Jennifer Muller

Effects of land use on surface–atmosphere exchanges of trace gases and energy in Borneo: comparing fluxes over oil palm plantations and a rainforest

This paper reports measurements of land–atmosphere fluxes of sensible and latent heat, momentum, CO2, volatile organic compounds (VOCs), NO, NO2, N2O and O3 over a 30 m high rainforest canopy and a 12 m high oil palm plantation in the same region of Sabah in Borneo between April and July 2008. The daytime maximum CO2 flux to the two canopies differs by approximately a factor of 2, 1200 mg C m−2 h−1 for the oil palm and 700 mg C m−2 h−1 for the rainforest, with the oil palm plantation showing a substantially greater quantum efficiency. Total VOC emissions are also larger over the oil palm than over the rainforest by a factor of 3. Emissions of isoprene from the oil palm canopy represented 80 per cent of the VOC emissions and exceeded those over the rainforest in similar light and temperature conditions by on average a factor of 5. Substantial emissions of estragole (1-allyl-4-methoxybenzene) from the oil palm plantation were detected and no trace of this VOC was detected in or above the rainforest. Deposition velocities for O3 to the rainforest were a factor of 2 larger than over oil palm. Emissions of nitrous oxide were larger from the soils of the oil palm plantation than from the soils of the rainforest by approximately 25 per cent. It is clear from the measurements that the large change in the species composition generated by replacing rainforest with oil palm leads to profound changes in the net exchange of most of the trace gases measured, and thus on the chemical composition of the boundary layer over these surfaces.

Sources of uncertainty in eddy covariance ozone flux measurements made by dry chemiluminescence fast response analysers

We present a systematic intercomparison study of eddy covariance ozone flux measurements made using two fast response dry chemiluminescence analysers. Ozone deposition was measured over a well characterised managed grassland near Edinburgh, Scotland, during August 2007. A data quality control procedure specific to these analysers is introduced. Absolute ozone fluxes were calculated based on the relative signals of the dry chemiluminescence analysers using three different methods and the results are compared for both analysers. It is shown that the error in the fitted analyser calibration parameters required for the flux calculations provides a substantial source of uncertainty in the fluxes. The choice of the calculation method itself can also constitute an uncertainty in the flux as the calculated fluxes by the three methods do not agree within error at all times. This finding highlights the need for a consistent and rigorous approach for comparable datasets, such as e.g. in flux networks. Ozone fluxes calculated by one of the methods were then used to compare the two analysers in more detail. This systematic analyser comparison reveals half-hourly flux values differing by up to a factor of two at times with the difference in mean hourly flux ranging from 0 to 23% with an error in the mean daily flux of± 12%. The comparison of analysers shows that the agreement in fluxes is excellent for some days but that there is an underlying uncertainty as a result of variable analyser performance and/or non-linear sensitivity. Correspondence to: J. B. A. Muller (jennifer.muller@postgrad.manchester.ac.uk)

The first UK measurements of nitryl chloride using a chemical ionization mass spectrometer in central London in the summer of 2012, and an investigation of the role of Cl atom oxidation

The first nitryl chloride (ClNO2) measurements in the UK were made during the summer 2012 ClearfLo campaign with a chemical ionization mass spectrometer, utilizing an I− ionization scheme. Concentrations of ClNO2 exceeded detectable limits (11 ppt) every night with a maximum concentration of 724 ppt. A diurnal profile of ClNO2 peaking between 4 and 5 A.M., decreasing directly after sunrise, was observed. Concentrations of ClNO2 above the detection limit are generally observed between 8 P.M. and 11 A.M. Different ratios of the production of ClNO2:N2O5 were observed throughout with both positive and negative correlations between the two species being reported. The photolysis of ClNO2 and a box model utilizing the Master Chemical Mechanism modified to include chlorine chemistry was used to calculate Cl atom concentrations. Simultaneous measurements of hydroxyl radicals (OH) using low pressure laser-induced fluorescence and ozone enabled the relative importance of the oxidation of three groups of measured VOCs (alkanes, alkenes, and alkynes) by OH radicals, Cl atoms, and O3 to be compared. For the day with the maximum calculated Cl atom concentration, Cl atoms in the early morning were the dominant oxidant for alkanes and, over the entire day, contributed 15%, 3%, and 26% toward the oxidation of alkanes, alkenes, and alkynes, respectively.

