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Nighttime atmospheric chemistry of iodine

Little attention has so far been paid to the nighttime atmospheric chemistry of iodine species. Current atmospheric models predict a buildup of HOI and I2 during the night that leads to a spike of IO at sunrise, which is not observed by measurements. In this work, electronic structure calculations are used to survey possible reactions that HOI and I2 could undergo at night in the lower troposphere, and hence reduce their nighttime accumulation. The new reaction NO3+ HOI  →  IO + HNO3 is proposed, with a rate coefficient calculated from statistical rate theory over the temperature range 260–300 K and at a pressure of 1000 hPa to be k(T) =  2.7  ×  10−12 (300 K/T)2.66 cm3 molecule−1 s−1. This reaction is included in two atmospheric models, along with the known reaction between I2 and NO3, to explore a new nocturnal iodine radical activation mechanism. The results show that this iodine scheme leads to a considerable reduction of nighttime HOI and I2, which results in the enhancement of more than 25 % of nighttime ocean emissions of HOI + I2 and the removal of the anomalous spike of IO at sunrise. We suggest that active nighttime iodine can also have a considerable, so far unrecognized, impact on the reduction of the NO3 radical levels in the marine boundary layer (MBL) and hence upon the nocturnal oxidizing capacity of the marine atmosphere. The effect of this is exemplified by the indirect effect on dimethyl sulfide (DMS) oxidation.
Exploiting stagnant conditions to derive robust emission ratio estimates for CO2, CO and volatile organic compounds in Paris

We propose an approach to estimate urban emission ratios that takes advantage of the enhanced local urban signal in the atmosphere at low wind speed. We apply it to estimate monthly ratios between CO2, CO and some VOCs from several atmospheric concentration measurement datasets acquired in the centre of Paris between 2010 and 2014. We find that this approach is not very sensitive to the regional background level definition and that, in the case of Paris, it samples all days (weekdays and weekends) and all hours of the day evenly. A large seasonal variability of the ΔCO ∕ ΔCO2 ratio in Paris is shown, with a difference of around 60 % between the extreme values and a strong anti-correlation (r2 = 0.75) with atmospheric temperature. The comparison of the ratios obtained for two short measurement campaigns conducted in two different districts and two different periods (autumn and winter) shows differences ranging from −120 to +63 %. A comparison with a highly resolved regional emission inventory suggests some spatial variations of the ratio within the city.
Measurements of biogenic volatile organic compounds at a grazed savannah grassland agricultural landscape in South Africa

Biogenic volatile organic compounds (BVOCs) play an important role in the chemistry of the troposphere, especially in the formation of tropospheric ozone (O3) and secondary organic aerosols (SOA). Ecosystems produce and emit a large number of BVOCs. It is estimated on a global scale that approximately 90 % of annual BVOC emissions are from terrestrial sources. In this study, measurements of BVOCs were conducted at the Welgegund measurement station (South Africa), which is considered to be a regionally representative background site situated in savannah grasslands. Very few BVOC measurements exist for savannah grasslands and results presented in this study are the most extensive for this type of landscape. Samples were collected twice a week for 2 h during the daytime and 2 h during the night-time through two long-term sampling campaigns from February 2011 to February 2012 and from December 2013 to February 2015, respectively. Individual BVOCs were identified and quantified using a thermal desorption instrument, which was connected to a gas chromatograph and a mass selective detector. The annual median concentrations of isoprene, 2-methyl-3-butene-2-ol (MBO), monoterpene and sesquiterpene (SQT) during the first campaign were 14, 7, 120 and 8 pptv, respectively, and 14, 4, 83 and 4 pptv, respectively, during the second campaign. The sum of the concentrations of the monoterpenes were at least an order of magnitude higher than the concentrations of other BVOC species during both sampling campaigns, with α-pinene being the most abundant species. The highest BVOC concentrations were observed during the wet season and elevated soil moisture was associated with increased BVOC concentrations. However, comparisons with measurements conducted at other landscapes in southern Africa and the rest of the world that have more woody vegetation indicated that BVOC concentrations were, in general, significantly lower for savannah grasslands. Furthermore, BVOC concentrations were an order of magnitude lower compared to total aromatic concentrations measured at Welgegund. An analysis of concentrations by wind direction indicated that isoprene concentrations were higher from the western sector that is considered to be a relatively clean regional background region with no large anthropogenic point sources, while wind direction did not indicate any significant differences in the concentrations of the other BVOC species. Statistical analysis indicated that soil moisture had the most significant impact on atmospheric levels of MBO, monoterpene and SQT concentrations, whereas temperature had the greatest influence on isoprene levels. The combined O3 formation potentials of all the BVOCs measured calculated with maximum incremental reactivity (MIR) coefficients during the first and second campaign were 1162 and 1022 pptv, respectively. α-Pinene and limonene had the highest reaction rates with O3, whereas isoprene exhibited relatively small contributions to O3 depletion. Limonene, α-pinene and terpinolene had the largest contributions to the OH reactivity of BVOCs measured at Welgegund for all of the months during both sampling campaigns.
Long-term observations of cloud condensation nuclei in the Amazon rain forest – Part 1: Aerosol size distribution, hygroscopicity, and new model parametrizations for CCN prediction

