Source Apportionment of Ammonia at Rocky Mountain National Park using Modeled Conservative Tracer Releases

摘要

All gaseous species show pronounced diurnal variability throughout the year. Typically minimum concentrations occur during evening hours and maximum in the late afternoon. NH_3 has the highest or greatest difference ratio, defined as (maximum median - minimum median)/(maximum median) of about 0.65, while NO_x and NO_y are also very high at about 0.50. The difference ratio for PM1.0 is lower but still high at 0.40. For the most part, the use of NH_x in the receptor model yielded higher correlations and variance explained than did URG data, suggesting either sources of reduced nitrogen other than ammonia or possibly the chemiluminescence measurement has a higher precision than the URG measurement. The relative apportionment between sources is approximately the same whether URG or chemiluminescence measurements are used. Two objectives of the study were to develop an approximate understanding of the relative contribution of ammonia sources in and outside the state of Colorado to measured reduced gaseous ammonia species in RMNP and to assess whether large ammonia emissions in the agricultural area of the Midwestern portion of the United States contributed significantly to ammonia in RMNP. In answer to the first question, the analysis suggests that slightly more ammonia measured at RMNP originates from instate sources. However, within in the uncertainty of the apportionment methodology, it is fair to say that it is about a 50:50 split between in-state versus out-of-state sources. In answer to the second question, it seems pretty clear that very little of measured ammonia at the receptor site had its origins in the Midwest. The model suggests that no more than about 10% of measured ammonia originates from sources east of eastern Colorado (102.3° longitude) Sources west of Colorado contribute on the order of 35-45% of measured ammonia, with California contributing on the order of 13%, eastern Utah 8%, and the Snake River valley of southern Idaho and Arizona contributing about 5-6%. The systematic correlation between diurnally varying local easterly winds measured at the receptor site and diurnal increases in measured NHx concentrations suggests Front Range transport into the receptor area. The inability of the WRF modeled winds to faithfully reproduce these systematic diurnal patterns suggests that the receptor model may be underestimating the contributions of Front Range and northeastern Colorado ammonia emissions. With that said, the receptor model suggests that, within the state of Colorado, on the average about an equal amount of ammonia is coming from western and central Colorado, and the Front Range including the Greeley area is at about 15%. Northeastern Colorado is estimated to contribute another 6%. The Yampa River valley is estimated to contribute about 4%. Concentrations of ammonia are highest during the summer and spring months when the northern Front Range of Colorado has its largest contributions. During the fall season, northeastern Colorado along with California and the Snake River valley contribute significantly. The winter season corresponded to the lowest measured concentrations, and source regions were primarily more local.