The Use of Optical Methods to Study Aerosols in the Paso del Norte Region

1. The Use of Optical Methods to Study Aerosols in the Paso del Norte Region Rosa M. Fitzgerald, Javier Polanco, Angel Esparza, Richard Medina Physics Department, UTEP El Paso, TX

2. Abstract The aerosol size distribution and the single scattering albedo are retrieved using multi-filter rotating shadow-band radiometers in conjunction with robust models. (I) An inverse reconstruction model for aerosol size distribution is used and applied to the Paso del Norte Region. The wavelengths used are in the visible and UV ranges. Results for the columnar aerosol size distribution for the Paso del Norte Airshed for two different pollution scenarios and at two different seasons are presented. In addition, HYSPLIT backward trajectories were used to identify pollution sources. (II) A methodology to retrieve Single Scattering Albedo (SSA) values employing the ratio of Direct to Diffuse Irradiances (DDR) is used and applied to the Paso del Norte region.The wavelengths used are 332 nm and 368 nm. The retrieved SSA values are presented. Furthermore, the variation of SSA values during the day is also analyzed for the Paso del Norte and linked to the flow of air masses into the region. These methodologies can be applied in any geographical location, and are particularly useful for cities that experience episodes of high PM concentrations.

3. Multi-filter Rotating Shadow Band Radiometers (MFRSR) Laser Particle Counters CI-150t

4. (I) Inverse Modeling to Obtain the Aerosol Size Distribution Angel E. Esparza, Rosa M. Fitzgerald , Thomas E. Gill and Javier Polanco, ‘Use of Light Extinction Method and Inverse Modeling to Study Aerosols in the Paso del Norte Airshed’, Atmospheric Environment, 45, 7360-7369, 2011.

5. The following general equation needs to be solved : where t is the aerosol optical depth, Cext is extinction cross-section, n is the height dependant aerosol number density in the radius range r to r + dr, m is the complex refractive index of the aerosols and l is the wavelength of the incident light.

6. The regularized solution is obtained by solving the following equation : where f is the size distribution array, A is the kernel matrix, g is the lagrange multiplier, H is the smoothing matrix and g is the optical depth data.

7. Results

10. for winter a = 2.986; for summer a = 0.758

12. Conclusions (I) The inverse reconstruction model (EPIRM) was successfully applied to the Paso del Norte Airshed. This model has an advantage over other techniques in that it can be used in the study of aerosols in situ. The methodology may be used in any area of the country, but is especially relevant to southwestern U.S cities such as El Paso, which experiences severe air pollution from fugitive dust source contaminants. In this work it was observed that for the clean day scenario the summer case exhibited a greater concentration of smaller size particles than the winter case. This is attributable in part to the different regions from which the air parcels originated. Moreover, the air parcels moved more slowly in the summer case than in the winter case. For the summer day the air parcels passed across the more industrialized region of the Paso del Norte Airshed, and this was interpreted as the cause of a higher concentration of small particles. It was also observed that for the polluted day scenario the winter case was ascribed to the predominance of smaller size particles due primarily to influences of the El Paso-Juarez urban region, whereas the summer observed the presence of more mineral dust as the air parcels passed across the Chihuahuan desert, a trajectory associated with dust events. In all four cases the correlation factors between the EPIRM model and the CI-150t laser particle instrument were above 0.98. The polymodality observed in the season inter-comparison graphs is expected for an interface and complex region, such as the Paso del Norte region.

13. (II) Retrieval of the Single Scattering Albedo Richard Medina, Rosa M. Fitzgerald and Qilong Min, ‘Retrieval of the Single Scattering Albedo in the El Paso-Juarez Airshed Using the TUV Model and a UV-MFRSR Radiometer’, Atmospheric Environment, vol. 46, pp 430-440, 2012.

14. PM2.5 Concentrations

15. is the source function given by: Radiative Transfer Equation- TUV model is the diffuse radiance, the optical depth, the cosine of the zenith angle, the single scattering albedo, incident solar flux, and is the axially-symmetric phase function defining the light incident at which scattered in the direction

16. Results

17. Retrieval of the Single Scattering Albedo SSA= bs/(bs + ba) , where bsis the aerosol scattering coefficient and ba is the aerosol absorption coefficient.

18. Evolution of The Single Scattering Albedo throughout the day

19. Evolution of The Single Scattering Albedo throughout the day

20. Sensitivity study of the Asymmetry Parameter The asymmetry parameter, g, represents the phase function, it is +1: forward direction and -1: backward direction

21. Conclusions (II) The retrieved values of SSA for the low polluted day (0.66 – 0.81 at 332 nm and 0.61 – 0.8 at 368 nm) and for the polluted day (0.56 – 0.7 at 332 nm and 0.53 – 0.66 at 368 nm) were successfully obtained for the El Paso-Juarez Airshed using the DDR irradiance method and are in agreement with previous studies. In addition, the presence of both moderately absorbing and absorptive particles was observed in the Airshed, which is symptomatic of a complex interface region such as the El Paso-Juarez Airshed, located between an urban and a rural site and surrounded by the Chihuahuan desert. An increase in the concentration of absorptive aerosols during the late morning and middle of the day is observed in the atmosphere, especially for the polluted day case, as shown in the graphs of the variation of the SSA values throughout the day, which correlated well with the analysis of the air masses flow in the same day. It was also observed in the uncertainty analysis that changes of 5% in g produced a variation of only 1% in the retrieved SSA values, denoting that g is a secondary effect in the retrieval of SSA. It was found that the TUV model can be used as a diagnostic model to interpret UV-MFRSR irradiance data and successfully to retrieve the single scattering albedo in this region.