Simulated aerosol emission and concentration

Emissions and transport of mineral dust and fly ash were simulated using the

CMAQ model, according to meteorological conditions generated from MM5 (Figure

3.4). As discussed earlier, we used iron mass fractions of 4.00% and 4.82% to convert

dust and ash emissions to iron emissions. Most (89%) of the dust iron in East Asia

originates in the desert areas of northern China and Mongolia, with the remainder (11%)

from nondesert areas including agricultural lands. Total iron emissions in this region are

26.0 Tg yr^–1from mineral dust and 7.2 Tg yr^–1from fly ash. Industrial coal-burning

accounts for 64% of total fly ash iron emissions, while residential use and power plants

contribute 27% and 9%, respectively. The large fraction originating from residential

sources appears anomalous, as only a minor fraction of total coal consumption is for

residential use (less than 14%, according to IEA 2007 statistics). This anomaly reflects

the fact that recovery devices are seldom used in the residential coal burning.

The emission and transport of mineral dust are controlled primarily by

meteorological factors. The East Asian monsoon system, in particular, determines the

emission strength and low-altitude transport, whereas the high-altitude westerlies

facilitate long-range transport in the free troposphere. Our goal was to evaluate the

deposition of atmospheric nutrient into the oceans, so we focused on the NWPO and

NSCS and excluded iron deposition on land. Figure 3.5 presents simulations of seasonal

variations in the near-surface mass concentration of mineral dust (all sizes) in spring

(MarchMay), summer (JuneAugust), autumn (SeptemberNovember), and winter

(DecemberFebruary). Due to their short lifetimes, aerosols tend not to move very far

from their geographically fixed sources. As a result, the general patterns in the four

seasons seem to be similar at a first glance. More obvious seasonal differences tend to

be observed when focusing on more distant areas, such as over open oceans. Over the

desert regions, airborne dust concentrations are generally greater in spring than in winter,

partly due to stronger synoptic activity and the reduced snow cover. Over the NWPO,

particularly the ECS, the highest dust concentrations occurred during spring. Over the

NSCS, the highest dust concentrations occurred during winter. This reflects the

characteristics of the continental cold-air outflow, which results in stronger southward

transport in winter and stronger eastward transport in spring. Dust concentrations tend

to be the lowest during summer over the land as well as the ocean, because seaward

transport of land pollutants tends to be limited under the southerly summer monsoon

winds. Average dust concentrations over the NWPO during the winter and the spring are

approximately 30 times higher than those during the summer, whereas over the NSCS

winter concentrations exceed summer concentrations by more than 2 orders of

magnitude.

Unlike mineral dust, fly ash emissions tend not to be very sensitive to seasonal

variations in the weather. As a result, seasonal variations in the concentration of fly ash

tend to be less pronounced, despite similarities in atmospheric transport and removal

conditions (Figure 3.6). Summertime concentrations over the NSCS and the NWPO are

good examples of this. Over the NWPO, fly ash concentrations are highest in the spring

and smallest in the summer, with seasonal contrast within a factor of 3. Over the NSCS,

fly ash seasonal concentrations are highest in the fall and winter, which are

approximately 10 times higher than those in the summer.

We evaluated the simulation results by comparing them with observational data.

Figure 3.7 shows the monthly mean PM₁₀concentrations at the Magong and Wanli

stations. In simulations, aerosols were divided into mineral dust, fly ash, and others

(including sulfates, nitrates, and sea salt). The fraction of mineral dust and fly ash in the

atmosphere was also estimated based on composition analysis obtained through direct

observation. Monthly variations in simulated PM₁₀resembled the seasonal trend

observed at the testing stations, with maximum values occurred in the winter and

minimum in the summer. A secondary peak occurred in spring, which was probably due

to the occurrence of dust storms in East Asia. Our simulations indicate that mineral dust

contributes a large fraction of PM₁₀in East Asia during the winter and spring, while the

contribution of fly ash was relatively small. However, the simulated summertime

concentrations of fly ash at Magong were comparable to those of mineral dust.

The absolute values of simulated PM10at Magong were in good agreement with

the observational data. The largest difference between simulation and observations

results (less than 45%) was obtained for August and September. In contrast, the

simulations at Wanli showed significant underestimation during the autumn, with a

deficit of up to 67%. These large discrepancies are likely due to inaccuracies in data

related to local emissions, specifically surf zone sea salt and secondary organic aerosols,

which are significantly lower than those reported by Chou et al. [2008; 2010].

Simulated mineral dust concentrations are in strong agreement with observed dust

concentrations, particularly at the Cape Fuguei station. Values at the Xiaomen station

were generally on par, except during summer and early autumn when the model tend to

underestimate the amount of dust. One likely cause of this underestimation is the fact

that local dust emissions from the Pescadores Islands or main island of Taiwan were not

accounted for in the simulation due to the coarse resolution of the model. Comparison

of simulated daily mean PM₁₀with measurements in 2007 had overall high correlations

(0.89 for Xiaomen Islet, 0.70 for Magong; 0.61 for Wanli, and 0.72 for Cape Fuguei; see

section 3.B), indicating that the model is overall simulating aerosol deposition in the

region well.