Submicron ambient aerosol was characterized in summer 2005 at an urban background site in Zurich, Switzer- land, during a three-week measurement campaign Highly time-resolved samples of non-refractory aerosol components were analyzed with an Aerodyne aerosol mass spectrometer (AMS) Positive matrix factorization (PMF) was used for the first time for aerosol mass spectra to identify the main com- ponents of the total organic aerosol and their sources The PMF retrieved factors were compared to measured reference mass spectra and were correlated with tracer species of the aerosol and gas phase measurements from collocated instru- ments Six factors were found to explain virtually all vari- ance in the data and could be assigned either to sources or to aerosol components such as oxygenated organic aerosol (OOA) Our analysis suggests that at the measurement site only a small (<10%) fraction of organic PM1 originates from freshly emitted fossil fuel combustion Other primary sources identified to be of similar or even higher importance are charbroiling (10-15%) and wood burning ( 10%) The fraction of all identified primary sources is considered as pri- mary organic aerosol (POA) This interpretation is supported by calculated ratios of the modelled POA and measured pri- mary pollutants such as elemental carbon (EC), NOx, and CO, which are in good agreement to literature values A high fraction (60-69%) of the measured organic aerosol mass is OOA which is interpreted mostly as secondary organic aerosol (SOA) This oxygenated organic aerosol can be sepa- rated into a highly aged fraction, OOA I, (40-50%) with low volatility and a mass spectrum similar to fulvic acid, and a more volatile and probably less processed fraction, OOA II (on average 20%) This is the first publication of a multiple component analysis technique to AMS organic spectral data and also the first report of the OOA II component

/pdf/source-apportionment-of-submicron-organic-aerosols-at-an-215o702oof.pdf

Source apportionment of submicron organic aerosols at an urban site by factor analytical modelling of aerosol mass spectra

. Source apportionment using the bilinear model through a multilinear engine (ME-2) was successfully applied to non-refractory organic aerosol (OA) mass spectra collected during the winter of 2011 and 2012 in Zurich, Switzerland using the aerosol chemical speciation monitor (ACSM). Five factors were identified: low-volatility oxygenated OA (LV-OOA), semivolatile oxygenated OA (SV-OOA), hydrocarbon-like OA (HOA), cooking OA (COA) and biomass burning OA (BBOA). A graphical user interface SoFi (Source Finder) was developed at PSI in order to facilitate the testing of different rotational techniques available within the ME-2 engine by providing a priori factor profiles for some or all of the expected factors. ME-2 was used to test the positive matrix factorization (PMF) model, the fully constrained chemical mass balance (CMB) model, and partially constrained models utilizing a values and pulling equations. Within the set of model solutions determined to be environmentally reasonable, BBOA and SV-OOA factor mass spectra and time series showed the greatest variability. This variability represents the uncertainty in the model solution and indicates that analysis of model rotations provides a useful approach for assessing the uncertainty of bilinear source apportionment models.

/pdf/sofi-an-igor-based-interface-for-the-efficient-use-of-the-5f5h07tgig.pdf

SoFi, an IGOR-based interface for the efficient use of the generalized multilinear engine (ME-2) for the source apportionment: ME-2 application to aerosol mass spectrometer data

[1] We present an approach to infer ground-level nitrogen dioxide (NO2) concentrations by applying local scaling factors from a global three-dimensional model (GEOS-Chem) to tropospheric NO2 columns retrieved from the Ozone Monitoring Instrument (OMI) onboard the Aura satellite. Seasonal mean OMI surface NO2 derived from the standard tropospheric NO2 data product (Version 1.0.5, Collection 3) varies by more than two orders of magnitude ( 10 ppbv) over North America. Two ground-based data sets are used to validate the surface NO2 estimate and indirectly validate the OMI tropospheric NO2 retrieval: photochemical steady-state (PSS) calculations of NO2 based on in situ NO and O3 measurements, and measurements from a commercial chemiluminescent NO2 analyzer equipped with a molybdenum converter. An interference correction algorithm for the latter is developed using laboratory and field measurements and applied using modeled concentrations of the interfering species. The OMI-derived surface NO2 mixing ratios are compared with an in situ surface NO2 data obtained from the U.S. Environmental Protection Agency's Air Quality System (AQS) and Environment Canada's National Air Pollution Surveillance (NAPS) network for 2005 after correcting for the interference in the in situ data. The overall agreement of the OMI-derived surface NO2 with the corrected in situ measurements and PSS-NO2 is −11–36%. A larger difference in winter/spring than in summer/fall implies a seasonal bias in the OMI NO2 retrieval. The correlation between the OMI-derived surface NO2 and the ground-based measurements is significant (correlation coefficient up to 0.86) with a tendency for higher correlations in polluted areas. The satellite-derived data base of ground level NO2 concentrations could be valuable for assessing exposures of humans and vegetation to NO2, supplementing the capabilities of the ground-based networks, and evaluating air quality models and the effectiveness of air quality control strategies.

