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Eddy covariance emission and deposition flux measurements using proton transfer reaction – time of flight – mass spectrometry (PTR-TOF-MS): comparison with PTR-MS measured vertical gradients and fluxes

During summer 2010, a proton transfer reaction – time of flight – mass spectrometer (PTR-TOF-MS) and a quadrupole proton transfer reaction mass spectrometer (PTR-MS) were deployed simultaneously for one month in an orange orchard in the Central Valley of California to collect continuous data suitable for eddy covariance (EC) flux calculations. The high time resolution (5 Hz) and high mass resolution (up to 5000 m/Δm) data from the PTR-TOF-MS provided the basis for calculating the concentration and flux for a wide range of volatile organic compounds (VOC). Throughout the campaign, 664 mass peaks were detected in mass-to-charge ratios between 10 and 1278. Here we present PTR-TOF-MS EC fluxes of the 27 ion species for which the vertical gradient was simultaneously measured by PTR-MS. These EC flux data were validated through spectral analysis (i.e., co-spectrum, normalized co-spectrum, and ogive). Based on inter-comparison of the two PTR instruments, no significant instrumental biases were found in either mixing ratios or fluxes, and the data showed agreement within 5% on average for methanol and acetone. For the measured biogenic volatile organic compounds (BVOC), the EC fluxes from PTR-TOF-MS were in agreement with the qualitatively inferred flux directions from vertical gradient measurements by PTR-MS. For the 27 selected ion species reported here, the PTR-TOF-MS measured total (24 h) mean net flux of 299 μg C m−2 h−1. The dominant BVOC emissions from this site were monoterpenes (m/z 81.070 + m/z 137.131 + m/z 95.086, 34%, 102 μg C m−2 h−1) and methanol (m/z 33.032, 18%, 72 μg C m−2 h−1). The next largest fluxes were detected at the following masses (attribution in parenthesis): m/z 59.048 (mostly acetone, 12.2%, 36.5 μg C m−2 h−1), m/z 61.027 (mostly acetic acid, 11.9%, 35.7 μg C m−2 h−1), m/z 93.069 (para-cymene + toluene, 4.1%, 12.2 μg C m−2 h−1), m/z 45.033 (acetaldehyde, 3.8%, 11.5 μg C m−2 h−1), m/z 71.048 (methylvinylketone + methacrolein, 2.4%, 7.1 μg C m−2 h−1), and m/z 69.071 (isoprene + 2-methyl-3-butene-2-ol, 1.8%, 5.3 μg C m−2 h−1). Low levels of emission and/or deposition (<1.6% for each, 5.8% in total flux) were observed for the additional reported masses. Overall, our results show that EC flux measurements using PTR-TOF-MS is a powerful new tool for characterizing the biosphere-atmosphere exchange including both emission and deposition for a large range of BVOC and their oxidation products.
OCS photolytic isotope effects from first principles: sulfur and carbon isotopes, temperature dependence and implications for the stratosphere

The isotopic fractionation in OCS photolysis is studied theoretically from first principles. UV absorption cross sections for OCS, OC33S, OC34S, OC36S and O13CS are calculated using the time-depedent quantum mechanical formalism and a recently developed ab-initio description of the photodissociation of OCS which takes into account the lowest four singlet and lowest four triplet electronic states. The calculated isotopic fractionations as a function of wavelength are in good agreement with recent measurements by Hattori et al. (2011) and indicate that photolysis leads to only a small enrichment of 34S in the remaining OCS. The photodissociation dynamics provide strong evidence that the photolysis quantum yield is unity at all wavelengths for atmospheric UV excitation, for all isotopologues. A simple stratospheric model is constructed taking into account the main sink reactions of OCS and it is found that overall stratospheric removal slightly favors light OCS in constrast to the findings of Leung et al. (2002). These results show, based on isotopic considerations, that OCS is an acceptable source of background stratosperic sulfate aerosol in agreement with a recent model study of of Brühl et al. (2012). The 13C isotopic fractionation due to photolysis of OCS in the upper stratosphere is significant and will leave a clear signal in the remaining OCS making it a candidate for tracing using the ACE-FTS and MIPAS data sets.
Investigation of the connections between atmospheric new particle formation and organics at an urban site of Beijing

