Comparison of AERONET inverted size distributions to measured distributions from the Aerodyne Aerosol Mass Spectrometer

Transcription

1 Comparison of inverted size distributions to measured distributions from the Aerodyne Aerosol Mass Spectrometer Peter DeCarlo Remote Sensing Project April 28, 23

2 Introduction The comparison of direct in-situ measurements to results from an inversion procedure are extremely important in verification, and testing of assumptions and models of aerosol behavior. The focus of this paper is to compare aerosol size distributions as derived from measurements of direct and diffuse radiation by the Aerosol Robotic Network () to measured size distributions by the Aerodyne Aerosol Mass Spectrometer () in Mexico City during the month of April 23. Since sites are located all over the world and provide verification for satellite measurements of Aerosol properties, this exercise helps to quantify the accuracy of satellite based instruments such as MODIS that also measure aerosol properties. Inversion Procedure Using measurements of direct and diffuse radiation at four wavelengths, the inversion procedure derives a complex refractive index at each of the four wavelengths used in measurements and a bimodal lognormal distribution (for a complete discussion of lognormal distributions see [1] Chapter 7) for fine and coarse aerosols. The inversion procedure is considered an ill-posed inverse problem, consequently constraining the solution to realistic values is necessary for a successful inversion procedure. The bimodal distribution, which is the focus of the current study, is divided into a fine and coarse mode. The fine mode is constrained to be between nm 1.2 µm physical diameter, with the coarse mode from 1.2- µm. Neglecting aerosols less than nm in diameter is valid since their contribution to extinction (scattering and absorption) is negligible at the wavelengths used in the inversion procedure based on Mie Theory. It will be important for later discussion to mention the assumptions inherent in the retrieval procedure. For the algorithm all particles are assumed to be spherical, internal mixtures. There is also an assumption of vertical homogeneity of the aerosol column. (For a complete discussion of the inversion procedure for Level 1.5

3 data see [2] ) Size Distribution Measurements The Aerodyne Aerosol Mass Spectrometer, uses an aerodynamic lens to focus particles into a tight beam where they are accelerated in a supersonic expansion. A chopper wheel with a 1.1% duty cycle allows the beam to pass through the vacuum where the particles are separated based on time-of-flight. The size resolved particles travel through a vacuum where they are vaporized on a heater at 6 C and ionized and quantified by a quadrupole mass spectrometer. A schematic of the instrument is shown below (Figure 1). Particles 7-5nm Aerodynamic Diameter have been shown to have a near % transmission efficiency through the lens, and transmission curves are known for particles 5nm 2 µm (See [3]). Analysis by Mass Spectrometry yields quantified data on sulfate, nitrate, chlorine, ammonia, and organic species in the aerosol. This information is important in order to convert the Aerodynamic Diameter measurements into a Physical Diameter and mass measurements into volume in order to compare with retrievals. Data is saved as µg/m3 for the different species and binned according to their detection by the Mass Spectrometer. Figure 1 (from [3])

4 Density Calculation Using the size resolved and speciated data, one can compute an approximate density for the particles of each size bin. To do this one needs to assume an internal mixture for the aerosol species. Based on our data from Mexico City we saw that all the particles were neutralized by ammonia, and so literature values for densities of ammonium nitrate (1.78g/cc), ammonium sulfate (1.72g/cc), and ammonium chloride (1.53g/cc) were used in the calculation. Organics were assumed to have a density of 1. g/cc, and water measurements have been neglected for this study since the measurements were extremely noisy, and comprised little of the mass of the aerosol. Conversions: Volume = Mass / Density (as calculated) Physical Diameter = Aerodynamic Diameter / Density (as calculated) Comparison Strategy Although is a column measurement, and the measures at a point, one can reasonably compare the measurements if we assume a well-mixed boundary layer, and have a good idea of the height of the boundary layer. For the following comparison, I have assumed a height of 1km for the boundary Layer, and as better data from LIDAR or soundings becomes available, I will incorporate that into the analysis. For now the 1km height can be considered a reasonable assumption. In addition to boundary layer assumptions, there is difference in time resolution between the two measurements. uses nearly a point in time. For size distributions an 8-minute average was used in computing a size distribution. This was done to reduce the noise associated with shorter time scale measurements. The distributions are still not as smooth as is desirable, but the shape is clear, and therefore good enough for comparison. Lastly, it should be noted that the comparison being done only addresses the small mode from nm-1.2µm diameter particles. derives a bimodal distribution, but the is incapable of measuring the larger coarse mode due to constraints on the aerodynamic lens. It should also be noted that future incorporation of coarse mode measurements will be done to fully compare with

5 the inversion products. Results April 12, 23 Both Almucantar (size distribution) retrievals from are plotted below with corresponding data plotted with them. In both cases the data seems to be under predicting the measurements taken by the. In addition there seems to be a bimodal distribution with the data that is most likely primarily an organic mode from early morning traffic. The later measurement from the still appears to have a smaller mode in addition to the large mode, but it is not as pronounced as the earlier mode. Times of the retrievals from and the averages from the are listed on the plots for reference. 8x April 12, 23 8: AM () 8:-8:6 AM () April 12, 23 :39 AM () :36-:44 AM (). Figure 2. Compared size distributions from April 12, 23 April 13, 23 This day illustrates the idea that there is most likely a bimodal distribution of aerosols in the fine mode that are not internal mixtures. The first plot in figure 3 indicates a smaller mode overlapping with the larger diameter mode. The second plot follows the first by only 45 minutes. Figure 4 clearly indicates a peak in the organic signal long before the secondary aerosols peak. At the times when these comparisons are being made, the organic signal is much larger than the nitrate or sulfate signals, which peak later in the day, presumably as the photochemistry increases with increasing radiance from the sun.

