Measurement and analysis of aerosol and black carbon in the southwestern United States and Panama and their dependence on air mass origin

Abstract

[1] Total aerosol mass loading, aerosol absorption, and black carbon (BC) content were determined from aerosol collected on 598 quartz fiber filters at a remote, semiarid site near Orogrande, New Mexico from December 1989 to October 1995. Aerosol mass was determined by weighing filters before and after exposure, and aerosol absorption was determined by measuring the visible light transmitted through loaded filter samples and converting these measurements to aerosol absorption. BC content was determined by measuring visible light transmitted through filter samples before and after firing and converting the absorption to BC mass, assuming a BC absorption cross section of 19 m 2 /g in the fiber filter medium. Two analyses were then performed on each of the logged variables: an autoregressive integrating moving average (ARIMA) analysis and a decomposition analysis using an autoregressive model to accommodate first-order autocorrelation. The two analyses reveal that BC mass has no statistically significant seasonal dependence at the 5% level of significance but only random fluctuations varying around an average annual value that has a long-term decreasing trend (from 0.16 to 0.11 mg/m 3 during 1990-1995). Aerosol absorption, which is dominated by BC, also displays random fluctuations about an average value, and decreases from 1.9 Mm À1 to 1.3 Mm À1 during the same period. Unlike BC, aerosol mass at the Orogrande site displays distinctly different character. The analyses reveal a pronounced seasonal dependence, but no long-term trend for aerosol mass. The seasonal indices resulting from the autoregression analysis have a minimum in January (À0.78) and maximum in June (+0.58). The geometric mean value over the 1990-1995 period for aerosol mass is 16.0 mg/m 3 . Since BC aerosol at the Orogrande site is a product of long-range atmospheric transport, a back trajectory analysis of air masses was conducted. Back trajectory analyses indicate that air masses traversing high population centres on the west coast of the USA can lead to significant BC loading, while air masses originating from the southeast of the site tend to have less BC. Additional aerosol measurements are reported for a remote maritime site located on the Atlantic side of the former Panama Canal Zone for the 1976-1979 period, although these data have been analyzed with less statistical rigor. The average geometric mean value of aerosol mass loading for this site is 9.7 mg/m 3 , with a significant decreasing trend of À19% per year. The mean value of aerosol absorption is 0.59 Mm À1 , with a decreasing trend of À15% per year.

Fluorescence spectra and elastic scattering characteristics of atmospheric aerosol in Las Cruces, New Mexico, USA: Variability of concentrations and possible constituents and sources of particles in various spectral clusters

“…The metroplex of El Paso, Texas e Ciudad Juarez, Mexico is located 80 km to the south. In May dusty conditions are common and mean total aerosol loadings are approximately 25 mg per cubic meter (Junker et al, 2004). A somewhat heavily traveled (4-lane) auto thoroughfare (University Avenue) is located 105 m north from the sample site, which is the same as the NM site reported by Pan et al (2007).…”

Aerosol light absorption in the North Atlantic: trends and seasonal characteristics during the period 1989 to 2003

“…The logarithmic mode corresponding to continental sector conditions of σ att ≈ 3.4 Mm -¹ (equivalent to C M,BC ≈ 179 ng/m³) is comparable to C M,BC reported for remote continental sites by Junker et al (2004) and by Echalar et al (1998). …”

“…These modes emerge when the data is divided into logarithmically sized bins and are therefore referred to in this text as logarithmic modes. The logarithmic mode corresponding to marine sector conditions of σ att 0.4 Mm −1 (equivalent to C M,BC 21 ng/m 3 ) is within the range of mean concentrations found at other marine sites in Panama, Junker et al (2004) and in Korea, Kim et al (2000), see Table 1. The logarithmic mode corresponding to continental sector conditions of σ att 3.4 Mm −1 (equivalent to C M,BC 179 ng/m 3 ) is comparable to C M,BC reported for remote continental sites by Junker et al (2004) and by Echalar et al (1998).…”

Decreases in elemental carbon and fine particle mass in the United States

“…These data indicate decreases in both fine particle mass and elemental carbon (EC) over wide areas of the United States, in places by more than 50 %. Trends in EC in the US at single sites and/or shorter periods (Junker et al, 2004;Husain et al, 2008;Kirchstetter et al, 2008) as well as trends in fine mass in Canada (Hidy et al, 2010) support these decreases.…”