Nonlinearities in source receptor relationships for sulfur and nitrogen compounds.

Abstract The relationship between emissions and deposition of air pollutants, both spatially and in time forms an important focus for science and for policy makers. In practice, this relationship may become nonlinear if the underlying processes change with time, or in space. Nonlinearities may also appear due to errors in emission or deposition data, and careful scrutiny of both data sources and their relationship provides a means of picking up such deficiencies. Nonlinearities in source receptor relationships for sulfur and nitrogen compounds in Europe have been identified in measurement data for the UK. In the case of sulfur, the dry deposition process has been shown to be strongly influenced by ambient concentrations of NH3, leading to substantial increases in deposition rate as SO2 concentrations decline and the ratio SO2/NH3 decreases. The field evidence extends to measurements over three different surfaces in three countries across Europe. A mechanistic understanding of the cause of this nonlinearity has been provided. Apparent nonlinearities also exist in the sulfur deposition field through the influence of shipping emissions. The effect is clear at west coast locations, where during a period in which land-based sulfur emissions declined by 50%, no significant decline in concentrations of SO4 2− in precipitation were observed. The sites affected are primarily the coastal regions of southwestern UK, where shipping sources contribute a substantial fraction of the deposited sulfur, but the effect is not detectable elsewhere. Full quantification of the spatially disaggregated emission and their changes in time will eliminate this apparent nonlinearity in the source—receptor data. For oxidized nitrogen emission and deposition in the UK, there is strong evidence of nonlinearity in the source—receptor relationship. The concentrations and deposition of NO3− in precipitation have declined little following a reduction in emissions of 45% during the period 1987 to 2001. The data imply a significant decrease in the average transport distance for oxidized nitrogen and most probably an increase in the average oxidation rate. However, the net effect of changes in aerosol chemistry due to changes in sulfur emissions and less competition for the main oxidants as a consequence of reductions in sulfur emission have not been separated. A quantitative explanation of the cause of this nonlinearity is lacking and the effects are therefore identified as an important uncertainty for the development of further protocols to control acidification, eutrophication and photochemical oxidants in Europe.

The first airborne comparison of N2O5 measurements over the UK using a CIMS and BBCEAS during the RONOCO campaign

Dinitrogen pentoxide (N2O5) plays a central role in nighttime tropospheric chemistry as its formation and subsequent loss in sink processes limits the potential for tropospheric photochemistry to generate ozone the next day. Since accurate observational data for N2O5 are critical to examine our understanding of this chemistry, it is vital also to evaluate the capabilities of N2O5 measurement techniques through the co-deployment of the available instrumentation. This work compares measurements of N2O5 from two aircraft instruments on board the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft during the Role of Nighttime Chemistry in Controlling the Oxidising Capacity of the Atmosphere (RONOCO) measurement campaigns over the United Kingdom in 2010 and 2011. A chemical ionisation mass spectrometer (CIMS), deployed for the first time for ambient N2O5 detection during RONOCO, measured N2O5 directly using I− ionisation chemistry and an aircraft-based broadband cavity enhanced absorption spectrometer (BBCEAS), developed specifically for RONOCO, measured N2O5 by thermally dissociating N2O5 and quantifying the resultant NO3 spectroscopically within a high finesse optical cavity. N2O5 mixing ratios were simultaneously measured at 1 second time resolution (1 Hz) by the two instruments for 8 flights during RONOCO. The sensitivity for the CIMS instrument was 52 ion counts per pptv with a limit of detection of 7.4 pptv for 1 Hz measurements. BBCEAS, a proven technique for N2O5 measurement, had a limit of detection of 2 pptv. Comparison of the observed N2O5 mixing ratios show excellent agreement between the CIMS and BBCEAS methods for the whole dataset, as indicated by the square of the linear correlation coefficient, R2 = 0.89. Even stronger correlations (R2 values up to 0.98) were found for individual flights. Altitudinal profiles of N2O5 obtained by CIMS and BBCEAS also showed close agreement (R2 = 0.93). Similarly, N2O5 mixing ratios from both instruments were greatest within pollution plumes and were strongly positively correlated with the NO2 concentrations. The transition from day to nighttime chemistry was observed during a dusk-to-dawn flight during the summer 2011 RONOCO campaign: the CIMS and BBCEAS instruments simultaneously detected the increasing N2O5 concentrations after sunset. The performance of the CIMS and BBCEAS techniques demonstrated in the RONOCO dataset illustrate the benefits that accurate, high-frequency, aircraft-based measurements have for improving understanding the nighttime chemistry of N2O5.

