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

Abstract

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 m2/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 μg/m3 during 1990–1995). Aerosol absorption, which is dominated by BC, also displays random fluctuations about an average value, and decreases from 1.9 Mm−1 to 1.3 Mm−1 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 (−0.78) and maximum in June (+0.58). The geometric mean value over the 1990–1995 period for aerosol mass is 16.0 μg/m3. 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 μg/m3, with a significant decreasing trend of −19% per year. The mean value of aerosol absorption is 0.59 Mm−1, with a decreasing trend of −15% per year.