov/.) and the fire counts imagery from LANCE FIRMS operated by NASA's Earth Science Data and Information System (ESDIS) with funding provided by NASA Headquarters. The UV aerosol index visualization used in this work was produced with the Giovanni online data system, developed and maintained by the NASA GES DISC. We also acknowledge the OMI mission scientists and associated NASA personnel for the production of the UV aerosol index data used in this study (Bhartia P. K. 2012, OMTO3d-OMI/Aura TOMS-Like Ozone, Aerosol Index, Cloud Radiance Fraction L3 1 day 1° x 1° V3).

30 pags., 13 figs., 4 tabs. ; We present the inter-comparison of delta slant column densities (SCDs) and vertical profiles of nitrous acid (HONO) derived from measurements of different multiaxis differential optical absorption spectroscopy (MAXDOAS) instruments and using different inversion algorithms during the Second Cabauw Inter-comparison campaign for Nitrogen Dioxide measuring Instruments (CINDI- 2) in September 2016 at Cabauw, the Netherlands (51.97° N, 4.93° E). The HONO vertical profiles, vertical column densities (VCDs), and near-surface volume mixing ratios are compared between different MAX-DOAS instruments and profile inversion algorithms for the first time. Systematic and random discrepancies of the HONO results are derived from the comparisons of all data sets against their median values. Systematic discrepancies of HONO delta SCDs are observed in the range of ±0:3×1015 molec. cm2, which is half of the typical random discrepancy of 0:6× 1015 molec. cm2. For a typical high HONO delta SCD of 2×1015 molec. cm2, the relative systematic and random discrepancies are about 15% and 30 %, respectively. The inter-comparison of HONO profiles shows that both systematic and random discrepancies of HONO VCDs and nearsurface volume mixing ratios (VMRs) are mostly in the range of ∼ ±0:5×1014 molec. cm2 and ∼ ±0:1 ppb (typically ∼ 20 %). Further we find that the discrepancies of the retrieved HONO profiles are dominated by discrepancies of the HONO delta SCDs. The profile retrievals only contribute to the discrepancies of the HONO profiles by ∼ 5 %. However, some data sets with substantially larger discrepancies than the typical values indicate that inappropriate implementations of profile inversion algorithms and configurations of radiative transfer models in the profile retrievals can also be an important uncertainty source. In addition, estimations of measurement uncertainties of HONO dSCDs, which can significantly impact profile retrievals using the optimal estimation method, need to consider not only DOAS fit errors, but also atmospheric variability, especially for an instrument with a DOAS fit error lower than ∼ 3×1014 molec. cm2. The MAX-DOAS results during the CINDI-2 campaign indicate that the peak HONO levels (e.g. near-surface VMRs of ∼ 0:4 ppb) often appeared in the early morning and below 0.2 km. The near-surface VMRs retrieved from the MAXDOAS observations are compared with those measured using a co-located long-path DOAS instrument. The systematic differences are smaller than 0.15 and 0.07 ppb during early morning and around noon, respectively. Since true HONO values at high altitudes are not known in the absence of real measurements, in order to evaluate the abilities of profile inversion algorithms to respond to different HONO profile shapes, we performed sensitivity studies using synthetic HONO delta SCDs simulated by a radiative transfer model with assumed HONO profiles. The tests indicate that the profile inversion algorithms based on the optimal estimation method with proper configurations can reproduce the different HONO profile shapes well. Therefore we conclude that the features of HONO accumulated near the surface derived from MAX-DOAS measurements. ; Funding for this study was provided by ESA through the CINDI-2 (ESA contract no. 4000118533/16/I-Sbo) and FRM4DOAS (ESA contract no. 4000118181/16/I-EF) projects, by the NSFC (grant no. 41805027), the Russian Foundation for Basic Research (grant no. 18-35-00682), the Russian Academy of Sciences (grant nos. 0150-2018-0052 and 0129-2019- 0002), NASA's Atmospheric Composition Program (grant no. NASA-16-NUP2016-0001), the US National Science Foundation (AGS-1620530 award), and the European Union's Horizon 2020 research and innovation programme through the ACTRIS-2 transnational access programme (grant no. 654109). The AIOFM group is grateful for the support by the NSFC (grant no. 41530644). The article processing charges for this open-access publication were covered by the Max Planck Society