Summary (149 words of referenced text): 46The climate impact of aerosols is highly uncertain owing primarily to their poorly quantified 47 influence on cloud properties. During 2014-15, a fissure eruption in Holuhraun (Iceland) 48 emitted huge quantities of sulphur dioxide, resulting in significant reductions in liquid cloud 49 droplet size. Using satellite observations and detailed modelling, we estimate a global mean 50 radiative forcing from the resulting aerosol-induced cloud brightening for the time of the 51 eruption of around -0.2 W.m -2 . Changes in cloud amount or liquid water path are 52 undetectable, indicating that these aerosol-cloud indirect effects are modest. It supports the 53 idea that cloud systems are well buffered against aerosol changes as only impacts on cloud 54 effective radius appear relevant from a climate perspective, thus providing a strong constraint 55 on aerosol-cloud interactions. This result will reduce uncertainties in future climate 56 projections as we are able to reject the results from climate models with an excessive liquid 57 water path response. 58 59Main Text: (3103 words of referenced text, including concluding paragraph) 60 The 2014-15 eruption at Holuhraun (486 words of referenced text): 61Anthropogenic emissions that affect climate are not just confined to greenhouse gases. 62Sulphur dioxide and other pollutants form atmospheric aerosols that can scatter and absorb 63 sunlight and can influence the properties of clouds, modulating the Earth-atmosphere energy 64 balance. Aerosols act as cloud condensation nuclei (CCN); an increase in CCN translates into 65 a higher number of smaller, more reflective cloud droplets that scatter more sunlight back to 66 space 1 (the ÔfirstÕ indirect effect of aerosols). Smaller cloud droplets decrease the efficiency 67 of collision-coalescence processes that are pivotal in rain initiation, thus aerosol-influenced 68 clouds may retain more liquid water and extend coverage/lifetime 2,3 (the ÔsecondÕ or Ôcloud 69 lifetimeÕ indirect effect). Aerosols usually co-vary with key environmental variables making 70 it difficult to disentangle aerosol-cloud impacts from meteorological variability [4][5][6] . 71Additionally, clouds themselves are complex transient systems subject to dynamical 72 feedbacks (e.g. cloud top entrainment/evaporation, invigoration of convection) which 73 influence cloud response [7][8][9][10][11][12] . These aspects present great challenges in evaluating and 74 constraining aerosol-cloud interactions (ACI) in General Circulation Models (GCM) 13-17 , 75 with particular contentious debate surrounding the relative importance of these feedback 76 mechanisms. 77Nonetheless, anthropogenic aerosol emissions are thought to cool the Earth via indirect 78 effects 17 , but the uncertainty ranges from -1.2 to -0.0 W.m -2 (90% confidence interval) due to 79 i) a lack of characterization of the pre-industrial aerosol state 15,18,19 , and ii) model parametric 80 and structural errors in representing cloud responses to aerosol chan...

In this paper, we describe a new flexible and robust NH3 retrieval algorithm from measurements of the Infrared Atmospheric Sounding Interferometer (IASI). The method is based on the calculation of a spectral hyperspectral range index (HRI) and subsequent conversion to NH3 columns via a neural network. It is an extension of the method presented in Van Damme et al. (2014a) who used lookup tables (LUT) for the radiance‐concentration conversion. The new method inherits the advantages of the LUT‐based method while providing several significant improvements. These include the following: (1) Complete temperature and humidity vertical profiles can be accounted for. (2) Third‐party NH3 vertical profile information can be used. (3) Reported positive biases of LUT retrieval are reduced, and finally (4) a full measurement uncertainty characterization is provided. A running theme in this study, related to item (2), is the importance of the assumed vertical NH3 profile. We demonstrate the advantages of allowing variable profile shapes in the retrieval. As an example, we analyze how the retrievals change when all NH3 is assumed to be confined to the boundary layer. We analyze different averaging procedures in use for NH3 in the literature, introduced to cope with the variable measurement sensitivity and derive global averaged distributions for the year 2013. A comparison with a GEOS‐Chem modeled global distribution is also presented, showing a general good correspondence (within ±3 × 1015 molecules.cm−2) over most of the Northern Hemisphere. However, IASI finds mean columns about 1–1.5 × 1016 molecules.cm−2 (∼50–60%) lower than GEOS‐Chem for India and the North China plain.

