Abstract. This paper presents extensive bias determination analyses of ozone observations from the Atmospheric Chemistry Experiment (ACE) satellite instruments: the ACE Fourier Transform Spectrometer (ACE-FTS) and the Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (ACE-MAESTRO) instrument. Here we compare the latest ozone data products from ACE-FTS and ACE-MAESTRO with coincident observations from nearly 20 satellite-borne, airborne, balloonborne and ground-based instruments, by analysing volume mixing ratio profiles and partial column densities. The ACE-FTS version 2.2 Ozone Update product reports more ozone than most correlative measurements from the upper troposphere to the lower mesosphere. At altitude levels from 16 to 44 km, the average values of the mean relative differences are nearly all within +1 to +8%. At higher altitudes (45-60 km), the ACE-FTS ozone amounts are significantly larger than those of the comparison instruments, with mean relative differences of up to +40% (about +20% on average). For the ACE-MAESTRO version 1.2 ozone data product, mean relative differences are within ±10% (average values within ±6%) between 18 and 40 km for both the sunrise and sunset measurements. At higher altitudes (∼35-55 km), systematic biases of opposite sign are found between the ACE-MAESTRO sunrise and sunset observations. While ozone amounts derived from the ACE-MAESTRO sunrise occultation data are often smaller than the coincident observations (with mean relative differences down to −10%), the sunset occultation profiles for ACE-MAESTRO show results that are qualitatively similar to ACE-FTS, indicating a large positive bias (mean relative differences within +10 to +30%) in the 45-55 km altitude range. In contrast, there is no significant systematic difference in bias found for the ACE-FTS sunrise and sunset measurements.

Published by Copernicus Publications on behalf of the European Geosciences Union. constructed from averaged ACE-FTS data over China and completed with TES data over the 5 same area below 6 km. The vertical sensitivity of each CO sounding type of instrument is also 6 reported on the right-hand side of this plot. MW and TIR refer to millimeter-wave and 7 thermal infrared spectral regions, respectively. 8 Fig. 1. Schematic plot of a standard atmospheric CO profile, with the different sources of production (blue) and destruction/sinks (red) as a function of altitude. The CO profile was constructed from averaged ACE-FTS data over China and completed with TES data over the same area below 6 km. The vertical sensitivity of each CO sounding type of instrument is also reported on the right-hand side of this plot. MW and TIR refer to millimeter-wave and thermal infrared spectral regions, respectively.Abstract. The Atmospheric Chemistry Experiment (ACE) mission was launched in August 2003 to sound the atmosphere by solar occultation. Carbon monoxide (CO), a good tracer of pollution plumes and atmospheric dynamics, is one of the key species provided by the primary instrument, the ACE-Fourier Transform Spectrometer (ACE-FTS). This instrument performs measurements in both the CO 1-0 and 2-0 ro-vibrational bands, from which vertically resolved CO concentration profiles are retrieved, from the mid-troposphere to the thermosphere. This paper presents an updated description of the ACE-FTS version 2.2 CO data product, along with a comprehensive validation of these profiles using available observations (February 2004 to December 2006. We have compared the CO partial columns with ground-based measurements using Fourier transform infrared spectroscopy and millimeter wave radiometry, and the volume mixing ratio profiles with airborne (both high-altitude balloon flight and airplane) observations. CO satellite observations provided by nadir-looking instruments (MOPITT and TES) as well as limb-viewing remote sensors (MIPAS, SMR and MLS) were also compared with the ACE-FTS CO products. We show that the ACE-FTS measurements provide CO profiles with small retrieval errors (better than 5% from the upper troposphere to 40 km, and better than 10% above). These observations agree well with the correlative measurements, considering the rather loose coincidence criteria in some cases. Based on the validation exercise we assess the following uncertainties to the ACE-FTS measurement data: better than 15% in the upper troposphere (8-12 km), than 30% in the lower stratosphere (12-30 km), and than 25% from 30 to 100 km.

Nitrous acid (HONO) is a major precursor of tropospheric hydroxyl radical (OH) that accelerates the formation of secondary pollutants. The HONO sources, however, are not well understood, especially in polluted areas. Based on a comprehensive winter field campaign conducted at a rural site of the North China Plain, a box model (MCM v3.3.1) was used to simulate the daytime HONO budget and nitrate formation. We found that HONO photolysis acted as the dominant source for primary OH with a contribution of more than 92%. The observed daytime HONO could be well explained by the known sources in the model. The heterogeneous conversion of NO2 on ground surfaces and the homogeneous reaction of NO with OH were the dominant HONO sources with contributions of more than 36% and 34% to daytime HONO, respectively. The contribution from the photolysis of particle nitrate and the reactions of NO2 on aerosol surfaces were found to be negligible in clean periods (2%) and slightly higher during polluted periods (8%). The relatively high OH levels due to fast HONO photolysis at the rural site remarkably accelerated gas-phase reactions, resulting in the fast formation of nitrate as well as other secondary pollutants in the daytime.

