Hygroscopicity of mineral dust particles: Roles of chemical mixing state and hygroscopic conversion timescale
Our laboratory investigations of mineral dust particle hygroscopicity are motivated by field observations of the atmospheric processing of dust. During ACE-Asia we observed sulphate and nitrate to be strongly segregated from each other in individual aged Asian dust particles. CCN activation curves of pure calcium minerals as proxies for fresh (calcium carbonate) and aged (calcium sulphate, nitrate, chloride) dust indicate that this mixing state would cause a large fraction of aged dust particles to remain poor warm cloud nucleation potential, contrary to previous assumptions. The enrichment of oxalic acid in calcium-rich dust particles could have similar effects due to the formation of insoluble calcium oxalate. Soluble calcium nitrate and chloride reaction products are hygroscopic and will transform mineral dust into excellent CCN. Generating insoluble mineral particles wet by atomization produced particles with much higher hygroscopicity then when resuspended dry. The atomized particles are likely composed of dissolved residuals and do not properly reflect the chemistry of dry mineral powders. Aerosol flow tube experiments were employed to study the conversion of calcium carbonate into calcium nitrate via heterogeneous reaction with nitric acid, with simultaneous measurements of the reacted particles' chemistry and hygroscopicity. The timescale for this hygroscopic conversion was found to occur on the order of a few hours under tropospheric conditions. This implies that the conversion of non-hygroscopic calcite- containing dust into hygroscopic particles will be controlled by the availability of nitric acid, and not by the atmospheric residence time. Results from recent investigations of the effect of secondary coatings on the ice nucleation properties of dust particles will also be presented. The cloud formation potential of aged dust particles depends on both the quantity and form of the secondary species that have reacted or mixed with the dust. These results have important implications for the treatment of mineral dust particles in global chemistry and climate models.
A Novel Method for Direct In Situ Measurements of N2O5 Reactivity on Ambient Aerosol Particles
An experimental approach for the direct measurement of trace gas reactivity on ambient aerosol particles has been developed. The method utilizes a newly designed entrained aerosol flow reactor coupled to a custom- built chemical ionization mass spectrometer. The experimental method is described via application to the measurement of N2O5 reactivity, γ(N2O5). Laboratory calibration on well characterized aerosol particles show that measurements of γ(N2O5), observed with this new technique, are in agreement with previous observations even though the new method utilizes atmospherically relevant particle surface area concentrations (0-1000 μm2cm-3) that are orders of magnitude lower than previous laboratory studies. Sources of uncertainty in the measured γ(N2O5) are discussed. Examples of the capabilities and utility of the retrieved data are included from both laboratory calibrations and field observations made in two locations. The field observations demonstrate that particulate organic matter and relative humidity exert strong controls on γ(N2O5).
Development of a New Calibration Method for an Ambient Ion Monitor Ion Chromatograph (AIM-IC)
Fine atmospheric aerosols play an important role in the atmosphere as they alter the radiative balance of the Earth through direct and indirect climate effects, reduce visibility, participate in acid rain formation and affect human health. The motivation for chemically and temporally resolved measurements of fine aerosol composition has lead to the development of the Ambient Ion Monitor Ion Chromatograph (AIM-IC) system by Dionex/URG. This instrument is capable of simultaneously monitoring fine aerosols (<2.5μm) and associated precursor gases on a nearly continuous basis with a time resolution of 1 hour. The instrument utilizes a parallel-plate wet denuder with a constantly regenerated surface for collection of gases and a particle condensation chamber for the collection of aerosols. AIM-IC is capable of monitoring HCl(g), HONO(g), HNO3(g), SO2(g), NH3(g), Cl-, NO2-, NO3-, SO42-, NH4+ , and some water soluble organic acids and amines. Standard calibration of the AIM-IC is carried out by injecting a series of mixed standards directly onto the ion chromatographs, bypassing the sampling component of the instrument. This results in calculated detection limits on the order of 10-200 pptv for gases and 10-500 of ng/m3 for individual particle constituents when collecting at 3 L/min for 55 minutes. In this work, we present a new method for the calibration of the AIM-IC for both gas and particle collection that enables us to evaluate the entire system from size-selection to detection. This external calibration method is assessed for the gases HNO3(g), SO2(g), and NH3(g), and for particles containing (NH4)2SO4, NH4NO3, and Na2SO4. Quantitative collection of SO2 is found to require careful optimization of the H2O2 concentration of the denuder liquid, while the replacement of a cyclone with an impactor improves the sampling efficiency of NH3 and HNO3.
