6 research outputs found
Implications of Organic Mass to Carbon Ratios Increasing Over Time in the Rural United States
The thermal evolution procedure used by most monitoring programs in the United States to determine carbonaceous aerosol concentrations is referred to as the thermal‐optical reflectance method, where an aerosol sample that has been collected on a quartz filter is heated and evolved carbon is characterized as either organic (OC) or light absorbing carbon (LAC). Evolved carbon assigned to OC is multiplied by a factor, Roc, to achieve an estimate of organic mass. Over the last 10–15 years, Roc, estimated through multiple linear regression analysis of data collected in the Interagency Monitoring of Protected Visual Environments (IMPROVE) program, has increased at about a rate of about 0.02 per year, reaching values above 2.0 in many regions of the United States. Analysis of evolved carbon concentration temporal trends suggests that thermal‐optical reflectance analysis, on the average, inaccurately bifurcates particulate carbon into the OC and LAC fractions with some LAC being inadvertently and wrongly assigned to the OC fraction. It is shown that misapportioned LAC assigned to OC is decreasing faster than true OC, resulting in a compensating increase in the Roc assigned to reported OC over time. A first‐order model is proposed to correct for the misapportioned carbon
Implications of Organic Mass to Carbon Ratios Increasing Over Time in the Rural United States
The thermal evolution procedure used by most monitoring programs in the United States to determine carbonaceous aerosol concentrations is referred to as the thermal‐optical reflectance method, where an aerosol sample that has been collected on a quartz filter is heated and evolved carbon is characterized as either organic (OC) or light absorbing carbon (LAC). Evolved carbon assigned to OC is multiplied by a factor, Roc, to achieve an estimate of organic mass. Over the last 10–15 years, Roc, estimated through multiple linear regression analysis of data collected in the Interagency Monitoring of Protected Visual Environments (IMPROVE) program, has increased at about a rate of about 0.02 per year, reaching values above 2.0 in many regions of the United States. Analysis of evolved carbon concentration temporal trends suggests that thermal‐optical reflectance analysis, on the average, inaccurately bifurcates particulate carbon into the OC and LAC fractions with some LAC being inadvertently and wrongly assigned to the OC fraction. It is shown that misapportioned LAC assigned to OC is decreasing faster than true OC, resulting in a compensating increase in the Roc assigned to reported OC over time. A first‐order model is proposed to correct for the misapportioned carbon
Aerosol decadal trends - Part 1: In-situ optical measurements at GAW and IMPROVE stations
Currently many ground-based atmospheric stations
include in-situ measurements of aerosol physical and
optical properties, resulting in more than 20 long-term
(>10 yr) aerosol measurement sites in the Northern Hemisphere
and Antarctica. Most of these sites are located at remote
locations and monitor the aerosol particle number concentration,
wavelength-dependent light scattering, backscattering,
and absorption coefficients. The existence of these
multi-year datasets enables the analysis of long-term trends
of these aerosol parameters, and of the derived light scattering
A° ngstro¨m exponent and backscatter fraction. Since the
aerosol variables are not normally distributed, three different
methods (the seasonal Mann-Kendall test associated with the
Sen’s slope, the generalized least squares fit associated with
an autoregressive bootstrap algorithm for confidence intervals,
and the least-mean square fit applied to logarithms of
the data) were applied to detect the long-term trends and their
magnitudes. To allow a comparison among measurement
sites, trends on the most recent 10 and 15 yr periods were calculated.
No significant trends were found for the three continental
European sites. Statistically significant trends were
found for the two European marine sites but the signs of the
trends varied with aerosol property and location. Statistically
significant decreasing trends for both scattering and absorption
coefficients (mean slope of −2.0%yr−1) were found for
most North American stations, although positive trends were
found for a few desert and high-altitude sites. The difference
in the timing of emission reduction policy for the Europe
and US continents is a likely explanation for the decreasing
trends in aerosol optical parameters found for most American
sites compared to the lack of trends observed in Europe. No
significant trends in scattering coefficient were found for the
Arctic or Antarctic stations, whereas the Arctic station had
a negative trend in absorption coefficient. The high altitude
Pacific island station of Mauna Loa presents positive trends
for both scattering and absorption coefficients
Thirty years of the Clean Air Act Amendments: Impacts on haze in remote regions of the United States (1990–2018)
Toward the improvement of total nitrogen deposition budgets in the United States
Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Program Total Deposition (NADP/TDep) data illustrates several aspects of current Nr deposition that motivate additional research. Averaged across the continental U.S., dry deposition contributes slightly more (55%) to total deposition than wet deposition and is the dominant process (>90%) over broad areas of the Southwest and other arid regions of the West. Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher degree of uncertainty relative to wet deposition which is routinely measured. As nitrogen oxide (NOx) emissions continue to decline, reduced forms of inorganic nitrogen (NHx = NH3 + NH4 +) now contribute >50% of total Nr deposition over large areas of the U.S. Expanded monitoring and additional process-level research are needed to better understand NHx deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems. Urban and suburban areas are hotspots where routine monitoring of oxidized and reduced Nr deposition is needed. Finally, deposition budgets have incomplete information about the speciation of atmospheric nitrogen; monitoring networks do not capture important forms of Nr such as organic nitrogen. Building on these themes, we detail the state of the science of Nr deposition budgets in the U.S. and highlight research priorities to improve deposition budgets in terms of monitoring and flux measurements, leaf- to regional-scale modeling, source apportionment, and characterization of deposition trends and patterns.</p