The European Union (EU)-funded project Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) investigates the relationship between weather, climate, and air pollution in southern West Africa—an area with rapid population growth, urbanization, and an increase in anthropogenic aerosol emissions. The air over this region contains a unique mixture of natural and anthropogenic gases, liquid droplets, and particles, emitted in an environment in which multilayer clouds frequently form. These exert a large influence on the local weather and climate, mainly owing to their impact on radiation, the surface energy balance, and thus the diurnal cycle of the atmospheric boundary layer. In June and July 2016, DACCIWA organized a major international field campaign in Ivory Coast, Ghana, Togo, Benin, and Nigeria. Three supersites in Kumasi, Savè, and Ile-Ife conducted permanent measurements and 15 intensive observation periods. Three European aircraft together flew 50 research flights between 27 June and 16 July 2016, for a total of 155 h. DACCIWA scientists launched weather balloons several times a day across the region (772 in total), measured urban emissions, and evaluated health data. The main objective was to build robust statistics of atmospheric composition, dynamics, and low-level cloud properties in various chemical landscapes to investigate their mutual interactions. This article presents an overview of the DACCIWA field campaign activities as well as some first research highlights. The rich data obtained during the campaign will be made available to the scientific community and help to advance scientific understanding, modeling, and monitoring of the atmosphere over southern West Africa.

The kinetics and mechanism of the reactions CCl3O2 + CCl3O2 → 2CCl3O + O2 (1), CHCl2O2 + CHCl2O2 → 2CHCl2O + O2 (2a), CHCl2O2 + CHCl2O2 → CHCl2OH + CCl2O + O2 (2b), CCl3O2 + HO2 → products (3), and CHCl2O2 + HO2 → products (4) have been investigated as a function of temperature at total pressures of 700−760 Torr. Two complementary techniques were used:  flash photolysis/UV absorption for kinetic measurements and continuous photolysis/FTIR spectroscopy for end-product analyses. The UV absorption spectra of CHCl2O2 and CCl3O2 were determined between 220 and 280 nm; they have shapes similar to those of other alkyl peroxy radicals, but with broader and less intense bands. The rate constant k 1 was determined between 273 and 460 K from the formation rate of CCl2O in the Cl atom initiated oxidation chain of chloroform, where reaction 1 was the rate-limiting step; k 1 = (3.3 ± 0.6) × 10-13 exp[(745 ± 58)K/T] cm3 molecule-1 s-1, where quoted (1σ) errors represent only statistical uncertainties. Reaction 2 proceeds predominately (≥90%) by channel 2a. While k 2 was not measured directly, satisfactory simulations in the CHCl2O2 + HO2 experiments could only be achieved with k 2 values comparable to those of the self-reactions of CCl3O2 and CH2ClO2 radicals. By averaging the kinetic data for the CH2ClO2 and CCl3O2 radical self-reactions, we derived k 2 = (2.6 ± 0.5) × 10-13 exp[(800 ± 60) K/T] cm3 molecule-1 s-1. The observation of a chain reaction at low temperature (250 K) showed that the CHCl2O radical produced in reaction 2 always reacts by Cl atom elimination so that CHClO is the major atmospheric oxidation product of CH2Cl2. The rate constants of reactions 3 and 4 were measured over the temperature range 286−440 K by generating simultaneously CCl3O2 (or CHCl2O2) and HO2; k 3 = (4.8 ± 0.5) × 10-13 exp[(706 ± 31) K/T], and k 4 = (5.6 ± 1.2) × 10-13 exp[(700 ± 64 K/T] cm3 molecule-1 s-1 (errors = 1σ). Two products were observed following the reaction of CHCl2O2 radicals with HO2 at 296 K in 700 Torr of air:  CHClO (71%) and CCl2O (29%). One product was observed following the reaction of CCl3O2 radicals with HO2:  CCl2O in a yield indistinguishable from 100%. In contrast to all other studies of peroxy radical reactions with HO2, there was no evidence of hydroperoxide formation. Ab initio quantum mechanical calculations (MP2/6-31G(d,p)) were used to derive Δf H°298(CHCl2OOH) = −46.3, Δf H°298(CCl3OOH) = −48.4, Δf H°298(CHCl2O2) = −6.3, Δf H°298(CCl3O2) = −8.0, and Δf H°298(CHClO) = −43.9 kcal mol-1. The mechanistic implications and the trends in the reactivity of chloromethyl peroxy radicals are discussed. As part of this work, the following reaction rate constants were measured (units of cm3 molecule-1 s-1) at room temperature:  k(CCl3O2 + CH3O2) = (6.6 ± 1.7) × 10-12, k(Cl + CHCl3) = (1.1 ± 0.1) × 10-13, k(Cl + CH2Cl2) = (3.5 ± 0.4) × 10-13, k(Cl + CHClO) = (7.0 ± 1.0) × 10-13, and k(F + CHCl3) = (5.4 ± 1.5) × 10-12.

