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.

According to current budget estimations the seasonal variation of carbonyl sulfide (COS) is governed by oceanic release and vegetation uptake. Its assimilation by plants is assumed to be similar to the photosynthetic uptake of CO2 but, contrary to the latter process, to be irreversible. Therefore, COS has been suggested as cotracer of the carbon cycle. Observations of COS, however, are sparse, especially in tropical regions. We use the comprehensive data set of spaceborne measurements of the Michelson Interferometer for Passive Atmospheric Sounding to analyze its global distribution. Two major features are observed in the tropical upper troposphere around 250 hPa: enhanced amounts over the western Pacific and the Maritime Continent, peaking around 550 parts per trillion by volume (pptv) in boreal summer, and a seasonally varying depletion of COS extending from tropical South America to Africa. The large‐scale COS depletion, which in austral summer amounts up to −40 pptv as compared to the rest of the respective latitude band, has not been observed before and reveals the seasonality of COS uptake through tropical vegetation. The observations can only be reproduced by global models, when a large vegetation uptake and a corresponding increase in oceanic emissions as proposed in several recent publications are assumed.

Abstract. The first concerted multi-model intercomparison of halogenated very short-lived substances (VSLS) has been performed, within the framework of the ongoing Atmospheric Tracer Transport Model Intercomparison Project (TransCom). Eleven global models or model variants participated (nine chemical transport models and two chemistry–climate models) by simulating the major natural bromine VSLS, bromoform (CHBr3) and dibromomethane (CH2Br2), over a 20-year period (1993–2012). Except for three model simulations, all others were driven offline by (or nudged to) reanalysed meteorology. The overarching goal of TransCom-VSLS was to provide a reconciled model estimate of the stratospheric source gas injection (SGI) of bromine from these gases, to constrain the current measurement-derived range, and to investigate inter-model differences due to emissions and transport processes. Models ran with standardised idealised chemistry, to isolate differences due to transport, and we investigated the sensitivity of results to a range of VSLS emission inventories. Models were tested in their ability to reproduce the observed seasonal and spatial distribution of VSLS at the surface, using measurements from NOAA's long-term global monitoring network, and in the tropical troposphere, using recent aircraft measurements – including high-altitude observations from the NASA Global Hawk platform. The models generally capture the observed seasonal cycle of surface CHBr3 and CH2Br2 well, with a strong model–measurement correlation (r  ≥  0.7) at most sites. In a given model, the absolute model–measurement agreement at the surface is highly sensitive to the choice of emissions. Large inter-model differences are apparent when using the same emission inventory, highlighting the challenges faced in evaluating such inventories at the global scale. Across the ensemble, most consistency is found within the tropics where most of the models (8 out of 11) achieve best agreement to surface CHBr3 observations using the lowest of the three CHBr3 emission inventories tested (similarly, 8 out of 11 models for CH2Br2). In general, the models reproduce observations of CHBr3 and CH2Br2 obtained in the tropical tropopause layer (TTL) at various locations throughout the Pacific well. Zonal variability in VSLS loading in the TTL is generally consistent among models, with CHBr3 (and to a lesser extent CH2Br2) most elevated over the tropical western Pacific during boreal winter. The models also indicate the Asian monsoon during boreal summer to be an important pathway for VSLS reaching the stratosphere, though the strength of this signal varies considerably among models. We derive an ensemble climatological mean estimate of the stratospheric bromine SGI from CHBr3 and CH2Br2 of 2.0 (1.2–2.5) ppt,  ∼  57 % larger than the best estimate from the most recent World Meteorological Organization (WMO) Ozone Assessment Report. We find no evidence for a long-term, transport-driven trend in the stratospheric SGI of bromine over the simulation period. The transport-driven interannual variability in the annual mean bromine SGI is of the order of ±5 %, with SGI exhibiting a strong positive correlation with the El Niño–Southern Oscillation (ENSO) in the eastern Pacific. Overall, our results do not show systematic differences between models specific to the choice of reanalysis meteorology, rather clear differences are seen related to differences in the implementation of transport processes in the models.