The role of low-volatility organic vapors in atmospheric new particle formation has been studied based on a data set of 17 nucleation events observed during the CAREBeijing 2008 campaign. The particle formation rates show good correlations with sulfuric acid and organic vapors implying that both play an important role in the atmospheric new particle formation. High correlation coefficients are observed in all investigated nucleation mechanisms. The best fit (R = 0.73, slope = 1.1) between the observed and modelled particle formation rates is achieved with the homogenous nucleation theory of sulfuric acid (both homomolecularly and hetermolecularly) with separate coefficients in J=KSA1[H2SO4]2+KSA2[H2SO4][Org]. The contributions of the sulfuric acid and the organics involving terms have been 43% and 57%, respectively. In addition, the higher particle formation rates are observed on polluted nucleation days, indicating the organic vapors should be involved in the new particle formation process in the polluted urban environment of Beijing with high background aerosol loading.
The spatial scale of ozone depletion events derived from an autonomous surface ozone network in coastal Antarctica

To probe the spatial extent of tropospheric ozone depletion events during Antarctic spring, a network of 10 autonomous ozone monitors was established around the Dronning Maud Land sector of Antarctica for a full calendar year. Together with manned stations in the area, the network covered a ~1200 km stretch of coast, as well as a transect ~300 km inland and to ~2000 m above sea level (a.s.l.). Here we present results from the spring period (August to October 2008). While some ozone depletion events were evident at only a single site, implying localised ozone destruction, others were evident across the network. The fact that, on occasions, ozone depletion events were observed at all coastal sites simultaneously, suggests the depleted air mass had a scale of at least 1200 km. As the ozone-poor air was advected from the Weddell Sea sea ice zone, the data imply that large areas over the Weddell Sea sea ice zone are significantly depleted in ozone on occasions during Antarctic spring.
How well do different tracers constrain the firn diffusivity profile?

Firn air transport models are used to interpret measurements of the composition of air in firn and bubbles trapped in ice in order to reconstruct past atmospheric composition. The diffusivity profile in the firn is usually calibrated by comparing modelled and measured concentrations for tracers with known atmospheric history. However, in most cases this is an under-determined inverse problem, often with multiple solutions giving an adequate fit to the data (this is known as equifinality). Here we describe a method to estimate the firn diffusivity profile that allows multiple solutions to be identified, in order to quantify the uncertainty in diffusivity due to equifinality. We then look at how well different combinations of tracers constrain the firn diffusivity profile. Tracers with rapid atmospheric variations like CH3CCl3, HFCs and 14CO2 are most useful for constraining molecular diffusivity, while &delta:15N2 is useful for constraining parameters related to convective mixing near the surface. When errors in the observations are small and Gaussian, three carefully selected tracers are able to constrain the molecular diffusivity profile well with minimal equifinality. However, with realistic data errors or additional processes to constrain, there is benefit to including as many tracers as possible to reduce the uncertainties. We calculate CO2 age distributions and their spectral widths with uncertainties for five firn sites (NEEM, DE08-2, DSSW20K, South Pole 1995 and South Pole 2001) with quite different characteristics and tracers available for calibration. We recommend moving away from the use of a firn model with one calibrated parameter set to infer atmospheric histories, and instead suggest using multiple parameter sets, preferably with multiple representations of uncertain processes, to assist in quantification of the uncertainties.
The effect of regional changes in anthropogenic aerosols on rainfall of the East Asian Summer Monsoon