Air Quality and Observations From Space: Using High-Resolution Atmospheric Chemistry Model Data for a Time-Resolved Tropospheric Chemistry Mission Study
To better capture the formation and impact of air pollution episodes from space, the NRC Decadal Survey recommended that NASA undertake an atmospheric chemistry mission from geostationary orbit, GEO-CAPE. Satellites from low Earth orbit sample at best once per day, with relatively coarse spatial resolutions (10s- to 100 km), limiting analysis of precursor development and emission sources. From geostationary orbit, GEO- CAPE is expected to deliver hourly data at 5-10 km horizontal spatial resolution. This study was performed to better understand spatial and temporal sampling strategies in the mission's design. We used atmospheric chemistry data generated from EPA's Community Multiscale Air Quality (CMAQ) modeling system during one week of the 2006 Air Quality Field Study (TexAQS II, Aug 30 - Sep 5, 2006) to optimize spatial and temporal resolution for each of the principal mission measurements (O3, CO, NO2, SO2, HCHO, and aerosols), using autocorrelation techniques. The CMAQ data that we used was generated every 15 minutes at a 4km spatial resolution with a domain of Eastern Texas. We also used the data coupled with available ozonesonde and radiosonde measurements, satellite observations, and surface observations to help characterize the 4 dimensional state of the atmosphere during that week. The results of this sampling study will help to define the next-generation air quality satellite mission, with the ultimate goal of improving air quality management and human health.
A New Satellite Measurement Capability for Assessing Damage to Crops from Regional Scale Ozone Pollution
High concentrations of ground-level ozone are frequently measured over farmland regions in many parts of the world. Since laboratory data show that ozone can significantly impact crop productivity if levels above a threshold concentration are reached, there is a consensus that crop yield should be impacted now and that the effects will become even more detrimental as global background concentrations continue to rise, as suggested by the latest IPCC report. Using the long-term record of tropospheric ozone derived from satellite measurements (http://asd-www.larc.nasa.gov/TOR/data.html), we present a methodology that can be used to assess the impact of regional ozone pollution on crop productivity. In this study, we use soybean crop yield data during a 5-year period over the Midwest of the United States and analyze the results using multiple linear regression statistical models. The results are consistent with findings using conventional ground-based measurements and with results obtained from an open-air experimental facility SoyFACE (Soybean Free Air Concentration Enrichment) in central Illinois. Our analysis suggests that the cost to the farmers globally is substantial, and supports other studies that calculate an economic loss to the farming community of more than 10 billion dollars annually.
Ground and Satellite Observation of NO2 in Southwestern Ontario During the 2007 BAQS-met Campaign
Examining spatial and temporal variability in air pollutant concentrations provides insight into sources, atmospheric transport patterns and potential impacts. NO2 is an important pollutant to track because it is strongly linked to combustion, the cycle of photochemical oxidants and the fate of anthropogenic pollutants from local to regional scales. Various measurement strategies offer spatial or temporal resolution for NO2, but combined highly-resolved spatiotemporal evaluations are rare. Chemiluminescence analyzers can provide high time resolution NO2 point measurements but are expensive. Passive monitors can be deployed in multiple locations simultaneously, due to their lower cost, but provide time-averaged concentration measurements. The Ozone Monitoring Instrument aboard the EOS Aura satellite allows NO2 column density snapshots of the troposphere to be taken with daily frequency in most locations at a maximum spatial resolution of 13 × 24 km2. Tropospheric NO2 column densities derived from OMI have been shown to be a useful data source in analyzing ground level air quality, however there have been relatively few detailed evaluations of the spatial patterns revealed by OMI over areas of transition from high to low concentrations. During the Border Air Quality and Meteorology study of 2007, surface NO2 was measured in Southwestern Ontario for a period of almost three months in June, July and August. High time resolution data were obtained at 5 sites while two week integrated concentrations were measured through a network of 18 passive monitors. Emissions from the urban centres of Detroit, MI and Windsor, ON impacted the monitoring sites to different extents. This region was chosen because of the influence of the Great Lakes on local meteorology coupled with the impacts of local and transboundary sources on air quality. The spatiotemporal trends in NO2 will be presented. These data will be used to explore the capabilities and limitations of the satellite as an observer of ground based concentrations and how different data interpretation strategies affect those limitations. The relationship between the spatial resolution obtainable through the satellite data and the degree of temporal averaging will be discussed.