Pristine Air Masses Research Articles

Aerosol-related effects on the occurrence of heterogeneous ice formation over Lauder, New Zealand ∕ Aotearoa

Abstract. The presented study investigates the efficiency of heterogeneous ice formation in natural clouds over Lauder, New Zealand / Aotearoa. Aerosol conditions in the middle troposphere above Lauder are subject to huge contrasts. Clean, pristine air masses from Antarctica and the Southern Ocean arrive under southerly flow conditions, while high aerosol loads can occur when air masses are advected from nearby Australia. This study assesses how these contrasts in aerosol load affect the ice formation efficiency in stratiform midlevel clouds in the heterogeneous freezing range (−40 to 0 ∘C). For this purpose, an 11-year dataset was analyzed from a dual-wavelength polarization lidar system operated by National Institute of Water and Atmospheric Research (NIWA), Taihoro Nukurangi, at Lauder in collaboration with the National Institute for Environmental Studies in Japan and the Meteorological Research Institute of the Japan Meteorological Agency. These data were used to investigate the efficiency of heterogeneous ice formation in clouds over the site as a function of cloud-top temperature as in previous studies at other locations. The Lauder cloud dataset was put into context with lidar studies from contrasting regions such as Germany and southern Chile. The ice formation efficiency found at Lauder is lower than in polluted midlatitudes (i.e., Germany) but higher than, for example, in southern Chile. Both Lauder and southern Chile are subject to generally low free-tropospheric aerosol loads, which suggests that the low ice formation efficiency at these two sites is related to low ice-nucleating-particle (INP) concentrations. However, Lauder sees episodes of continental aerosol, more than southern Chile does, which seems to lead to the moderately increased ice formation efficiency. Trajectory-based tools and aerosol model reanalyses are used to relate this cloud dataset to the aerosol load and the air mass sources. Both analyses point clearly to higher ice formation efficiency for clouds which are more strongly influenced by continental aerosol and to lower ice formation efficiency for clouds which are more influenced by Antarctic/marine aerosol and air masses.

Role of South Asian outflow on the oxidative potential of marine aerosols over the Indian Ocean

Oxidative potential (OP) of fine marine aerosols (PM2.5) over the northern Indian Ocean (N_IO) and equatorial Indian Ocean (E_IO) were studied using shipborne measurements conducted as part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB-2018). During the study, an enhanced concentration of PM2.5 was found over N_IO (27.22 ± 14.29 μg.m−3) compared with E_IO (15.91 ± 2.58 μg.m−3), as N_IO experiences continental outflow from anthropogenically dominated South Asian region. However, E_IO received pristine air masses from the middle of the Arabian Sea, implying a reduced concentration. The OP of PM2.5 was evaluated using a dithiothreitol (DTT) assay. The mass (DTTm or intrinsic OP) and volume (DTTv or extrinsic OP) normalized DTT exhibited a significant spatial variation over the Indian Ocean (IO). Intrinsic OP showed ∼2 times higher values over N_IO than E_IO, indicating aging of aerosols during long-range transport impacts OP of marine aerosol. Similarly, increased concentrations of anthropogenic species such as non-sea sulfate (nssSO42−), nitrate (NO3−), ammonium (NH4+), non-sea potassium ion (nssK+), water-soluble transition metals (Fe, Ti, Zn, Cu, Mn, Cr), elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC), were also observed over N_IO compared with E_IO. Pearson correlation and multiple linear regression (MLR) analysis revealed that combustion sources, chemical processing and co-transportation of anthropogenic species during long-range transport are the main drivers of intrinsic OP in the outflow region.

Airborne glyoxal measurements in the marine and continental atmosphere: comparison with TROPOMI observations and EMAC simulations

