Nighttime Science Questions
- How do nighttime chemical transformations involving NO3, N2O5 and O3 influence the composition and evolution of smoke plumes? Fires are known to emit a wide array of volatile organic compounds, including highly reactive VOCs such as monoterpenes, oxygenated aromatics and furans. These compounds should undergo rapid chemical reactions with the major nighttime oxidants, NO3 and O3, but there has been little investigation of this chemistry in either a laboratory or a field setting. Fire plumes should also produce ample NOx that provides a ready source of NO3, as well as a large amount of aerosol surface area that may drive heterogeneous reactions of N2O5. Experiments at the fire laboratory could refine estimates of the emissions of these very highly reactive species. Subsequent chamber investigations could target the chemistry of individual compounds, mixtures of compounds and laboratory generated smoke. Field investigations could then examine the nighttime evolution of smoke plumes using a mobile laboratory, light aircraft or the NOAA WP-3D.
- What is the spatial distribution of fire plumes and fire emissions during day and night? A common pattern for western wildfire smoke is to accumulate in valleys overnight and during much of the day and often fires "blow up" in late afternoon/evening. Concentrated regions of emissions from these sudden intensifications are often visible in satellite images hundreds of miles downwind of their source on the following day. Nighttime dilution and chemistry will play a role in downwind impacts of these plumes. The smoke that accumulates in valleys leads to impacts and exposure closer to the sources. These varying scenarios need investigation with a combination of mobile laboratory and aircraft platforms.
- How important is nighttime chemistry for production of secondary organic aerosol and brown carbon aerosol in smoke? Reactions of O3 and NO3 with monoterpenes are a known source of secondary organic aerosol. The speciation of monoterpenes in fire emissions is weighted toward compounds such as limonene, which have higher SOA yields via reaction with these oxidants than other monoterpenes. The SOA yields via nighttime oxidation from other reactive VOCs present in fire plumes is essentially uknown. Gas phase reaction with NO3 or heterogeneous uptake of N2O5 produces nitrated compounds that may be a source of brown carbon aerosol. In addition, oxygenated organic compounds may react with ammonium in BB aerosol to form BrC. Yields and optical properties of such compounds are little studied or unknown. Nighttime conditions tend to favor smoldering combustion, which is more enriched in reduced N compounds that may be chromophores, but missed by traditional day-time sampling.
- What is the diurnal cycle of free radical and oxidant production in fire plumes, and how important are reactions with different oxidants at various times of day? Major sources of radicals in fire plumes include photolysis of HONO and CH2O, as well as production of NO3. The latter typically proceeds only at night, but may be active in fire plumes during daytime if there is sufficient attenuation of sunlight by fire smoke. Similarly, HONO and CH2O photolyze at longer UV wavelengths and in the presence of UV-absorbing BrC, may have different diurnal cycles for radical generation in fire smoke than in a polluted, urban setting.
- What are the mechanisms that lead to PAN formation in fire plumes during daytime and nighttime? Peroxy acetyl nitrates are a major component of reactive nitrogen in fire plumes. How do these species form at different times of day? How does the PAN/HNO3 ratio and the distribution of PAN analogs vary from that seen in urban plumes as a function of time of day.