3.1. Frequency and Categorization of Events
Of the total number of days when data were available in the time ranges in
Table 1 (
i.e., 1431–1664 depending on site), between 76 and 120 total days were characterized by some type of extreme event (
i.e., PM
10, PM
2.5, and/or fine soil) depending on the site (
Table 2). This number of days of extreme events corresponds to between 5% and 7% of the total days examined. Grand Canyon exhibited the most extreme event days (120,
i.e., 7% of its total days), which is coincident with it being one of the most recognized tourism spots in the Southwest.
Relative to the total days with extreme PM2.5 levels, Grand Canyon exhibited the highest percentage in the High EC category (47% versus 13%–25% for other sites). Of the total number of days with extreme fine soil (54–69 days depending on the site), the number of these events being linked to Asian dust ranged from 19% to 29% (i.e., 10–20 days). The total number of days with extreme events classified as Other (i.e., not High EC or fine soil events) ranged from 17 to 30 days, which represents between 21% and 33% of the total extreme days depending on the site. The fact that the highest percentage of Other days were at Phoenix (31%) and Saguaro West (33%), the most urban-impacted sites among those studied, suggests that anthropogenic pollution, including anthropogenic dust, contributes to these events. Queen Valley also reached 31%, reflective of possible impact from transported pollution from the major nearby urban center Phoenix. Between 7 and 22 days in the Other category also registered extreme values of Coarse Mass (CM = PM10 – PM2.5), supporting the possibility of influence from locally generated dust.
To gain a sense of the spatial extent of pollution registering as extreme events,
Table 3 shows how many sites experienced an extreme event for a specific pollutant category on the same day. Locally produced aerosol would not be expected to impact multiple sites at an extreme level as compared to a transported plume such as from Asia. Grand Canyon is farther removed from the other seven sites that are clustered closer in Southern Arizona, and, thus, Grand Canyon exhibits the highest number of days where an extreme event only impacted that site. Computed as a percentage of all extreme days registered for a particular pollutant type, Grand Canyon was the only site impacted out of 65%, 64%, 51%, 100%, and 84% of its extreme events for PM
10, PM
2.5, fine soil, High EC, and Other, respectively. The categories with the least number of extreme events impacting five or more sites were High EC (0 days for all sites) and Other (0–1 day depending on site). This result is thought to be due to locally generated pollution from either (i) some combination of biomass burning and anthropogenic activity (for High EC) or (ii) dust (for Other) that was not regional in nature. Fine soil events conversely impacted five or more sites on between 13 and 23 days, accounting for between 16% and 38% of all fine soil extreme events, depending on the site. Therefore, for the study region, fine soil is the pollutant type that most successfully impacts multiple sites at once at an extreme level.
The region-wide average for the PM2.5:PM10 ratio was 0.37, 0.35, and 0.23 for non-Asian dust, Asian dust, and Other-CM, respectively. Unexpectedly, the ratios for non-Asian dust events exceeded those for Asian dust events for half the sites (i.e., Chiricahua, Phoenix, Saguaro National Monument, Saguaro West). Also of interest is that non-Asian dust event averages for PM2.5:PM10 were well above 0.35 at the two sites in Tucson, Arizona (Saguaro National Monument = 0.57 ± 0.36; Saguaro West = 0.46 ± 0.26). A plausible explanation for these unexpected results is interference of background anthropogenic emissions at these urban-impacted sites in Tucson (Saguaro National Monument and Saguaro West), in addition to Phoenix which exhibited an average ratio of 0.35. The same explanation can be applied to the Fe:Ca results, which do not show a clear reduction in value for Asian dust events as compared to the more locally-relevant pollution categories for Chiricahua, Saguaro National Monument, and Saguaro West. However, the region-wide average for Fe:Ca was lowest for Asian dust (0.88), followed by non-Asian dust (0.96), and Other-CM (1.03). The values were generally low and close to the threshold value applied by past work to classify dust as purely Asian dust. These results suggest that caution should be exercised with the use of such ratios to distinguish between dust sources owing to mixing between distant and local sources.
Due to the nature of Asian dust pollution being more geographically widespread than other forms of pollution, this category registered the highest frequency of its events impacting ≥5 sites. Depending on the site, 30%–57% of extreme Asian dust events (i.e., 3–8 days) impacted ≥5 sites.
