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Positive sampling artifact of carbonaceous aerosols and its influence on the thermal-optical split of OC/EC

Accurate measurement of carbonaceous aerosols is challenging, due to the sampling artifact and the problems of the split of OC/EC. Two approaches have been used to account for the positive artifact: backup quartz approach in which a backup quartz filter is placed either behind a front quartz filter (QBQ) or in a parallel port behind a Teflon filter (QBT), and organic denuder approach in which an organic denuder is placed upstream of the quartz filter. Both approaches were evaluated in Beijing, China, from January to February 2009. 10% of the OC captured by the bare quartz filter was from the positive artifact. The origin of backup OC was quantitatively evaluated by the denuder-based method. All of the QBQ OC was from gaseous organics passing through the front filter, but the QBQ had not reached equilibrium with gas phase due to the relative small sampling volume resulting in an undercorrection of the positive artifact by 3.7%. QBT OC was from both gaseous organics passing through the front filter (82%) and the evaporated organic carbon (18%), thus overcorrecting the positive artifact by 6.3%. Even the positive artifact-contributed QBT OC was found to overestimate the positive artifact, perhaps due to the difference in the adsorption properties of the loaded filter and the filter without particle loading. Re-partitioning of PC and EC was performed by the multiple linear regression approach. The attenuation coefficient of PC was twofold higher than that of EC, indicating PC was darker than EC, resulting in the underestimation of native EC by TOT-split-EC. It was also found that PC formed on the bare quartz filter (45.56 m2/g) was darker than that formed on the denuded filter (38.64 m2/g), indicating that the underestimation for the bare quartz filter was more significant.
Constraints on inorganic gaseous iodine in the tropical upper troposphere and stratosphere inferred from balloon-borne solar occultation observations

We report upper limits of IO and OIO in the tropical upper troposphere and stratosphere inferred from solar occultation spectra recorded by the LPMA/DOAS (Limb Profile Monitor of the Atmosphere/Differential Optical Absorption Spectroscopy) payload during two stratospheric balloon flights from a station in Northern Brazil (5.1° S, 42.9° W). In the tropical upper troposphere and lower stratosphere, upper limits for both, IO and OIO, are below 0.1 ppt. Photochemical modelling is used to estimate the compatible upper limits for the total gaseous inorganic iodine burden (Iy) amounting to 0.09 to 0.16 (+0.10/−0.04) ppt in the tropical lower stratosphere (21.0 km to 16.5 km) and 0.17 to 0.35 (+0.20/−0.08) ppt in the tropical upper troposphere (16.5 km to 13.5 km). In the middle stratosphere, upper limits increase with altitude as sampling sensitivity decreases. Our findings imply that the amount of gaseous iodine transported into the stratosphere through the tropical tropopause layer is small. Thus, iodine-mediated ozone loss plays a minor role for contemporary stratospheric photochemistry but might become significant in the future if source gas emissions or injection efficiency into the upper atmosphere are enhanced. However, photochemical modelling uncertainties are large and iodine might be transported into the stratosphere in particulate form.
Evolution of Asian aerosols during transpacific transport in INTEX-B

Measurements of aerosol composition were made with an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) on board the NSF/NCAR C-130 aircraft as part of the Intercontinental Chemical Transport Experiment Phase B (INTEX-B) field campaign over the Eastern Pacific Ocean. The HR-ToF-AMS measurements of non-refractory submicron aerosol mass are shown to compare well with other aerosol instrumentation in the INTEX-B field study. Two case studies are described for pollution layers transported across the Pacific from the Asian continent, intercepted 3–4 days and 7–10 days downwind of Asia, respectively. Aerosol chemistry is shown to be a robust tracer for air masses originating in Asia, specifically the presence of sulfate dominated aerosol is a distinguishing feature of Asian pollution layers that have been transported to the Eastern Pacific. We examine the time scales of processing for sulfate and organic aerosol in the atmosphere and show that our observations confirm a conceptual model for transpacific transport from Asia proposed by Brock et al. (2004). Our observations of both sulfate and organic aerosol in aged Asian pollution layers are consistent with fast formation near the Asian continent, followed by washout during lofting and subsequent transformation during transport across the Pacific. Our observations are the first atmospheric measurements to indicate that although secondary organic aerosol (SOA) formation from pollution happens on the timescale of one day, the oxidation of organic aerosol continues at longer timescales in the atmosphere. Comparisons with chemical transport models of data from the entire campaign reveal an under-prediction of organic aerosol mass in the MOZART model, but much smaller discrepancies with the GEOS-Chem model than found in previous studies over the Western Pacific. No evidence is found to support a previous hypothesis for significant secondary organic aerosol formation in the free troposphere.
Ship emitted NO2 in the Indian Ocean: comparison of model results with satellite data