Abstract. We report on airborne limb and nadir measurements of vertical profiles and total vertical column densities (VCDs) of glyoxal (C2H2O2) in the troposphere, which were performed aboard the German research aircraft HALO (High Altitude and LOng Range) in different regions and seasons around the globe between 2014 and 2019. The airborne nadir and integrated limb profiles agree excellently among each other. Our airborne observations are further compared to collocated glyoxal measurements of the TROPOspheric Monitoring Instrument (TROPOMI), with good agreement between both data sets for glyoxal observations in (1) pristine terrestrial, (2) pristine marine, (3) mixed polluted, and (4) biomass-burning-affected air masses with high glyoxal concentrations. Exceptions to the overall good agreement are observations of (1) faint and aged biomass burning plumes over the oceans and (2) of low-lying biomass burning or anthropogenic plumes in the terrestrial or marine boundary layer, both of which contain elevated glyoxal that is mostly not captured by TROPOMI. These differences in airborne and satellite-detected glyoxal are most likely caused by the overall small contribution of plumes of a limited extent to the total glyoxal absorption in the atmosphere and the difficulty in remotely detecting weak absorbers located close to low reflective surfaces (e.g. the ocean in the visible wavelength range) or within dense aerosol layers. Observations of glyoxal in aged biomass burning plumes (e.g. observed over the tropical Atlantic off the coast of West Africa in summer 2018, off the coast of Brazil by the end of the dry season 2019, and the East China Sea in spring 2018) could be traced back to related wildfires, such as a plume crossing over the Drake Passage that originated from the Australian bushfires in late 2019. Our observations of glyoxal in such aged biomass burning plumes confirm recent findings of enhanced glyoxal and presumably secondary organic aerosol (SOA) formation in aged wildfire plumes from yet-to-be-identified, longer-lived organic precursor molecules (e.g. aromatics, acetylene, or aliphatic compounds) co-emitted in the fires. Furthermore, elevated glyoxal (median 44 ppt – parts per trillion), as compared to other marine regions (median 10–19 ppt), is observed in the boundary layer over the tropical oceans, which is well in agreement with previous reports. The airborne data sets are further compared to glyoxal simulations performed with the global atmosphere chemistry model EMAC (ECHAM/MESSy Atmospheric Chemistry). When using an EMAC set up that resembles recent EMAC studies focusing on complex chemistry, reasonable agreement is found for pristine air masses (e.g. the unperturbed free and upper troposphere), but a notable glyoxal overestimation of the model exists for regions with high emissions of glyoxal and glyoxal-producing volatile organic compounds (VOCs) from the biosphere (e.g. the Amazon). In all other investigated regions, the model underpredicts glyoxal to varying degrees, in particular when probing mixed emissions from anthropogenic activities (e.g. over continental Europe, the Mediterranean, and East China Sea) and potentially from the sea (e.g. the tropical oceans). Also, the model tends to largely underpredict glyoxal in city plumes and aged biomass burning plumes. The potential causes for these differences are likely to be multifaceted, but they all point to missing glyoxal sources from the degradation of the mixture of potentially longer-chained organic compounds emitted from anthropogenic activities, biomass burning, and from the organic microlayer of the sea surface.

Enhanced Deposition of Atmospheric Soluble Iron by Intrusions of Marine Air Masses to East Antarctica

AbstractBio‐essential iron can relieve nutrient limitation and stimulate marine productivity in the Southern Ocean. The fractional iron solubility of aerosol iron is an important variable determining iron availability for biological uptake. However, estimates of dissolved iron (dFe; iron < 0.2 μm) and the factors driving the variability of fractional iron solubility in pristine air masses are largely unquantified. To constrain inputs of fractional iron solubility to remote East Antarctic waters, dFe, total dissolvable iron (TDFe), trace elements and refractory black carbon were analyzed in a 9‐year‐old snow pit (2005–2014) from a new ice core site at Aurora Basin North (ABN) in Wilkes Land, East Antarctica. Extremely low annual dFe deposition fluxes were estimated (0.2 × 10−6 g m−2 y−1), while annual TDFe deposition fluxes (70 × 10−6 g m−2 y−1) were comparable to other Antarctic sites. TDFe is dominantly sourced from mineral dust. Unlike coastal Antarctic sites where the variability of fractional iron solubility in modern snow is explained by a mixture of dust and biomass burning sources, dFe deposition and fractional iron solubility at ABN (ranging between 0.1% and 6%) is enhanced in episodic high precipitation events from synoptic warm air masses. Enhanced fractional iron solubility reaching the high elevation site at ABN is suggested through the mechanism of cloud processing of background mineral dust that modifies the dust chemistry and increases iron dissolution during long‐range transport. This study highlights a complex interplay of sources and processes that drive fractional iron solubility in pristine air masses.