It is of interest to compare the Asian dust extreme event data to criteria used previously to distinguish Asian dust events in the study region, including mass concentration ratios of both Fe:Ca and PM
2.5:PM
10. Previous work showed that Fe:Ca ratios below 1 are considered to be 100% Asian dust and values above 2 are 100% local dust [
38]. A threshold ratio value of 0.35 for PM
2.5:PM
10 has been applied in other work to remove contamination of non-local dust sources in the study region [
39]. This ratio generally increases with dust plume age and, thus, values higher than 0.35 are assumed to be contaminated with sources such as transported Asian dust. Values between 0.15 and 0.26 are associated with soil dust emissions from human activities according to the EPA [
39].
Table 4 examines statistics associated with the two aforementioned ratios for non-Asian dust, Asian dust, and also the subset of Other extreme events that also had extreme values of CM (PM
10–PM
2.5). The latter are presumed to be due to locally generated dust.
Table 2.
Statistics associated with the number of days with extreme events observed in the date range shown in
Table 1 for each site. (NAAPS global data were unavailable for the following dates in 2001 that are omitted from categorization into Asian and non-Asian dust: *16 October 2001, 26 August 2001, 9 November 2001; **16 October 2001, 31 October 2001, 21 November 2001; ***16 October 2001, 9 November 2001; ****9 November 2001). Values in the Total category represent days with any type of extreme event (
i.e., PM
10, PM
2.5, fine soil).
Table 2.
Statistics associated with the number of days with extreme events observed in the date range shown in Table 1 for each site. (NAAPS global data were unavailable for the following dates in 2001 that are omitted from categorization into Asian and non-Asian dust: *16 October 2001, 26 August 2001, 9 November 2001; **16 October 2001, 31 October 2001, 21 November 2001; ***16 October 2001, 9 November 2001; ****9 November 2001). Values in the Total category represent days with any type of extreme event (i.e., PM10, PM2.5, fine soil).
Site Name | Total Days Data Available | Extreme Event Types |
---|
Total | PM10 | PM2.5 | Fine Soil | High EC | Non-Asian Dust | Asian Dust | Other |
---|
Chiricahua * | 1664 | 89 | 61 | 56 | 54 | 14 | 41 | 10 | 23 |
Grand Canyon | 1664 | 120 | 82 | 64 | 69 | 30 | 49 | 20 | 25 |
Organ Pipe | 1431 | 76 | 45 | 55 | 54 | 7 | 41 | 13 | 17 |
Phoenix ** | 1628 | 98 | 54 | 58 | 63 | 9 | 50 | 10 | 30 |
Queen Valley *** | 1628 | 98 | 65 | 60 | 60 | 9 | 43 | 15 | 30 |
Saguaro NM *** | 1628 | 85 | 49 | 56 | 56 | 8 | 40 | 14 | 23 |
Saguaro West **** | 1563 | 91 | 56 | 55 | 55 | 10 | 44 | 10 | 30 |
Tonto *** | 1664 | 103 | 67 | 65 | 67 | 12 | 47 | 18 | 25 |
Table 3.
Percentage breakdown (represented as fractions; i.e., 0.1 = 10%) of the extreme events for different pollutant categories in terms of how many sites registered an extreme event for a particular pollutant on the same day. Each pollution category is separated into three columns representing extreme events occurring only at that site (1), 2–4 total sites, or 5–8 total sites.
Table 3.
Percentage breakdown (represented as fractions; i.e., 0.1 = 10%) of the extreme events for different pollutant categories in terms of how many sites registered an extreme event for a particular pollutant on the same day. Each pollution category is separated into three columns representing extreme events occurring only at that site (1), 2–4 total sites, or 5–8 total sites.