The inventory of NOx emission from international shipping has been evaluated by comparing NO2 tropospheric columns derived from the satellite instruments SCIAMACHY (January 2003 to February 2008), GOME (January 1996 to June 2003), and GOME-2 (March 2007 to February 2008) to NO2 columns calculated with the atmospheric chemistry general circulation model ECHAM5/MESSy1 (January 2000 to October 2005). For both measurements and model consistently the tropospheric excess method was used to obtain mean NO2 columns over the shipping lane from India to Indonesia, and over two ship free regions, the Bay of Bengal and the central Indian Ocean. The long-term data set from SCIAMACHY yields the first monthly analysis of ship induced NO2 enhancements in the Indian Ocean. Comparison of data from the three instruments and in addition OMI reveals differences between the datasets which are discussed with respect to the diurnal cycle of NO2 and the increase in shipping traffic over the time period studied. In general, the model simulates the differences between the regions affected by ship pollution and ship free regions reasonably well. Minor discrepancies between model results and satellite data were identified during biomass burning seasons in March to May over India and the Indochinese Peninsula and August to October over Indonesia. We conclude that the NOx ship emission inventory used in this study is a good approximation of NOx ship emissions in the Indian Ocean for the years 2002 to 2007. It assumes that around 6 Tg(N) yr−1 are emitted by international shipping globally, resulting in 90 Gg(N) yr−1 in the region of interest when using Automated Mutual Assistance Vessel Rescue System (AMVER) as spatial proxy. A second model run using lower ship emissions estimates of 3–4 Tg(N) yr−1 globally results in poorer agreement with the satellite data.
Influence of line mixing on the retrievals of atmospheric CO2 from spectra in the 1.6 and 2.1 μm regions

We present the first study of the influence of line mixing among CO2 lines on the remote sensing retrieval of atmospheric carbon dioxide. This is done in the bands near 1.6 and 2.1 μm which will be used by the Greenhouse Gases Observatory Satellite (GOSAT) instrument and eventual successors of the Orbiting Carbon Observatory (OCO). A purely theoretical analysis is first made, based on simulations of atmospheric spectra. It shows that line mixing cannot be neglected since disregarding this process induces significant errors in the calculated absorption coefficients, leading to systematic structures in the spectral fit residuals and airmass-dependent biases in the retrieved CO2 amounts. These theoretical predictions are then confirmed by using atmospheric solar-absorption spectra measured by a ground-based Fourier transform spectrometer. It is first shown that including line mixing in the forward model used for the inversion leads to a very significant reduction of the residuals in the 2.1 μm region. Secondly, the inclusion of line mixing reduces the dependence of the retrieved CO2 on the airmass and greatly improves the consistency between values obtained independently from spectra in the 1.6 and 2.1 μm bands. These results open promising prospects for various ground-based and space-borne experiments monitoring the carbon dioxide atmospheric amounts.
Concurrent observations of air pollutants at two sites in the Pearl River Delta and the implication of regional transport

