1. Preliminary Results: HONO and HNO3 from the MC/IC.
Investigation of Reactive Nitrogen Chemistry in Smoke Plumes Emitted from Western Wildfires
1Hannah R. Munro, 1Eric M. Scheuer, 1Eric W. Heim, 2Jiajue Chai, 2Wendell W. Walters, 2Meredith Hastings, 1Jack E. Dibb
1University of New Hampshire Durham NH, USA, 2Brown University Providence RI, USA
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Background:
As a part of Western Wildfire Experiments for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) 2018 we employed ground based mobile sampling to investigate the influence of western wildfires on atmospheric chemistry.
Reactive nitrogen species are of interest as they play crucial roles in photolysis driven chemistry and are precursors to several reservoir species that can be transported long distances (Figure 1).
Nitrous Acid (HONO) is an important primary emission, on the order of 5-20% of NOx emission with additional production in fresh plumes (Figure 2).
Nitric Acid (HNO3), peroxyacyl nitrates (PAN), and nitrate (p-NO3- ), are secondary emissions, formed from the interactions of nonmethane hydrocarbons (NMOC’s), NOx, water vapor and sunlight, and play important roles in smoke plume chemistry (Figure 1).
Preliminary Results: HONO and HNO3 from the MC/IC.
Figure 1: Important reactions involving reactive nitrogen species in day and night time settings
Figure 6: Complete timeseries of HONO and HNO3 starting moving south from Missoula to Challis (7th-8th) , to Mackay (8th- 9th), to North Fork (9th-12th) and to Challis (12th – 17th) Gaps in the time series are indicative of lost power or calibration time.
Figure 9: Day vs. night ratio of HONO/HNO3 for the dataset. Note the nighttime ratio is ~10:1 HONO/HNO3, and daytime ratio is much smaller
Methods:
Dual mobile and stationary sampling approaches allow targeting of specific smoke plumes while also providing power for auxiliary measurements (Figure 3).
Gas phase data was collected using a Mist Chamber Ion Chromatograph (MC/IC) at 5-minute resolution and samples were analyzed in situ.
Emissions from the Rabbit Foot Fire in the Challis-Salmon National Forest were targeted (August 6th –August 19th, 2018), via Morgan Creek Road (MCR) (Figure 4).
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Figure 7: Subset of timeseries from the 12th – 13th showing an interval of interest. Spike in HONO concentration correlates with Mobile sampling up MCR, ~3.2 kilometers from the Rabbit Foot Fire.
Figure 10: Day vs. night ratio of HONO/HNO3 from the 12th – 13th . Nighttime ratio of HONO/HNO3 is of interest due to the extremely low concentration of HNO3.
HNO3 pptv
HONO pptv
Figure 2: Concentrations of HONO and HNO3 at the onset of combustion for different fuel types. Subset of data from the FIREX 2016 tests at the Missoula MT Fire Lab.
Research Objectives:
Track the primary emissions, secondary formation pathways, and diurnal cycles of NOx ,HNO3, HONO, and p-NO3-.
Assess the relative abundances of p-NO3- , HONO, HNO3 and NOx in aged plumes and in background, regional air during no/low burning influence, and during day and night time plumes.
Figure 8: Subset of timeseries from the 14th – 15th. The first large spike in HONO is consistent with expected nighttime chemistry. The second large spike has the same pattern but occurs in full sunlight. Smaller spikes from 0:00-3:00 correspond to driving past the entrance to MCR, where all the smoke from the fire funnels down into the valley.
Figure 11: Day vs. night ratio of HONO/HNO3 from the 14th – 15th . Nighttime ratios of HONO/HNO3 seem more consistent to the expected 10:1 HONO/HNO3. Daytime ratios are very low in HONO, with a much less scattered trend than the overall trend shown in figure 9.
Figure 4: Subset of the navigational data for the mobile lab centered on Challis Idaho, and Morgan Creek Road (MCR).
Discussion:
Low HNO3 on the morning of the 13th is an interval of interest. Potentially there is a lack of O3 due to high amounts of NOx that is preventing the formation of nighttime NO3- and N2O5 and subsequent formation of HNO3 on particles.
Daytime chemistry does not seem to start immediately with sunrise in thick fresh plumes.
HONO/HNO3 for 12th-13th has two peaks at 95 and 160. HONO/HNO3 for the 14th -15th peaks at only 19.
Figure 5: Progression of Rabbit Foot Fire growth from August 3rd – August 23rd
Auxiliary Data:
Common ions from the inlet filter on the MC/IC.
CO, CO2, CH4, H2O, navigation and meteorological data from the NOAA instrument suite.
Trace Metal Filters from J. Bryce’s Lab at UNH.
NOx and NOy Data from Brown’s NOx BOX.
NH3 and NH4 from W. Walter’s denuders at Brown University.
Future Directions:
Continue to investigate MC/IC time series data.
Compare common ions to MC/IC data.
Utilize CO and CO2 measurements to calculate Modified Combustion Efficiency (MCE.)
Estimate age of smoke from meteorological data.
Figure 6: Ratio of HONO/HNO3 for burn #6 and #7 of FIREX 2016, peaks 12 minutes into the burn time ~ 93 before rapidly dropping to a ratio ~25.
Figure 3: NOAA Mobile Lab at Spike Camp taking mobile measurements ~3.2 km from the Rabbit Foot Fire.