Optical properties of aerosol particles in the atmospheric boundary layer in regions with and without sea ice

The optical properties of aerosol particles must be clearly understood to reduce uncertainty in the prediction of the global climate change. Simultaneous measurements of scattering and absorption coefficients of aerosol particles were conducted in the atmospheric boundary layer over the western North Pacific and the Southern Ocean. For the scattering coefficient, total scattering and backscattering coefficients were measured at three different wavelengths. Over the western North Pacific, significant differences in the Ångström exponent and absorption coefficient were found between polluted and pristine air masses. Over the parts of the Southern Ocean free of sea ice, the Ångström exponent and the ratio of the back-to total-scattering coefficients were uniform. However, over the sea-ice regions, the scattering coefficients were an order of magnitude lower, and the Ångström exponent an order of magnitude higher, than over the parts of the ocean free of sea ice. These differences appear to be due to the fact that the emission of aerosol precursors and sea salt particles is suppressed by sea ice. This study represents the difference in aerosol optical properties as a function of source region and sea condition. These results will contribute to improving the accuracy of estimates of the direct radiative forcing effects due to aerosol particles.

Assessing local CO2 contamination revealed by two near-by high altitude records at Jungfraujoch, Switzerland

Remote research stations are guarantor of high-quality atmospheric measurements as they are essentially exposed to pristine air masses. However, in a context of increasing touristic pressure for certain sites, attention should be paid to the local anthropogenic emission related to the infrastructure itself. Among emissions, carbon dioxide (CO2) is the most important anthropogenic greenhouse gas and a major contributor to the current global warming. Here, we compared two years of CO2 dry air mole fraction records from Jungfraujoch (Swiss Alps) measured at the Sphinx Laboratory (3580 m a.s.l.; JFJ) and the East Ridge facility (3705 m a.s.l.; JER; horizontal distance of ∼1 km), respectively. Both stations show an overall increase of the annual mean CO2 mole fraction in line with current global trends. On a daily basis, values during the night (00h00–06h00) show robust coherence with variability ranging within the measurement uncertainties matching the WMO compatibility goal of 0.1 ppm, which we considered to be background air CO2 mole fraction for Central and Western Europe. However, JFJ record shows superimposed short-term variability with diurnal CO2 spikes centered around noon. Whereas the variability occurring during time intervals ranging from days to weeks seem to be driven by inputs of air masses from the planetary boundary layer, we suppose that the super-imposed diurnal CO2 spikes occurring essentially in summer are explained by local emission sources related to the infrastructure (visitors, tourism, etc). Nevertheless, we cannot point to a single triggering cause for those spikes as it probably results from a combination of factors. In order to minimize these local emissions, smooth collaboration between all the involved stakeholders is required.

O3 and CO in the South Asian outflow over the Bay of Bengal: Impact of monsoonal dynamics and chemistry

Abstract Monsoonal dynamics strongly impacts the composition over the South Asian region, however, in situ measurements remain limited mostly to the continental region. Whereas, remote oceanic regions of the Bay of Bengal (BoB) remain largely unexplored where shipping emissions can infuse into the South Asian outflow. In this direction, we performed ship-based measurements of ozone (O3) and carbon monoxide (CO) over the BoB during July‒August 2018 and corroborated the analyses with model results and satellite retrievals. Surface level mixing ratios of O3 and CO varied in the range of 14–45 and 50–164 ppbv, respectively during the expedition. Elevated O3 and CO levels were observed in coastal region downwind of a tropical megacity (Chennai), referred to as the Chennai-plume. Analysis of the Copernicus Atmosphere Monitoring Service (CAMS) model simulations suggests volatile organic compound (VOC)-limited O3 production in morning over most of the plume and east coast followed by gradual shift to NOx-limited regime over the BoB in afternoon. While the southern BoB (7–10° N) received generally pristine airmasses from the equatorial Indian Ocean, impacts of shipping emissions near 6° N are observed. Consequently, higher values of CO (as much as 120 ppbv) and net O3 production by 1.1–1.3 ppbv h−1 with a diurnal amplitude of 4–8 ppbv are observed over the southern BoB. Satellite-based observations from the Measurements of Pollution in the Troposphere (MOPITT) and results from CAMS model show decreasing trend (4.1–5.2 ± 1 ppbv decade−1) of surface CO, as also inferred from in situ measurements. Zonal fluxes of CO across the east coast show the outflow over the BoB with higher values (2–3 × 1019 molecules m−2 s−1) over the northern BoB. Positive vertical fluxes (~2 × 1016 molecules m−2 s−1) show the transport of CO from marine boundary layer to the free-troposphere. Due to strong updrafts, CO levels in upper and lower troposphere are seen to be comparable during monsoon. These findings in conjunction with higher methane and nitrogen dioxide in the upper-troposphere manifest the influences of monsoonal convection and trapping of trace gases in the anticyclone.

