Characterization of summertime coarse particulate matter in the Desert Southwest—Arizona, USA

Abstract

A year-long study was conducted in Pinal County, AZ, to characterize coarse (2.5 -10 µm aerodynamic diameter, AD) and fine (< 2.5 µm AD) particulate matter (PMc and PMf, respectively) to further understand spatial and temporal variations in ambient PM concentrations and composition in rural, arid environments. Measurements of PMc and PMf mass, ions, elements, and carbon concentrations at one-in-six day resolution were obtained at three sites within the region. Results from the summer of 2009 and specifically the local monsoon period are presented.The summer monsoon season (July -September) and associated rain and/or high wind events, has historically had the largest number of PM 10 NAAQS exceedances within a year. Rain events served to clean the atmosphere, decreasing PMc concentrations resulting in a more uniform spatial gradient among the sites. The monsoon period also is characterized by high wind events, increasing PMc mass concentrations, possibly due to increased local wind-driven soil erosion or transport. Two PM 10 NAAQS exceedances at the urban monitoring site were explained by high wind events and can likely be excluded from PM 10 compliance calculations as exceptional events. At the more rural Cowtown site, PM 10 NAAQS exceedances were more frequent, likely due to the impact from local dust sources.PM mass concentrations at the Cowtown site were typically higher than at the Pinal County Housing and Casa Grande sites. Crustal material was equal to 52-63% of the PMc mass concentration on average. High concentrations of phosphate and organic carbon found at the rural Cowtown were associated with local cattle feeding operations. A relatively high correlation between PMc and PMf (R 2 = 0.63) indicated that the lower tail of the coarse particle fraction often impacts the fine particle fraction, increasing the PMf concentrations. Therefore, reductions in PMc sources will likely also reduce PMf concentrations, which also are near the value of the 24-hr PM 2.5 NAAQS.Implications: In the desert southwest, summer monsoons are often associated with above average PM 10 (<10 µm AD) mass concentrations. Competing influences of monsoon rain and wind events showed that rain suppresses ambient concentrations while high wind increase them. In this region, the PMc fraction dominates PM 10 and crustal sources contribute 52-63% to local PMc mass concentrations on average. Cattle feedlot emissions are also an important source and a unique chemical signature was identified for this source. Observations suggest monsoon wind events alone cannot explain PM 10 NAAQS exceedances, thus requiring these values to remain in compliance calculations rather than being removed as exceptional wind events.

Source identification of coarse particles in the Desert Southwest, USA using Positive Matrix Factorization

“…This area experiences some of the highest PM 10 concentrations in the region (U.S. EPA, 2014). This paper expands on work already published from this study (Clements et al, 2013;Upadhyay et al, 2015) by detailing the PMF modeling results for PM c and PM f within the region including source profiles, source identification, and spatially resolved yearly averaged and seasonally resolved source contributions to PM f and PM c mass within the region.…”

“…After optimization based on the above criteria, the optimal number of factors was determined by PMF through trial and error including reducing and increasing the number of factors modeled. Following these initial model runs, the data set was further refined by removing four out of the 64 sampling days, that appeared to have significant outliers (i.e., greater than three standard deviations of the mean) primarily due to extreme wind-blown dust events (Clements et al, 2013). Bootstrap analysis of the number of factors was performed to provide error estimates of each profile.…”

“…The Desert Southwest Coarse Particulate Matter Study was conducted in and around the town of Casa Grande in Pinal County, Arizona (Clements et al 2013;. This area experiences some of the highest PM 10 concentrations in the region (U.S. EPA, 2014).…”

Spatial differences in ambient coarse and fine particles in the Monterrey metropolitan area, Mexico: Implications for source contribution

“…(50-70%) of PM 10 (Clements et al 2013;Sprovieri et al 2011;Sprovieri and Pirrone 2008). This is relevant for the Monterrey Metropolitan Area (MMA), a region with a semiarid climate that could suggest a relevant contribution of geological material (Al, Ca, Si, and Fe) to PM c .…”

The characterization of haboobs and the deposition of dust in Tempe, AZ from 2005 to 2014

“…The size of PM determines the extent of penetration into the respiratory tract and therefore the adverse health risk: PM 10 can penetrate to the bronchi passages while the finer and more hazardous fraction, PM 2.5 , is able to penetrate fully into the alveolar recesses of the lungs (WHO, 2006). Their mass concentrations, c(PM 10 ) and c(PM 2.5 ) respectively, both increase during haboobs; peak c(PM 10 ) can be in the thousands of µg m -3 for several hours and c(PM 2.5 ) increases although to a somewhat lesser extent (e.g., tens to hundreds of µg m -3 ; Clements et al, 2013;Lei and Wang, 2014). The U.S. National Ambient Air Quality Standards (NAAQS) are 12 µg m -3 for c(PM 2.5 ) and 150 µg m -3 for c(PM 10 ) over a 24-hour period (U.S. Environmental Protection Agency (EPA), 2013).…”