/pdf/ground-level-nitrogen-dioxide-concentrations-inferred-from-2cwjf056oc.pdf

Ground-level nitrogen dioxide concentrations inferred from the satellite-borne Ozone Monitoring Instrument

Data from a recent field campaign in Mexico City are used to evaluate the performance of the EPA Federal Ref- erence Method for monitoring the ambient concentrations of NO2. Measurements of NO2 from standard chemilumines- cence monitors equipped with molybdenum oxide converters are compared with those from Tunable Infrared Laser Differ- ential Absorption Spectroscopy (TILDAS) and Differential Optical Absorption Spectroscopy (DOAS) instruments. A significant interference in the chemiluminescence measure- ment is shown to account for up to 50% of ambient NO2 con- centration during afternoon hours. As expected, this interfer- ence correlates well with non-NOx reactive nitrogen species (NOz) as well as with ambient O3 concentrations, indicat- ing a photochemical source for the interfering species. A combination of ambient gas phase nitric acid and alkyl and multifunctional alkyl nitrates is deduced to be the primary cause of the interference. Observations at four locations at varying proximities to emission sources indicate that the per- centage contribution of HNO3 to the interference decreases with time as the air parcel ages. Alkyl and multifunctional alkyl nitrate concentrations are calculated to reach concen-

/pdf/evaluation-of-nitrogen-dioxide-chemiluminescence-monitors-in-i62sm2bf4g.pdf

Evaluation of nitrogen dioxide chemiluminescence monitors in a polluted urban environment

In many large cities of Europe standard air quality limit values of particulate matter (PM) are exceeded. Emis- sions from road traffic and biomass burning are frequently reported to be the major causes. As a consequence of these exceedances a large number of air quality plans, most of them focusing on traffic emissions reductions, have been imple- mented in the last decade. In spite of this implementation, a number of cities did not record a decrease of PM levels. Thus, is the efficiency of air quality plans overestimated? Do the road traffic emissions contribute less than expected to am- bient air PM levels in urban areas? Or do we need a more specific metric to evaluate the impact of the above emissions on the levels of urban aerosols? This study shows the results of the interpretation of the 2009 variability of levels of PM, Black Carbon (BC), aerosol number concentration (N) and a number of gaseous pollu- tants in seven selected urban areas covering road traffic, ur- ban background, urban-industrial, and urban-shipping envi- ronments from southern, central and northern Europe. The results showed that variations of PM and N levels do not always reflect the variation of the impact of road traf-

/pdf/new-considerations-for-pm-black-carbon-and-particle-number-2iscfg8np6.pdf

New considerations for PM, Black Carbon and particle number concentration for air quality monitoring across different European cities

[1] Nitrogen oxides (NOx = NO + NO2) in the atmosphere are often measured using instruments equipped with molybdenum converters. NO2 is catalytically converted to NO on a heated molybdenum surface and subsequently measured by chemiluminescence after reaction with ozone. The drawback of this technique is that other oxidized nitrogen compounds such as peroxyacetyl nitrate and nitric acid are also partly converted to NO. Thus such NO2 measurements are really surrogate NO2 measurements because the resultant values systematically overestimate the true value because of interferences of these compounds, especially when sampling photochemically aged air masses. However, molybdenum converters are widely used, and a dense network of surrogate NO2 measurements exists. As an alternative with far less interference, photolytic converters using ultraviolet light are nowadays applicable also for long-term measurements. This work presents long-term collocated NO2 measurements using molybdenum and photolytic converters at two rural sites in Switzerland. On a relative scale, the molybdenum converter instruments overestimate the NO2 concentrations most during spring/summer because of prevalent photochemistry. On a monthly basis, only 70–83% of the “surrogate” NO2 can be attributed to “real” NO2 at the non-elevated site and even less (43–76%) at the elevated one. The observed interferences have to be taken into account for monitoring and regulatory issues and to be considered when using these data for ground-truthing of satellite data or for validation of chemical transport models. Alternatively, an increased availability of artifact-free data would also be beneficial for these issues.

/pdf/nitrogen-oxide-measurements-at-rural-sites-in-switzerland-4xmv1lisxj.pdf

Nitrogen oxide measurements at rural sites in Switzerland : Bias of conventional measurement techniques

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Nitrogen oxide measurements at rural sites in Switzerland : Bias of conventional measurement techniques