Site Name | PM10 | PM2.5 | Fine Soil | High EC | Non-Asian Dust | Asian Dust | Other |
---|
1 | 2–4 | 5–8 | 1 | 2–4 | 5–8 | 1 | 2–4 | 5–8 | 1 | 2–4 | 5–8 | 1 | 2–4 | 5–8 | 1 | 2–4 | 5–8 | 1 | 2–4 | 5–8 |
---|
Chiricahua | 0.38 | 0.34 | 0.28 | 0.39 | 0.38 | 0.23 | 0.43 | 0.33 | 0.24 | 0.79 | 0.21 | 0.00 | 0.30 | 0.40 | 0.30 | 0.49 | 0.34 | 0.17 | 0.74 | 0.26 | 0.00 |
Grand Canyon | 0.65 | 0.24 | 0.11 | 0.64 | 0.33 | 0.03 | 0.51 | 0.33 | 0.16 | 1.00 | 0.00 | 0.00 | 0.50 | 0.20 | 0.30 | 0.51 | 0.39 | 0.10 | 0.84 | 0.12 | 0.04 |
Organ Pipe | 0.31 | 0.40 | 0.29 | 0.44 | 0.36 | 0.20 | 0.37 | 0.30 | 0.33 | 0.71 | 0.29 | 0.00 | 0.08 | 0.38 | 0.54 | 0.46 | 0.27 | 0.27 | 0.71 | 0.29 | 0.00 |
Phoenix | 0.39 | 0.37 | 0.24 | 0.47 | 0.34 | 0.19 | 0.41 | 0.40 | 0.19 | 0.89 | 0.11 | 0.00 | 0.00 | 0.60 | 0.40 | 0.48 | 0.38 | 0.14 | 0.83 | 0.17 | 0.00 |
Queen Valley | 0.25 | 0.45 | 0.31 | 0.27 | 0.47 | 0.27 | 0.22 | 0.40 | 0.38 | 0.56 | 0.44 | 0.00 | 0.00 | 0.47 | 0.53 | 0.30 | 0.40 | 0.30 | 0.57 | 0.40 | 0.03 |
Saguaro NM | 0.20 | 0.45 | 0.35 | 0.27 | 0.50 | 0.23 | 0.27 | 0.36 | 0.38 | 0.75 | 0.25 | 0.00 | 0.21 | 0.21 | 0.57 | 0.30 | 0.43 | 0.28 | 0.74 | 0.22 | 0.04 |
Saguaro West | 0.25 | 0.43 | 0.32 | 0.35 | 0.36 | 0.29 | 0.38 | 0.36 | 0.25 | 0.50 | 0.50 | 0.00 | 0.10 | 0.40 | 0.50 | 0.45 | 0.36 | 0.18 | 0.60 | 0.37 | 0.03 |
Tonto | 0.18 | 0.51 | 0.31 | 0.20 | 0.57 | 0.23 | 0.13 | 0.55 | 0.31 | 0.75 | 0.25 | 0.00 | 0.11 | 0.44 | 0.44 | 0.15 | 0.62 | 0.23 | 0.60 | 0.36 | 0.04 |
Table 4.
Average (±standard deviation) of two mass concentration ratios often applied to distinguish local dust from non-local dust (i.e., Asian dust). Statistics are calculated for three extreme event categories: non-Asian dust, Asian dust, and both Other and CM simultaneously.
Table 4.
Average (±standard deviation) of two mass concentration ratios often applied to distinguish local dust from non-local dust (i.e., Asian dust). Statistics are calculated for three extreme event categories: non-Asian dust, Asian dust, and both Other and CM simultaneously.
Site Name | PM10:PM2.5 | Fe:Ca |
---|
Non-Asian Dust | Asian Dust | Other and CM | Non-Asian Dust | Asian Dust | Other and CM |
---|
Chiricahua | 0.32 ± 0.17 | 0.30 ± 0.10 | 0.19 ± 0.08 | 0.82 ± 0.35 | 0.88 ± 0.24 | 0.95 ± 0.30 |
Grand Canyon | 0.38 ± 0.11 | 0.43 ± 0.11 | 0.34 ± 0.06 | 0.83 ± 0.27 | 0.79 ± 0.21 | 0.92 ± 0.19 |
Organ Pipe | 0.27 ± 0.10 | 0.39 ± 0.08 | 0.20 ± 0.09 | 0.98 ± 0.35 | 0.81 ± 0.16 | 0.96 ± 0.43 |
Phoenix | 0.35 ± 0.16 | 0.30 ± 0.08 | 0.20 ± 0.07 | 1.27 ± 0.28 | 1.02 ± 0.25 | 1.22 ± 0.26 |
Queen Valley | 0.27 ± 0.17 | 0.30 ± 0.10 | 0.18 ± 0.05 | 1.04 ± 0.35 | 0.84 ± 0.13 | 1.09 ± 0.40 |
Saguaro NM | 0.57 ± 0.36 | 0.39 ± 0.13 | 0.27 ± 0.04 | 0.79 ± 0.28 | 0.80 ± 0.15 | 0.89 ± 0.24 |
Saguaro West | 0.46 ± 0.26 | 0.31 ± 0.12 | 0.21 ± 0.06 | 0.95 ± 0.48 | 1.04 ± 0.38 | 1.02 ± 0.44 |
Tonto | 0.35 ± 0.19 | 0.38 ± 0.14 | 0.23 ± 0.08 | 1.00 ± 0.31 | 0.86 ± 0.15 | 1.16 ± 0.38 |
3.2. “Other” Events
The Other category was investigated in more detail to gain insight about the source of these extreme events (
Table 5). Between 47% and 84% of the Other events exhibited extreme PM
10 levels, which is suggestive of the presence of locally-generated coarse matter (
i.e., dust) since fine soil levels did not reach extreme levels. To gain confidence in this reasoning, the percent frequency of extreme CM days was calculated and is similar to the percent frequency of extreme PM
10 days (
i.e., within two days) with the exception of Grand Canyon and Phoenix, which had nine and seven fewer extreme CM days as compared to PM
10, respectively. Since the ratio of extreme CM:Other days ranges from 41% (Organ Pipe) to as high as 74% (Chiricahua), with the average among all sites being 56%, locally generated CM (
i.e., dust) accounted for a significant amount of the Other events.