Size-resolved long-term measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations and hygroscopicity were conducted at the remote Amazon Tall Tower Observatory (ATTO) in the central Amazon Basin over a 1-year period and full seasonal cycle (March 2014–February 2015). The measurements provide a climatology of CCN properties characteristic of a remote central Amazonian rain forest site. The CCN measurements were continuously cycled through 10 levels of supersaturation (S  =  0.11 to 1.10 %) and span the aerosol particle size range from 20 to 245 nm. The mean critical diameters of CCN activation range from 43 nm at S  =  1.10 % to 172 nm at S  =  0.11 %. The particle hygroscopicity exhibits a pronounced size dependence with lower values for the Aitken mode (κAit  =  0.14 ± 0.03), higher values for the accumulation mode (κAcc  =  0.22 ± 0.05), and an overall mean value of κmean  =  0.17 ± 0.06, consistent with high fractions of organic aerosol. The hygroscopicity parameter, κ, exhibits remarkably little temporal variability: no pronounced diurnal cycles, only weak seasonal trends, and few short-term variations during long-range transport events. In contrast, the CCN number concentrations exhibit a pronounced seasonal cycle, tracking the pollution-related seasonality in total aerosol concentration. We find that the variability in the CCN concentrations in the central Amazon is mostly driven by aerosol particle number concentration and size distribution, while variations in aerosol hygroscopicity and chemical composition matter only during a few episodes. For modeling purposes, we compare different approaches of predicting CCN number concentration and present a novel parametrization, which allows accurate CCN predictions based on a small set of input data.
Airborne observations of far-infrared upwelling radiance in the Arctic

The first airborne measurements of the Far-InfraRed Radiometer (FIRR) were performed in April 2015 during the panarctic NETCARE campaign. Vertical profiles of spectral upwelling radiance in the range 8–50 µm were measured in clear and cloudy conditions from the surface up to 6 km. The clear sky profiles highlight the strong dependence of radiative fluxes to the temperature inversion typical of the Arctic. Measurements acquired for total column water vapour from 1.5 to 10.5 mm also underline the sensitivity of the far-infrared greenhouse effect to specific humidity. The cloudy cases show that optically thin ice clouds increase the cooling rate of the atmosphere, making them important pieces of the Arctic energy balance. One such cloud exhibited a very complex spatial structure, characterized by large horizontal heterogeneities at the kilometre scale. This emphasizes the difficulty of obtaining representative cloud observations with airborne measurements but also points out how challenging it is to model polar clouds radiative effects. These radiance measurements were successfully compared to simulations, suggesting that state-of-the-art radiative transfer models are suited to study the cold and dry Arctic atmosphere. Although FIRR in situ performances compare well to its laboratory performances, complementary simulations show that upgrading the FIRR radiometric resolution would greatly increase its sensitivity to atmospheric and cloud properties. Improved instrument temperature stability in flight and expected technological progress should help meet this objective. The campaign overall highlights the potential for airborne far-infrared radiometry and constitutes a relevant reference for future similar studies dedicated to the Arctic and for the development of spaceborne instruments.
Antarctic ozone depletion between 1960 and 1980 in observations and chemistry–climate model simulations