Summertime HONO concentrations were synchronously measured at two (an agricultural and a non-agricultural) sites in the North China Plain (NCP). Daytime HONO (1.4±0.6 ppbv) and HONO/NO2 ((12±8)%) over the agricultural field after fertilization were found to be remarkably higher than those before fertilization, implying strong HONO emission from the fertilized fields.Synchronous enhancements of HONO and O3 after fertilization at both sites suggested that the emitted HONO accelerated the local and the regional O3 pollution. HONO budget analysis further revealed that its emission was significantly enhanced after fertilization. Soil HONO emission flux and its uncertainty were estimated and discussed. The estimated emission flux exhibited a distinct diurnal variation with a noontime maximum. Net OH production rate from HONO photolysis greatly exceeded that from O3 photolysis over the agricultural field, and their maximum ratio of 4.7 was obtained after fertilization. We provide field evidence that fertilized fields in the NCP act as a strong HONO source, which accelerates daytime photochemistry, leading to an increase of regional photo-oxidants such as O3. Considering the severe O3 pollution in the summer NCP and that the large area of the agricultural field is regularly treated with high fertilization amount in this region, HONO emission should be taken into account in the regional air quality deterioration.

Marine-produced short-lived trace gases such as dibromomethane (CH2Br2), bromoform (CHBr3), methyliodide (CH3I) and dimethyl sulfide (DMS) significantly impact tropospheric and stratospheric chemistry. Describing their marine emissions in atmospheric chemistry models as accurately as possible is necessary to quantify their impact on ozone depletion and Earth's radiative budget. So far, marine emissions of trace gases have mainly been prescribed from emission climatologies, thus lacking the interaction between the actual state of the atmosphere and the ocean. Here we present simulations with the chemistry climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry) with online calculation of emissions based on surface water concentrations, in contrast to directly prescribed emissions. Considering the actual state of the model atmosphere results in a concentration gradient consistent with model real-time conditions at the ocean surface and in the atmosphere, which determine the direction and magnitude of the computed flux. This method has a number of conceptual and practical benefits, as the modelled emission can respond consistently to changes in sea surface temperature, surface wind speed, sea ice cover and especially atmospheric mixing ratio. This online calculation could enhance, dampen or even invert the fluxes (i.e. deposition instead of emissions) of very short-lived substances (VSLS). We show that differences between prescribing emissions and prescribing concentrations (−28 % for CH2Br2 to +11 % for CHBr3) result mainly from consideration of the actual, time-varying state of the atmosphere. The absolute magnitude of the differences depends mainly on the surface ocean saturation of each particular gas. Comparison to observations from aircraft, ships and ground stations reveals that computing the air–sea flux interactively leads in most of the cases to more accurate atmospheric mixing ratios in the model compared to the computation from prescribed emissions. Calculating emissions online also enables effective testing of different air–sea transfer velocity (k) parameterizations, which was performed here for eight different parameterizations. The testing of these different k values is of special interest for DMS, as recently published parameterizations derived by direct flux measurements using eddy covariance measurements suggest decreasing k values at high wind speeds or a linear relationship with wind speed. Implementing these parameterizations reduces discrepancies in modelled DMS atmospheric mixing ratios and observations by a factor of 1.5 compared to parameterizations with a quadratic or cubic relationship to wind spee

Abstract. Bromoform is one of the most abundant halogenated Very Short-Lived Substances (VSLS) that possibly contributes, when degradated, to the inorganic halogen loading in the stratosphere. In this paper we present a detailed modelling study of the transport and the photochemical degradation of bromoform and its product gases (PGs) in a tropical convective cloud. The aim was to explore the transport and chemistry of bromoform under idealised conditions at the cloud scale. We used a 3-D cloud-resolving model coupled with a chemistry model including gaseous and aqueous chemistry. In particular, our model features explicit partitioning of the PGs between the gas phase and the aqueous phase based on newly calculated Henry's law coefficients using theoretical methods. We ran idealised simulations for up to 10 days that were initialised using a tropical radiosounding of atmospheric conditions and using outputs from a global chemistry-transport model for chemical species. Two simulations were run with stable atmospheric conditions with a bromoform initial mixing ratio of 40 pptv (part per trillion by volume) and 1.6 pptv up to 1 km altitude. The first simulation corresponds to high bromoform mixing ratios that are representative of real values found near strong localised sources (e.g. tropical coastal margins) and the second to the global tropical mean mixing ratio from observations. Both of these simulations show that the sum of bromoform and its PGs significantly decreases with time because of dry deposition, and that PGs are mainly in the form of HBr after 2 days of simulation. Two further simulations are conducted; these are similar to the first two simulations but include perturbations of temperature and moisture leading to the development of a convective cloud reaching the tropical tropopause layer (TTL). Results of these simulations show an efficient vertical transport of the bromoform from the boundary layer to the upper troposphere and the TTL. The bromoform mixing ratio in the TTL is up to 45 % of the initial boundary layer mixing ratio. The most abundant organic PGs, which are not very soluble, are also uplifted efficiently in both simulations featuring the convective perturbation. The inorganic PGs are more abundant than the organic PGs, and their mixing ratios in the upper troposphere and in the TTL depend on the partitioning between inorganic soluble and insoluble species in the convective cloud. Important soluble species such as HBr and HOBr are efficiently scavenged by rain. This removal of Br y by rain is reduced by the release of Br 2 (relatively insoluble) to the gas phase due to aqueous chemistry processes in the cloud droplets. The formation of Br 2 in the aqueous phase and its subsequent release to the gas phase makes a non negligible contribution to the high altitude bromine budget in the case of the large bromoform (40 pptv) initial mixing ratios. In this specific, yet realistic case, this Br 2 production process is important for the PG budget in the upper troposphere and in the TTL above con...