We present a new data set of sulfur dioxide (SO 2 ) vertical columns from observations of the Ozone Monitoring Instrument (OMI)/AURA instrument between 2004 and 2013. The retrieval algorithm used is an advanced Differential Optical Absorption Spectroscopy (DOAS) scheme combined with radiative transfer calculation. It is developed in preparation for the operational processing of SO 2 data product for the upcoming TROPOspheric Monitoring Instrument/Sentinel 5 Precursor mission. We evaluate the SO 2 column results with those inferred from other satellite retrievals such as Infrared Atmospheric Sounding Interferometer and OMI (Linear Fit and Principal Component Analysis algorithms). A general good agreement between the different data sets is found for both volcanic and anthropogenic SO 2 emission scenarios. We show that our algorithm produces SO 2 columns with low noise and is able to provide accurate estimates of SO 2 . This conclusion is supported by important validation results over the heavily polluted site of Xianghe (China). Nearly 4 years of OMI and ground-based multiaxis DOAS SO 2 columns are compared, and an excellent match is found. We also highlight the improved performance of the algorithm in capturing weak SO 2 sources by detecting shipping SO 2 emissions in long-term averaged data, an unreported measurement from space.

In this paper we investigate a severe pollution episode that occurred in Beijing, Tianjin, and the Hebei province in January 2013. The episode was caused by the combination of anthropogenic emissions and a high-pressure system that trapped pollutants in the boundary layer. Using IASI (Infrared Atmospheric Sounding Interferometer) satellite measurements, high concentrations of key trace gases such as carbon monoxide (CO), sulfur dioxide (SO 2 ), and ammonia (NH 3 ) along with ammonium sulfate aerosol ((NH 4 ) 2 SO 4 ) are found. We show that IASI is able to detect boundary layer pollution in case of large negative thermal contrast combined with high levels of pollution. Our findings demonstrate that anthropogenic key pollutants, such as CO and SO 2 , can be monitored by IASI in the North China Plain during wintertime in support of air quality evaluation and management.

Emission of volcanic gas is thought to be the dominant process by which volatiles transit from the deep earth to the atmosphere. Volcanic gas emissions, remain poorly constrained, and volcanoes of Peru are entirely absent from the current global dataset. In Peru, Sabancaya and Ubinas volcanoes are by far the largest sources of volcanic gas. Here, we report the first measurements of the compositions and fluxes of volcanic gases emitted from these volcanoes. The measurements were acquired in November 2015. We determined an average SO 2 flux of 15.3 ± 2.3 kg s −1 (1325-ton day −1 ) at Sabancaya and of 11.4 ± 3.9 kg s −1 (988-ton day −1 ) at Ubinas using scanning ultraviolet spectroscopy and dual UV camera systems. In-situ Multi-GAS analyses yield molar proportions of H 2 O, CO 2 , SO 2 , H 2 S and H 2 gases of 73, 15, 10 1.15 and 0.15 mol% at Sabancaya and of 96, 2.2, 1.2 and 0.05 mol% for H 2 O, CO 2 , SO 2 and H 2 S at Ubinas. Together, these data imply cumulative fluxes for both volcanoes of 282, 30, 27, 1.2 and 0.01 kg s −1 of H 2 O, CO 2 , SO 2 , H 2 S and H 2 respectively. Sabancaya and Ubinas volcanoes together contribute about 60% of the total CO 2 emissions from the Central Volcanic zone, and dominate by far the total revised volatile budget of the entire Central Volcanic Zone of the Andes.