knitr::opts_chunk$set(echo = TRUE)

# globals
num_species <- 41
num_sources <- 20
sources <- c("AG", "AIRC", "BIOG", "COAL", "DUST", "FIRE", "FOIL", "MEAT", 
             "METAL", "NG", "NRDIE", "NRGAS", "NROTH", "ORDIE", "ORGAS", "OT", 
             "OTHCMB", "SOLVENT", "SSALT", "WOOD")

# load data for examples
library(readr)
library(magrittr)

Introduction

This document seeks to illustrate how CMAQ DDM values are pre-processed, then used in hybrid source apportionment, and then post-processed for subsequent analysis and interpretation of results.

The order in which this overview covers processing functionality, is:

  1. Conversion of MATLAB data to R objects (convert_mat.R)
  2. Re-arranging of R objects for hybrid calculations (data_prep.R, reorder_rds_files.R)

In particular, this document elaborates on what is performed in the first two parts of new_rj_script.R, the script used to produce Rj values for a single year. For an explanation of how the Rj values are calculated, and the post-processing of those values, see a subsequent vignette.

Steps in the Analysis

What is helpful to keep in mind, is that the following structure for processing/manipulating the data were developed to facilitate computations with limited access to RAM. That is, with the following manipulations, all of the processing for a single year's data can be performed on an $8$Gb RAM laptop. This is not trivial, since loaded into R uses up RAM, then there's is a limitation on how much data can be used at any one time. Also, we want to minimize having to read in different files during computations, and to use as much of the data loaded into working memory as possible.

To start, we begin with matlab files.

Converting .mat files

The CMAQ-DDM data is originally stored in .mat files. This is because, although the raw DDM output is in netCDF format, some post-processing of those raw results is conducted with matlab.

Since there are r num_species chemical species being used, then that is how many matlab files there initially are. Each matlab file consists of a list of sensitivity values, averaged by day. Each list contains r num_sources + 1 elements, and each element of the list is a three-dimensional array. The dimensions of the array correspond to: x- and y- coordinates, and time. The coordinates identify the cell's location in the grid, and each element of the time dimension corresponds to a different day.

Ultimately, each .mat file is converted into a list of arrays, stored as an .rds file. Using this native (to R) file format significantly reduces time needed to read in the data from a single file, from about five minutes to around ten seconds. In turn, it simplifies subsequent computations with the data.

Renaming Scheme

No consistent naming scheme has (as of this writing) yet been adopted among the team members generating the DDM results. So, to at least have consistency in the R portion of the analysis, the names of the list elements (i.e., sources) are always a subset of the following:

sources

and if they are originally otherwise in the .mat files, then those names are converted to the above sources which they correspond to.

Re-ordering of Sources

Along with renaming the sources, the list elements of the .mat files should be (i.e., this is as of yet not implemented) in the same order. This limits the number of changes that need to be made later on in the hybridSA processing, out of accomodating different years' data. For example, the order of uncertainties $\sigma_{\ln(Rj)}$ for the Rj values wouldn't need to be changed in the hybrid optimization.

Interval and Aggregated-Interval Files

The way the sensitivity values are arranged in the .mat files, is not conducive to generating hybridSA results with R most efficiently. Also, given memory and time constraints, ideally all the data needed to generate Rj values for any single day, could be contained in one file.

Thus, the files are converted so they are grouped temporally instead of by sensitivities of sources to species. (???) That is, the data is re-arranged so that in the end, thirty-seven "aggregate interval" files are generated, one file for each ten-day period in the year (except for the last). So, for example, the first aggregate interval file contains all the CMAQ data for days $1-10$ in the year, the second for days $11-20$ in the year, and so on.

Each aggregate interval file contains a list, where each element corresponds to a single day.

aa <- read_rds("/Volumes/My Passport for Mac/gpfs:pace1:/data_2005/Agg_intv_files/Intv_1_agg.rds")
names(aa)

In turn, the list element for a day is itself a list of forty-one elements (one for each species).

aa[[1]] %>% str

And each array in that day-list has dimensions corresponding to: x-coordinate, y-coordinate, and source. (The third dimension here is one greater than the number of sources, because the first contains simulated concentrations for that grid cell).

Finalizing Hybrid Inputs

Up to this stage, the original CMAQ-DDM results have only been re-arranged. Now, we want to take those values, and organize them into a format that can readily be used for processing for hybrid source apportionment.

The hybrid optimization requires two kinds of data as inputs: concentration data, and CMAQ sensitivities. In the following, we describe how to obtain each of these two inputs.

Preparing Concentration Data

As for the concentration data, it consists of two types: observed values and simulated values. The observed values are obtained directly from the EPA website, while the simulated values are contained in the CMAQ DDM. So, to combine all the concentration data together, it's necessary to extract the simulated values from the aggregate interval files.

Since the simulated concentrations are contained in the first index of the "Sources" dimension of the array, then for Day1 (which was shown above), we can obtain just the simulated concentrations by the following:

library(plyr)
day_sim_values <- aa[[1]] %>% llply(function(x) x[, , 1]) 
str(day_sim_values)

Since we only took one "slice" from the third dimension, the resulting arrays are each two-dimensional. The interpretation for day_sim_values would then be: For the first day of the year, the simulated concentrations for Ag over the entire spatial domain, is given by the first array, and for aluminum, by the second array, etc.

There is already a script which downloads and combines all of the observed concentrations over the entire spatial and temporal domain. So, all that is needed after extracting these simulated values is to combine them with the previously-generated observed values. And that is exactly what the aggregate_concentrations function does.

Preparing Sensitivity Data

While the each day's simulated concentrations are contained in the first slice of the arrays, the sensitivity values of the species to the sources, are contained in the rest. That is,

day_sens_values <- aa[[1]] %>% llply(function(x) x[, , -1]) 
str(day_sens_values)

The interpretation for this is as follows: for the first day of the year, in list element Ag, the value at element (x, y, z) is the sensitivity of Silver to the $z$th source at grid cell (x, y). Note that for $2005$, r num_sources are used for the hybrid optimization. From $2007$ onwards, though, only sixteen sources are used.

From here, the sensitivities for each day of the year are extracted, and combined into a single file. This is performed by the gen_sens_mats function, so named because it generates the sensitivity matrices for each grid cell location.

Conclusion

Up to this stage, we have (hopefully) shown how the original matlab files are converted to R objects, which are then re-arranged and manipulated to a format amenable to being analyzed with the hybridSA package. For details on how the Rj values are calculated, please see the next vignette in the series.



nabilabd/hybridSA documentation built on May 23, 2017, 9:34 a.m.