An intensive field measurement study was conducted simultaneously at a site within the inland Pearl River Delta (PRD) region (WQS) and a site in Hong Kong (TC) between 22 October and 1 December 2007. Ambient air pollutants measured included O3, NOx, CO, SO2, NMHCs, and carbonyls. The purpose is to improve our understanding of the interplay among local and regional air pollutants in the Hong Kong area, and the influence of regional transport on local air pollutants. The results indicate that the mean levels of air pollutants at the WQS site were much higher than those at the TC site, except NOx. Thirteen O3 episode days (daily O3 peak in excess of 122 ppbv) were monitored at WQS during the study period, while only 2 days were recorded at TC. Diurnal variations of O3 showed higher nighttime levels of O3 at TC than at WQS as well as more photochemical activity at WQS than TC. Remarkable differences in diurnal variations were also found between high and low O3 pollution days at each site, implying that Hong Kong is more acutely VOC-limited than the inland PRD region. Ratio analyses for trace gases and VOCs and back trajectory calculation revealed that the air masses arriving at WQS were more aged due to regional influence, whereas the air masses at TC were mainly affected by local emissions and/or regional transport. In addition, the influence of regional transport from Eastern China on the primary pollutants of Hong Kong was noticeable, whereas the air masses from the inland PRD region (e.g. Dongguan and Huizhou) had significant influence on the air pollutants at WQS, and the anthropogenic emissions in Eastern PRD (e.g. Shenzhen) played an important role on the photochemical ozone pollution in Western Hong Kong. These results confirm that regional and sub-regional transport of air pollution has a complex and significant impact on local air pollutants in this region.
Error correlation between CO2 and CO as constraint for CO2 flux inversions using satellite data

Inverse modeling of CO2 satellite observations to better quantify carbon surface fluxes requires a chemical transport model (CTM) to relate the fluxes to the observed column concentrations. CTM transport error is a major source of uncertainty. We show that its effect can be reduced by using CO satellite observations as additional constraint in a joint CO2-CO inversion. CO is measured from space with high precision, is strongly correlated with CO2, and is more sensitive than CO2 to CTM transport errors on synoptic and smaller scales. Exploiting this constraint requires statistics for the CTM transport error correlation between CO2 and CO, which is significantly different from the correlation between the concentrations themselves. We estimate the error correlation globally and for different seasons by a paired-model method (comparing GEOS-Chem CTM simulations of CO2 and CO columns using different assimilated meteorological data sets for the same meteorological year) and a paired-forecast method (comparing 48- vs. 24-h GEOS-5 CTM forecasts of CO2 and CO columns for the same forecast time). We find strong error correlations (r2>0.5) between CO2 and CO columns over much of the extra-tropical Northern Hemisphere throughout the year, and strong consistency between different methods to estimate the error correlation. Application of the averaging kernels used in the retrieval for thermal IR CO measurements weakens the correlation coefficients by 15% on average (mostly due to variability in the averaging kernels) but preserves the large-scale correlation structure. We present a simple inverse modeling application to demonstrate that CO2-CO error correlations can indeed significantly reduce uncertainty on surface carbon fluxes in a joint CO2-CO inversion vs. a CO2-only inversion.
Eddy covariance flux measurements of pollutant gases in urban Mexico City

Eddy covariance (EC) flux measurements of the atmosphere/surface exchange of gases over an urban area are a direct way to improve and evaluate emissions inventories, and, in turn, to better understand urban atmospheric chemistry and the role that cities play in regional and global chemical cycles. As part of the MCMA-2003 study, we demonstrated the feasibility of using eddy covariance techniques to measure fluxes of selected volatile organic compounds (VOCs) and CO2 from a residential district of Mexico City (Velasco et al., 2005a, b). During the MILAGRO/MCMA-2006 field campaign, a second flux measurement study was conducted in a different district of Mexico City to corroborate the 2003 flux measurements, to expand the number of species measured, and to obtain additional data for evaluation of the local emissions inventory. Fluxes of CO2 and olefins were measured by the conventional EC technique using an open path CO2 sensor and a Fast Isoprene Sensor calibrated with a propylene standard. In addition, fluxes of toluene, benzene, methanol and C2-benzenes were measured using a virtual disjunct EC method with a Proton Transfer Reaction Mass Spectrometer. The flux measurements were analyzed in terms of diurnal patterns and vehicular activity and were compared with the most recent gridded local emissions inventory. In both studies, the results showed that the urban surface of Mexico City is a net source of CO2 and VOCs with significant contributions from vehicular traffic. Evaporative emissions from commercial and other anthropogenic activities were significant sources of toluene and methanol. The results show that the emissions inventory is in reasonable agreement with measured olefin and CO2 fluxes, while C2-benzenes and toluene emissions from evaporative sources are overestimated in the inventory. It appears that methanol emissions from mobile sources occur, but are not reported in the mobile emissions inventory.
Lightning-produced NOx during the Northern Australian monsoon; results from the ACTIVE campaign