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The ultraviolet spectrum of the ethylperoxy radical (C2H502) and the reactions C2H5O2 + C2H5O2 -products (1) and C2H5O2 + H02 -C2H502H + 0 2 ( 5 ) have been studied using the flash photolysis/UV absorption technique. The spectrum was taken between the wavelengths of 210 and 290 nm and at the temperatures of 298 and 600 K. The room temperature spectrum is found to be in good agreement with previous determinations, with a maximum cross section umax = (4.89 f 0.60) X lo-'* cm2 molecule-' at 240 nm. The temperature dependence of the broadness of the spectrum as well as the value of umax is analyzed by fitting the data to a Gaussian function that predicts the temperature behavior of broad, structureless UV absorptions. Our results on the C2H5O2 self-reaction are also in good agreement with previous studies, with kl/cm3 molecule-' s-I = (6.7 f 0.6) X exp((60 f 40)/TJ for the temperature range 248-460 K. At higher temperatures, we observe non-second-order kinetic behavior which can be attributed to the thermal decomposition of the ethoxy radical, a product of reaction 1. Our results for the reaction C2H5O2 + HO2 are significantly different from the only previous determination of its temperature dependence, especially at and below room temperature, with ks/cm3 molecule-' s-I = (1.6 f 0.4) X lO-I3 expi( 1260 f 130)/ 7') over the temperature range of 248-480 K; our room temperature rate constant is about a factor of 2 greater than the currently accepted value of k5, with k5(298)/cm3 molecule-' s-I = (1.10 f 0.21) X lo-". This result holds implications for the understanding of the reactivity of R02 species with H02, which is important for the chemical modeling of the troposphere.

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), on-board the European ENVIronmental SATellite (ENVISAT) launched on 1 March 2002, is a middle infrared Fourier Transform spectrometer measuring the atmospheric emission spectrum in limb sounding geometry. The instrument is capable to retrieve the vertical distribution of temperature and trace gases, aiming at the study of climate and atmospheric chemistry and dynamics, and at applications to data assimilation and weather forecasting.Correspondence to: U. Cortesi (u.cortesi@ifac.cnr.it) MIPAS operated in its standard observation mode for approximately two years, from July 2002 to March 2004, with scans performed at nominal spectral resolution of 0.025 cm −1 and covering the altitude range from the mesosphere to the upper troposphere with relatively high vertical resolution (about 3 km in the stratosphere). Only reduced spectral resolution measurements have been performed subsequently. MIPAS data were re-processed by ESA using updated versions of the Instrument Processing Facility (IPF v4.61 and v4.62) (and, to a lesser extent, v4.62) O 3 VMR profiles and a comprehensive set of correlative data, including observations from ozone sondes, ground-based lidar, FTIR and microwave radiometers, remote-sensing and in situ instruments on-board stratospheric aircraft and balloons, concurrent satellite sensors and ozone fields assimilated by the European Center for Medium-range Weather Forecasting.A coordinated effort was carried out, using common criteria for the selection of individual validation data sets, and similar methods for the comparisons. This enabled merging the individual results from a variety of independent reference measurements of proven quality (i.e. well characterized error budget) into an overall evaluation of MIPAS O 3 data quality, having both statistical strength and the widest spatial and temporal coverage. Collocated measurements from ozone sondes and ground-based lidar and microwave radiometers of the Network for the Detection Atmospheric Composition Change (NDACC) were selected to carry out comparisons with time series of MIPAS O 3 partial columns and to identify groups of stations and time periods with a uniform pattern of ozone differences, that were subsequently used for a vertically resolved statistical analysis. The results of the comparison are classified according to synoptic and regional systems and to altitude intervals, showing a generally good agreement within the comparison error bars in the upper and middle stratosphere. Significant differences emerge in the lower stratosphere and are only partly explained by the larger contributions of horizontal and vertical smoothing differences and of collocation errors to the total uncertainty. Further results obtained from a purely statistical analysis of the same data set from NDACC ground-based lidar stations, as well as from additional ozone soundings at middle latitudes and from NDACC ground-based FTIR measurements, confirm the validity of MIPAS O 3 profil...