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Technical Note: In-situ quantification of aerosol sources and sinks over regional geographical scales

In order to obtain the source/sink functions for atmospheric particulates located on the planetary surface or elevated in the atmosphere; direct aerosol emission measurements are required. For this purpose, the performance of an airborne aerosol flux measurement system with an improved 3-kilometer (km) spatial resolution is evaluated in this study. Eddy covariance method was used in flux calculations. A footprint for airborne flux sampling with the increased resolution becomes comparable in area to the footprint for tower sampling (with the footprint length being 2 to 10 km). The improvement in spatial resolution allows the quantification of emission rates from individual sources located several kilometers apart such as highway segments, city blocks, and remote and industrial areas. The advantage is a moving platform that allows scanning of aerosol emissions or depositions over regional geographic scales. Airborne flux measurements with the improved spatial resolution were conducted in various environments ranging from clean to partly polluted marine to polluted continental environment with low (<500 m) mixed boundary layer heights. The upward and downward fluxes from the clean marine environment were smaller than 0.5×106 particles m−2 s−1 in absolute value. The effective emissions measured from a ship plume ranged from 2×108 to 3×108 m−2 s−1, and effective fluxes measured crossing cities plumes with populations of 10 000 to 12 000 inhabitants were in the range of 2×108 to 3×108 m−2 s−1. Correlations between heat and aerosol fluxes are evaluated.
Size-dependent activation of aerosols into cloud droplets at a subarctic background site during the second Pallas Cloud Experiment (2nd PaCE): method development and data evaluation

In situ measurements of aerosol water uptake and activation of aerosols into cloud droplets provide information on how aerosols influence the microphysical properties of clouds. Here we present a computational scheme that can be used in connection with such measurements to assess the influence of the particle hygroscopicity and mixing state (in terms of the water uptake) on the cloud nucleating ability of particles. Additionally, it provides an estimate for the peak supersaturation of water vapour reached during the formation of the observed cloud(s). The method was applied in interpreting results of a measurement campaign that focused on aerosol-cloud interactions taking place at a subarctic background site located in Northern Finland (second Pallas Cloud Experiment, 2nd PaCE). A set of case studies was conducted, and the observed activation behavior could be successfully explained by a maximum supersaturation that varied between 0.18 and 0.26% depending on the case. In these cases, the diameter corresponding to the activated fraction of 50% was in the range of 110–140 nm, and the particles were only moderately water soluble with hygroscopic growth factors varying between 1.1 and 1.4. The conducted analysis showed that the activated fractions and the total number of particles acting as CCN are expected to be highly sensitive to the particle hygroscopic growth properties. For example, the latter quantity varied over a factor between 1.8 and 3.1, depending on the case, when the mean hygroscopic growth factors were varied by 10%. Another important conclusion is that size-dependent activation profiles carries information on the mixing state of particles.
Observations of NOx, ΣPNs, ΣANs, and HNO3 at a Rural Site in the California Sierra Nevada Mountains: summertime diurnal cycles

Observations of NO, NO2, total peroxy nitrates (ΣPNs), total alkyl nitrates (ΣANs), HNO3, CO, O3, and meteorological parameters were obtained from October 2000 through February 2002 at 1315 m a.s.l., 38.9° N, 120.6° W on Sierra Pacific Industries land, adjacent to the University of California Blodgett Forest Research Station (UC-BFRS). We describe the data set with emphasis on the diurnal cycles during summertime 2001. We show that transport of the Sacramento urban plume is a primary factor responsible for diurnal variation in total reactive nitrogen mixing ratios as well as in NOx, ΣPNs and ΣANs, all of which exhibit a late afternoon/early evening peak. In contrast, HNO3 has a peak just after local noon indicating that HNO3 is in near steady state during the day with production due to photochemistry and removal by deposition and mixing with the background free troposphere. Boundary layer dynamics influence mixing ratios of all species in the early morning. Analysis of the morning feature suggests that higher mixing ratios of NOx and HNO3 persist in the residual layer than in the nocturnal boundary layer indicating the presence of nocturnal sinks of both species. Nighttime observations also indicate large HNO3 and ΣANs production through oxidation of alkenes by NO3.
Detection of pollution transport events southeast of Mexico City using ground-based visible spectroscopy measurements of nitrogen dioxide