Snapshot atmospheric parameters for the Istanbul metropolitan area during the course of two major wind regimes

At present scientific community is greatly concerned with extreme weather events due to their potential adverse effects that is associated with mesoscale weather events which can often be embedded in synoptic scale events that may be influenced by climatic variabilities. Extreme weather events can also change associated chemical compositions of the atmospheric parameters over metropolitan region. In this paper, we have used two major wind directions (N and SW) to characterize the upper and lower limits of atmospheric pollutants (PM10 and SO2) over Istanbul/Turkey rather than the wind or precipitation events that are associated with such events. In contrast to expectations, northerly air mass that originates from polar regions and traverses the industrialized or semi-industrialized countries and the Black Sea sets the lower limits for both PM10 and SO2 as 13 and 2.9 μg/m3, while southwesterly winds with its Saharan dust load set upper limits for PM10 as an average 300 μg/m3 reaching to 800 μg/m3 as peak hourly averages. SO2 levels during SW winds were also below 15 μg/m3. It has been further shown that the pristine northerly air mass can be used to delineate anthropogenic hot spots that can otherwise impossible to identify due to local perturbations.

Aeolian transport and deposition of carbonaceous aerosols over the Northwest Pacific Ocean in spring

The present study provides insight into the long-range transport of carbonaceous aerosols over the Northwest Pacific Ocean (NWP) based on marine aerosol samples collected onboard a research vessel in the spring of 2015. Organic carbon (OC) and elemental carbon (EC) concentrations showed maxima in proximity to land, with high levels also observed in advective air masses. The decoupled spatial variations in OC and EC levels in relatively pristine air masses from the Pacific, in which aerosol n-alkanes of marine origin were relatively high in abundance, revealed the influence of heterogeneity in air masses and suggested the release of n-alkanes from local marine environments. This was confirmed by the enhanced particle-bound fraction of marine organic matter-sourced n-alkanes, resulting in non-significant relationships between the gas–particle partition coefficient of n-alkanes and the corresponding sub-cooled liquid vapor pressure in some marine air masses. High OC/EC ratios over the NWP indicated secondary organic aerosol formation, possibly as a result of marine emissions followed by gas-to-particle conversion and/or aerosol aging during long-range transport from East Asia. Similar molecular profiles of n-alkanes were found in air and seawater particles in regions dominated by either marine or continental influence, reflecting the strengths of continental and/or marine input and more importantly, suggesting the occurrence of air–sea exchange through wind-induced marine emissions or atmospheric deposition. Relative to the East China Sea, dry particle deposition after long-range transport acted as a more important source for terrestrial lipids in the open NWP.

Influence of rainfall on atmospheric deposition fluxes of 7Be and 210Pb in Mangaluru (Mangalore) at the Southwest Coast of India

The depositional fluxes of 7Be and 210Pb were measured at Mangalore (12.82° N, 74.92° E) on the South West Coast region, which is one of the highest rainfall regions in India. This region receives pristine air masses from the Southern Indian Ocean (oceanic) and polluted air from Asia (continental). The activity concentration for individual rain events collected during the monsoon season varied between 0.15 and 4.3 Bq L−1 (with geometric mean, GM = 1.1 Bq L−1, geometric standard deviation, GSD = 2.6) and 0.01–0.71 Bq L−1 (GM = 0.12 Bq L−1, GSD = 2.8) for 7Be and 210Pb, respectively. The GM value for monthly (dry period) 7Be and 210Pb concentrations in air were 6.9 mBq m−3 (GSD = 1.3) and 1.2 mBq m−3 (GSD = 2.1), respectively. The dry deposition flux of 7Be varied in the range of 0.081–0.184 Bq m−2 d−1 (GM = 0.12 Bq m−2 d−1, GSD = 1.3) and that of 210Pb varied in the range of 0.026–0.293 Bq m−2 d−1 (GM = 0.11 Bq m−2 d−1, GSD = 1.8). The GM value of the wet deposition flux of 7Be was higher by two orders of magnitude when compared with that of the dry deposition flux, whereas, that in the case of 210Pb it was higher by an order of magnitude. The mean annual deposition fluxes of 7Be and 210Pb were 2026 Bq m−2 y−1 and 221 Bq m−2 y−1, respectively. The large variability observed in the wet deposition fluxes was attributed to the large variations in the intensity of the rainfall. The wet deposition velocities of the aerosols, estimated based on the concentrations of these radionuclides in air and their corresponding deposition fluxes, were found to be 6.8 cm s−1 for 7Be and 6.5 cm s−1 for 210Pb. Although the two radionuclides originate from distinct sources, a strong positive correlation was observed between the temporal variability of their deposition fluxes. In addition to this, a high degree of correlation was observed between the deposition fluxes of both 7Be and 210Pb and rainfall. This points at the dominating effect of scavenging by rainfall in controlling their fluxes.

