Anoop S. Mahajan

Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean

Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; the prediction of future trends in tropospheric ozone will require a full understanding of both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models, but thus far has not been verified by observations. Here we report eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low-level aircraft observations shows that the mean daily observed ozone loss is ∼50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future.

Studies of the Formation and Growth of Aerosol from Molecular Iodine Precursor

Abstract The formation and growth of iodine oxide particles (IOPs), originating from molecular iodine precursor, has been studied at room temperature as a function of water vapour, and sulphuric and oxalic acid vapours. A linear variation in total IOP mass was observed over a wide range of iodine atom production rates under both dry and humid formation conditions. Particle formation was also observed in the absence of ozone, and was found to be temperature sensitive, with elevated temperatures resulting in reduced particle number and mass. Electronic structure calculations are used to show that particle formation is initiated by polymerization of I2O4 with I2O3, or with itself. Formation of IOPs in humid conditions results in lower numbers and smaller particles than formed in the absence of water vapour, because H2O forms relatively stable complexes with molecules such as I2O3 and I2O4, inhibiting their polymerization. Addition of H2O to particles formed under dry conditions shows the collapse of fractal-like, aggregate particle structures. The uptake of sulphuric acid vapour onto humidified particles was studied over a wide range of relative humidity (RH) at room temperature, with the calculated accommodation coefficient (α) for this process increasing with RH to a value of 0.75±0.05 at RH = 90%. In contrast, growth of particles exposed to oxalic acid vapour was not observed on the experimental timescales employed, indicating an upper limit for α of 10−3.

Glyoxal observations in the global marine boundary layer

Glyoxal is an important intermediate species formed by the oxidation of common biogenic and anthropogenic volatile organic compounds such as isoprene, toluene, and acetylene. Although glyoxal has been shown to play an important role in urban and forested environments, its role in the open ocean environment is still not well understood, with only a few observations showing evidence for its presence in the open ocean marine boundary layer (MBL). In this study, we report observations of glyoxal from 10 field campaigns in different parts of the worlds oceans. These observations together represent the largest database of glyoxal in the MBL. The measurements are made with similar instruments that have been used in the past, although the open ocean values reported here, average of about 25 parts per trillion by volume (pptv) with an upper limit of 40 pptv, are much lower than previously reported observations that were consistently higher than 40 pptv and had an upper limit of 140 pptv, highlighting the uncertainties in the differential optical absorption spectroscopy method for the retrieval of glyoxal. Despite retrieval uncertainties, the results reported in this work support previous suggestions that the currently known sources of glyoxal are insufficient to explain the average MBL concentrations. This suggests that there is an additional missing source, more than a magnitude larger than currently known sources, which is necessary to account for the observed atmospheric levels of glyoxal. Therefore, it could play a more important role in the MBL than previously considered.

Quantifying the impacts of an updated global dimethyl sulfide climatology on cloud microphysics and aerosol radiative forcing

One of the critical parameters in assessing the global impacts of dimethyl sulfide (DMS) on cloud properties and the radiation budget is the estimation of phytoplankton-induced ocean emissions, which are derived from prescribed, climatological surface seawater DMS concentrations. The most widely used global ocean DMS climatology was published 15 years ago and has recently been updated using a much larger database of observations. The updated climatology displays significant differences in terms of the global distribution and regional monthly averages of sea surface DMS. In this study, we use the ECHAM5-HAMMOZ aerosol-chemistry-climate general circulation model to quantify the influence of the updated DMS climatology in computed atmospheric properties, namely, the spatial and temporal distributions of atmospheric DMS concentration, sulfuric acid concentration, sulfate aerosols, number of activated aerosols, cloud droplet number concentration, and the aerosol radiative forcing at the top of the atmosphere. Significant differences are observed for all the modeled variables. Comparison with observations of atmospheric DMS and total sulfate also shows that in places with large DMS emissions, the updated climatology shows a better match with the observations. This highlights the importance of using the updated climatology for projecting future impacts of oceanic DMS emissions, especially considering that the relative importance of the natural sulfur fluxes is likely to increase due to legislation to “clean up” anthropogenic emissions. The largest estimated differences are in the Southern Ocean, Indian Ocean, and parts of the Pacific Ocean, where the climatologies differ in seasonal concentrations over large geographical areas. The model results also indicate that the former DMS climatology underestimated the effect of DMS on the globally averaged annual aerosol radiative forcing at the top of the atmosphere by about 20%.

Particles and iodine compounds in coastal Antarctica

Aerosol particle number concentrations have been measured at Halley and Neumayer on the Antarctic coast, since 2004 and 1984, respectively. Sulphur compounds known to be implicated in particle formation and growth were independently measured: sulphate ions and methane sulphonic acid in filtered aerosol samples and gas phase dimethyl sulphide for limited periods. Iodine oxide, IO, was determined by a satellite sensor from 2003 to 2009 and by different ground-based sensors at Halley in 2004 and 2007. Previous model results and midlatitude observations show that iodine compounds consistent with the large values of IO observed may be responsible for an increase in number concentrations of small particles. Coastal Antarctica is useful for investigating correlations between particles, sulphur, and iodine compounds, because of their large annual cycles and the source of iodine compounds in sea ice. After smoothing all the measured data by several days, the shapes of the annual cycles in particle concentration at Halley and Neumayer are approximated by linear combinations of the shapes of sulphur compounds and IO but not by sulphur compounds alone. However, there is no short-term correlation between IO and particle concentration. The apparent correlation by eye after smoothing but not in the short term suggests that iodine compounds and particles are sourced some distance offshore. This suggests that new particles formed from iodine compounds are viable, i.e., they can last long enough to grow to the larger particles that contribute to cloud condensation nuclei, rather than being simply collected by existing particles. If so, there is significant potential for climate feedback near the sea ice zone via the aerosol indirect effect.