This work presents ground based differential optical absorption spectroscopy (DOAS) measurements of nitrogen dioxide (NO2) during the MILAGRO field campaign in March 2006 at the Tenango del Aire research site located to the southeast of Mexico City. The DOAS NO2 column density measurements are used in conjunction with ceilometer, meteorological and surface nitric oxide (NO), nitrogen oxides (NOx) and total reactive nitrogen (NOy) measurements to analyze pollution transport events to the southeast of Mexico City during the MILARGO field campaign. The study divides the data set into three case study pollution transport events that occurred at the Tenango del Aire research site. The unique data set is then used to provide an in depth analysis of example days of each of the pollution transport events. An in depth analysis of 13 March 2006, a Case One day, shows the transport of several air pollution plumes during the morning through the Tenango del Aire research site when southerly winds are present and demonstrates how DOAS tropospheric NO2 vertical column densities (VCD), surface NO2 mixing ratios and ceilometer data are used to determine the vertical homogeneity of the pollution layer. The analysis of 18 March 2006, a Case Two day, shows that when northerly winds are present for the entire day, the air at the Tenango del Aire research site is relatively clean and no major pollution plumes are detected. Case 3 days are characterized by relatively clean air throughout the morning with large DOAS NO2 enhancements detected in the afternoon. The analysis of 28 March 2006 show the DOAS NO2 enhancements are likely due to lightning activity and demonstrate how suitable ground-based DOAS measruements are for monitoring anthropogenic and natural pollution sources that reside above the mixing layer.
On the extraction of wind information from the assimilation of ozone profiles in Météo–France 4-D-Var operational NWP suite

By applying four-dimensional variational data-assimilation (4-D-Var) to a combined ozone and dynamics Numerical Weather Prediction model (NWP), ozone observations generate wind increments through the ozone-dynamics coupling. The dynamical impact of Aura/MLS satellite ozone profiles is investigated using Météo-France operational ARPEGE NWP 4-D-Var assimilation system for a period of 3 months. A data-assimilation procedure has been designed and run on 6-h windows. The procedure includes: (1) 4-D-Var assimilating both ozone and operational NWP standard observations, (2) ARPEGE transporting ozone as a passive-tracer, (3) MOCAGE, the Météo–France chemistry and transport model re-initializing the ARPEGE ozone background at the beginning time of the assimilation window. Using observation minus forecast statistics, it is found that the ozone assimilation reduces the wind bias in the lower stratosphere. Moreover, the Degrees of Freedom for Signal diagnostics show that the MLS data covering the 68.1–31.6 hPa vertical pressure range are the most informative and their information content is nearly of the same order as tropospheric humidity-sensitive radiances. Furthermore, with the help of error variance reduction diagnostics, the ozone contribution to the reduction of the horizontal divergence background-error variance is shown to be better than tropospheric humidity-sensitive radiances.
NASA LaRC airborne high spectral resolution lidar aerosol measurements during MILAGRO: observations and validation

The NASA Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) measures vertical profiles of aerosol extinction, backscatter, and depolarization at both 532 nm and 1064 nm. In March of 2006 the HSRL participated in the Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign along with several other suites of instruments deployed on both aircraft and ground based platforms. This paper presents high spatial and vertical resolution HSRL measurements of aerosol extinction and optical depth from MILAGRO and comparisons of those measurements with similar measurements from other sensors and model predictions. HSRL measurements coincident with airborne in situ aerosol scattering and absorption measurements from two different instrument suites on the C-130 and G-1 aircraft, airborne aerosol optical depth (AOD) and extinction measurements from an airborne tracking sunphotometer on the J-31 aircraft, and AOD from a network of ground based Aerosol Robotic Network (AERONET) sun photometers are presented as a validation of the HSRL aerosol extinction and optical depth products. Regarding the extinction validation, we find bias differences between HSRL and these instruments to be less than 3% (0.01 km−1) at 532 nm, the wavelength at which the HSRL technique is employed. The rms differences at 532 nm were less than 50% (0.015 km−1). To our knowledge this is the most comprehensive validation of the HSRL measurement of aerosol extinction and optical depth to date. The observed bias differences in ambient aerosol extinction between HSRL and other measurements is within 15–20% at visible wavelengths, found by previous studies to be the differences observed with current state-of-the-art instrumentation (Schmid et al., 2006).
Secondary organic material formed by methylglyoxal in aqueous aerosol mimics – Part 2: Product identification using Aerosol-CIMS

We used chemical ionization mass spectrometry with a volatilization flow tube inlet (Aerosol-CIMS) to characterize secondary organic material formed by methylglyoxal with ammonium sulfate in aqueous aerosol mimics. Bulk reaction mixtures were diluted and atomized to form submicron aerosol particles. Organics were detected using Aerosol-CIMS in positive and negative ion mode using I− and H3O+·(H2O)n as reagent ions. The results are consistent with aldol condensation products, carbon-nitrogen species, sulfur-containing compounds, and oligomeric species up to 759 amu. These results support previous observations by us and others that ammonium sulfate plays a critical role in the SOA formation chemistry of dicarbonyl compounds.
Tracking the emission and transport of pollution from wildfires using the IASI CO retrievals: analysis of the summer 2007 Greek fires