6 6x -3 5x April 13, 23 7:5 AM () 7:58-8:6 AM () April 13, 23 8:34 AM () 8:32-8:4 AM () Figure 3. Compared size distributions from April 13, 23 Figure 4. A time series plot from the plotting the different aerosol components over the period of a day. April 14, 23 All the plots in Figure 5 appear to have better agreement between the data, and the data. The median diameter and standard deviation of the retrieval seem to match up very well with the data. The total concentration does not match up as well, but that could be due to a variety of factors (see discussion section).

7 4x April 14, 23 7:5 AM () 7:58-8:6 AM () 6x April 14, 23 8:33 AM () 8:26-8:34 AM () 8x April 14, 23 9: AM () 8:54-9:4 AM () Figure 5. Compared size distributions from April 14, 23 April 15, 23 The last day of comparisons has some promising comparisons. For example the third plot in Figure 6 looks like a very good match. The first plot has an interesting artifact just below nm. This is possibly noise from short time averages with the, although this should be further investigated. The second plot shows fairly good agreement between and the measurements. The fourth plot shows the distribution shifted slightly left from what the measured. All plots on this day seem to indicate the presence of a small organic mode in the data.

8 6x April 15, 23 7:5 AM () 7:58-8:6 AM () 6x April 15, 23 8:32 AM () 8:28-8:36 AM () 5x April 15, 23 8:59 AM () 8:52-9: AM () April 15, 23 11:38 AM () 11:3-11:38 AM () Figure 6. Compared size distributions from April 15, 23 Discussion Overall agreement between and the was better than expected. More work should be done however to fully quantify the agreement, and the availability of more data from different instruments will certainly help in this quantification. In general, however, it can be seen that tends to over predict the column concentration. In addition, the retrieval does not have the ability to resolve two different modes that seem to be overlapping in the data. This is an understandable shortcoming since deriving a third mode which overlaps with another would be nearly impossible given the nature of the inversion problem. So the mode given is more of a composite of the two modes seen by the in the fine mode range. This points to a potential problem since internal mixtures are assumed for the retrieval. The effects of this likely external mixture have not been determined here, but should be looked at. The addition of aerosol size measurements of 1-5µm (the coarse mode) will be

9 extremely helpful in fully quantifying the retrieval. Errors in deriving fine mode contributions may be balanced by opposite errors in deriving the coarse mode. Comparing the full retrieval product with comparable size measurement data will help us to fully evaluate the accuracy of the retrieval. There is data on the coarse mode being taken with an instrument colocated with the. Future integration of coarse mode data is planned. Lastly the ability to resolve the fine mode relies on Mie Theory. The Mie Size parameter is calculated at a wavelength and is the ratio of the circumference of a spherical aerosol to the wavelength used (Mie Size Parameter = 2π r / λ). The smallest wavelength used in the retrieval is 44nm, and the smallest radius used is 5nm. This translates to a Mie Size parameter of less than 1, where scattering and absorption efficiency is very low. It seems that this would lead to a poor, or at least, less accurate measure of the aerosols in the low end of the fine mode. is limited in wavelengths since 44nm is the shortest wavelength used on the instruments. Taking into the account the constraints on the retrieval method, the results were surprisingly in good agreement, and future work on better quantifying the column concentration of the could possibly yield better agreement. Future Work There is much more work that will be done on this particular project. Firstly, incorporation of black carbon (BC) data into the fine mode measurements will be done to fully characterize the composition of the fine mode aerosols. Current data from Mexico City indicates that BC exists in a ratio of 2 parts organic carbon to one part black carbon. Adding this data, could potentially improve the agreement between the to the retrievals, since in general the is slightly lower in column concentration than. It should also be noted that the has not undergone a full ionization energy calibration. Once this has been done, the concentrations measured will be

10 adjusted slightly either up or down based on the new calibration. This could potentially make the measurements agree or disagree more in terms of concentration, but will not change the size measurements of the. In addition when data becomes available for accurate measures of boundary layer height, those will also be incorporated into the analysis, to yield better solutions for the extrapolation from a point measurement, to a column quantity. Data regarding the coarse mode was unavailable at the time of analysis, but has since become available, and will be added to the fine mode measurements so that both retrieved modes can be looked at in concert. Finally, in the future, I would also like to compute an average complex refractive index based on our composition measurements with the black carbon data incorporated, and compare the composition-averaged refractive index to the retrieved refractive index. References [1] Seinfeld, J.H. and Pandis, S.N., Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. John Wiley & Sons, inc, New York, [2] Dubovik, O. and M. D. King, 2: A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements," J. Geophys. Res., 5, [3] Jimenez, J.L., Jayne, J.T., Shi, Q., Kolb, C.E., Worsnop, D.R., Yourshaw, I., Seinfeld, J.H., Flagan, R.C., Zhang, X., Smith, K.A., Morris, J., and Davidovits, P. Ambient Aerosol Sampling with an Aerosol Mass Spectrometer. Journal of Geophysical Research - Atmospheres,8(D7), 8425, doi:.29/21jd1213, 23.