Summertime Primary and Secondary Contributions to Southern Ocean Cloud Condensation Nuclei

Atmospheric aerosols in clean remote oceanic regions contribute significantly to the global albedo through the formation of haze and cloud layers; however, the relative importance of ‘primary’ wind-produced sea-spray over secondary (gas-to-particle conversion) sulphate in forming marine clouds remains unclear. Here we report on marine aerosols (PM1) over the Southern Ocean around Antarctica, in terms of their physical, chemical, and cloud droplet activation properties. Two predominant pristine air masses and aerosol populations were encountered: modified continental Antarctic (cAA) comprising predominantly sulphate with minimal sea-salt contribution and maritime Polar (mP) comprising sulphate plus sea-salt. We estimate that in cAA air, 75% of the CCN are activated into cloud droplets while in mP air, 37% are activated into droplets, for corresponding peak supersaturation ranges of 0.37–0.45% and 0.19–0.31%, respectively. When realistic marine boundary layer cloud supersaturations are considered (e.g. ~0.2–0.3%), sea-salt CCN contributed 2–13% of the activated nuclei in the cAA air and 8–51% for the marine air for surface-level wind speed < 16 m s−1. At higher wind speeds, primary marine aerosol can even contribute up to 100% of the activated CCN, for corresponding peak supersaturations as high as 0.32%.

Investigation of a regional ozone reduction event over eastern India by integrating in situ and satellite measurements with WRF-Chem simulations

This study examines various physical and chemical processes contributing towards an anomalous low surface ozone (O3) event at a tropical urban site Bhubaneswar (20.29° N, 85.83° E) located in eastern India. Surface O3 generally peaks during winter (~ 40 ppbv) at Bhubaneswar. However, during 5–11 December 2010, surface O3 decreased rapidly from 48°ppbv on December 4 to 14°ppbv on December 6. Average surface O3 concentration before and after the event was 50.4 ± 5.8 ppbv and 47.8 ± 2.4 ppbv, respectively, with two-day running mean ranging from 15.5 to 30.5 ppbv during the event. This event is analyzed using in situ surface O3, meteorological parameters, backward air trajectories, and simulations using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). Our analysis shows that this low O3 event was driven by advection of cleaner low O3 marine masses to Bhubaneswar associated with a rare low-pressure event over Bhubaneswar (occurred only once in winter during 2005–2014). Analysis of modeled spatial distribution of surface O3 showed that this event was not confined to Bhubaneswar only and significantly reduced surface O3 over a large area comprising most of the Indian land mass, the Bay of Bengal (BOB), and Burma as well. The study elucidates the importance of transport processes in controlling trace gas levels and highlights the advantage of Bhubaneswar as a strategic location to study atmospheric chemistry in contrasting air masses, i.e., polluted air masses from the Indo-Gangetic Plain (IGP) versus pristine air masses from the BOB.

Long-term (2001–2012) trends of carbonaceous aerosols from a remote island in the western North Pacific: an outflow region of Asian pollutants