Abstract. We present a global carbonyl sulfide (OCS) data set covering the period June 2002 to April 2012, derived from FTIR (Fourier transform infrared) limb emission spectra measured with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the ENVISAT satellite. The vertical resolution is 4-5 km in the height region 6-15 km and 15 at 40 km altitude. The total estimated error amounts to 40-50 pptv between 10 and 20 km and to 120 pptv at 40 km altitude. MIPAS OCS data show no systematic bias with respect to balloon observations, with deviations mostly below ±50 pptv. However, they are systematically higher than the OCS volume mixing ratios of the ACE-FTS instrument on SCISAT, with maximum deviations of up to 100 pptv in the altitude region 13-16 km. The data set of MIPAS OCS exhibits only moderate interannual variations and low interhemispheric differences. Average concentrations at 10 km altitude range from 480 pptv at high lat-

Aerosols from the Sarychev volcano eruption (Kuril Islands, northeast of Japan) were observed in the Arctic lower stratosphere a few days after the strongest SO₂ injection which occurred on 15 and 16 June 2009. From the observations provided by the Infrared Atmospheric Sounding Interferometer (IASI) an estimated 0.9 Tg of sulphur dioxide was injected into the upper troposphere and lower stratosphere (UTLS). The resultant stratospheric sulphate aerosols were detected from satellites by the Optical Spectrograph and Infrared Imaging System (OSIRIS) limb sounder and by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and from the surface by the Network for the Detection of Atmospheric Composition Changes (NDACC) lidar deployed at OHP (Observatoire de Haute-Provence, France). By the first week of July the aerosol plume had spread out over the entire Arctic region. The Sarychev-induced stratospheric aerosol over the Kiruna region (north of Sweden) was measured by the Stratospheric and Tropospheric Aerosol Counter (STAC) during eight balloon flights planned in August and September 2009. During this balloon campaign the Micro Radiomètre Ballon (MicroRADIBAL) and the Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NOx (SALOMON) remote-sensing instruments also observed these aerosols. Aerosol concentrations returned to near-background levels by spring 2010. The effective radius, the surface area density (SAD), the aerosol extinction, and the total sulphur mass from STAC in situ measurements are enhanced with mean values in the range 0.15–0.21 μm, 5.5–14.7 μm² cm⁻³, 5.5–29.5 × 10⁻⁴ km⁻¹, and 4.9–12.6 × 10⁻¹⁰ kg[S] kg⁻¹[air], respectively, between 14 km and 18 km. The observed and modelled e-folding time of sulphate aerosols from the Sarychev eruption is around 70–80 days, a value much shorter than the 12–14 months calculated for aerosols from the 1991 eruption of Mt Pinatubo. The OSIRIS stratospheric aerosol optical depth (AOD) at 750 nm is enhanced by a factor of 6, with a value of 0.02 in late July compared to 0.0035 before the eruption. The HadGEM2 and MIMOSA model outputs indicate that aerosol layers in polar region up to 14–15 km are largely modulated by stratosphere–troposphere exchange processes. The spatial extent of the Sarychev plume is well represented in the HadGEM2 model with lower altitudes of the plume being controlled by upper tropospheric troughs which displace the plume downward and upper altitudes around 18–20 km, in agreement with lidar observations. Good consistency is found between the HadGEM2 sulphur mass density and the value inferred from the STAC observations, with a maximum located about 1 km above the tropopause ranging from 1 to 2 × 10⁻⁹ kg[S] kg⁻¹[air], which is one order of magnitude higher than the background level