PM Size Distribution Research Articles

Field evaluation of a modified DataRAM MIE scattering monitor for real-time PM 2.5 mass concentration measurements

In this paper, we investigated the feasibility of using a modified DataRAM nephelometer (RAM-1, MIE Inc., Billerica, MA) as a continuous PM 2.5 monitor to measure concentrations of ambient and concentrated aerosols in real time. The DataRAM operated with a diffusion dryer tube in its inlet in order to reduce the relative humidity of the sampled air to less than 50%. A total of 39 field tests were conducted in which the average dry DataRAM concentration was compared to the gravimetrically determined mass concentration, corrected for nitrate losses. Tests were conducted over one calendar year (from January to December 1999) in order to capture maximum seasonal variations in the levels of relative humidity, PM size distribution and chemical composition in the Los Angeles Basin. Our experimental results indicated that the aerosol mass median diameter (MMD) is the single, most important parameter in affecting the response of the DataRAM. As the MMD increases from 0.3 to 1.1 μm, the DataRAM-to-MOUDI ratio increases from approximately 0.7 to about 1.6. The DataRAM-to-MOUDI ratio subsequently decreases to about 1.0, as the MMD further increases to 1.5 μm. For MMD values in the range of 0.4–0.7 μm (i.e., the MMD size range that is most commonly associated with urban aerosols), the DataRAM and gravimetrically measured mass concentrations (corrected for nitrate losses) agree within ±20%. Based only on ambient data, the average DataRAM-to-gravimetric concentration ratio was 0.93 (±0.17), whereas the average DataRAM-to-gravimetric concentration ratio for concentrated PM 2.5 aerosols was 1.23 (±0.20). Our field evaluation also indicated that the effect of particle chemical composition on the DataRAM-to-gravimetric concentration ratio is much less important than that of particle size distribution.

Modeling urban and regional aerosols near acid neutrality—application to the 24–25 June scaqs episode

Mathematically predicting the size and composition distribution of atmospheric aerosols can help to elucidate the complex link between emissions and particulate air quality. Wexler et al. (1994)identified and analyzed the atmospheric aerosol processes and estimated the relative importance of each term using parameters typical in South Coast Air Basin (SoCAB). The result was a general dynamic equation including only the relevant terms. In a previous paper we described practical difficulties integrating these equations under the acid-neutral conditions common in the SoCAB and introduced an acid equilibrium assumption, that is, the aerosol hydrogen ion concentration can be assumed to be in equilibrium with the gas-phase acidity. In this paper we use the model to predict the size and composition distribution of PM during the 24–25 June 1987 SCAQS episode. The predicted size distribution is compared to the SCAQS measurement data by John et al. (1989a, b, 1990).