In this paper, we analyze the performance of the Infrared Atmospheric Sounding Interferometer (IASI), launched in October 2006 on board METOP-A, for the monitoring of carbon monoxide (CO) during extreme fire events, focusing on the record-breaking fires which devastated thousands of square kilometers of forest in Greece during the last week (23–30) of August 2007. After an assessment of the quality of the profiles retrieved using the Fast Optimal Retrievals on Layers for IASI (FORLI) algorithm, the information provided on fire emissions and subsequent pollution outflow is discussed. Large CO plumes were observed above the Mediterranean Basin and North Africa, with total CO columns exceeding 24×1018 molecules/cm2 and absolute volume mixing ratios up to 4 ppmv on the 25 August. Up to 30×1018 molecules/cm2 and 22 ppmv in the lower troposphere are retrieved close to the fires above the Peloponnese, but with larger uncertainty. The average root-mean-square (RMS) difference between simulated and observed spectra is close to the estimated radiometric noise level, slightly increasing (by ~14%) in the fresh fire plumes. CO profiles are retrieved with a vertical resolution of about 8 km, with ~1.7 pieces of independent information on the vertical in the region considered and a maximum sensitivity in the free troposphere (~4–5 km). Using the integrated total amount, the increase in CO burden due to these fires is estimated to 0.321 Tg, ~40% of the total annual anthropogenic emissions in Greece. The patterns of these CO enhancements are in good agreement with the aerosol optical depth (AOD) retrieved from the MODIS measurements, highlighting a rapid transport of trace gases and aerosols across the Mediterranean Basin (less than one day). While the coarse vertical resolution will not allow the location of the exact plume height, the large CO enhancements observed in the lower troposphere are consistent with the maximum aerosol backscatter coefficient at ~2 km detected by the CALIPSO lidar in space (CALIOP).
A new physically-based quantification of marine isoprene and primary organic aerosol emissions

The global marine sources of organic carbon (OC) are estimated here using a physically-based parameterization for the emission of marine isoprene and primary organic matter. The marine isoprene emission model incorporates new physical parameters such as light sensitivity of phytoplankton isoprene production and dynamic euphotic depth to simulate hourly marine isoprene emissions totaling 0.92 Tg C yr−1. Sensitivity studies using different schemes for the euphotic zone depth and ocean phytoplankton speciation produce the upper and the lower range of marine-isoprene emissions of 0.31 to 1.09 Tg C yr−1, respectively. Established relationships between sea spray fractionation of water-insoluble organic carbon (WIOC) and chlorophyll-a concentration are used to estimate the total primary sources of marine sub- and super-micron OC of 2.9 and 19.4 Tg C yr−1, respectively. The consistent spatial and temporal resolution of the two emission types allow us, for the first time, to explore the relative contributions of sub- and super-micron organic matter and marine isoprene-derived secondary organic aerosol (SOA) to the total OC fraction of marine aerosol. Using a fixed 3% mass yield for the conversion of isoprene to SOA, our emission simulations show minor (<0.2%) contribution of marine isoprene to the total marine source of OC on a global scale. However, our model calculations also indicate that over the tropical oceanic regions (30° S to 30° N), marine isoprene SOA may contribute over 30% of the total monthly-averaged sub-micron OC fraction of marine aerosol. The estimated contribution of marine isoprene SOA to hourly-averaged sub-micron marine OC emission is even higher, approaching 50% over the vast regions of the oceans during the midday hours when isoprene emissions are highest. As it is widely believed that sub-micron OC has the potential to influence the cloud droplet activation of marine aerosols, our findings suggest that marine isoprene SOA could play critical role in modulating properties of shallow marine clouds and influencing the climate.
Biomass burning and urban air pollution over the Central Mexican Plateau

Observations during the 2006 dry season of highly elevated concentrations of cyanides in the atmosphere above Mexico City (MC) and the surrounding plains demonstrate that biomass burning (BB) significantly impacted air quality in the region. We find that during the period of our measurements, fires contribute more than half of the organic aerosol mass and submicron aerosol scattering, and one third of the enhancement in benzene, reactive nitrogen, and carbon monoxide in the outflow from the plateau. The combination of biomass burning and anthropogenic emissions will affect ozone chemistry in the MC outflow.