The year 1980 has often been used as a benchmark for the return of Antarctic ozone to conditions assumed to be unaffected by emissions of ozone-depleting substances (ODSs), implying that anthropogenic ozone depletion in Antarctica started around 1980. Here, the extent of anthropogenically driven Antarctic ozone depletion prior to 1980 is examined using output from transient chemistry–climate model (CCM) simulations from 1960 to 2000 with prescribed changes of ozone-depleting substance concentrations in conjunction with observations. A regression model is used to attribute CCM modelled and observed changes in Antarctic total column ozone to halogen-driven chemistry prior to 1980. Wintertime Antarctic ozone is strongly affected by dynamical processes that vary in amplitude from year to year and from model to model. However, when the dynamical and chemical impacts on ozone are separated, all models consistently show a long-term, halogen-induced negative trend in Antarctic ozone from 1960 to 1980. The anthropogenically driven ozone loss from 1960 to 1980 ranges between 26.4 ± 3.4 and 49.8 ± 6.2 % of the total anthropogenic ozone depletion from 1960 to 2000. An even stronger ozone decline of 56.4 ± 6.8 % was estimated from ozone observations. This analysis of the observations and simulations from 17 CCMs clarifies that while the return of Antarctic ozone to 1980 values remains a valid milestone, achieving that milestone is not indicative of full recovery of the Antarctic ozone layer from the effects of ODSs.
Trends and variability of atmospheric PM2.5 and PM10–2.5 concentration in the Po Valley, Italy

The Po Valley is one of the largest European regions with a remarkably high concentration level of atmospheric pollutants, both for particulate and gaseous compounds. In the last decade stringent regulations on air quality standards and on anthropogenic emissions have been set by the European Commission, including also for PM2.5 and its main components since 2008. These regulations have led to an overall improvement in air quality across Europe, including the Po Valley and specifically PM10, as shown in a previous study by Bigi and Ghermandi (2014). In order to assess the trend and variability in PM2.5 in the Po Valley and its role in the decrease in PM10, we analysed daily gravimetric equivalent concentration of PM2.5 and of PM10–2.5 at 44 and 15 sites respectively across the Po Valley. The duration of the times series investigated in this work ranges from 7 to 10 years. For both PM sizes, the trend in deseasonalized monthly means, annual quantiles and in monthly frequency distribution was estimated: this showed a significant decreasing trend at several sites for both size fractions and mostly occurring in winter. All series were tested for a significant weekly periodicity (a proxy to estimate the impact of primary anthropogenic emissions), yielding positive results for summer PM2.5 and for summer and winter PM10–2.5. Hierarchical cluster analysis showed moderate variability in PM2.5 across the valley, with two to three main clusters, dividing the area in western, eastern and southern/Apennines foothill sectors. The trend in atmospheric concentration was compared with the time series of local emissions, vehicular fleet details and fuel sales, suggesting that the decrease in PM2.5 and in PM10 originates from a drop both in primary and in precursors of secondary inorganic aerosol emissions, largely ascribed to vehicular traffic. Potentially, the increase in biomass burning emissions in winter and the modest decrease in NH3 weaken an otherwise even larger drop in atmospheric concentrations.
Influence of the spatial distribution of gravity wave activity on the middle atmospheric dynamics