PM
2.5 levels reached extreme levels in 30%–76% of the Other extreme events, with Organ Pipe being the only site with a higher percentage for PM
2.5 being extreme
versus PM
10. The PM
2.5 constituents only reached extreme levels in an average of 10% (OC), 11% (K), 17% (nitrate), and 22% (sulfate) of the Other events. Among these four PM
2.5 constituents, sulfate reached extreme levels in 48% and 35% of the Other events in Tonto and Organ Pipe, respectively, which were the highest values among all species and sites. This is likely due to anthropogenic emissions near those two sites such as from smelting [
40,
41,
42]. Between 0% and 28% of Other events exhibited extreme levels of nitrate, OC, and potassium, which are all associated with wintertime pollution and fine soil emissions. These relatively low percentages for PM
2.5 constituents are consistent with the majority of the Other events being due to CM.
Table 5.
Percentage frequency summary (represented as fractions; i.e., 0.1 = 10%) of how many of the Other events at each site exhibited extreme levels of PM10, PM2.5, coarse mass (CM = PM10 - PM2.5) and individual PM2.5 constituents (potassium, organic carbon, nitrate, sulfate).
Table 5.
Percentage frequency summary (represented as fractions; i.e., 0.1 = 10%) of how many of the Other events at each site exhibited extreme levels of PM10, PM2.5, coarse mass (CM = PM10 - PM2.5) and individual PM2.5 constituents (potassium, organic carbon, nitrate, sulfate).
Species | Chiricahua (N = 23) | Grand Canyon (N = 25) | Organ Pipe (N = 17) | Phoenix (N = 30) | Queen Valley (N = 30) | Saguaro NM (N = 23) | Tonto (N = 25) |
---|
PM10 | 0.78 | 0.84 | 0.47 | 0.70 | 0.63 | 0.65 | 0.73 |
PM2.5 | 0.30 | 0.52 | 0.76 | 0.50 | 0.43 | 0.43 | 0.30 |
CM | 0.74 | 0.48 | 0.41 | 0.47 | 0.63 | 0.57 | 0.73 |
K | 0.09 | 0.24 | 0.12 | 0.17 | 0.10 | 0.09 | 0.03 |
OC | 0.04 | 0.04 | 0.12 | 0.23 | 0.13 | 0.00 | 0.07 |
NO3− | 0.13 | 0.20 | 0.18 | 0.17 | 0.10 | 0.17 | 0.10 |
SO42− | 0.17 | 0.08 | 0.35 | 0.07 | 0.20 | 0.26 | 0.13 |
3.3. Temporal Nature of Events
Figure 2 displays the monthly distribution of cumulative (
i.e., summed for all years and sites) extreme events broken into the various pollutant categories. The month of August experienced the highest number of extreme events in the study region for PM
10, PM
2.5, fine soil, non-Asian dust, High EC, and Other (also had an equal peak in June). In contrast to all other pollutant categories, Asian dust events mainly occurred in the spring months of March–June (41 out of 42 days,
i.e., 98%) with only one event in February. The Other category exhibited a relatively constant amount in each month (12–16 days). Unlike PM
10, PM
2.5 exhibited a secondary mode in the winter month of January, driven mostly by High EC and Other events, suggestive of the importance of anthropogenic emissions and biomass burning, and secondary production of aerosol species that are favorably produced in wintertime conditions such as ammonium nitrate.