Abstract. The present study reports on long-term trends of carbonaceous aerosols in total suspended particulate (TSP) samples collected at Chichijima in the western North Pacific during 2001–2012. Seasonal variations of elemental carbon (EC), organic carbon (OC), and water-soluble organic carbon (WSOC) concentrations showed maxima in winter to spring and minima in summer. These seasonal differences in the concentrations of carbonaceous aerosols were associated with the outflows of polluted air masses from East Asia, which are clearly distinguishable from pristine air masses from the central Pacific. The higher concentrations of carbonaceous aerosols during winter to spring are associated with long-range atmospheric transport of East Asian continental polluted air masses, whereas lower concentrations may be due to pristine air masses from the central Pacific in summer. The annual trends of OC ∕ EC (+0.46 % yr−1), WSOC (+0.18 % yr−1) and WSOC ∕ OC (+0.08 % yr−1) showed significant (p &lt; 0.05) increases during the period of 2001–2012, suggesting that photochemical formation of WSOC and its contributions to secondary organic aerosols (SOAs) have increased over the western North Pacific via long-range atmospheric transport. We found a significant increase (+0.33 % yr−1) in nss-K+ ∕ EC ratios, demonstrating that concentrations of biomass-burning-derived carbonaceous aerosols have increased, while those of primary fossil-fuel-derived aerosols have decreased over the western North Pacific. Further, secondary biogenic emissions are also important over the western North Pacific as inferred from a significant increase (+0.14 % yr−1) in the concentrations of methanesulfonate (MSA−, a tracer for biogenic sources). This point was further supported by a moderate correlation (r = 0.40) between WSOC and MSA−. We also found a significant increase in OC ∕ TC (total carbon) and WSOC ∕ TC ratios, further suggesting that photochemical formation of WSOC and its contributions to SOAs have increased over the western North Pacific during 2001–2012 via long-range atmospheric transport from East Asia.

Evidence for secondary ice production in Southern Ocean open cellular convection

Ice particles present at temperatures warmer than −9 °C were encountered in unexpectedly high number concentrations (up to 54 L−1) by an instrumented aircraft over the Southern Ocean (SO), off the southwest coast of Tasmania, Australia, on 7 September 2013. The sampled clouds were precipitating, characterized by mixed‐phase, open‐cellular shallow convection. These clouds were present within a large‐scale environment characterized by cold air advection, in a pristine air mass for over 72 h. Using a Cloud and Aerosol Spectrometer, aerosol particles (diameters &gt; 0.6 µm) size and number concentrations were measured and ice nucleating particle (INP) number concentrations were estimated with a recognized ice nuclei parametrization scheme. The estimated INP number concentrations were in the range of 10−5–10−1 L−1 at temperature above −9 °C, which is up to three orders of magnitude less than the ice number concentrations typically observed. The high ice number concentrations are largely consistent with the theoretical values when ice crystals are produced via a splinter production. The evidence suggests that secondary ice processes (likely the Hallett–Mossop mechanism) were playing a key role in generating the high ice number concentrations observed. Satellite observations from an A‐Train overpass in the neighbourhood during the flight period reveal a qualitatively consistent story, with patchy, mixed‐phase (but predominantly supercooled liquid water) clouds observed at cloud‐top temperatures around −6 °C. Using back trajectory calculations, these clouds are tracked over 23 and 46 h with A‐Train observations. The presence of these clouds is found to be common over the SO during this period of time. This suggests that the ice particles present in a relatively warm temperature range could potentially be commonplace, within the widespread (up to thousands of kilometres) shallow convective cloud fields over the SO. These clouds may have important implication for the energy budget and precipitation production over this climatically important region.

Hydroxy fatty acids in marine aerosols as microbial tracers: 4-year study on β- and ω-hydroxy fatty acids from remote Chichijima Island in the western North Pacific

To better understand the long-range atmospheric transport of microbial aerosols from Asia to the western North Pacific, marine aerosols were collected from Chichijima Island (27°04′N; 142°13′E) on a biweekly basis during 1990–1993. These samples were investigated for β– and ω–hydroxy fatty acids (FAs) as terrestrial biomarkers of Gram-negative bacteria (GNB) and higher plants, respectively. The average concentrations of β–hydroxy (C8–C31) and ω–hydroxy (C11–C28) FAs show pronounced seasonal variability with maxima in spring (300 ± 70 pg m−3) and winter (650 ± 330 pg m−3), respectively. Airmass back trajectories clearly indicate the continental outflow from Asia during winter to spring, whereas maritime airmasses dominate in summer to autumn over Chichijima. It is noteworthy that atmospheric abundances of β–hydroxy FAs and, thus, the estimated mass concentration of GNB have not been significantly varied between polluted (continental) and pristine (oceanic) airmasses during the study period. However, the relative source strength observed from cluster analysis of β–hydroxy FAs in the polluted continental airmassess vary significantly among seasons (winter: 98%, spring: 63%, summer; 11%, autumn: 26%). In addition, there were distinguishable differences between polluted continental and pristine maritime airmasses with regard to C-number predominance. The even C-number predominance of β– and ω–hydroxy FAs (∼80 and 98% of total mass concentration, respectively) in marine aerosols could be due to their significant contribution from GNB, terrestrial plants and soil microorganisms. These results have implications towards assessing the atmospheric transport of bacterial and plant lipids in the continental outflow over the open ocean.