The response of East Asian Summer Monsoon (EASM) precipitation to long term changes in regional anthropogenic aerosols (sulphate and black carbon) is explored in an atmospheric general circulation model, the atmospheric component of the UK High-Resolution Global Environment Model v1.2 (HiGAM). Separately, sulphur dioxide (SO2) and black carbon (BC) emissions in 1950 and 2000 over East Asia are used to drive model simulations, while emissions are kept constant at year 2000 level outside this region. The response of the EASM is examined by comparing simulations driven by aerosol emissions representative of 1950 and 2000. The aerosol radiative effects are also determined using an off-line radiative transfer model. During June, July and August, the EASM was not significantly changed as either SO2 or BC emissions increased from 1950 to 2000 levels. However, in September, precipitation is significantly decreased by 26.4% for sulphate aerosol and 14.6% for black carbon when emissions are at the 2000 level. Over 80% of the decrease is attributed to changes in convective precipitation. The cooler land surface temperature over China in September (0.8 °C for sulphate and 0.5 °C for black carbon) due to increased aerosols reduces the surface thermal contrast that supports the EASM circulation. However, mechanisms causing the surface temperature decrease in September are different between sulphate and BC experiments. In the sulphate experiment, the sulphate direct and the 1st indirect radiative effects contribute to the surface cooling. In the BC experiment, the BC direct effect is the main driver of the surface cooling, however, a decrease in low cloud cover due to the increased heating by BC absorption partially counteracts the direct effect. This results in a weaker land surface temperature response to BC changes than to sulphate changes. The resulting precipitation response is also weaker, and the responses of the monsoon circulation are different for sulphate and black carbon experiments. This study demonstrates a mechanism that links regional aerosol emission changes to the precipitation changes of the EASM, and it could be applied to help understand the future changes in EASM precipitation in CMIP5 simulations.
Sources of anions in aerosols in northeast Greenland during late winter

The knowledge of climate effects of atmospheric aerosols is associated with large uncertainty, and a better understanding of their physical and chemical properties is needed, especially in the Arctic environment. The objective of the present study is to improve our understanding of the processes affecting the composition of aerosols in the high Arctic. Therefore size-segregated aerosols were sampled at a high Arctic site, Station Nord (Northeast Greenland), in March 2009 using a Micro Orifice Uniform Deposit Impactor. The aerosol samples were extracted in order to analyse three water-soluble anions: chloride, nitrate and sulphate. The results are discussed based on possible chemical and physical transformations as well as transport patterns. The total concentrations of the ions at Station Nord were 53–507 ng m−3, 2–298 ng m−3 and 535–1087 ng m−3 for chloride (Cl−), nitrate (NO3−) and sulphate (SO42−), respectively. The aerosols in late winter/early spring, after polar sunrise, are found to be a mixture of long-range transported and regional to local originating aerosols. Fine particles, smaller than 1 μm, containing SO42−, Cl− and NO3−, are hypothesized to originate from long-range transport, where SO42− is by far the dominating anion accounting for 50–85% of the analyzed mass. The analysis suggests that Cl− and NO3− in coarser particles (> 1.5 μm) originate from local/regional sources. Under conditions where the air mass is transported over sea ice at high wind speeds, very coarse particles (> 18 μm) are observed, and it is hypothesized that frost flowers on the sea ice are a source of the very coarse nitrate particles.
Tropospheric NO2 vertical column densities over Beijing: results of the first three years of ground-based MAX-DOAS measurements (2008–2011) and satellite validation

Ground-based measurements of scattered sunlight by the Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) have been carried out at an urban site (39.95° N, 116.32° E) in Beijing megacity since 6 August 2008. In this study, we retrieved the tropospheric NO2 vertical column densities (VCDs) over Beijing from these MAX-DOAS observations from August 2008 to September 2011. Over this period, the daytime (08:00–17:00 Beijing Time (BJT, which equals UTC + 8)) mean tropospheric NO2 VCDs varied from 0.5 to 13.3 with an average of 3.6 during summertime, and from 0.2 to 16.8 with an average of 5.8 during wintertime, all in units of 1016 molecules cm−2. The average diurnal variation patterns of tropospheric NO2 over Beijing appeared to be rather different from one season to another, indicating differences in the emission strength and atmospheric lifetime. In contrast to previous studies, we find a small weekly cycle of the tropospheric NO2 VCD over Beijing. The NO2 VCD in the late afternoon was the largest on Saturday and the lowest on Sunday, and in the morning it reached a clear maximum on Wednesday. We also find a post-Olympic Games effect, with 39–54% decrease in the tropospheric NO2 VCD over Beijing estimated for August of 2008, compared to the following years. The tropospheric NO2 VCDs derived by our ground MAX-DOAS measurements show a good correlation with SCIAMACHY and OMI satellite data. However, compared with the MAX-DOAS measurements, the satellite observations underestimate the tropospheric NO2 VCDs over Beijing systematically, by 43% for SCIAMACHY and 26–38% for OMI (DOMINO v2.0 and DOMINO v1.02). Based on radiative transfer simulations, we show that the aerosol shielding effect can explain this underestimation, while the gradient smoothing effect caused by the coarse spatial resolution of the satellite observations could play an additional role.
Ten-year trends of atmospheric mercury in the high Arctic compared to Canadian sub-Arctic and mid-latitude sites