Measurements of nitrogen oxides onboard a high altitude aircraft were carried out for the first time during the Northern Australian monsoon in the framework of the Aerosol and Chemical Transport in Tropical Convection (ACTIVE) campaign, in the area around Darwin, Australia. During one flight on 22 January 2006, average NOx volume mixing ratios (vmr) of 984 and 723 parts per trillion (ppt) were recorded for both in and out of cloud conditions, respectively. The in-cloud measurements were made in the convective outflow region of a storm 56 km south-west of Darwin, whereas those out of cloud were made due south of Darwin and upwind from the storm sampled. This storm produced a total of only 8 lightning strokes, as detected by an in-situ lightning detection network, ruling out significant lightning-NOx production. 5-day backward trajectories suggest that the sampled airmasses had travelled over convectively-active land in Northern Australia during that period. The low stroke count of the sampled storm, along with the high out-of-cloud NOx concentration, suggest that, in the absence of other major NOx sources during the monsoon season, a combination of processes including regional transport patterns, convective vertical transport and entrainment may lead to accumulation of lightning-produced NOx, a situation that contrasts with the pre-monsoon period in Northern Australia, where the high NOx values occur mainly in or in the vicinity of storms. These high NOx concentrations may help start ozone photochemistry and OH radical production in an otherwise NOx-limited environment.
Performance of the line-by-line radiative transfer model (LBLRTM) for temperature and species retrievals: IASI case studies from JAIVEx

Presented here are comparisons between the Infrared Atmospheric Sounding instrument (IASI) and the "Line-By-Line Radiative Transfer Model" (LBLRTM). Spectral residuals from radiance closure studies during the IASI JAIVEx validation campaign provide insight into a number of spectroscopy issues relevant to remote sounding of temperature, water vapor and trace gases from IASI. In order to perform quality IASI trace gas retrievals, the temperature and water vapor fields must be retrieved as accurately as possible. In general, the residuals in the CO2 ν2 region are of the order of the IASI instrument noise. However, outstanding issues with the CO2 spectral regions remain. There is a large residual ~−1.7 K in the 667 cm−1 Q-branch, and residuals in the CO2 ν2 and N2O/CO2 ν3 spectral regions that sample the troposphere are inconsistent, with the N2O/CO2 ν3 region being too negative (warmer) by ~0.7 K. Residuals on this lower wavenumber side of the CO2 ν3 band will be improved by line parameter updates, while future efforts to reduce the residuals reaching ~−0.5 K on the higher wavenumber side of the CO2 ν3 band will focus on addressing limitations in the modeling of the CO2 line shape (line coupling and duration of collision) effects. Brightness temperature residuals from the radiance closure studies in the ν2 water vapor band have standard deviations of ~0.2–0.3 K with some large peak residuals reaching ±0.5–1.0 K. These are larger than the instrument noise indicating that systematic errors still remain. New H2O line intensities and positions have a significant impact on the retrieved water vapor, particularly in the upper troposphere where the water vapor retrievals are 10% drier when using line intensities compared with HITRAN 2004. In addition to O3, CH4, and CO, of the IASI instrument combined with an accurate forward model allows for the detection of minor species with weak atmospheric signatures in the nadir radiances, such as HNO3 and OCS.