High-resolution (375 m) cloud microstructure as seen from the NPP/VIIRS satellite imager

Abstract. VIIRS (Visible Infrared Imaging Radiometer Suite), onboard the Suomi NPP (National Polar-orbiting Partnership) satellite, has an improved resolution of 750 m with respect to the 1000 m of the Moderate Resolution Imaging Spectroradiometer for the channels that allow retrieving cloud microphysical parameters such as cloud drop effective radius (re). VIIRS also has an imager with five channels of double resolution of 375 m, which was not designed for retrieving cloud products. A methodology for a high-resolution retrieval of re and microphysical presentation of the cloud field based on the VIIRS imager was developed and evaluated with respect to MODIS in this study. The tripled microphysical resolution with respect to MODIS allows obtaining new insights for cloud–aerosol interactions, especially at the smallest cloud scales, because the VIIRS imager can resolve the small convective elements that are sub-pixel for MODIS cloud products. Examples are given for new insights into ship tracks in marine stratocumulus, pollution tracks from point and diffused sources in stratocumulus and cumulus clouds over land, deep tropical convection in pristine air mass over ocean and land, tropical clouds that develop in smoke from forest fires and in heavy pollution haze over densely populated regions in southeastern Asia, and for pyro-cumulonimbus clouds. It is found that the VIIRS imager provides more robust physical interpretation and refined information for cloud and aerosol microphysics as compared to MODIS, especially in the initial stage of cloud formation. VIIRS is found to identify significantly more fully cloudy pixels when small boundary layer convective elements are present. This, in turn, allows for a better quantification of cloud–aerosol interactions and impacts on precipitation-forming processes.

Aerosol, Ozone and Water Vapour Distributions Observed over Land and Sea Regions during INDOEX Field Phases

Extensive land- and ocean-based experiments involving active and passive remote sensing of aerosols and minor constituents have been conducted simultaneously over Pune and over the Indian Ocean and Arabian Sea during the INDian Ocean EXperiment (INDOEX) field phases with an aim to study the interface between the polluted continental air-mass and pristine marine air-mass, and the underlying circulat ion and Inter Tropical Convergence Zone (ITCZ) phenomena, which mixes the polluted air mass from Northern hemisphere with the pristine air mass from the Southern hemisphere. In this communicat ion, we report some important findings of the special observational programs which were carried out during the First Field Phase (FFP) i.e. fro m 17 February to 31 March 1998 and Intense Field Phase (IFP) i.e. fro m 1 January to 31 March 1999 of the INDOEX. These programs involve synchronous measurements of aerosol, ozone and precipitable water vapor using ground-based lidar, sunphotometer / spectroradiometer and ozone / water vapor monitor at the Indian Institute of Tropical Meteorology (IITM ), Pune, India, and aerosol characterization over the Indian Ocean and Arabian Sea utilizing a mult i-channel solar radio meter installed onboard Ocean Research Vessel (ORV) Sagar Kanya ship. Besides significant deviations between aerosol features over the land and oceanic regions, the lidar derived aerosol colu mn content (ACC) in the boundary layer, and radio meter derived height-integrated aerosol optical depth (AOD) show significant trend and characteristic changes in aerosol size distribution over land during both FFP and IFP. Moreover, the IFP measurements indicate higher aerosol ext inction (particularly in the initial period) as co mpared to those of FFP. On some experimental days, the land AODs appear to be of the magnitude similar to those of marine AODs observed close to the Indian subcontinent and adjoining South Asian regions. The variations in AOD over land also show an association with those observed over ocean, particularly near the coast. Besides an increasing trend in total colu mn ozone throughout the IFP, it exhib its a specific relat ionship with those observed in AOD and water vapor over land. Co mparison between the boundary layer and total column AODs reveals that the observed increasing trend in AOD is not confined to the boundary layer alone but extends to higher altitudes with significant contribution fro m the former to the latter.