Analysing GPS radio occultation density profiles, we have recently pointed out a localised area of enhanced gravity wave (GW) activity and breaking in the lower stratosphere of the east Asian–northwestern Pacific (EA/NP) region. With a mechanistic model of the middle and upper atmosphere, experiments are performed to study the possible effect of such a localised GW breaking region on large-scale circulation and transport and, more generally, a possible influence of the spatial distribution of gravity wave activity on middle atmospheric dynamics. The results indicate the important role of the spatial distribution of GW activity for polar vortex stability, formation of planetary waves and for the strength and structure of zonal-mean residual circulation. Furthermore, a possible effect of a zonally asymmetric GW breaking in the longitudinal variability of the Brewer–Dobson circulation is analysed. Finally, consequences of our results for a variety of research topics (e.g. sudden stratospheric warming, atmospheric blocking, teleconnection patterns and a compensation mechanism between resolved and unresolved drag) are discussed.
Dynamic climate emulators for solar geoengineering

Climate emulators trained on existing simulations can be used to project project the climate effects that result from different possible future pathways of anthropogenic forcing, without further relying on general circulation model (GCM) simulations. We extend this idea to include different amounts of solar geoengineering in addition to different pathways of greenhouse gas concentrations, by training emulators from a multi-model ensemble of simulations from the Geoengineering Model Intercomparison Project (GeoMIP). The emulator is trained on the abrupt 4 × CO2 and a compensating solar reduction simulation (G1), and evaluated by comparing predictions against a simulated 1 % per year CO2 increase and a similarly smaller solar reduction (G2). We find reasonable agreement in most models for predicting changes in temperature and precipitation (including regional effects), and annual-mean Northern Hemisphere sea ice extent, with the difference between simulation and prediction typically being smaller than natural variability. This verifies that the linearity assumption used in constructing the emulator is sufficient for these variables over the range of forcing considered. Annual-minimum Northern Hemisphere sea ice extent is less well predicted, indicating a limit to the linearity assumption.
Integrated studies of a regional ozone pollution synthetically affected by subtropical high and typhoon system in the Yangtze River Delta region, China

Severe high ozone (O3) episodes usually have close relations to synoptic systems. A regional continuous O3 pollution episode was detected over the Yangtze River Delta (YRD) region in China during 7–12 August 2013, in which the O3 concentrations in more than half of the cities exceeded the national air quality standard. The maximum hourly concentration of O3 reached 167.1 ppb. By means of the observational analysis and the numerical simulation, the characteristics and the essential impact factors of the typical regional O3 pollution are comprehensively investigated. The observational analysis shows that the atmospheric subsidence dominated by the western Pacific subtropical high plays a crucial role in the formation of high-level O3. The favorable weather conditions, such as extremely high temperature, low relative humidity and weak wind speed, caused by the abnormally strong subtropical high are responsible for the trapping and the chemical production of O3 in the boundary layer. In addition, when the YRD cities are at the front of Typhoon Utor, the periphery circulation of typhoon system can enhance the downward airflows and cause worse air quality. However, when the typhoon system weakens the subtropical high, the prevailing southeasterly surface wind leads to the mitigation of the O3 pollution. The integrated process rate (IPR) analysis incorporated in the Community Multi-scale Air Quality (CMAQ) model is applied to further illustrate the combined influence of subtropical high and typhoon system in this O3 episode. The results show that the vertical diffusion (VDIF) and the gas-phase chemistry (CHEM) are two major contributors to O3 formation. During the episode, the contributions of VDIF and CHEM to O3 maintain the high values over the YRD region. On 10–12 August, the cities close to the sea are apparently affected by the typhoon system, with the contribution of VDIF increasing to 28.45 ppb h−1 in Shanghai and 19.76 ppb h−1 in Hangzhou. In contrast, the cities far away from the sea can hardly be affected by the periphery circulation of typhoon system. When the typhoon system significantly weakens the subtropical high, the contribution values of all individual processes decrease to a low level in all YRD cities. These results provide an insight for the O3 pollution synthetically impacted by the western Pacific subtropical high and the tropical cyclone system.