Nighttime Radical NOx Chemistry by CRD
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Abstract
The following work presents measurements of NO2, NO3, N2O5, and organic nitrates (ΣPNs (R(O)O2NO2;
and ΣANs (RONO2)) made by 3 years of measurements by a 5-channel cavity ringdown spectroscopy (5-
CRD) instrument over the course of two field campaigns and multiple laboratory experiments. The results
of these two campaigns are discussed within.
The 2017 AQABA ship campaign reports measurements of NO2, NO3 and N2O5 from a mixture of highlypolluted and clean marine environments around the Arabian Peninsula and eastern Mediterranean Sea.
NO3 and N2O5 mixing ratios over the detection limits seen over 30 of a possible 60 nights during the nights,
with mixing ratios of 12 ± 18 ppt for NO3 (max: 146 ppt) and 23 ± 35 ppt (max: 295 ppt) for N2O5,
respectively. This corresponds to a median NO3 lifetime of 65.5 seconds (~0.0153 s-1
reactivity) across the
entire campaign, although this was found to be highly variable according to the region. DMS was found to
be the single largest contributor (20 – 25%) in each region to controlling the lifetime of NO3, though
reactions with anthropogenic VOCs, while generally negligible were a regionally important sink in the
Persian Gulf. Heterogeneous uptake onto the surface of particles was limited by high temperatures and
contributed only a small fraction to reactivity (~5%). Most reactivity (40 – 80%, depending on region) could
not be accounted for in the measurements made during AQABA, though an analysis within suggests that
ship emissions may contribute strongly to this and shows that SO2 can be used marker to estimate
reactivity.
Results further suggest that nighttime removal of NOx was considerably more efficient (factor 2 – 3) than
during the day in 4 out of 5 of the AQABA regions, despite longer days, high [OH] concentrations and
approximately similar NO2 loss rates with OH and O3.
The 2018 SAPHIR campaign reports measurements of NO2, NO3, N2O5, ΣPNs and ΣANs from a series of
chamber experiments studying the reaction of NO3 + isoprene. Measurements of NO2, NO3 and N2O5 are
presented in a statistical intercomparison against other instruments. NO2 measurements from three
instruments, including the 5-CRD, report strong agreement in linear regression analysis with high slopes
(> 0.9), high R2
values (> 0.9) and generally insignificant intercepts. NO3 and N2O5 comparisons report high
levels of agreement with slopes within instrument uncertainties (1.0 and 0.95); high R2
(> 0.9) and
intercepts generally consistent with limits of detection.
ΣPNs and ΣANs data show possibility of thermal dissociation of isoprene nitrates at temperatures far lower
than previously expected (beginning at 430 K), the implications of which are discussed. Gas phase yields
of RONO2 from the reaction of NO3 + isoprene were determined as 0.81 ± 0.17 when taken all together
but yields from individual experiments were found to be highly variable with significant uncertainty.