The detection of post-monsoon tropospheric ozone variability over south Asia using IASI data

Abstract. The ozone (O3) variability over south Asia during the 2008 post-monsoon season has been assessed using measurements from the MetOP-A/IASI instrument and O3 profiles retrieved with the SOftware for a Fast Retrieval of IASI Data (SOFRID). The information content study and error analyses carried out in this paper show that IASI Level 1 data can be used to retrieve tropospheric O3 columns (TOC, surface-225 hPa) and UTLS columns (225–70 hPa) with errors smaller than 20%. Validation with global radiosonde O3 profiles obtained during a period of 6 months show the excellent agreement between IASI and radiosonde for the UTLS with correlation coefficient R &gt; 0.91 and good agreement in the troposphere with correlation coefficient R &gt; 0.74. For both the UTLS and the troposphere Relative Standard Deviations (RSD) are lower than 23%. Comparison with in-situ measurements from the MOZAIC program around Hyderabad demonstrates that IASI is able to capture the TOC inter and intra-seasonal variability in central India. Nevertheless, the agreement is mitigated by the fact that the smoothing of the true O3 profiles by the retrieval results in a reduction of the TOC variability detected by IASI relative to the variability observed by in situ instruments. The post-monsoon temporal variability of the vertical profile of O3 around Hyderabad has been investigated with MOZAIC observations. These observations from airborne instruments show that tropospheric O3 is steadily elevated during most of the studied period with the exception of two sharp drops following the crossing of tropical storms over India. Lagrangian simulations with the FLEXPART model indicate that elevated O3 concentrations in the middle troposphere near Hyderabad are associated with the transport of UTLS air-masses that have followed the Subtropical Westerly Jet (SWJ) and subsided over northern India together with boundary layer polluted air-masses transported from the Indo-gangetic plain by the north-easterly trades. Low O3 concentrations result from the uplift and westward transport of pristine air-masses from the marine boundary layer of the Bay of Bengal by tropical storms. In order to extend the analysis of tropospheric O3 variability to the whole of south Asia, we have used IASI-SOFRID O3 data. We show that IASI O3 data around Hyderabad were able to capture the fast variability revealed by MOZAIC. Furthermore, their spatio-temporal coverage demonstrates that the behaviour of tropospheric O3 observed near Hyderabad extended over most of central and south India and part of the Bay of Bengal. This result highlights the ability of the IASI sensor to capture fast changes in chemical composition related to dramatic tropical weather conditions.

Anomalous geographical gap in carbon monoxide mixing ratios over Hokkaido (Japan) in summer 2004

An anomalously low carbon monoxide (CO) episode was observed at Cape Ochi-ishi located in northern Japan in summer 2004. The low CO episode persisted for as long as twelve days from July 27 to August 7, 2004, and the CO levels at Cape Ochi-ishi during this episode were comparable to at Hateruma Island, a southernmost island of Japan. We explored regional distributions of CO and roles of synoptic-scale meteorology in this episode by using a regional chemistry-transport model. The model reproduced the observed levels and day-to-day variations of CO very well, and indicated that the low CO episode widely occurred around Japan. The model simulations revealed the presence of steep gradient in central Hokkaido, explaining the observed differences in CO mixing ratios between Cape Ochi-ishi and Rishiri Island. The eleven-year model results and backward trajectories suggested that the low CO event observed in 2004 was due to rapid transport of pristine air masses from the Pacific Ocean, driven by anomalously stronger easterly flows in the summer of 2004. This mechanism well explained the anomalous patterns observed for methane, carbon dioxide, and molecular hydrogen associated with low CO.

The Chemistry of Intercepted Clouds in Northern Arizona during the North American Monsoon Season

Cloudwater samples have been collected for the first time at a high-elevation site in the US interior Southwest. Cloud samples were collected at the summit of Mt. Elden near Flagstaff, Arizona. The samples were analyzed for pH, ionic composition, trace metals, organic carbon content, and volatile organic compounds. All of the samples showed high pH values (5.12–6.66), which appear to be the result of soil/crustal acid-neutralizing components. Ammonium and nitrate were the dominant ionic species. Organic carbon concentrations ranged from 3 to 18 mg/l. Volatile aromatic compounds (toluene, ethylbenzene, and xylenes) were detected, although they did not contribute significantly to the dissolved organic matter (<1% of dissolved organic carbon). Still, their aqueous-phase concentrations were substantially higher than equilibrium partitioning from the gas phase would suggest. Metal concentrations were high when compared to other cloud studies in remote areas. Overall, with the exception of pH, the cloud chemistry showed marked inter-event variability. The source of the variability was investigated using NOAA HYSPLIT dispersion calculations. Like the cloud composition, the air mass back trajectories differed widely from event to event, and consistently, air masses that passed over highly urbanized areas had higher trace metal, organic, and ion concentrations than more pristine air masses.