Large inter annual variation in air quality during the annual festival 'Diwali' in an Indian megacity.

A network of air quality and weather monitoring stations was established under the System of Air Quality Forecasting and Research (SAFAR) project in Delhi. We report observations of ozone (O3), nitrogen oxides (NOx), carbon monoxide (CO) and particulate matter (PM2.5 and PM10) before, during and after the Diwali in two consecutive years, i.e., November 2010 and October 2011. The Diwali days are characterised by large firework displays throughout India. The observations show that the background concentrations of particulate matter are between 5 and 10 times the permissible limits in Europe and the United States. During the Diwali-2010, the highest observed PM10 and PM2.5 mass concentration is as high as 2070µg/m3 and 1620μg/m(3), respectively (24hr mean), which was about 20 and 27 times to National Ambient Air Quality Standards (NAAQS). For Diwali-2011, the increase in PM10 and PM2.5 mass concentrations was much less with their peaks of 600 and of 390μg/m(3) respectively, as compared to the background concentrations. Contrary to previous reports, firework display was not found to strongly influence the NOx, and O3 mixing ratios, with the increase within the observed variability in the background. CO mixing ratios showed an increase. We show that the large difference in 2010 and 2011 pollutant concentrations is controlled by weather parameters.

A high-resolution time-depth view of dimethylsulphide cycling in the surface sea

Emission of the trace gas dimethylsulphide (DMS) from the ocean influences the chemical and optical properties of the atmosphere, and the olfactory landscape for foraging marine birds, turtles and mammals. DMS concentration has been seen to vary across seasons and latitudes with plankton taxonomy and activity, and following the seascape of ocean’s physics. However, whether and how does it vary at the time scales of meteorology and day-night cycles is largely unknown. Here we used high-resolution measurements over time and depth within coherent water patches in the open sea to show that DMS concentration responded rapidly but resiliently to mesoscale meteorological perturbation. Further, it varied over diel cycles in conjunction with rhythmic photobiological indicators in phytoplankton. Combining data and modelling, we show that sunlight switches and tunes the balance between net biological production and abiotic losses. This is an outstanding example of how biological diel rhythms affect biogeochemical processes.

Evidence of atmospheric nanoparticle formation from emissions of marine microorganisms

Earth, as a whole, can be considered as a living organism emitting gases and particles into its atmosphere, in order to regulate its own temperature. In particular, oceans may respond to climate change by emitting particles that ultimately will influence cloud coverage. At the global scale, a large fraction of the aerosol number concentration is formed by nucleation of gas-phase species, but this process has never been directly observed above oceans. Here we present, using semicontrolled seawater-air enclosures, evidence that nucleation may occur from marine biological emissions in the atmosphere of the open ocean. We identify iodine-containing species as major precursors for new particle clusters’ formation, while questioning the role of the commonly accepted dimethyl sulfide oxidation products, in forming new particle clusters in the region investigated and within a time scale on the order of an hour. We further show that amines would sustain the new particle formation process by growing the new clusters to larger sizes. Our results suggest that iodine-containing species and amines are correlated to different biological tracers. These observations, if generalized, would call for a substantial change of modeling approaches of the sea-to-air interactions.

Small-scale variability patterns of DMS and phytoplankton in surface waters of the tropical and subtropical Atlantic, Indian, and Pacific Oceans

High-resolution surface measurements of dimethylsulfide (DMS), chlorophyll a fluorescence, and the efficiency of photosystem II were conducted together with temperature and salinity along five eastward sections in the tropical and subtropical Atlantic, Indian, and Pacific Oceans. Analysis of variability length scales revealed that much of the variability in DMS concentrations occurs at scales between 15 and 50 km, that is, at the lower edge of mesoscale dynamics, decreasing with latitude and productivity. DMS variability was found to be more commonly related to that of phytoplankton-related variables than to that of physical variables. Unlike phytoplankton physiological data, DMS did not show any universal diel pattern when using the normalized solar zenith angle as a proxy for solar time across latitudes and seasons. The study should help better design sampling and computing schemes aimed at mapping surface DMS and phytoplankton distributions, taking into account latitude and productivity.

On the variability of ozone in the equatorial eastern Pacific boundary layer

Observations of surface ozone (O3) mixing ratios carried out during two ground-based field campaigns in the Galapagos Islands are reported. The first campaign, Primera Investigacion sobre la Quimica, Evolucion y Reparto de Ozono, was carried out from September 2000 to July 2002. The second study, Climate and HAlogen Reactivity tropicaL EXperiment, was conducted from September 2010 to March 2012. These measurements complement the Southern Hemisphere ADditional OZonesonde observations made with weekly to monthly frequency at Galapagos. In this work, the daily, intraseasonal, seasonal and interannual variability of O3 in the marine boundary layer are described and compared to those observed in other tropical locations. The O3 diurnal cycle shows two regimes: (i) photochemical destruction followed by nighttime recovery in the cold season (July to November) and (ii) daytime advection and photochemical loss followed by nighttime depositional loss associated to windless conditions in the warm season (February to April). Wavelet spectral analysis of the intraseasonal variability of O3 reveals components with periods characteristic of tropical instability waves. The O3 seasonal variation in Galapagos is typical of the Southern Hemisphere, with a maximum in August and a minimum in February–March. Comparison with other measurements in remote tropical ocean locations shows that the change of the surface O3 seasonal cycle across the equator is explained by the position of the Intertropical Convergence Zone and the O3 levels upwind.