Isotopic evaluation on relative contributions of major NOx sources to nitrate of PM2.5 in Beijing.

Nitrate (NO3-) is a key component of secondary inorganic aerosols and PM2.5. However, the contributions of nitrogen oxides (NOx) emission sources to NO3- in PM2.5 remain poorly constrained. This study measured nitrogen (N) isotopes of NO3- (hereafter as δ15N-NO3-) in PM2.5 collected at Beijing in 2014. We observed that δ15N-NO3- values in PM2.5 (-2.3‰â€¯- 19.7‰; 7.3 ±â€¯5.4‰ annually) were significantly higher in winter (11.9 ±â€¯4.4‰) than in summer (2.2 ±â€¯2.5‰). The δ15N differences between source NOx and NO3- in PM2.5 (hereafter as Δ values) were estimated by a computation module as 7.8 ±â€¯2.2‰â€¯- 10.4 ±â€¯1.6‰ (8.8 ±â€¯2.4‰). Using the Δ values and δ15N values of NOx from major fossil (coal combustion, vehicle exhausts) and non-fossil sources (biomass burning, microbial N cycle), contributions of major NOx sources to NO3- in PM2.5 were further estimated by the SIAR model. We found that seasonal variations of δ15N-NO3- values in PM2.5 of Beijing were mainly caused by those of NOx contributions from coal combustion (38 ±â€¯10% in winter, 20 ±â€¯9% in summer). Annually, NOx from coal combustion, vehicle exhausts, biomass burning, and microbial N cycle contributed 28 ±â€¯12%, 29 ±â€¯17%, 27 ±â€¯15%, and 16 ±â€¯7% to NO3- in PM2.5, respectively, showing actually comparable contributions between non-fossil NOx (43 ±â€¯16%) and fossil NOx (57 ±â€¯21%). These results are useful for planning the reduction of NOx emissions in city environments and for elucidating relationships between regional NOx emissions and atmospheric NO3- pollution or deposition.

Nitrogen isotope variations of ammonium across rain events: Implications for different scavenging between ammonia and particulate ammonium.

Enhanced ammonia (NH3) emissions and deposition caused negative effects on air quality and ecosystems. Precipitation is an efficient pathway to remove NH3 and particulate ammonium (p-NH4+) from the atmosphere into ecosystems. However, precipitation scavenging of p-NH4+ in chemical transport models has often considered fine p-NH4+, with inadequate constraints on NH3 and coarse p-NH4+. Based on distinct δ15N values between NH3 and NH4+ in PM2.5 (particulate matters with aerodynamic diameters ≤ 2.5 µm) or TSP (total suspended particulates), this paper interpreted intra-event variations of precipitation NH4+ concentrations and δ15N values (δ15N-NH4+ values) at Guiyang (Xiao et al., 2015). Generally decreased NH4+ concentrations across rain events reflected decreasing scavenging of NH3 and p-NH4+. Using a Bayesian isotope mixing model, we found that differing contributions between 15N-depleted NH3 and 15N-enriched p-NH4+ were responsible for the three-stage variations of intra-event δ15N-NH4+ values. The decreases of δ15N-NH4+ values across the first and third stages indicated more decreases in scavenging p-NH4+ than NH3, while the increases of δ15N-NH4+ values across the second stage were resulted primarily from more increases in scavenging p-NH4+ (particularly fine p-NH4+) than NH3. These results stressed influences of differing scavenging between NH3 and p-NH4+ on precipitation δ15N-NH4+ values, which should be considered in modeling precipitation scavenging of atmospheric p-NH4+.

Inter-species and intra-annual variations of moss nitrogen utilization: Implications for nitrogen deposition assessment.

Moss nitrogen (N) concentrations and natural 15N abundance (δ15N values) have been widely employed to evaluate annual levels and major sources of atmospheric N deposition. However, different moss species and one-off sampling were often used among extant studies, it remains unclear whether moss N parameters differ with species and different samplings, which prevented more accurate assessment of N deposition via moss survey. Here concentrations, isotopic ratios of bulk carbon (C) and bulk N in natural epilithic mosses (Bryum argenteum, Eurohypnum leptothallum, Haplocladium microphyllum and Hypnum plumaeforme) were measured monthly from August 2006 to August 2007 at Guiyang, SW China. The H. plumaeforme had significantly (P < 0.05) lower bulk N concentrations and higher δ13C values than other species. Moss N concentrations were significantly (P < 0.05) lower in warmer months than in cooler months, while moss δ13C values exhibited an opposite pattern. The variance component analyses showed that different species contributed more variations of moss N concentrations and δ13C values than different samplings. Differently, δ15N values did not differ significantly between moss species, and its variance mainly reflected variations of assimilated N sources, with ammonium as the dominant contributor. These results unambiguously reveal the influence of inter-species and intra-annual variations of moss N utilization on N deposition assessment.

Stable isotope analyses of precipitation nitrogen sources in Guiyang, southwestern China.

To constrain sources of anthropogenic nitrogen (N) deposition is critical for effective reduction of reactive N emissions and better evaluation of N deposition effects. This study measured δ15N signatures of nitrate (NO3-), ammonium (NH4+) and total dissolved N (TDN) in precipitation at Guiyang, southwestern China and estimated contributions of dominant N sources using a Bayesian isotope mixing model. For NO3-, the contribution of non-fossil N oxides (NOx, mainly from biomass burning (24 ± 12%) and microbial N cycle (26 ± 5%)) equals that of fossil NOx, to which vehicle exhausts (31 ± 19%) contributed more than coal combustion (19 ± 9%). For NH4+, ammonia (NH3) from volatilization sources (mainly animal wastes (22 ± 12%) and fertilizers (22 ± 10%)) contributed less than NH3 from combustion sources (mainly biomass burning (17 ± 8%), vehicle exhausts (19 ± 11%) and coal combustions (19 ± 12%)). Dissolved organic N (DON) accounted for 41% in precipitation TDN deposition during the study period. Precipitation DON had higher δ15N values in cooler months (13.1‰) than in warmer months (-7.0‰), indicating the dominance of primary and secondary ON sources, respectively. These results newly underscored the importance of non-fossil NOx, fossil NH3 and organic N in precipitation N inputs of urban environments.