Figure 3 represents the interannual distribution of extreme events for different pollutant categories. It is cautioned that the time range with full years of data at all eight sites is from 2003 to 2013 (refer to
Table 1 for data time ranges for each site). All categories exhibited the most events in either 2002 or 2003 with the exception of Asian dust which reached 10 events in 2007 and followed a distinctly different temporal pattern than all other categories due to its distant source. An interesting feature of
Figure 3 is the cyclical pattern of there being a peak every few years for PM
10, PM
2.5, fine soil, non-Asian dust, and Other, specifically in the years 2002–2003, 2006–2007, 2009, and 2011–2012. It is unclear with the dataset as to what explains these recurring peaks, and future work is warranted, with a longer term record, to identify what an explanation could be for these features in the data.
Figure 2.
Monthly distribution of extreme events (cumulative for all years and sites) for different pollution categories.
Figure 2.
Monthly distribution of extreme events (cumulative for all years and sites) for different pollution categories.
Figure 3.
Time series of total extreme events as a function of year for different pollution categories. It is cautioned that the time range with full years of data at all eight sites is from 2003 to 2013 (refer to
Table 1 for data time ranges for each site).
Figure 3.
Time series of total extreme events as a function of year for different pollution categories. It is cautioned that the time range with full years of data at all eight sites is from 2003 to 2013 (refer to
Table 1 for data time ranges for each site).
A simple linear regression was used to obtain the best-fit line for each pollutant type in
Figure 3 using data between 2003 and 2013 when data were available for all sites for full years. Most all slopes were negative except for the Other category, which was only barely positive. The slopes, reported in units of number of events per year, and p values (in parenthesis) are as follows: PM
10 = −0.19 (0.80), PM
2.5 = −1.13 (0.23), fine soil = −0.95 (0.29), High EC = −0.83 (0.03), non-Asian dust = −0.85 (0.25), Asian dust = −0.11 (0.76), Other = 0.03 (0.97). The only statistically significant trend at 95% confidence was for the High EC category. This is thought to be due to reduced anthropogenic emissions since other work for the study region examining 2005–2009 has shown that the fastest rate of decline in EC levels was in Phoenix [
43], which is the most populated area. Another study analyzing IMPROVE data between 1990 and 2004 across the United States, including the Southwest, showed that there has been a ~25% reduction in EC attributed mostly to emissions controls, with the reduction being most dramatic in the winter as compared to summer [
44].
The distribution of extreme events across days of the week is of interest for a few reasons. For example, EC rooted in anthropogenic emissions is thought to lead to higher concentrations around Thursday with minimum values on the weekend [
44], and thus examining the frequency of High EC events as a function of the day of the week could help determine if the source of these events is anthropogenic in nature
versus biomass burning. When normalized by total number of days on either the weekend (Saturday-Sunday) or weekday (Monday-Friday), High EC events occurred more frequently during weekdays (13.2
versus 10.5). All other categories exhibited more events during weekdays too. No air pollutant category exhibited a statistically significant difference in the number of events (normalized by number of either weekend or weekday days) on either the weekend or weekdays (or on any specific day) using a chi-square statistical test at the 95% confidence level. The day of the week with the most extreme events for the various pollutant categories was either Wednesday or Thursday.
Table 6.
Day of week distribution of extreme events combining data from all eight sites over the entire time duration of the study. The number of extreme events on weekends and weekdays are shown with the values in parenthesis being the normalized values relative to the total number of weekend (2) and weekday (5) days. The day of the week with the most and least events are also shown with values in parenthesis being the actual number of occurrences on that particular day.
Table 6.
Day of week distribution of extreme events combining data from all eight sites over the entire time duration of the study. The number of extreme events on weekends and weekdays are shown with the values in parenthesis being the normalized values relative to the total number of weekend (2) and weekday (5) days. The day of the week with the most and least events are also shown with values in parenthesis being the actual number of occurrences on that particular day.
-- | Total | PM10 | PM2.5 | Fine Soil | High EC | Non-Asian Dust | Asian Dust | Other |
---|
Weekend (Saturday–Sunday) | 57 (28.5) | 65 (32.5) | 60 (30) | 21 (10.5) | 52 (26) | 9 (4.5) | 44 (22) | 57 (28.5) |
Weekday (Monday–Friday) | 201 (40.2) | 208 (41.6) | 197 (39.4) | 66 (13.2) | 162 (32.4) | 33 (6.6) | 122 (24.4) | 201 (40.2) |
Day With Most Events | W (53) | Th (55) | W/Th (50/50) | Th (19) | Th (42) | W (11) | W (35) | W (53) |
Day with Least Events | Tu (23) | Tu (25) | Tu (21) | M/F (11) | Tu (19) | Tu (1) | Tu (11) | Tu (23) |