Global emissions of mercury continue to change at the same time as the Arctic is experiencing ongoing climatic changes. Continuous monitoring of atmospheric mercury provides important information about long-term trends in the balance between transport, chemistry, and deposition of this pollutant in the Arctic atmosphere. Ten-year records of total gaseous mercury (TGM) from 2000 to 2009 were analyzed from two high Arctic sites at Alert (Nunavut, Canada) and Zeppelin Station (Svalbard, Norway); one sub-Arctic site at Kuujjuarapik (Nunavik, Québec, Canada); and three temperate Canadian sites at St. Anicet (Québec), Kejimkujik (Nova Scotia) and Egbert (Ontario). Five of the six sites examined showed a decreasing trend over this time period. Overall trend estimates at high latitude sites were: −0.9% yr−1 (95% confidence limits: −1.4, 0) at Alert and no trend (−0.5, +0.7) at Zeppelin Station. Faster decreases were observed at the remainder of the sites: −2.1% yr−1 (−3.1, −1.1) at Kuujjuarapik, −1.9% yr−1 (−2.1, −1.8) at St. Anicet, −1.6% yr−1 (−2.4, −1.0) at Kejimkujik and −2.2% yr−1 (−2.8, −1.7) at Egbert. Trends at the sub-Arctic and mid-latitude sites agree with reported decreases in background TGM concentration since 1996 at Mace Head, Ireland, and Cape Point, South Africa, but conflict with estimates showing an increase in global anthropogenic emissions over a similar period. Trends in TGM at the two high Arctic sites were not only less negative (or neutral) overall but much more variable by season. Possible reasons for differences in seasonal and overall trends at the Arctic sites compared to those at lower latitudes are discussed, as well as implications for the Arctic mercury cycle. The first calculations of multi-year trends in reactive gaseous mercury (RGM) and total particulate mercury (TPM) at Alert were also performed, indicating increases from 2002 to 2009 in both RGM and TPM in the spring when concentrations are highest.
The effects of timing and rate of marine cloud brightening aerosol injection on albedo changes during the diurnal cycle of marine stratocumulus clouds

The marine-cloud brightening geoengineering technique has been suggested as a possible means of counteracting the positive radiative forcing associated with anthropogenic atmospheric CO2 increases. The focus of this study is to quantify the albedo response to aerosols injected into marine stratocumulus cloud from a point source at different times of day. We use a cloud-resolving model to investigate both weakly precipitating and non-precipitating regimes. Injection into both regimes induces a first indirect aerosol effect. Additionally, the weakly precipitating regime shows evidence of liquid water path gain associated with a second indirect aerosol effect that contributes to a more negative radiative forcing, and cloud changes indicative of a regime change to more persistent cloud. This results in a cloud albedo increase up to six times larger than in the non-precipitating case. These indirect effects show considerable variation with injection at different times in the diurnal cycle. For the weakly precipitating case, aerosol injection results in domain average increases in cloud albedo of 0.28 and 0.17 in the early and mid morning (03:00:00 local time (LT) and 08:00:00 LT respectively) and 0.01 in the evening (18:00:00 LT). No cloud develops when injecting into the cloud-free early afternoon (13:00:00 LT). However, the all-sky albedo increases (which include both the indirect and direct aerosol effects) are highest for early morning injection (0.11). Mid-morning and daytime injections produce increases of 0.06, with the direct aerosol effect compensating for the lack of cloud albedo perturbation during the cloud-free early afternoon. Evening injection results in an increase of 0.04. For the weakly precipitating case considered, the optimal injection time for planetary albedo response is the early morning. Here, the cloud has more opportunity develop into a more persistent non-precipitating regime prior to the dissipative effects of solar heating. The effectiveness of the sea-spray injection method is highly sensitive to diurnal injection time and the direct aerosol effect of an intense aerosol point source. Studies which ignore these factors could overstate the effectiveness of the marine cloud brightening technique.