Measurements of δ13C in CH4 and using particle dispersion modeling to characterize sources of arctic methane within an air mass

Abstract A stratified air mass enriched in methane (CH4) was sampled at ~600 m to ~2000 m altitude, between the north coast of Norway and Svalbard as part of the Methane in the Arctic: Measurements and Modelling campaign on board the UKs BAe‐146‐301 Atmospheric Research Aircraft. The approach used here, which combines interpretation of multiple tracers with transport modeling, enables better understanding of the emission sources that contribute to the background mixing ratios of CH4 in the Arctic. Importantly, it allows constraints to be placed on the location and isotopic bulk signature of the emission source(s). Measurements of δ13C in CH4 in whole air samples taken while traversing the air mass identified that the source(s) had a strongly depleted bulk δ13C CH4 isotopic signature of −70 (±2.1)‰. Combined Numerical Atmospheric‐dispersion Modeling Environment and inventory analysis indicates that the air mass was recently in the planetary boundary layer over northwest Russia and the Barents Sea, with the likely dominant source of methane being from wetlands in that region.

Evaluation of coated QCM for the detection of atmospheric ozone

A coated acoustic wave sensor has been developed to selectively detect atmospheric ozone. The selective detection has been assessed using a variety of coatings: beeswax, gallic acid, indigo carmine, polybutadiene, potassium iodide and sodium nitrite. Polybutadiene was the most sensitive with a limit of detection of 55 ppb. The sensitivity was improved by operating at higher harmonics and was shown to increase linearly with harmonic up to the 11th harmonic. This novel work shows that ozone detection can be improved by operating at the crystals harmonic frequencies and in conjunction with a suitable flow rate, a potentially highly sensitive and fast response sensor can be created based on acoustic wave technology.

Seasonality of Formic Acid (HCOOH) in London during the ClearfLo Campaign

Following measurements in the winter of 2012, formic acid (HCOOH) and nitric acid (HNO3) were measured using a chemical ionization mass spectrometer (CIMS) during the Summer Clean Air for London (ClearfLo) campaign in London, 2012. Consequently, the seasonal dependence of formic acid sources could be better understood. A mean formic acid concentration of 1.3 ppb and a maximum of 12.7 ppb was measured which is significantly greater than that measured during the winter campaign (0.63 ppb and 6.7 ppb, respectively). Daily calibrations of formic acid during the summer campaign gave sensitivities of 1.2 ion counts s-1 parts per trillion (ppt) by volume-1 and a limit of detection of 34 ppt. During the summer campaign, there was no correlation between formic acid and anthropogenic emissions such as NOx and CO or peaks associated with the rush hour as was identified in the winter. Rather, peaks in formic acid were observed that correlated with solar irradiance. Analysis using a photochemical trajectory model has been conducted to determine the source of this formic acid. The contribution of formic acid formation through ozonolysis of alkenes is important but the secondary production from biogenic VOCs could be the most dominant source of formic acid at this measurement site during the summer.